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1
Eizadjou, Mehdi. "Design of Advanced High Strength Steels." Thesis, The University of Sydney, 2017. http://hdl.handle.net/2123/17315.
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A new advanced high strength steels (AHSS) is designed based on Fe-C-Mn-Al composition. Martensitic steel is processed in intercritical region to achieve an ultrafine-grained duplex γ–(α + α') microstructure. The focus was on tuning the degree of austenite plasticity via controlling its stability, called austenite engineering. Interest in austenite engineering stems from transformation-induced plasticity (TRIP) effect, which is known to enhance ductility. The thermodynamic and kinetic analyses were used to optimize the annealing condition. The evolution of microstructure and mechanical properties was studied using different techniques. Due to high heating rate, the austenite reversion occurred before recrystallization of the ferrite. The final microstructure was duplex steel with globular-shaped grains. High volume fraction of the austenite phase was obtained (f_γ>40%) in very short time annealing. By increasing annealing temperature and time, austenite fraction and grain size increased. However, due to dilution of the austenite from stabilizers elements, the stability of the austenite dropped and transformed into martensite during quenching. This led in variety of austenite stabilities that resulted in different combination of mechanical properties. The critical factors influencing the onset of TRIP effect is studied and it was found that both early and delayed onset of the TRIP effect will lead to worse ductility. Hence, to achieve ultrahigh strength and excellent ductility, austenite stability shall be controlled to precisely trigger out TRIP. This study find out that discontinuous yielding or Lüders bands phenomenon can be used in ultrafine duplex steels to improve ductility. The results showed that superb combination of strength (σ_YS>1.0GPa and σ_UTS>1.4GPa) and ductility (ε_t≥20%) could be achieved in short time annealing of less than 10 minutes. This work evidence that tuning the austenite to a marginal stability enables us to design strong and ductile steels.
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Sarma, Abhijit. "High strain properties of advanced high strength spot welded steels." Diss., Columbia, Mo. : University of Missouri-Columbia, 2007. http://hdl.handle.net/10355/5997.
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Thesis (M.S.)--University of Missouri-Columbia, 2007.The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on April 14, 2008) Includes bibliographical references.
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Thompson, Alan. "High Strain Rate Characterization of Advanced High Strength Steels." Thesis, University of Waterloo, 2006. http://hdl.handle.net/10012/2831.
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The current research has considered the characterization of the high strain rate constitutive response of three steels: a drawing quality steel (DDQ), a high strength low alloy steel (HSLA350), and a dual phase steel (DP600). The stress-strain response of these steels were measured at seven strain rates between 0. 003 s-1 and 1500 s-1 (0. 003, 0. 1, 30, 100, 500, 1000, and 1500 s-1) and temperatures of 21, 150, and 300 °C. In addition, the steels were tested in both the undeformed sheet condition and the as-formed tube condition, so that tube forming effects could be identified. After the experiments were performed, the parameters of the Johnson-Cook and Zerilli-Armstrong constitutive models were fit to the results. In order to determine the response of the steels at strain rates of 30 and 100 s-1, an intermediate rate tensile experiment was developed as part of this research using an instrumented falling weight impact facility (IFWI). An Instron tensile apparatus was used to perform the experiments at lower strain rates and a tensile split-Hopkinson bar was used to perform the experiments at strain rates above 500 s-1
A positive strain rate sensitivity was observed for each of the steels. It was found that, as the nominal strength of the steel increased, the strain rate sensitivity decreased. For an increase in strain rate from 0. 003 to 100 s-1, the corresponding increase in strength at 10% strain was found to be approximately 170, 130, and 110 MPa for DDQ, HSLA350, and DP600, respectively.
The thermal sensitivity was obtained for each steel as well, however no correlation was seen between strength and thermal sensitivity. For a rise in temperature from 21 to 300 °C, the loss in strength at 10% strain was found to be 200, 225, and 195 MPa for DDQ, HSLA350, and DP600, respectively for the 6 o?clock tube specimens.
For all of the alloys, a difference in the stress ? strain behaviour was seen between the sheet and tube specimens due to the plastic work that was imparted during forming of the tube. For the DP600, the plastic work only affected the work-hardening response.
It was found that both the HSLA350 and DDQ sheet specimens exhibited an upper/lower yield stress that was amplified as the strain rate increased. Consequently the actual strength at 30 and 100 s-1 was obscured and the data at strain rates above 500 s-1 to be unusable for constitutive modeling. This effect was not observed in any of the tube specimens or the DP600 sheet specimens
For each of the steels, both the Johnson-Cook and Zerilli-Armstrong models fit the experimental data well; however, the Zerilli-Armstrong fit was slightly more accurate. Numerical models of the IFWI and the TSHB tests were created to assess whether the experimental results could be reproduced using the constitutive fits. Both numerical models confirmed that the constitutive fits were applied correctly.
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Qu, Hao. "ADVANCED HIGH STRENGTH STEEL THROUGH PARAEQUILIBRIUM CARBON PARTITIONING AND AUSTENITE STABILIZATION." Case Western Reserve University School of Graduate Studies / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=case1346250505.
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Qu, Hao. "Advanced High Strength Steel Through Paraequilibrium Carbon Partitioning and Austenite Stabilization." Case Western Reserve University School of Graduate Studies / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=case1283353953.
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Wang, Yueyue. "Theoretical experiment of GISSMO failure model for Advanced High Strength Steel." Thesis, Högskolan Väst, Avdelningen för produktionssystem (PS), 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:hv:diva-11658.
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When developing an electric vehicle, it is essential to evaluate the deformation in and around the battery box for different crash scenarios, and it is necessary to develop a more advanced model that would take into account all the stress modes. Thanks to the excellent properties of Advanced High Strength Steel (AHSS) combine with high strength for more safety and weight reduction for less exhaust emission, AHSS is more and more commonly used in automobile industry. The material employed in this project is DOCOL 900M and it is a martensitic steel with yield strength higher than 700MPa. The focus of the current work is to describe the experimental setup for the GISSMO model used in LS-DYNA. A number of experimental methods and theories have been reviewed. Different geometries of the test specimens under different stress triaxialities have been discussed. The study also compares the accuracy and robustness of each of the testing methods and setups. The effect of anisotropy of materials on the mechanical properties was studied. Some summaries about how to reduce errors in the experiment under the conditions of low costing and high efficiency have been discussed. According to the stress-strain response of ductile materials, the parameters of plasticity model can be calibrated. The material can be implemented in finite element software to calibrate the parameters of damage and the prediction of material failure can be achieved. The experiment and simulation are always good to be used together in the research.
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Grantab, Rassin. "Interaction Between Forming and Crashworthiness of Advanced High Strength Steel S-Rails." Thesis, University of Waterloo, 2006. http://hdl.handle.net/10012/2882.
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This thesis presents the results of experimental and numerical investigations carried out to assess the effects of tube bending and hydroforming on the crash performance of s-rail structures manufactured from three different advanced high strength steels, namely DDQ, HSLA350, and DP600. The main impetus for this project is to reduce vehicle weight through material substitution and, in order to do so, the effects of material strength on crashworthiness, as well as the interaction between forming processes and crash response must be well understood. To this end, in the current research, s-rails were fabricated through tube bending and hydroforming experiments conducted on DDQ, HSLA350, and DP600 steels with a nominal wall thickness of 1. 8mm, as well as HSLA350 steel with a nominal wall thickness of 1. 5mm. Impact experiments were subsequently performed on non-hydroformed and hydroformed s-rails to examine the effects of the forming processes and material substitution on the crushing loads and levels of absorbed energy. All forming and crash experiments were simulated using numerical finite element methods which provide additional insight into various aspects of the crash response of these structures. In particular, crash simulations were used to show the effects of work-hardening, material thickness changes, and residual stresses incurred during the forming operations. The numerical tube bending simulations accurately predict the results of the tube bending and hydroforming processes for all materials, particularly for the DP600; the predictions for the DDQ material are the least accurate. Both simulations and experiments show that material thinning occurs on the tensile side of the bend, and material thickening on the compressive side of the bend; the level of thickness change is unaffected by material strength or initial material thickness. The low-pressure hydroforming process does not greatly affect the thickness and strain distributions of s-rails.
The crash simulations provide predictions that are in excellent accord with the measured results, with a maximum error of ±10% in the peak loads and energies; simulations of DP600 s-rails are the most accurate, while simulations of DDQ s-rails are the least accurate. Through simulations and experiments, it is shown that material thickness has the greatest effect on the crash performance of s-rail structures, while material strength plays a secondary role. A 20% increase in the wall thickness of HSLA350 s-rails amounts to a 47% increase in energy absorption. Substituting HSLA350 and DP600 steels in place of DDQ steel leads to increases in energy absorption of 31% and 64%, respectively, for corresponding increases in strength of 30% and 76%. Neglecting material strain-rate effects in the numerical models lowers the predicted peak loads and energies by roughly 15%. By performing a numerical parametric study, it is determined that a weight reduction of 22% is possible by substituting thinner-gauge DP600 s-rails in place of DDQ s-rails while maintaining the energy absorption of the structures.
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Kim, Hyunok. "Prediction and elimination of galling in forming galvanized advanced high strength steels (AHSS)." Columbus, Ohio : Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1204515296.
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Hanhold, Brian J. "Weldability Investigations of Advanced High Strength Steels Produced by Flash Processing." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1337795659.
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Keating, Elspeth. "Lightweighting of stiffness critical advanced high strength steel structures using fibre reinforced plastics." Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/89185/.
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In the drive for lightweighting in many industries, optimum material selection is at the forefront of research. Many solutions are being investigated, including the fabrication of multi-material components. Following a state of the art review of the literature, it has been shown that there is an opportunity to improve basic knowledge and understanding of the characteristics of hybrid steel-FRP materials for lightweight applications. This dissertation explores the potential for designing lightweight automotive steel structures through novel use of lower gauges combined with local reinforcement by fibre-reinforced plastics to achieve desired stiffness performances. The main focus of the work is to provide underpinning research to enable the further understanding of the stiffness performance of hybrid steel-FRP materials, both experimentally and in simulation. This thesis focuses on the characterisation of high strength automotive grade steel (DP600) reinforced with a fibre reinforced polyamide (PA6 GF60) laminate, however, the results are readily applicable for other combinations. The project was achieved through two main phases; each phase consisting of an iteration loop between experimentation and simulation validations. Initial characterisation was achieved using coupon samples in quasi-static three-point bend, cross-validated in simulation providing a trusted material model. Correlating experimental and simulated results showed a potential lightweighting of up to 30 % of a hybrid DP600-GFRP over a DP600 counterpart with a matched stiffness performance. Further characterisation was performed using an idealised automotive component in flexure, confirming a potential lightweighting of up to 30 %. The simulation investigation demonstrated the effect of localised hybrid reinforcements, and identified difficulties in predicting the local geometrical effects of plastic hinging. For an overall application to an automotive body-in-white, these would require further investigating. This thesis has proven that downgauging steel whilst locally reinforcing (intelligent deployment) with FRP patches provides a significant lightweight solution with a matched stiffness performance. A hybrid material model has been validated and the application to an automotive component investigated. This work provides the basic understanding for a direct application in lightweight automotive designs using computer aided engineering (CAE).
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Allen, Quentin Scott. "Microstructural Evaluation of Hydrogen Embrittlement and Successive Recovery in Advanced High Strength Steel." BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6617.
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Advanced high strength steels (AHSS) have high susceptibility to hydrogen embrittlement, and are often exposed to hydrogen environments in processing. In order to study the embrittlement and recovery of steel, tensile tests were conducted on two different types of AHSS over time after hydrogen charging. Concentration measurements and hydrogen microprinting were carried out at the same time steps to visualize the hydrogen behavior during recovery. The diffusible hydrogen concentration was found to decay exponentially, and equations were found for the two types of steel. Hydrogen concentration decay rates were calculated to be -0.355 /hr in TBF steel, and -0.225 /hr in DP. Hydrogen concentration thresholds for embrittlement were found to be 1.04 mL/100 g for TBF steel, and 0.87 mL/100g for DP steel. TBF steel is predicted to recover from embrittlement within 4.1 hours, compared to 7.2 hours in DP steel. A two-factor method of evaluating recovery from embrittlement, requiring hydrogen concentration threshold and decay rate, is explained for use in predicting recovery after exposure to hydrogen. Anisotropic hydrogen diffusion rates were also observed on the surface of both steels for a short time after charging, as hydrogen left the surface through <001> and <101> grains faster than grains with <111> orientations. This could be explained by differences in surface energies between the different orientations.
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Hedman, Daniel. "Casting and Characterization of Advanced High Strength Steels." Thesis, Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-81098.
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The Latin American steel making company Ternium S.A. aims to develop and produce a new type of advanced high strength steel (AHSS) in which the main alloying elements are carbon, aluminium, manganese, and silicon. The present work is the first phase of the development project and it involves casting and characterization of four steel compositions with varying amounts of the aforementioned elements. The results revealed that the Mn-content had a large impact on the development of hard phases during solidification. A steel with a Mn-content of 2 %wt. had almost completely transformed to pearlite during cooling, while a steel with a 4 %wt. Mn-content consisted of primarily martensite and retained austenite. Only the impact of the Mn-content is evaluated. The columnar grain size for two of the four steel compositions were in the range of 20-30 mm, which is similar to those observed from continuous casting. This indicate that the heat transfer rate was slow enough to allow these grains to grow. Measurements during casting showed an initial cooling rate of 10-20°C/min at a distance of 10 mm inside the ingot, which is much slower than the surface cooling rate during continuous casting (100-150°C/min). It was assumed that the cooling rate was similar for all castings since the methodology was identical. However, the steel used for cooling rate measurements was not characterized, why a correlation between cooling rate and composition could not be obtained. A heat transfer model was developed to gain further knowledge of the solidification process. As a reference to the heat transfer model, a eutectic Bi-42Sn alloy was cast with temperaturemonitoring using a casting setup identical to that of the steel castings. A similar cooling rate tothe Bi-42Sn reference casting was obtained where the cooling was faster from above of the ingot than below. Thus, the last part of the metal to solidify during the simulation was situated in the lower half of the ingot. This provides a model for testing future steel compositions.
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Kim, Kyungbo. "Evaluation of Bendability of Advanced High Strength Steels." The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1244008200.
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Srinivasan, Ganapathy. "Flanging and Bending of Advanced High Strength Steels." The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1408716079.
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Karki, Utsab. "Experimental and Numerical Study of High-Speed Friction Stir Spot Welding of Advanced High-Strength Steel." BYU ScholarsArchive, 2015. https://scholarsarchive.byu.edu/etd/5521.
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With the desire to lighten the frame while keeping or increasing the strength, Advanced High-Strength Steels (AHSS) have been developed for use in the automotive industry. AHSS meet many vehicle functional requirements because of their excellent strength and acceptable ductility. But joining AHSS is a challenge, because weldability is lower than that of mild steels. Friction stir spot welding (FSSW) is a solid state joining process that can provide a solution to the weldability issues in AHSS, but FSSW has not been studied in great detail for this application. In this work, Si3N4 tools were used for FSSW experiments on DP 980 steel with 1.2mm thickness. Joint strength was measured by lap shear tension testing, while thermocouples were used for the temperature measurements. A finite element model was developed in order to predict material flow and temperatures associated with FSSW. Since a 3D model of the process is very time consuming, a novel 2D model was developed for this study. An updated Lagrangian scheme was employed to predict the flow of sheet material, subjected to the boundary conditions of the fixed backing plate and descending rotating tool. Heat generation by friction was computed by including the rotational velocity component from the tool in the thermal boundary conditions. Material flow was calculated from a velocity field while an isotropic, viscoplastic Norton-Hoff law was used to compute the material flow stress as a function of temperature, strain and strain rate. Shear stress at the tool/sheet interface was computed using the viscoplastic friction law. The model predicted welding temperatures to within 4% of the experiments. The welding loads were significantly over predicted. Comparison with a 3D model of FSSW showed that frictional heating and the proportion of total heat generated by friction were similar. The position of the joint interface was reasonably well predicted compared to experiment.
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Chen, Meng-Hsien. "A STUDY OF SELECTIVE SURFACE AND INTERNAL OXIDATION OF ADVANCED HIGH STRENGTH STEEL GRADES." Case Western Reserve University School of Graduate Studies / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=case1401380512.
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Squires, Lile P. "Friction Bit Joining of Dissimilar Combinations of Advanced High-Strength Steel and Aluminum Alloys." BYU ScholarsArchive, 2014. https://scholarsarchive.byu.edu/etd/4104.
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Friction bit joining (FBJ) is a new method that enables lightweight metal to be joined to advanced high-strength steels. Weight reduction through the use of advanced high-strength materials is necessary in the automotive industry, as well as other markets, where weight savings are increasingly emphasized in pursuit of fuel efficiency. The purpose of this research is twofold: (1) to understand the influence that process parameters such as bit design, material type and machine commands have on the consistency and strength of friction bit joints in dissimilar metal alloys; and (2) to pioneer machine and bit configurations that would aid commercial, automated application of the system. Rotary broaching was established as an effective bit production method, pointing towards cold heading and other forming methods in commercial production. Bit hardness equal to the base material was found to be highly critical for strong welds. Bit geometry was found to contribute significantly as well, with weld strength increasing with larger bit shaft diameter. Solid bit heads are also desirable from both a metallurgical and industry standpoint. Cutting features are necessary for flat welds and allow multiple material types to be joined to advanced high-strength steel. Parameters for driving the bit were established and relationships identified. Greater surface area of contact between the bit and the driver was shown to aid in weld consistency. Microstructure changes resulting from the weld process were characterized and showed a transition zone between the bit head and the bit shaft where bit hardness was significantly increased. This zone is frequently the location of fracture modes. Fatigue testing showed the ability of FBJ to resist constant stress cycles, with the joined aluminum failing prior to the FBJ fusion bond in all cases. Corrosion testing established the use of adhesive to be an effective method for reducing galvanic corrosion and also for protecting the weld from oxidation reactions.
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Laarich, Abdellatif. "Designing a Heat Treatment to Achieve Ductile Advanced High Strength Steels." Thesis, Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-79754.
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Heat treatment is a way to significantly change materials properties. When presented with materials that lack certain mechanical properties, it is possible to change its chemical properties and microstructures by applying heat. This can help achieve better yield strength, ductility and toughness. This project discusses the effects of multiple distinct heat treatment methods for several materials in order to improve ductility and elongation without diminishing strength. The materials in question are High Aluminum Steel and Strenx 700MC steel, the first being under development and the second being a commercially available steel. These steels show promise to be used as high ductility, high strength, and 3rd generation steels. The heat treatments can change the mechanical proprieties of the base materials in order to optimize these steels for applications in vertical access solutions. The heat treatments in this project were Quenching and Partitioning (QP), Quenching and Tempering (QT), Austempering (AUST), Intercritical Heat Treatment (IHT) and other usual heat treatments such as Double normalizing (D-Norm). First, the most beneficial type of the above mentioned heat treatments was selected for each steel and series of heat treatments were performed in order to identify and optimize the best method for each steel. Then, heat treated samples underwent a series of tests to numerically quantify their properties and compare them to the existing steels in Alimak’s applications. The results show that Quenching and Partitioning is the most promising heat treatment for optimizing strength and ductility in High Aluminum Steel, with elongation values up to 19% together with yield strengths of 700 MPa. For Strenx 700MC a combination of temperature and time was found that gave an elongation of above 25% with a yield strength of 450 MPa. The explanation for the good properties was partly grain refinement and phase transformations during heat treatments.
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Cluff, Stephen Roy. "Characterization and Modeling of the Martensite Transformation in Advanced High-Strength Steels." BYU ScholarsArchive, 2019. https://scholarsarchive.byu.edu/etd/9051.
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Multiple studies on the microstructures of advanced high-strength steels are presented here that seek to add to the already substantial body of knowledge on martensite in steel. These studies seek to gain additional insight into the role that the martensite transformation has on the observed mechanical properties of modern steels. Crystallographic Reconstruction of Parent Austenite Twin Boundaries in a Lath Martensitic Steel The study of post-transformation microstructures and their properties can be greatly enhanced by studying their dependence on the grain boundary content of parent microstructures. Recent work has extended the crystallographic reconstruction of parent austenite in steels to include the reconstruction of special boundaries, such as annealing twins. These reconstructions present unique challenges, as twinned austenite grains share a subset of possible daughter variant orientations. This gives rise to regions of ambiguity in a reconstruction. A technique for the reconstruction of twin boundaries is presented here that is capable of reconstructing 60 degree <1 1 1> twins, even in the case where twin regions are comprised entirely of variants that are common between the twin and the parent. This technique is demonstrated in the reconstruction of lath martensitic steels. The reconstruction method utilizes a delayed decision-making approach, where a chosen orientation relationship is used to define all possible groupings of daughter grains into possible parents before divisive decisions are made. These overlapping, inclusive groupings (called clusters) are compared to each other individually using their calculated parent austenite orientations and the topographical nature of the overlapping region. These comparisons are used to uncover possible locations of twin boundaries present in the parent austenite. This technique can be applied to future studies on the dependence of post-transformation microstructures on the special grain boundary content of parent microstructures. Coupling Kinetic Monte Carlo and Implicit Finite Element Methods for Predicting the Strain Path Sensitivity of the Mechanically Induced Martensite Transformation The kinetic Monte Carlo method is coupled with a finite-element solver to simulate the nucleation of martensite inside the retained austenite regions of a TRIP (transformation induced plasticity) assisted steel. Nucleation kinetics are expressed as a function of load path and kinematic coupling between retained austenite regions. The model for martensite nucleation incorporates known elements of the kinetics and crystallography of martensite. The dependence of martensite transformation on load path is simulated and compared to published experimental results. The differences in transformation rates of retained austenite are shown to depend on load path through the Magee effect. The effects of average nearest neighbor distance between austenite grains is shown to affect the rate at which martensite nucleates differently depending on load path. Ductility and Strain Localization of Advanced High-Strength Steel in the Presence of a Sheared Edge The localization of strain in the microstructures of DP 980 and TBF 980 is quantified and compared. Of particular interest is the difference in final elongation observed for both materials in the presence of a sheared edge. Scanning electron micrographs of etched microstructures near the sheared edge are gathered for both materials at varying amounts of macroscopic strain. These micrographs are used to generate strain maps using digital image correlation. A two point statistical measure for strain localization is developed that utilizes strain map data to quantify the degree to which strain localizes around the hard phase of both materials. The DP steel exhibits higher strain localization around the martensite phase. Reasons for differences in strain localization and shear banding between the two materials are suggested, and the role played by the mechanically induced martensite transformation is speculated.
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Mallick, Dwaipayan. "Hydrogen behavior in first and second generation of advanced high strength steels." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEI052.
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Les aciers AHSS sont largement utilisés pour les caisses des véhicules, en raison de leurs bonnes propriétés mécaniques et de leurs capacités à réduire l'empreinte carbone. Toutefois, leur utilisation est limitée par leur sensibilité à la fragilisation par l'hydrogène (HE). La présente étude vise à comprendre l'influence de l'hydrogène sur quatre aciers AHSS : deux biphasés (DP), un phase complexe (CP) et un à plasticité induite (TWIP). Les résultats montrent une forte susceptibilité à l’HE pour les aciers DP et TWIP par rapport aux aciers CP. Le comportement de CP est attribuée à une microstructure plus homogène, une densité de piège plus petite (mais à énergie élevé) et une concentration en H plus faible. Dans les aciers DP, la forte densité de pièges à faible énergie et la forte absorption de H augmentent la susceptibilité à l’HE. Les dislocations et les joints de grains sont les principaux sites de piégeage pour tous ces aciers, ainsi que la cémentite dans les aciers CP et les particules AlN et l’austénite pour les aciers TWIP. Sous chargement mécanique, la désorption de l'hydrogène s’accélère avec l'expansion du réseau cristallin et les mouvements des dislocations (jusqu'à la limite d'élasticité), alors qu'elle diminue en raison de la génération de défauts dans la domaine plastique. Pour l'acier CP, l'hydrogène piégé fortement désorbe à l'UTS alors que dans l'acier TWIP, la génération de défauts libère l'hydrogène. Pour l’acier DP galvanisés, la couche de Zn se comporte une couche barrière à l’hydrogène sous polarisation fortement cathodiques, tandis qu'à potentiels cathodique plus faible, elle favorise la perméation de l'hydrogène en raison de sa dissolutionAdvanced High Strength Steels (AHSS) are increasingly used as fabrication material for vehicle Body In White (BIW), owing to their superior properties and ability to reduce carbon footprint. However, its susceptibility to hydrogen embrittlement (HE) restricts the use of AHSS. The present study aims to understand the H influence on four commercial-grade AHSS steels, two Dual Phase (DP), one Complex Phase (CP), and one Twinning Induced Plasticity (TWIP) steel. Results show high HE susceptibility for DP and TWIP steel compared to CP steel. The superior HE resistance in CP steel was attributed to a more homogeneous microstructure, smaller yet stronger trap density, and lower H concentration. In DP steels, a high density of weak traps and high H uptake increased HE susceptibility. During charging, H preferentially adsorbed along the grain boundaries and interfaces for all steels along with grain interior in TWIP steels. Dislocations and grain boundaries were the main trap sites for all steels, along with cementite particles in CP steels and AlN particles and austenitic grain interior in TWIP steels. For all steels under stress, hydrogen desorption increased up to yield point due to lattice expansion and dislocation movement, whereas decreased in the plastic region due to defect generation. For CP steel, strongly trapped hydrogen desorbed at UTS whereas in TWIP steel, generation of deformation twinning released hydrogen. The study of the galvanized layer showed that at higher cathodic overpotential, the Zn layer behaved as a barrier layer protecting the steel, while at a lower potential, it increased the HE susceptibility due to Zn layer dissolution. Overall, CP steel was the most resistant steel to HE, followed by TWIP and DP steels
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Al-Obaidi, Amar Baker Salim. "Induction Assisted Single Point Incremental Forming of Advanced High Strength Steels." Universitätsverlag der Technischen Universität Chemnitz, 2018. https://monarch.qucosa.de/id/qucosa%3A31527.
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Induction Assisted Single Point Incremental Forming (IASPIF) is a die-less hot sheet metal forming. The IASPIF does not apply characteristic complex tooling like those applied in deep drawing and bending. In this thesis, induction heating was used to heat up the sheet while simultaneously forming with a tool. The research goal is to improve the formability of high strength steels by heating. The IASPIF consists of non-complicated set up that allows induction heating to be utilized through the coil inductor moved under the sheet and synchronized with the forming tool that moves on the upper side of the sheet. The advanced high strength steel alloys, DP980, DP600 and 22MnB5 steels, were investigated. The influence of induction heating on formability was evaluated by the maximum wall angle that can be achieved in a single pass. Additionally, tool diameter and tool feed rate was also varied. The most influencing parameters were tool feed rate, induction power, and the profile depth. A new forming strategy was also developed by control the heating temperature through coupling the formed profile depth with a successively increased tool feed rate. The forming forces of DP980 steel sheet, were reduced from 7 kN to 2.5 kN when forming process was performed at room and elevated temperature, respectively. Stretching stresses were developed during forming process causing a high reduction in the resulting wall part thickness. New findings in this investigation were the reverse relationship between the step-down depth and the thickness reduction percentage. The smaller the tool diameter, the better was the formability. The finite element simulation of the investigated forming process showed that the increase in heating temperature has a direct effect on rising the plastic strain from 0.2 at room temperature to 1.02 at 800 ◦ C. The maximum true strain achieved in the resulting wall part thickness was determined by FEM simulations and validated with experimental trials. The part shape accuracy was measured and the highest deflection was founded when the part was formed by the highest step-down depth. Moreover, the minimum deflection in the part shape was achieved by utilizing a high induction power in the experiments. Finally, the resulting mechanical properties of the 22MnB5 alloy sheet material were tailored during IASPIF. For this purpose, the sheets were locally heated by induction during the forming process and subsequently quenched at different rates. As a result, the produced tailored parts consist of three different regions, which consist of a ductile, transitional and hardened region. The proposed procedure allows forming and quenching at the same time without transfer and thus, process time was reduced.Die induktionsgestützte, inkrementelle Blechumformung (englisch: Induction Assisted Single-Point Incremental Forming IASPIF) ist Warmumformprozess, bei dem keine komplexen Werkzeuge wie beim Tiefziehen und Biegen benötigt werden. Inhalt dieser Arbeit ist die inkrementelle Umformung eines Bleches mit gleichzeitig ablaufender induktiver Erwärmung. Das Forschungsziel bestand in der Verbesserung der Umformbarkeit von hochfesten Stahlwerkstoffen wie DP600, DP980 und 22MnB5 durch eine gezielte partielle Erwärmung. Der prinzipielle Aufbau des Versuchsstandes besteht aus einem Spuleninduktor, der unterhalb des umzuformenden Blechs platziert ist, und der synchron mit dem Werkzeug – einem Drückdorn – während des Umformvorganges verfährt. Ein wesentlicher Untersuchungsschwerpunkt bestand in der Ermittlung der Einflussgrößen auf den untersuchten IASPIF-Prozess. Für die Bewertung der Umformbarkeit wurden hierbei der maximal erreichbare Teilwandwinkel und die Profiltiefe, die in einem Umformdurchgang herstellbar waren, ermittelt und ausgewertet. Darüber hinaus konnten im Rahmen der Arbeit die Induktionsleistung des Generators, der Werkzeugdurchmesser und die Werkzeugvorschubgeschwindigkeit als relevante Prozessparameter identifiziert werden. Im Ergebnis der durchgeführten Untersuchungen zeigten die Werkzeugvorschubgeschwindigkeit und die Induktionsleistung einen wesentlichen Einfluss auf die erreichbare Profiltiefe. Aufbauend auf den erzielten Ergebnissen konnte eine prozessangepasste Umformstrategie entwickelt werden, bei der eine konstante Erwärmungstemperatur durch das Koppeln der momentanen Profiltiefe mit einer sukzessiv steigenden Werkzeugvorschubgeschwindigkeit erreicht wird. Weiterhin ließen sich die Kräfte bei der Umformung eines Stahlbleches aus DP980 von 7 kN (bei Raumtemperatur) auf 2,5 kN (bei erhöhter Temperatur) reduzieren. Aufgrund des mit einem Streckziehvorgang vergleichbaren Spannungszustandes während des Umformprozesses war eine starke Verringerung der resultierenden Wanddicke zu beobachten. Als neue Erkenntnis in dieser Untersuchung konnte die umgekehrte Beziehung zwischen der Zustelltiefe und dem Dickenreduktionsprozentsatz abgleitet werden. Aus der Finite - Elemente - Simulation des vorgestellten Umformprozesses wurde erkennbar, dass die Erhöhung der Erwärmungstemperatur einen direkten Einfluss auf die plastische Dehnung von 0,2 (bei Raumtemperatur) auf 1,02 (bei 800 °C) hat. Mittels der numerischen Simulation und der nachfolgenden experimentellen Validierung erfolgte darüber hinaus die Bestimmung der maximalen wahren Dehnung, die in der resultierenden Wanddicke erreicht wurde. Bei den Versuchen mit der größten Zustellung ließ sich durch die Bestimmung der Teileformgenauigkeit die höchste Abweichung von der Sollgeometrie CAD Modell feststellen. Abschließend wurde nachgewiesen, dass der IASPIF Prozess auch zur Einstellung maßgeschneiderter Bauteileigenschaften wie der resultierenden mechanischen Eigenschaften des Blechmaterials aus 22MnB5 einsetzbar ist. Zu diesem Zweck wurden die Bleche während des Umformprozesses lokal induktiv erwärmt und anschließend zur Einstellung des gewünschten Gefüges bei unterschiedlichen Abkühlgeschwindigkeiten abgeschreckt.
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Bardelcik, Alexander. "Effect of Pre-Bending and Hydroforming Parameters on the Formability of Advanced High Strength Steel Tube." Thesis, University of Waterloo, 2006. http://hdl.handle.net/10012/2829.
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With increasing fuel costs and the current drive to reduce greenhouse gas emissions and fuel consumption, a need to reduce vehicle weight is apparent. Weight reduction can be achieved by replacing conventionally stamped structural members with hydroformed parts. The weight reduction can be further enhanced by reducing the thickness of the hydroformed members through the use of advanced high strength steel (AHSS). A primary limitation in hydroforming AHSS, is the limited ductility or formability of these materials. This limitation becomes acute in multi-stage forming operations in which strain path changes become large making it difficult to predict formability. Thus, the focus of the current work is to study the effects of pre-bending on the subsequent hydroformability of Dual-Phase DP600 steel tubes. As part of this effort, the effect of key bending and hydroforming process parameters, bending boost and hydroforming end-feed, have been studied in a parametric fashion. Multi-step pre-bending and hydroforming experiments were performed on 76. 2 mm (3. 0") OD tubes with a wall-thickness of 1. 85mm (DP600). Experiments were also performed on 1. 74mm Interstitial Free (IF) steel tube, which provided a low strength, high formability baseline material for comparison purposes. A fully instrumented servo-hydraulic mandrel-rotary draw tube bender was used in the pre-bending experiments in which various levels of boost were applied. The results showed that increased boost reduced the major (tensile) strain and thinning at the outside of the bend. At the inside of the bend, the compressive minor strain became larger and thickening increased.
Hydroforming of the straight and pre-bent tubes was conducted using various levels of load-control end-feed (EF). For both straight and pre-bend tube hydroforming, an increase in hydroforming EF resulted in increased burst pressure and corner-fill expansion (CFE). The effect of bending boost on CFE was also measured. For a given hydroforming EF case, a tube bent with greater boost achieved a higher burst pressure and consequently a greater CFE which increased the hydroformability of the material. Pre-bending was shown to consume a considerable amount of the formability of the tube in the hydroforming experiments. For the same EF case, the pre-bent tubes could only achieve a fraction of the straight tube CFE at burst.
The pre-bending and hydroforming experiments were complimented by finite element simulation in the hope of providing additional insight into these processes. The finite element (FE) models were able to accurately predict the strain and thickness changes imposed during pre-bending. The models were able to accurately predict the CFE, EF displacement, and strain and thickness distributions after hydroforming.
The extended stress-based forming limit curve (XSFLC) failure criterion was applied to predict failure (onset of necking) during hydroforming, which was measured as the burst pressure in the experiments. For straight tube hydroforming, the XSFLC predicted the correct failure pressure versus hydroforming EF load trend, but over predicted the failure pressures. In pre-bend hydroforming, the models were able to capture the effect of bending boost and hydroforming EF on the hydroformability of the tubes. The XSFLC was able to capture the drop in formability for bending versus straight tube hydroforming, but was unable to capture the failure pressure versus hydroforming EF load trend or magnitude. Further work is required to make the XSFLC applicable to straight and pre-bend hydroforming.
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Aykas, Berk. "Evaluation of Edge Fracture in Flanging Advanced High Strength Steel (AHSS) Using the Double Bending Test." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1557210237260137.
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Ahmed, Essam Ahmed Ali [Verfasser]. "Laser Welding of Advanced High Strength Steels / Essam Ahmed Ali Ahmed." Aachen : Shaker, 2011. http://d-nb.info/1074087704/34.
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Östlund, Rickard. "Modelling and characterisation of fracture properties of advanced high strength steels." Licentiate thesis, Luleå tekniska universitet, Material- och solidmekanik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-26695.
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Growing demands for passenger safety, vehicle performance and fueleconomy is a continuous driving force for the increase in use of advancedhigh strength steels (AHSS) in the automotive industry. Thesesteels area characterised by improved formability and crash worthinesscompared to conventional steel grades. An important prerequisite of theapplication of new material grades is the characterization of its mechanicalproperties. Post-localization and fracture predictive technologiesgreatly facilitate the design of components which make optimal use ofthese steel grades. In this thesis, press hardened boron alloyed steelsubjected to differential thermo-mechanical processing is characterized.Fracture properties in relation to the different microstructures obtainedis studied. Furthermore a dual phase (DP) cold forming steel is chosenfor evaluation of ductility limit in shear loading. throughout thiswork a strategy for modelling post-localization response and predictingductility limit using shell elements larger then the typical width of thelocalized neck is used. The studied material is assumed to be in a stateof plane stress. Mesh dependency is alleviated by the introduction of aelement size dependent parameter into the constitutive description. Thisparameter acts as a hardening parameter, controlling the evolution ofthe yield surface depending on loading, strain history and shell elementsize. Model calibration relies on a full field measurement technique, DigitalSpeckle Photography (DSP), to record the plane deformation field oftensile specimens. Quantitative measurements of the severely localizeddeformation preceding crack initiation are feasible. With the proposedstrategy, mesh sensitivity in terms of post localization load responseand fracture elongation predictions is reduced significantly compared toresults obtained without the element size dependent parameter. It wasfound that high strain hardening favours strain localization of shear band type, and accelerates the formation of a localized neck. The hardeningcharacteristics is determinant to which deformation mode dissipates theminimum energy. For the DP steel, the Tresca yield surface more accuratelydescribes the yielding point compared to the von Mises planestress elipse. Furthermore, the exponential ductility function dependenton the stress triaxiality parameter agrees well with experimental fracturedata in the ductile loading regime for both DP and boron steel.In shear loading, the maximum shear (MS) stress criterion successfullydescribes the ductility limit. Due to the significantly different ductilityof the various microstructures obtainable by the thermo-mechanicalprocessing of boron alloyed steel, a modelling strategy is needed. It wasfound that in ductile loading, local equivalent fracture strain can be relatedto the hardness of that material point. An exponential decrease inductility with increased hardness describes experimental data collectedfor five different microstructures.Godkänd; 2011; 20110927 (ricost); LICENTIATSEMINARIUM Ämnesområde: Hållfasthetslära/Solid Mechanics Examinator: Professor Mats Oldenburg, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Diskutant: Dr, Forskningsingenjör Greger Bergman, Gestamp Hardtech AB, Luleå Tid: Torsdag den 3 november 2011 kl 10.00 Plats: E246, Luleå tekniska universitet
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Jiang, Menglei S. M. Massachusetts Institute of Technology. "Resetting microstructures and properties in TRIP-assisted advanced high strength steels." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/118713.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 53-57).
Metals are widely used structural materials in automotive, packaging, construction, and machines. Driven by demands to decrease greenhouse gas emissions, the reuse, re-forming, and re-manufacturing of metals draws great attention. However, current processes such as mechanical joining, welding, coating, etc. have key practical and theoretical limitations. Recently, a new reuse strategy is proposed, which aims to reset the microstructures of materials to maintain performance and increase lifetime. We refer to alloys that demonstrate this capability as resettable alloys. One resettable alloy is the transformation-induced plasticity-maraging (TRIP-maraging) steel. However, current resettable TRIP-maraging steels require long and unfeasible resetting treatments. The limit of resetting kinetics has not been reached and the microstructure resetting mechanism has not been fully understood. Here we focus on providing a deeper understanding of the resetting mechanism in TRIP-maraging steel, such as the effects of composition and pre-strain, to increase the kinetics of the underlying transformations. This study demonstrates that with proper microstructure design, the resetting process could be completed within minutes following a critical level of deformation.
by Menglei Jiang.
S.M.
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Taylor, Thomas James. "New generation advanced high strength steels for automotive hot stamping technologies." Thesis, Swansea University, 2014. https://cronfa.swan.ac.uk/Record/cronfa43085.
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Morata, Royes Joan. "Wear resistance of heat-treated Advanced High Strength Steels and casting." Thesis, Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-80526.
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The necessity to improve the durability of the machinery used in the milling industry has lead to several types of research. One study is focused on the plates that are located in the stationary and moving jaws of crushers to diminish particles sizes of Gneiss and Diabase rocks. Thus, one type of approach to increase its durability is by reducing the material loss of the plates. Amongst all the parameters to reduce the wear ratio that includes inputs from geometry to load, just the hardness input of the material can be in constant development. Consequently, there are two well-known types of heat treatment that can produce this change in hardness and are named Carbide Free Bainite (CFB) and Quenching and Partitioning (QP). In this master thesis the topic is to perform the QP heat treatments for two compositions A and B to obtain the microstructure of the steels that consist in a mix of austenite, bainite and martensite which considerably increase the hardness while the toughness is not drastically reduced due to the austenite soft phase. Five samples have been studied at four different partitioning temperatures: QP250 A, QP180 B, QP210 B, QP240 B and Mn Steel as it is the composition most used nowadays in the industry. In order to characterise both mechanical properties and microstructural features, different analysis had been performed with Micro-indentations, Charpy-V, Gouging Abrasion Tests, Optical Microscopy, Scanning Electron Microscopy and X-Ray Diffraction. These analysis had been done in the samples before and after wear as a result in change of the microstructure. As the abrasive-impact of the rocks collide with the sample, austenite transforms to martensite by induced plasticity called TRIP effect. Thus, the surface of the alloy is harder than the bulk material as no austenite is found and the wear ratio is seen to be improved. The results have shown several behaviours. Austenite transforms in its majority to fresh martensite which is an unstable martensitic phase but harder than tempered martensite that is the stable martensitic phase. Moreover, the difference in hardness between the bulk and the surface produce an affected depth layer as a consequence of the abrasive-impact penetration of rocks in which the microstructure has fully transformed to martensite on the surface and the austenite phase increases as it goes further inside the steel. The thinner this layer is, the better wear ratio presents the alloy. From all the samples, the best combination of hardness and toughness is for QP210 B.
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Miller, Christopher Carl Edward. "The joining of advanced high strength steels using resistance spot welding." Thesis, Swansea University, 2008. https://cronfa.swan.ac.uk/Record/cronfa42252.
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Weight reduction of the automotive body-in-white structure is increasingly necessary to improve fuel efficiency and reduce the carbon emissions and environmental impact of motor vehicles. This must be achieved without compromising the strength of the body-in-white structure. Steel manufacturers are continuing to develop advanced high strength steels (AHSS) which not only exhibit high strength and excellent energy absorbing characteristics but also retain a comparable degree of formability to low alloy grades. The specific properties of advanced high strength steels such as dual phase and TRIP are derived via the addition of specific alloying elements and careful control of thermomechanical processing routes in order to develop the required microstructures in the final product. The utilisation of such grades could yield significant reductions in the weight of body-in-white structures since they offer the automotive design engineer the opportunity to fabricate components out of thinner sheet whilst retaining or even improving on the structural strength and impact performance of components fabricated from thicker mild steel sheets. A major barrier to the widespread acceptance and implementation of AHSS by the automotive industry is the perceived complexities associated with the joining of these materials using resistance spot welding, which remains the dominant joining process in modem automobile construction. The complex alloy chemistries of these grades coupled with the extremely high cooling rates generated by the resistance welding process can give rise to weld microstructures with properties differing greatly from the parent microstructure. Of particular concern for automotive manufacturers is the potential for the formation of martensite in the weld nugget and heat affected zones, since its high hardness and low ductility are thought to result in poor weld performance. This research programme has investigated the weldability of six AHSS grades in comparison to low alloy grades typical of those currently used in the automotive industry, using a basic single pulse weld schedule. Simple modifications to welding parameters such as increasing electrode force and electrode tip diameter were investigated as well as the effects of advanced weld schedules on weld microstructures, microhardness and strength. The effect of joining selected advanced high strength steels to low carbon mild steel has also been studied.
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Smith, Anthony Justin. "Procedure and Results for Constitutive Equations for Advanced High Strength Steels Incorporating Strain, Strain Rate, and Temperature." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1343150464.
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Forouzan, Farnoosh [Verfasser], and Mücklich Esa Vuorinen [Akademischer Betreuer] Frank. "Increasing phase transformation rate in advanced high strength steel applications / Farnoosh Forouzan ; Betreuer: Esa Vuorinen Frank Mücklich." Saarbrücken : Saarländische Universitäts- und Landesbibliothek, 2019. http://d-nb.info/1192754824/34.
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Frómeta, Gutiérrez David. "On the measurement of fracture toughness to understand the cracking resistance of advanced high strength steel sheets." Doctoral thesis, TDX (Tesis Doctorals en Xarxa), 2021. http://hdl.handle.net/10803/672379.
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Automotive designers are constantly facing new challenges to meet the more and more stringent safety and CO2 emission legislations. Concerning the latter, vehicle lightweighting has become one of the main goals of the automotive industry, not only to reduce fuel consumption in fuel-powered cars but also to enhance the battery range in electric vehicles. At the same time, weight reduction cannot be attained at the expense of passenger’s safety in case of a crash. Hence, it is important to select the best-suited strategies to find the optimum balance between weight reduction and crashworthiness. In this sense, Advanced High Strength Steels (AHSS) have been positioned as one of the most effective solutions to this demand. AHSS present very high strength and high crash performance, which allows reducing vehicle mass while maintaining the safety of the occupants. These outstanding mechanical properties have promoted their widespread implementation for structural and crash-relevant automobile components. However, the application of AHSS have introduced new challenges related to their limited ductility and cracking resistance. Premature cracking during edge forming operations (edge cracking) or the occurrence and propagation of cracks under impact loading are some of the common cracking related issues in processing and implementation of AHSS.
To face these problems, the development of new approaches to properly characterize the cracking resistance of AHSS has become unavoidable since conventional failure criteria based on uniaxial tensile properties and forming limit curves fail to describe cracking related phenomena. In this thesis, a fracture mechanics-based approach is proposed to rationalize and understand the crack initiation and propagation resistance of AHSS. Results have been correlated with edge cracking resistance and crash behaviour of a broad range of advanced high strength sheet steels.
Fracture toughness is evaluated in the frame of fracture mechanics through different testing methods, such as the essential work of fracture, the J-integral and the Kahn-type tear tests. The relationship between the obtained fracture toughness parameters as well as the limitations of the different methods have been discussed. High-resolution video extensometry and Digital Image Correlation (DIC) techniques were used to investigate the fracture behaviour of the different steels. Edge cracking resistance is characterized by standard hole expansion tests and DIC-assisted hole tension tests. Crashworthiness is assessed through laboratory impact resistance tests. The influence of microstructural constituents on the crack propagation resistance of AHSS is also assessed.
The results show that fracture toughness, in particular the specific essential work of fracture (we), is a suitable material property to understand the cracking behaviour of AHSS and rank the material’s resistance to different crack-related failures, such as edge fracture or crack propagation during a crash event. These conclusions are based on the good correlation established between we and the results from edge cracking and impact resistance tests. On the other hand, the experimental observations show that we can be used to discern the role of microstructural constituents on the fracture behaviour of AHSS.
It is pointed out that proper microstructural design cannot be only focused on tensile properties since they do not inform about cracking resistance.
According to all the experimental findings, the fracture toughness is considered as a relevant material property for AHSS design and performance classification. In line with this, a new classification system, considering global ductility and fracture toughness, is proposed for a more comprehensive description of the overall formability and fracture behaviour of AHSS.Los diseñadores de automóviles se enfrentan constantemente a nuevos desafíos para cumplir con las cada vez más estrictas legislaciones de seguridad y emisiones de CO2. Con respecto a esto último, el aligeramiento de los vehículos se ha convertido en uno de los principales objetivos de la industria automotriz, no solo para reducir el consumo en los automóviles de combustión interna, sino también para mejorar la autonomía de los vehículos eléctricos. Al mismo tiempo, la reducción de peso no se puede lograr a expensas de la seguridad del pasajero en caso de accidente. Por lo tanto, es importante seleccionar las estrategias más adecuadas para encontrar el equilibrio óptimo entre reducción de peso y resistencia al impacto. En este sentido, los aceros avanzados de alta resistencia (AHSS) se han posicionado como una de las soluciones más efectivas. Los AHSS presentan una elevada resistencia y un buen comportamiento en caso de impacto, lo que permite reducir el peso del vehículo manteniendo la seguridad de los ocupantes. Estas excepcionales propiedades mecánicas han contribuido a su extensa implementación en componentes estructurales y de seguridad en el automóvil. Sin embargo, estos aceros también han introducido nuevos problemas relacionados con su limitada ductilidad y resistencia la fisuración, como la aparición prematura de fisuras durante el conformado (edge cracking) o la generación de fisuras durante el impacto. Para hacer frente a estos problemas, se ha hecho inevitable el desarrollo de nuevos enfoques para caracterizar la resistencia a la fisuración de los AHSS, ya que los criterios convencionales basados en ensayos de tracción y curvas límite de conformabilidad no son adecuados. En esta tesis doctoral se propone un enfoque basado en la mecánica de la fractura para explicar este tipo de fracturas relacionadas con la resistencia a la iniciación y propagación de grietas en el material. Con este fin, se investiga la correlación entre las mediciones de tenacidad de fractura y la resistencia al edge cracking y el comportamiento en caso de impacto en una amplia gama de chapas de acero avanzado de alta resistencia. La tenacidad de fractura se evalúa en el marco de la mecánica de la fractura mediante distintos métodos como el trabajo esencial de fractura, la integral J o los ensayos tipo Kahn y se discute la relación entre los parámetros obtenidos, así como las limitaciones de los diferentes métodos. Se utilizan técnicas de video de alta resolución y correlación de imágenes digitales para investigar el comportamiento de fractura de los diferentes aceros. La resistencia edge cracking se caracteriza mediante ensayos de expansión de orificios (hole expansion tests). La resistencia al impacto se evalúa mediante ensayos de impacto de laboratorio. Finalmente, se analiza brevemente la influencia de la microestructura en la resistencia a la propagación de grietas de los AHSS. Los resultados muestran que la tenacidad de fractura, en concreto el trabajo esencial de fractura (we) es una herramienta útil para comprender fenómenos de fisuración en los AHSS. Estas conclusiones se basan en la buena correlación establecida entre we y los resultados de las pruebas de resistencia al impacto y al edge cracking. Por otro lado, las observaciones experimentales muestran el gran potencial del parámetro we para discernir el efecto de la microestructura en la resistencia a la fractura de los AHSS. Se destaca que el diseño microestructural no debe centrarse sólo en las propiedades de tracción, ya que éstas no aportan información sobre la resistencia a la propagación de fisuras. De acuerdo con esto, la tenacidad de fractura se considera una propiedad del material relevante para el diseño y clasificación de los AHSS y se propone un nuevo método de clasificación para una descripción más completa de la conformabilidad y la resistencia a la fractura de los aceros AHSS.
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Dunand, Matthieu. "Effect of strain rate on the ductile fracture of Advanced High Strength Steel Sheets : Experiments and modeling." Palaiseau, Ecole polytechnique, 2013. http://pastel.archives-ouvertes.fr/pastel-00838906.
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L'industrie automobile emploie massivement les Aciers à Haute Performance (AHP) pour la fabrication des caisses en blanc, en raison de leur rapport résistance/masse élevé. Ils sont utilisés afin d'augmenter la sécurité des occupants en cas de crash, ou de réduire la masse du véhicule grâce à une diminution des sections utiles. En parallèle, le prototypage virtuel est omniprésent dans le processus de conception des nouveaux véhicules. En prenant l'exemple d'une caisse en blanc automobile, la conception de la structure globale et des procédés de mise en forme de ses composants nécessite des modèles prédictifs et fiables décrivant le comportement et la rupture des matériaux utilisés. Des efforts soutenus ont été entrepris ces cinq dernières années pour développer des modèles prédisant la rupture des AHP sous chargement statique. Pourtant les taux de déformations rencontrés lors d'opération de mise en forme sont de l'ordre de 10 s-1, et peuvent atteindre 103 s-1 lors de crashs. Le but de cette thèse est de développer une méthode fiable permettant d'évaluer l'influence du taux de déformation et de l'état de contrainte sur la rupture ductile d'AHP initialement non-fissurés. Une procédure expérimentale est conçue pour caractériser le comportement et l'initiation de la rupture dans des tôles chargées en traction à grande vitesse de déformation. La précision du dispositif est évaluée grâce à des validations numériques et expérimentales. Par la suite, une série d'expériences est réalisée à petite, moyenne et grande vitesse de déformation sur différents types d'éprouvettes de traction, afin de couvrir un spectre étendu d'états de contraintes. Une analyse détaillée de chaque expérience par la méthode des Éléments Finis permet de déterminer le trajet de chargement et l'état de déformation et de contrainte à la rupture dans chaque éprouvette, tout en prenant en compte les phénomènes de striction. La déformation à la rupture est significativement plus élevée à grande vitesse de déformation qu'à basse vitesse. De plus, les résultats montrent que l'influence du taux de déformation sur la ductilité ne peut pas être découplée de l'état de contrainte. Le modèle de comportement constitue un élément essentiel de cette approche hybride expérimentale-numérique. Un modèle de plasticité dépendant du taux de déformation est proposé pour prédire la réponse mécanique des AHP sur toute la plage de déformation, taux de déformation et état de contrainte couverte par le programme expérimental. La précision du modèle est validée par comparaison de mesures expérimentales globales et locales aux prédictions numériques correspondantes. De plus, l'influence de la discrétisation spatiale utilisée dans les simulations par Eléments Finis sur la précision de l'approche hybride expérimentale-numérique est quantifiée. Il est montré qu'un maillage fin d'éléments hexaédriques est nécessaire pour obtenir des prédictions précises jusqu'à la rupture. Ce type de maillage n'est pas compatible avec des applications industrielles à grande échelle pour des raisons évidentes d'efficacité numérique. C'est pourquoi une méthode de remaillage dynamique d'éléments coque vers des éléments solides est présentée et évaluée. Cette méthode permet d'obtenir des prédictions fiables de l'initiation de la rupture dans des tôles sans compromettre dramatiquement l'efficacité numérique obtenue grâce aux éléments coque. La seconde partie de ce travail s'intéresse aux micro-mécanismes responsables de la rupture ductile du matériau étudié. Une analyse micrographique du matériau soumis à différents niveaux de déformation permet d'identifier l'enchainement des mécanismes d'endommagement. Ces observations suggèrent que le mécanisme critique conduisant à la rupture est la localisation de la déformation plastique dans une bande de cisaillement à l'échelle du grain. Un model numérique reposant sur la déformation d'une cellule élémentaire 3D contenant une cavité est développé pour modéliser ce phénomène. Il est montré que le mécanisme de localisation à l'échelle micro de l'écoulement plastique dans une bande de cisaillement permet d'expliquer la dépendance de la ductilité à l'état de contrainte et au taux de déformation observée à l'échelle macroThe automotive industry has widely incorporated Advanced High Strength Steels sheets (AHSS) in vehicle structures due to their high strength to weight ratio: they are used to improve the vehicle safety or to reduce the vehicle weight through the use of thinner gages. At the same time, new vehicle design relies heavily on virtual prototyping practices. In the specific example of automotive structures, both the engineering of the production process and of the final product require reliable models of plasticity and fracture. Consequently, great efforts have been undertaken during the last five years to develop models that can predict the fracture of AHSS under static conditions. However, rates of deformation encountered in sheet metal forming operations are typically of the order of 10s-1, while they can be as high as 103 s-1 under accidental crash loading. Therefore, there is a need to investigate the effect of strain rate on deformation behavior and fracture of AHSS, and to assess whether models developed for static loading conditions can satisfactorily be used in industrial applications. The present research work consists of two main parts. The first part aims at developing a reliable methodology for evaluating the influence of strain rate as well as stress state on the ductile fracture properties of initially uncracked Advanced High Strength Steel sheets. An experimental procedure is designed to characterize the deformation behavior and the onset of fracture of sheet materials under tensile loading at high strain rate. Numerical and experimental validations of the proposed setup are performed to evaluate its accuracy. Then an experimental program is carried out at low, intermediate and high strain rates on different type of tensile specimens, thereby covering a range of stress states. Detailed Finite Element analyses of each experiment are used to determine the loading history and the material state at fracture in each experiment. A key component of this hybrid experimental-numerical approach is the constitutive model: a rate-dependent plasticity model is proposed to predict the mechanical response of AHSS over all the range of strains, strain rates and stress states reached in the experiments. The model accuracy is validated by comparing global and local test measurements to the corresponding simulation predictions. In addition, the influence of the geometric discretization used in Finite Element analysis on the accuracy of the hybrid experimental-numerical approach is evaluated. It is shown that fine meshes of brick elements are required for accurate fracture predictions, but cannot be used in industrial applications because of inadequate computational efficiency. A technique of shell-to-solid re-meshing is presented and evaluated, that allows for accurate predictions of the onset of ductile fracture in sheet materials without compromising the numerical efficiency of shell elements. The second part of this work is concerned with the micro-mechanisms responsible for ductile failure. Micrographs of specimens corresponding to different stages of loading prior to failure are analyzed to identify the sequence of damage processes leading to fracture. Observations suggest that the governing failure mechanism is the localization of plastic deformation into shear bands at the grain level. A numerical model based on three dimensional unit cell calculations is developed to assess whether the mechanism of shear localization of the plastic flow at the micro-scale can explain the dependence of the material ductility to both stress state and strain rate that was observed at the macro-scale
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Gardner, Rebecca. "An Experimental Investigation of Friction Bit Joining in AZ31 Magnesium and Advanced High-Strength Automotive Sheet Steel." BYU ScholarsArchive, 2010. https://scholarsarchive.byu.edu/etd/2159.
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Friction Bit Joining (FBJ) is a recently developed spot joining technology capable of joining dissimilar metals. A consumable bit cuts through the upper layer of metal to be joined, then friction welds to the lower layer. The bit then snaps off, leaving a flange. This research focuses on FBJ using DP980 or DP590 steel as the lower layer, AZ31 magnesium alloy as the top layer, and 4140 or 4130 steel as the bit material. In order to determine optimal settings for the magnesium/steel joints, experimentation was performed using a purpose-built computer controlled welding machine, varying factors such as rotational speeds, plunge speed, cutting and welding depths, and dwell times. It was determined that, when using 1.6 mm thick coupons, maximum joint strengths would be obtained at a 2.03 mm cutting depth, 3.30 mm welding depth, and 2500 RPM welding speed. At these levels, the weld is stronger than the magnesium alloy, resulting in failure in the AZ31 rather than in the FBJ joint in lap shear testing.
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Krishna, Chalavadi Sai. "Parameter identification of GISSMO damage model for DOCOL 900M high strength steel alloy : Usage of a general damage model coupled with material modeling in LS-DYNA for Advanced high strength steel crashworthiness simulations." Thesis, Högskolan Väst, Avdelningen för avverkande och additativa tillverkningsprocesser (AAT), 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:hv:diva-11745.
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Tarar, Wasim Akram. "A New Finite Element Procedure for Fatigue Life Prediction and High Strain Rate Assessment of Cold Worked Advanced High Strength Steel." The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1204575243.
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Portillo, Martínez Oscar. "Impact modeling of spot-welded columns fabricated with advanced high strength steels." Thesis, McGill University, 2005. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=83924.
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In order to increase vehicle fuel efficiency while improving safety and performance and maintaining affordability, the global steel industry has initiated the use of advanced high strength steels that ultimately result in the design of stronger, lighter and more energy efficient vehicles. Tubular steel columns are extensively used in the automotive body structure due to their inherent capacity to absorb energy on impact. Hence, the objective of this research work is to study the crashworthiness performance of columns made from advanced high strength steels.To conduct this study, computational simulations are developed to accurately assess the influence exerted in the crush response of the columns by different types of materials as well as geometrical characteristics of the thin-walled structural sections.
Furthermore, it is well known that spot-welding is the primary method of joining in the ground vehicle industry. Therefore, the strength of the spot weld under impact is extremely important to the safety design and durability of automobiles. Thus, in this project such an essential factor is addressed by developing a reliable and practical finite element model to predict the dynamic failure of spot-welds in the sheet metal structures.
It is shown that numerical results of the developed robust finite element model give fairly good agreement with experimental data in terms of collapse profile, deformed column shape, final crush length, and impact peak force.
Throughout the investigation, the finite element model permits the study of several structural and material variables that can be validated by a moderate set of destructive tests. Moreover, the current finite element crash model and the findings in this work can eventually be used to improve the crashworthiness efficiency of steel column specimens and to help meet society's demands for affordable, fuel efficient, environmentally responsible, and safe vehicles.
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Falk, Johannes. "Fracture prediction of stretched shear cut edges in sheets made of Dual-Phase steel." Thesis, Blekinge Tekniska Högskola, Institutionen för maskinteknik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:bth-13956.
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Dual-Phase (DP) steels, part of the group of Advanced High Strength Steels (AHSS), are used by car manufactures due to its large strength to weight ratio. The high strength of the DP steel does have a negative impact on the formability during sheet metal forming and stretch forming, e.g. fractures often appear in shear cut edges during forming of blanks made of DP steel. The main objective with this thesis is to develop a new punch for Volvo Cars that concentrates the strain to the sheared edges of a test specimen made from different types of DP steel. This is done to be able to measure and obtain maximum fracture strain during stretch forming tests in a press. The newly developed test method is called CTEST (Concentrated Trim Edge Strain Test). The tests are performed with DP steel specimens with three different qualities of the shear cut edges; fine cut, medium cut and worn cut. DP steels tested are DP600GI, DP600UC and DP800GI from three different suppliers. 10 different types of DP steels are tested in this study with different thickness. Thickness of specimens tested are 1 mm, 1.1 mm, 1.5 mm and 2 mm and all specimens tested have a lengthwise (RD) rolling direction. The quality of the sheared cut edge has a great impact to the formability and maximum fracture strain of the specimen. A specimen with a fine cut endures higher fracture strain than medium cut and a worn cut for all types of DP steel with different thickness. A 1 mm thick specimen endures a lower fracture strain than 1.5 mm and 2 mm specimen for all cut qualities. Further, the impact of the orientation of the burr zone of a shear cut edge is studied. With the burr zone facing upwards from the CTEST punch the formability of the specimens is decreased compared to a burr zone facing downwards, especially for a worn cut specimen with micro cracks and imperfections in the edge surface. ARAMIS Digital Image Correlation (DIC) system is used to analyze the specimen edges during press experiments. The ARAMIS results unveil that several small fractures appear in the sheared edges of a specimen just before the specimens split into two pieces. This phenomenon was seen for specimen with worn and medium shear cut qualities. Finite Element (FE) simulations of the CTEST is performed in AutoForm to determine maximum values of the true strain for the three different cut qualities. The simulation in AutoForm does show a slightly higher value of the force and press depth than the value from the press test before maximum fracture strain in reached. The small fractures seen in ARAMIS just before the specimen split into two pieces cannot be seen in the simulation in AutoForm.
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Dorribo, Dorribo Daniel. "Development of mechanism-based models for resistance spot weld failure simulation of multi-material advanced high strength steel sheets." Doctoral thesis, Universitat Politècnica de Catalunya, 2017. http://hdl.handle.net/10803/461803.
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The automotive industry is constantly involved in the development of new projects aimed at reducing weight, fuel consumptions and costs while improving passengers¿ safety. In order to achieve these increasing demands, Advanced High Strength Steels (AHSS) have been introduced in recent years reducing vehicle structure weights and improving the crashworthiness. With the increase in the bearing capacity of crash-relevant structural components, the sheet metal joining techniques such as adhesive bonding and resistance spot welding (RSW) become critical. In order to develop the vehicle structure in these new projects, full-vehicle crash finite element simulations are usually performed. Simplified beam-like models are currently used in these simulations (with thousands of spot welds) to represent RSW joints response. The maximum bearing force of these models are fitted using large experimental campaigns, considering all the main factors that have the highest influence on the fracture response of a welded joint. The objective of this thesis is twofold: (1) to develop a model that is able to partially replace the extensive experimental campaign in providing parameters for the crash simulation simplified spot weld models, and (2) to gain understanding of spot weld joints failure response in order to improve the current simplified models.
To achieve these objectives, a detailed spot weld model for the prediction of spot weld failure in joints in AHSS sheets is presented. The presented model includes a definition of the local material properties as well as the geometry features of a spot weld. In addition, an industrially suited fracture criterion, i.e. robust and without a long-term calibration, is used for the prediction of maximum force. An energetic fracture criterion based on the use of elastic-plastic fracture-mechanics is identified as the better suited for the prediction of spot weld failure and joint bearing capacity. The J-integral is evaluated in the weld notch and this value is compared with a material parameter, the fracture toughness, in order to obtain the joint maximum force.
The presented detailed FE spot weld model is validated to joints of two different steel grades of the AHSS family usually present in the current vehicle structure, a hot formed martensitic boron-alloyed steel (22MnB5) and a cold formed dual phase steel (DP 980). The validation is performed comparing the results obtained with the finite element model and the experimental results extracted from an extensive loading test experimental campaign where the main factors that have an influence in the spot weld fracture response are considered. The obtained simulated critical forces of the loading tests present good agreement with the experimental ones in all tested configurations.
Finally, based on the presented finite element spot weld model, some recommendations are exposed for extending the model for new combinations and loading conditions. The proposed procedure can be used to reduce the long-term characterization campaigns used to calibrate the joints of a new AHSS grade, where fracture is triggered by stress concentration ahead of a notch. Furthermore, some recommendations for the future structure design are given taking into account the information obtained with the present model.La industria del automóvil está constantemente involucrada en el desarrollo de nuevos proyectos persiguiendo la reducción de pesos, consumos de combustible y costes, así como mejoras en seguridad. Par alcanzar estas demandas, en los últimos años los llamados aceros avanzados de muy alta resistencia (AHSS) se han introducido reduciendo el peso de la estructura de los vehículos y mejorando su respuesta en caso de accidente. Con la mejora de la resistencia de los componentes estructurales relevantes durante un impacto a alta velocidad, las técnicas de unión de chapas de metal, como los adhesivos o los puntos de soldadura por resistencia (RSW), han pasado a tener un papel crucial. Para desarrollar la estructura de estos nuevos proyectos, se realizan habitualmente simulaciones de elementos finitos de vehículo completo. En estas simulaciones, con miles de puntos de soldadura, se usan modelos simplificados en los que la fuerza máxima según la unión y el caso de carga es obtenida mediante extensas campañas experimentales. Esta tesis tiene dos objetivos principales: (1) desarrollo de un modelo capaz de reemplazar parcialmente las extensas campañas experimentales que proveen de parámetros a los modelos simplificados de puntos de soldadura usados para la simulación de choque, (2) mejor comprensión de la respuesta a fallo de las uniones soldadas por puntos para mejorar los actuales modelos simplificados. Par cumplir estos objetivos se presenta un modelo detallado de elementos finitos. El modelo incluye la definición de las propiedades mecánicas locales así como las características geométricas de un punto de soldadura. Además, para la predicción de la fuerza máxima se aplica un criterio de fractura adecuado a la industria, es decir, robusto y a la vez sin la necesidad de una larga calibración. Se identifica un criterio energético de fractura basado en la mecánica de fractura elasto-plástica como el más adecuado para obtener la capacidad de carga de las uniones. La fuerza máxima de las uniones se obtiene al evaluar la concentración de tensiones alrededor de la entalla de soldadura mediante la integral J y comparándola con un parámetro del material (tenacidad de fractura). El modelo presentado es validado para uniones soldadas de dos tipos de aceros de la familia de los AHSS presentes habitualmente en la estructura de los vehículos modernos, un acero martensítico al boro estampado en caliente (22MnB5) y un acero de fase dual de estampación en frio (DP 980). Esta validación se realiza comparando los resultados de fuerza máximas obtenidos por el modelo de elementos finitos con los resultados experimentales obtenidos de una extensa campaña experimental donde se tienen en cuenta los principales factoras que tienen influencia en la fractura. Los resultados de las fuerzas críticas obtenidas de los ensayos experimentales de carga presentan una gran concordancia con la simulación para todas las configuraciones testeadas. Finalmente, basándose en el modelo detallado presentado se proponen algunas recomendaciones para extenderlo para nuevas combinaciones y condiciones de carga, así como recomendaciones sobre el diseño de estructuras teniendo en cuenta como se cargas los puntos de soldadura en distintas condiciones. El procedimiento propuesto puede ser usado para reducir las extensas campañas experimentales empleadas en la caracterización de uniones en nuevos tipos de aceros de alta resistencia, donde la fractura es desencadenada por concentración de tensiones alrededor de la entalla de soldadura.
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Godha, Anshul. "Microstructural effects on fatigue damage evolution in advanced high strength sheet (AHSS) steels." Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53510.
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An understanding of the damage evolution prior to crack initiation in advanced structural materials is of vital importance to the fatigue community in both academia and industry. Features known as the Persistent Slip Bands (PSBs) play an integral role in this damage evolution. Therefore, PSBs have been the focus of a lot of science-based investigations over the years. However, most existing studies in this area are restricted to analysis of PSBs in single crystal face centered cubic (FCC) materials. Moreover, these studies lack a quantitative analysis of the evolution of the fatigue damage (or PSBs) as a function of the material microstructure. This is especially true for relatively modern materials such as the Advanced High Strength Structural (AHSS) steels that are gaining a lot of importance in the automotive sector. Accordingly, the objective of this research is to quantitatively characterize evolution of PSBs in three AHSS steels having different microstructures as a function of number of fatigue cycles and strain amplitude. For this purpose strain controlled interrupted fatigue tests have been performed on two dual phase steels (DP-590 and DP-980) having different relative amounts of tempered martensite and a ferritic high strength low alloy steel (HR-590). Digital image analysis and Stereology have been used for unbiased quantitative characterization of the evolution of global geometry of the PSB colonies as function of number of fatigue cycles and strain amplitude. Evolution of PSB colonies has been couched in terms of variation of PSB colony volume fraction and total surface area unit volume, and total surface area of individual PSBs per unit volume and three-dimensional angular orientation distribution of the PSBs. For this purpose, new stereological techniques have been developed for estimation of the three-dimensional angular orientation distribution. The stereological data reveal that during strain controlled in these AHSS steels, volume fraction of the PSB colonies varies linearly with the their total surface area per unit volume. Detailed analysis of the stereological data leads to a simple geometric model for evolution of the PSB colonies in the three AHSS steels, which accounts for all observed data trends.
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Medvedeva, Anna. "Performance of advanced tool steels for cutting tool bodies." Doctoral thesis, Karlstads universitet, Avdelningen för maskin- och materialteknik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-5630.
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Performance of indexable insert cutting tools is not only about the performance of cutting inserts. It is also about the cutting tool body, which has to provide a secure and accurate insert positioning as well as its quick and easy handling under severe working conditions. The common damage mechanisms of cutting tool bodies are fatigue and plastic deformation. Cutting tools undergo high dynamic stresses going in and out cutting engagement; as a result, an adequate level of fatigue strength is the essential steel property. Working temperatures of tool bodies in the insert pocket can reach up to 600°C, why the tool steel requires high softening resistance to avoid plastic deformation. Machinability is also essential, as machining of the steel represents a large fraction of the production cost of a cutting tool. The overall aim of the study is to improve the tool body performance by use of an advanced steel grade with an optimized combination of all the demanding properties. Due to the high-temperature conditions, the thesis concerns mostly hot-work tool steels increasing also the general knowledge of their microstructure, mechanical properties and machinability. Knowing the positive effect of sulphur on machinability of steels, the first step was to indentify a certain limit of the sulphur addition, which would not reduce the fatigue strength of the tool body below an acceptable level. In tool bodies, where the demand on surface roughness was low and a geometrical stress concentrator was present, the addition of sulphur could be up to 0.09 wt%. Fatigue performance of the cutting tools to a large extent depended on the steel resistance to stress relaxation under high dynamic loading and elevated temperatures. The stress relaxation behaviour, material substructure and dislocation characteristics in low-alloyed and hot-work tool steels were studied using X-ray diffraction under thermal and mechanical loading. Different tool steels exhibited different stress relaxation resistance depending on their microstructure, temper resistance and working temperature. Hot-work tool steels showed to be more preferable to low-alloyed tool steels because of their ability to inhibit the rearrangement and annihilation of induced dislocations. High-temperature softening resistance of the hot-work tool steels was investigated during high-temperature hold-times and isothermal fatigue and discussed with respect to their microstructure. Carbide morphology and precipitation were determined using scanning and transmission electron microscopy. Machinability of a prehardened hot-work tool steel of varying nickel content from 1 to 5 wt% was investigated in end milling and drilling operations. Machining the higher nickel containing steels resulted in longer tool life and generated lower cutting forces and tool/workpiece interface temperature. The difference in machinability of the steels was discussed in terms of their microstructure and mechanical properties.
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Peterson, Rebecca Hilary. "Friction Bit Joining of Dissimilar Combinations of DP 980 Steel and AA 7075." BYU ScholarsArchive, 2015. https://scholarsarchive.byu.edu/etd/6030.
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Friction Bit Joining (FBJ) is a new technology that allows lightweight metals to be joined to advanced high-strength steels (AHSS). Joining of dissimilar metals is especially beneficial to the automotive industry because of the desire to use materials such as aluminum and AHSS in order to reduce weight and increase fuel efficiency. In this study, FBJ was used to join 7075 aluminum and DP980 ultra-high-strength steel. FBJ is a two-stage process using a consumable bit. In the first stage, the bit cuts through the top material (aluminum), and in the second stage the bit is friction welded to the base material (steel). The purpose of the research was to examine the impact a solid head bit design would have on joint strength, manufacturability, and ease of automation. The solid head design was driven externally. This design was compared to a previous internally driven head design. Joint strength was assessed according to an automotive standard established by Honda. Joints were mechanically tested in lap-shear tension, cross-tension, and peel configurations. Joints were also fatigue tested, cycling between loads of 100 N and 750 N. The failure modes that joints could experience during testing include: head, nugget, material, or interfacial failure. All tested specimens in this research experienced interfacial failure. Welds were also created and examined under a microscope in order to validate a simulation model of the FBJ process. The simulation model predicted a similar weld shape and bond length with 5 percent accuracy. Joints made with external bits demonstrated comparable joint strength to internal bits in lap-shear tension and cross-tension testing. Only external bits were tested after lap-shear tension, because it was determined that external bits would perform comparably to internal bits. Joints made with external bits also exceeded the standard for failure during fatigue testing. Peel tested specimens did not meet the required strength for the automotive standard. Examining specimens under a microscope revealed micro-cracks in the weld. These defects have been shown to decrease joint strength. Joint strength, especially during peel testing, could be increased by reducing the presence of micro-cracks. The external bit design is an improvement from the internal bits for manufacturability and ability to be automated, because of the less-expensive processes used to form the bit heads and the design that lends to ease of alignment.
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Demiralp, Yurdaer. "Determination of Material Properties and Prediction of Springback in Air Bending of Advance High Strength Steel (AHSS) and Commercially Pure Titanium (CP) Sheet Materials." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1339768136.
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Kardes, Sever Nimet. "Investigation of Lubrication and Springback in Forming of Draw Quality and Advanced High Strength Steels." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1332117974.
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Huin, Thibaut. "Experimental and numerical investigation of the mechanical behaviour of dissimilar arc and spot welds of advanced high strength steels." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSEI055/document.
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De nos jours, la politique écologique encourage les constructeurs automobiles à réduire le poids global du véhicule. Des tôles d'acier fines d'épaisseur différente optimisant chaque partie de l'assemblage sont utilisées et les sidérurgistes développent des aciers de plus en plus résistants à savoir l'Acier Haute Résistance Avancé (AHSS) avec un bon compromis entre résistance mécanique et ductilité (emboutissage). Lors des essais mécaniques de soudage hétérogène AHSS, des modes de fractures inhabituels sont observés, notamment le long de l'interface entre la zone affectée par la chaleur (ZAT) et la zone de fusion ou zone fondue (ZF). Ces fractures se produisent généralement avec une résistance inférieure à celle attendue pour ces soudures. Les objectifs de l'étude sont de comprendre les mécanismes de rupture au cours des essais mécaniques et de créer un modèle mécanique de FE conçu pour prédire la résistance mécanique des assemblages soudés. Tout d'abord, une étude de soudage hétérogène constituée de deux nuances d'acier bien connues d'ArcelorMittal vise à comprendre le mécanisme de défaillance et les paramètres affectant les modes de défaillance. Différentes configurations sont étudiées avec l'épaisseur. Le modèle FE est construit avec une réponse mécanique identifiée de chaque zone (matériaux de base, zones affectées par la chaleur et zone de fusion), en utilisant des modèles d'ArcelorMittal et des données expérimentales. Des critères de défaillance basés sur des dommages ductiles tenant compte de l'influence de la triaxialité sont utilisés et certains éléments cohésifs sont utilisés pour simuler une défaillance interfaciale. Deux configurations d'essais mécaniques dans le cas du soudage par résistance par points (essais de traction transversale et de traction) sont considérées. Les prédictions du modèle étaient très précises avec les modes de défaillance et les forces expérimentaux. Ensuite, cette méthode de modélisation FE a été appliquée avec succès à un boîtier de soudage par points très hétérogène comprenant un nouveau concept AHSS basse densité de troisième génération à forte teneur en aluminium et en manganèse. Les modes d'échec et les forces obtenues étaient comparables. De plus, la méthode de modélisation FE a été appliquée sur des configurations plus complexes, en particulier sur un assemblage soudé par points triple épaisseur. La robustesse du modèle pour prédire les modes de défaillance partielle et les forces d'une soudure par points triple épaisseur a été démontrée. En outre, la méthodologie de modélisation FE a été étendue à un autre type de soudage: le soudage à l'arc. Dans ce cas, deux feuilles sont soudées en configuration de chevauchement ab avec un fil d'apport. Le modèle FE permet de prédire la zone de rupture et la résistance de l'assemblage soudéNowadays, ecological policy encourages carmakers to reduce the global vehicle weight. Fine steel sheets assemblies with different thickness optimizing each part of the assembly are used and steelmakers develop steels which are more and more resistant namely Advanced High Strength Steel (AHSS) with a good compromise between mechanical strength and ductility (stamping). During the mechanical tests of heterogeneous AHSS welding, unusual fracture modes are observed, in particular along the interface between the Heat Affected Zone (HAZ) and the Fusion Zone or molten zone (FZ). These fractures generally occur with lower strength than expected for these welding. The objectives of the study are to understand fracture mechanisms during mechanical testing and create a mechanical FE model is developed to be able to predict mechanical strength of the welded assemblies. Firstly, a study of heterogeneous welding constituted of two well-known steel grades of ArcelorMittal aims at understanding failure mechanism and parameters affecting the failure modes. Different configurations are studied with thickness. FE model is built with mechanical response identified of each zone (base materials, heat affected zones and fusion zone), using ArcelorMittal models and experimental data. Failure criteria based on ductile damage taking into account the influence of the triaxiality are used and some cohesive elements are used to simulate interfacial failure. Two configurations of mechanical testing in the case of Resistance Spot Welding (cross tension and tensile shear tests) are considered. Model predictions were very accurate with experimental failure modes and strengths. Then, this FE modelling method was successfully applied to a highly heterogeneous spot welding case including a new third generation low density AHSS concept with high aluminum and manganese content. Failure modes and strengths obtained were comparable. Moreover, FE modelling method was applied on more complex configurations, in particular on a triple thick spot welded assembly. The robustness of the model to predict partial failure modes and strengths of a triple thick spot weld has been demonstrated. In addition, FE modelling methodology was extended to another welding type: arc welding. In this case, two sheets are welded in ab overlap configuration with a filler wire. FE model allows predicting the failure zone and strength of welded assembly
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Inacio, Da Rosa Gregory. "Mechanisms and consequences of boron segregation at austenite grain boundaries in advanced high strength steels." Thesis, Aix-Marseille, 2018. http://www.theses.fr/2018AIXM0041/document.
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L’objectif de cette thèse est d’aboutir à une meilleure compréhension des mécanismes de ségrégation du bore aux joints de grains austénitiques (γGB) et de leur effet sur la décomposition de l’austénite. En effet, l’addition de très faibles quantités de bore comme élément d’alliage permet d’augmenter de manière remarquable la résistance mécanique des aciers à très haute résistance. Cet effet est lié à l’état du bore aux γGBs qui décale la cinétique de décomposition de l’austénite.Tout d’abord, la distribution du bore dans la microstructure a été identifiée de manière très précise à l’aide des analyses de la même zone par Nano-SIMS et par MEB. De plus, le couplage de la sonde atomique tomographique et du Nano-SIMS a apporté une meilleure quantification de l’état du bore dans la microstructure. Ces études ont été réalisées après différents traitements thermiques qui ont été conçus spécifiquement pour étudier séparément chaque mécanisme. L’ensemble de ces résultats permet d’écarter la contribution de la ségrégation hors équilibre et confirme la présence d’un équilibre local entre les γGBs et la solution solide dans leurs voisinages. Par conséquent, le niveau de ségrégation du bore aux γGBs est contrôlé par l’état de précipitation des borures qui définit la concentration du bore en solution solide.Par ailleurs, des mesures de DRX in situ et de dilatomètrie ont été effectuées afin de suivre les cinétiques de formation de la bainite. Les résultats montrent que la cinétique de formation de la bainite est retardée en augmentant la quantité de bore ségrégé, par contre l’augmentation de la taille de grain austénitique l’accélèreThe aim of this thesis is to lead to a better understanding of the mechanisms of boron segregation at austenite grain boundaries (γGB) and its effect on the austenite decomposition. Indeed, the small quantity of boron as alloying element remarkably improves the mechanical resistance of the advanced high strength steels. This effect is related to the boron state at γGBs, which delays the kinetics of austenite decomposition.The boron distribution in the microstructure was precisely identified thanks to the analyses of the same field by using correlative nano-SIMS and SEM. In addition, the coupling of APT and nano-SIMS provided a better quantification of the boron state in the microstructure. These studies were performed after different heat treatments, which were specifically designed to study separately each mechanism. The results excludes the contribution of non-equilibrium segregation mechanism on boron segregation at γGBs and confirm the local equilibrium between the γGBs and the solid solution at the γGBs vicinity. Consequently, the level of boron segregation at γGBs is controlled by boride precipitation, which controls the concentration of boron in solid solution.Measurements of in situ XRD and the dilatometry were performed in order to follow the kinetics of bainite formation. The specific heat treatments were applied before bainite formation in order to study the effect of boron segregated amount at γGBs and the austenite grain size. These results show that the kinetics of bainitic transformation is delayed by the increase of boron segregated amount. Whereas, the increasing of austenite grain size accelerates the kinetics of bainitic transformation
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Aydin, Huseyin. "Effect of microstructure on static and dynamic mechanical properties of third generation advanced high strength steels." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119617.
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The recent studies on steels have largely focused on the development of new advanced high strength sheet steels (AHSS), particularly for automotive applications. "First generation AHSS" are steels that primarily possess ferrite-based microstructures with tensile strength (in the as-rolled condition) in the range of 450 to 700 MPa, and "second generation AHSS" are austenitic steels with high manganese content in the range of 900 MPa to 1100 MPa tensile strength. Recently, there has been interest in the development of a "third generation" of AHSS, which are steels with strength-ductility combinations better than the first generation AHSS with a range of 20 000 MPa x %ε but at a cost significantly less than that of the second generation AHSS as a result of reducing expensive alloying elements. Therefore, the current approach to the development of third generation AHSS is to combine all the aspects of first and second generation steels in unique alloy/microstructure combinations to achieve the desired properties. Thus, the third generation of AHSS microstructures consists of a high strength phase (e.g., martensite or bainite) and a significant amount of ductility and work hardening from an austenite that exhibits deformation induced plasticity through transformation or twinning. In this thesis, four different steel compositions, centered on Mn as the main alloying element, are designated as candidates for third generation AHSS grades. The design of these steels is based on controlling the deformation behavior of the retained austenite. Thus, heat treatment process parameters are determined in order to obtain different amounts and morphologies of retained austenite. The evolution of the microstructure, during processing as well as deformation, is characterized by using optical and electron microscopy techniques and mechanical tests. The effect of alloy composition and processing parameters on the deformation mechanisms of these steels is discussed.Les études récentes sur les aciers se sont surtout concentrées sur le développement avancé de nouvelles feuilles d'acier à haute résistance (AHSS, advanced high strength sheet steels, en anglais), particulièrement pour les applications automobiles. Les "AHSS de première génération" sont des aciers qui possèdent principalement des microstructures à base de ferrite ayant une résistance à la traction (à l'état brut de laminage) de l'ordre de 450 à 700 MPa tandis que les "AHSS de seconde génération" sont des aciers austénitiques à haute teneur en manganèse ayant une résistance à la traction de l'ordre de 900 à 1100 MPa. Récemment, un intérêt s'est manifesté pour le développement "d'AHSS de troisième génération" qui sont des aciers ayant une résistance et une ductilité combinées supérieures aux AHSS de Première Génération de l'ordre de 20 000 MPa x ε%, mais à un coût nettement moindre que celui requis pour les AHSS de seconde génération, réduisant ainsi le recours à des éléments d'alliage coûteux. Conséquemment, l'approche actuelle pour le développement d'AHSS de Troisième Génération est d'unir tous les aspects de la première et de la seconde génération d'aciers en des combinaisons uniques d'alliages et de microstructures qui permettront d'atteindre les propriétés désirées. Ainsi, les microstructures d'AHSS de troisième génération sont constituées d'une phase à haute résistance (e.g. Martensite ou bainite) et d'austénite dont la ductilité et l'écrouissage sont importants et qui possède une plasticité induite par déformation suite à une transformation ou un maclage. Dans cette thèse, quatre compositions d'aciers différents, centrés sur le Mn comme principal élément d'alliage, sont désignés comme candidats pour les grades d'AHSS de Troisième Génération. La conception de ces aciers est basée sur le contrôle du comportement à la déformation de l'austénite résiduelle. Par conséquent, les paramètres du procédé de traitement thermique sont déterminés de façon à obtenir différentes quantités et morphologies d'austénite résiduelle. L'évolution de la microstructure, au cours du traitement et de la déformation, est caractérisée par microscopie optique et électronique et des tests mécaniques. L'effet de la composition de l'alliage et des paramètres de traitement sur les mécanismes de déformation des aciers est discuté.
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Groseclose, Adam Richard. "Forming of AHSS using Servo-Presses." The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1408548321.
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Atwood, Lorne Steele. "Friction Bit Joining of Dissimilar Combinations of GADP 1180 Steel and AA 7085 – T76 Aluminum." BYU ScholarsArchive, 2016. https://scholarsarchive.byu.edu/etd/6400.
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Friction Bit Joining (FBJ) is a method used to join lightweight metals to advanced high-strength steels (AHSS). The automotive industry is experiencing pressure to improve fuel efficiency in their vehicles. The use of AHSS and aluminum will reduce vehicle weight which will assist in reducing fuel consumption. Previous research achieved joint strengths well above that which was required in three out of the four standard joint strength tests using DP980 AHSS and 7075 aluminum. The joints were mechanically tested and passed the lap-shear tension, cross-tension, and fatigue cycling tests. The t-peel test configuration never passed the minimum requirements. The purpose of continuing research was to increase the joint strength using FBJ to join the aluminum and AHSS the automotive industry desires to use specifically in the t-peel test. In this study FBJ was used to join 7085 aluminum and GADP1180 AHSS. The galvanic coating on the AHSS and its increased strength with the different aluminum alloy required that all the tests be re-evaluated and proven to pass the standard tests. FBJ is a two-step process that uses a consumable bit. In the first step the welding machine spins the bit to cut through the aluminum, and the second step applies pressure to the bit as it comes in contact with the AHSS to create a friction weld.
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Bergmann, Clemens [Verfasser], Michael [Gutachter] Pohl, and Werner [Gutachter] Theisen. "Hydrogen embrittlement resistance evaluation of advanced high strength steel grades in automotive applications / Clemens Bergmann ; Gutachter: Michael Pohl, Werner Theisen ; Fakultät für Maschinenbau." Bochum : Ruhr-Universität Bochum, 2020. http://d-nb.info/1219736635/34.
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