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Journal articles on the topic "Backstress"
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Servatan, M., S. M. Hashemi, and A. Varvani-Farahani. "Ratcheting Simulation of Additively Manufactured Aluminum 4043 Samples through Finite Element Analysis." Applied Sciences 13, no. 20 (2023): 11553. http://dx.doi.org/10.3390/app132011553.
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This study presents a finite element-based ratcheting assessment of additively manufactured aluminum 4043 samples undergoing asymmetric loading cycles. The Chaboche material model in ANSYS was utilized and the effects of mesh and element type were examined. Different element numbers were used in a thorough convergence study to obtain independent meshing structures. The coefficients of this model were defined through stress–strain hysteresis loops determined from the strain-controlled tests. The backstress evolution and the corresponding yield surface translation in the deviatoric stress space were discussed as three different mesh elements of linear brick, quadratic tetrahedron, and quadratic brick were adopted. The magnitude of backstress was affected as different element types were employed. The first-order brick elements resulted in the highest backstress increments, while the lowest backstresses were determined when quadratic brick elements were taken. Backstress increments are positioned in an intermediate level with the use of quadratic tetrahedron elements. The choice of the element type, shape, and number influenced material ratcheting response over the loading process. The use of quadratic brick elements elevated the simulated ratcheting curves. The quadratic tetrahedron and linear brick elements, however, suppressed ratcheting level as compared with those of experimental data. The closeness of the simulated ratcheting results to those of the measured values was found to be highly dependent on these finite element variables.
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McDowell, D. L. "A Bounding Surface Theory for Cyclic Thermoplasticity." Journal of Engineering Materials and Technology 114, no. 3 (1992): 297–303. http://dx.doi.org/10.1115/1.2904176.
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A nonisothermal, rate and time independent generalization of nonlinear kinematic hardening theory for cyclic plasticity is introduced. The model includes decomposition of backstress and of isotropic hardening between the yield surface radius and the backstress amplitude. A purely temperature dependent component of yield surface radius is assumed in addition to an isotropic hardening component. Issues of thermoplastic material stability and temperature history independence are clearly distinguished and addressed via implications of temperature rate terms. Correlations are reported for OFHC copper subjected to thermomechanical cyclic loading.
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Kassner, M. E., P. Geantil, L. E. Levine, and B. C. Larson. "Backstress, the Bauschinger Effect and Cyclic Deformation." Materials Science Forum 604-605 (October 2008): 39–51. http://dx.doi.org/10.4028/www.scientific.net/msf.604-605.39.
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Backstresses or long range internal stresses (LRIS) in the past have been suggested by many to exist in plastically deformed crystalline materials. Elevated stresses can be present in regions of elevated dislocation density or dislocation heterogeneities in the deformed microstructures. The heterogeneities include edge dislocation dipole bundles (veins) and the edge dipole walls of persistent slip bands (PSBs) in cyclically deformed materials and cell and subgrain walls in monotonically deformed materials. The existence of long range internal stress is especially important for the understanding of cyclic deformation and also monotonic deformation. X-ray microbeam diffraction experiments performed by the authors using synchrotron x-ray microbeams determined the elastic strains within the cell interiors. The studies were performed using, oriented, monotonically deformed Cu single crystals. The results demonstrate that small long-range internal stresses are present in cell interiors. These LRIS vary substantially from cell to cell as 0 % to 50 % of the applied stress. The results are related to the Bauschinger effect, often explained in terms of LRIS.
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Borstad, C. P., E. Rignot, J. Mouginot, and M. P. Schodlok. "Creep deformation and buttressing capacity of damaged ice shelves: theory and application to Larsen C ice shelf." Cryosphere Discussions 7, no. 4 (2013): 3567–610. http://dx.doi.org/10.5194/tcd-7-3567-2013.
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Abstract. Around the perimeter of Antarctica, much of the ice sheet discharges to the ocean through floating ice shelves. The buttressing provided by ice shelves is critical for modulating the flux of ice into the ocean, and the presently observed thinning of ice shelves is believed to be reducing their buttressing capacity and contributing to the acceleration and thinning of the grounded ice sheet. However, relatively little attention has been paid to the role that fractures play in the flow and stability of ice shelves and their capacity to buttress the flow of grounded ice. Here, we develop an analytical framework for describing the role that fractures play in the creep deformation and buttressing capacity of ice shelves. We apply principles of continuum damage mechanics to derive a new analytical relation for the creep of an ice shelf as a function of ice thickness, temperature, material properties, resistive backstress and damage. By combining this analytical theory with an inverse method solution for the spatial rheology of an ice shelf, both backstress and damage can be calculated. We demonstrate the applicability of this new theory using satellite remote sensing and Operation IceBridge data for the Larsen C ice shelf, finding damage associated with known crevasses and rifts. We find that increasing thickness of mélange between rift flanks correlates with decreasing damage, with some rifts deforming coherently with the ice shelf as if completely healed. We quantify the stabilizing backstress caused by ice rises and lateral confinement, finding high backstress associated with two ice rises that likely stabilize the ice front in its current configuration. Though overall the ice shelf appears stable at present, the ice in contact with the Bawden ice rise is weakened by fractures, and additional damage or thinning in this area could portend significant change for the shelf. Using this new approach, field and remote sensing data can be utilized to monitor the structural integrity of ice shelves, their ability to buttress the flow of ice at the grounding line, and thus their indirect contribution to ice sheet mass balance and global sea level.
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Benn, Douglas I., Adrian Luckman, Jan A. Åström, et al. "Rapid fragmentation of Thwaites Eastern Ice Shelf." Cryosphere 16, no. 6 (2022): 2545–64. http://dx.doi.org/10.5194/tc-16-2545-2022.
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Abstract. Ice shelves play a key role in the dynamics of marine ice sheets by buttressing grounded ice and limiting rates of ice flux to the oceans. In response to recent climatic and oceanic change, ice shelves fringing the West Antarctic Ice Sheet (WAIS) have begun to fragment and retreat, with major implications for ice-sheet stability. Here, we focus on the Thwaites Eastern Ice Shelf (TEIS), the remaining pinned floating extension of Thwaites Glacier. We show that TEIS has undergone a process of fragmentation in the last 5 years, including brittle failure along a major shear zone, formation of tensile cracks on the main body of the shelf, and a release of tabular bergs on both the eastern and western flanks. Simulations with the Helsinki Discrete Element Model (HiDEM) show that this pattern of failure is associated with high backstress from a submarine pinning point at the distal edge of the shelf. We show that a significant zone of shear, upstream of the main pinning point, developed in response to the rapid acceleration of the shelf between 2002 and 2006, seeding damage on the shelf. Subsequently, basal melting and positive feedback between damage and strain rates weakened TEIS, allowing damage to accumulate. Thus, although backstress on TEIS has likely diminished over time as the pinning point shrunk, accumulation of damage has ensured that the ice in the shear zone remained the weakest link in the system. Experiments with the BISICLES ice-sheet model indicate that additional damage to or unpinning of TEIS is unlikely to trigger significantly increased ice loss from WAIS, but the calving response to the loss of TEIS remains highly uncertain. It is widely recognised that ice-shelf fragmentation and collapse can be triggered by hydrofracturing and/or unpinning from ice-shelf margins or grounding points. Our results indicate a third mechanism, backstress triggered failure, that can occur if and when an ice shelf is no longer able to withstand stress imposed by pinning points. In most circumstances, pinning points are essential for ice-shelf stability, but as ice shelves thin and weaken, the concentration of backstress in damaged ice upstream of a pinning point may provide the seeds of their demise.
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Wolff, M., M. Böhm, M. Dalgic, G. Löwisch, N. Lysenko, and J. Rath. "Parameter identification for a TRIP model with backstress." Computational Materials Science 37, no. 1-2 (2006): 37–41. http://dx.doi.org/10.1016/j.commatsci.2005.12.007.
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Voyiadjis, G. Z., and P. I. Kattan. "Phenomenological evolution equations for the backstress and spin tensors." Acta Mechanica 88, no. 1-2 (1991): 91–111. http://dx.doi.org/10.1007/bf01170595.
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Foga, Steve, Leigh A. Stearns, and C. J. van der Veen. "Application of Satellite Remote Sensing Techniques to Quantify Terminus and Ice Mélange Behavior at Helheim Glacier, East Greenland." Marine Technology Society Journal 48, no. 5 (2014): 81–91. http://dx.doi.org/10.4031/mtsj.48.5.3.
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AbstractIceberg calving is an efficient mechanism for ice mass loss, and rapidly calving glaciers are often considered to be inherently unstable. However, the physical controls on calving are not well understood. Recent studies hypothesize that the presence of a rigid ice mélange (composed of icebergs, bergy bits, and sea ice) can reduce iceberg calving by providing “backstress” to the terminus. To test this hypothesis we use remote sensing techniques to construct a time series model of calving rate and size and composition of the adjacent ice mélange. We describe a semi-automated routine for expediting the digitization process and illustrate the methods for Helheim Glacier, East Greenland, using 2008 data. Ice velocities of the glacier terminus and ice mélange are derived with feature-tracking software applied to radar imagery, which is successfully tracked year-round. Object-based image analysis (OBIA) is used to inventory icebergs and sea ice within the ice mélange. We find that the model successfully identifies the calving rate and ice mélange response trends associated with seasonal increases in terminus retreat and advance and shows seasonal trends of ice mélange potentially providing seasonal backstress on the glacier terminus.
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Wierzba, Bartek, Tsutomu Mashimo, and Marek Danielewski. "Competition between Chemical and Gravity Forces in Binary Alloys." High Temperature Materials and Processes 37, no. 3 (2018): 285–88. http://dx.doi.org/10.1515/htmp-2016-0194.
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AbstractWe present the first experimental results of the growth of Cu6Sn5 and Cu3Sn under an external force field – during the sedimentation process. The results are compared with the proposed model under the combined influence of the Kirkendall effect, backstress and sedimentation. It is shown that Cu3Sn consumes the preformed Cu6Sn5 phase during sedimentation under a strong gravitational field of 779,000 G at 200 °C.
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Brahme, Abhijit P., Kaan Inal, Raja K. Mishra, and S. Saimoto. "The backstress effect of evolving deformation boundaries in FCC polycrystals." International Journal of Plasticity 27, no. 8 (2011): 1252–66. http://dx.doi.org/10.1016/j.ijplas.2011.02.006.
Full textDissertations / Theses on the topic "Backstress"
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Lamar, Andrew. "Bounding surface plasticity theory with backstress decomposition and material memory." Thesis, Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/15857.
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Jimenez, Reyes Jose. "Experimental and numerical contribution to the study of transformation induced plasticity (TRIP)." Electronic Thesis or Diss., Normandie, 2024. http://www.theses.fr/2024NORMIR20.
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Ce travail a été réalisé dans le but d'apporter une contribution à l'étude de la plasticité induite par transformation (TRIP) à travers des analyses expérimentales et numériques. Pour ce faire, du point de vue expérimental, un programme expérimental original sur l'acier 35NCD16 observant les comportements des transformations austénite → martensite est réalisé sous différentes conditions de chargement. Les résultats indiquent que : (i) le TRIP n'a pas d'effet mémoire significatif, (ii) le matériau peut présenter une récupération de l'écrouissage pendant la transformation martensite → austénite, (iii) une dissymétrie entre les réponses du TRIP en tension et en compression a été observée, (iv) l'existence d'une contre-contrainte du TRIP a été révélée, (v) la quantité de TRIP final peut être considérée comme proportionnelle à la norme de la contrainte appliquée tant que cette dernière ne dépasse pas la limite d'élasticité, et (vi) une légère orientation physique a toutefois été observée lors de l'analyse microstructurale de notre matériau, ce qui pourrait suggérer une faible contribution du mécanisme de Magee dans le TRIP. Quant au côté numérique, les capacités prédictives du modèle Leblond – Taleb – Sidoroff (LTS) et de sa version actualisée (U – LTS) sont mises à l'épreuve en simulant les essais expérimentaux réalisés dans le cadre de cette étude. Nos résultats montrent que (i) le modèle LTS surestime les résultats expérimentaux, (ii) le modèle LTS ne parvient pas à décrire la dissymétrie entre les réponses TRIP en tension et en compression, (iii) en utilisant le modèle U – LTS, les résultats comparatifs montrent d'excellentes capacités à simuler la base de données expérimentale réalisée dans ce travail, en particulier par rapport au modèle LTS. Bien que nos résultats expérimentaux aient montré des résultats significatifs, en particulier lorsqu'ils sont comparés aux modèles observés dans notre étude, il est recommandé de poursuivre les recherches en prenant en compte d'autres aciers et d'autres transformations<br>This thesis has been created with the objective of providing a contribution to the study of transformation induced plasticity (TRIP) via experimental and numerical analyses. To do so, from the experimental point of view, an original experimental program on 35NCD16 steel observing the behaviors of austenite → martensite transformations were performed under different loading conditions. The results indicate that: (i) TRIP does not hold any significant memory effect, (ii) the material may present recovery from strain hardening during the martensite → austenite transformation, (iii) a dissymmetry between the TRIP responses under tension and compression were observed, (iv) the existence of TRIP backstress was revealed, (v) the amount of final TRIP may be considered proportional to the norm of the applied stress as long as the latter does not exceed the yield stress, and (vi) a slight physical orientation has however been observed when carrying out microstructural analyses on our material, which could suggest a small contribution of the Magee mechanism in TRIP. As for the numerical side, the predictive capabilities of the Leblond – Taleb – Sidoroff (LTS) model, and its updated version (U – LTS) are put to the test by simulating the experimental tests performed for this study. Our results show that: (i) the LTS model overestimates the experimental results, (ii) the LTS model fails in describing the dissymmetry between the TRIP responses under tension and compression, (iii) when using the U – LTS model, the comparative results show excellent capabilities in simulating the experimental data base performed in this work, especially compared to the LTS model. While our experimental results showed significant results, especially when put up against to the models observed in our study, further research considering other steels and other transformations is recommended
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Jones, Benjamin David. "Digital butterflies of the backstreets : participatory art and the digital divide." Thesis, University of Newcastle upon Tyne, 2015. http://hdl.handle.net/10443/3063.
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Through this practice-based research I interrogate the terms community art, activism, social and digital media, as well as community and place. It is an investigation into both theoretical and practical aspects of community art practice and its connection to national and local policies on community, arts and digital media. It considers the increasing role digital technology and social media have in communities and community organisations, in particular under the guise of austerity, and how community organisations (do not) use social media and digital technology to encourage participation. It considers my position and role as an artist, curator and resident within the community that I live in and how, through becoming active and engaged with the place, I can develop a strategy for sustainable and long-term social engagement. This practice based Ph.D. takes as a starting point the stalled housing regeneration, due to the halting of the Housing Market Renewal Pathfinder, in the community where I live. The research builds upon my experience of working as an artist, curator and arts educator since the turn of the millennium where, under consecutive governments the purpose of contemporary art and its educational use has either been to effect social change (New Labour) or its economic value (current coalition government). Through the creation of a series of participatory and digital engagement events and workshops the research interrogates and considers the connections and conflict between the ‘physical’ (public space) and the digital and supposedly ‘open’ (the online). The research will be of use to those who feel an increasing and urgent need to engage with their own community as practitioners as well as community members.
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Wang, Yi-Ting, and 汪怡婷. "Edge of the situation ─ The “backstreet” notes." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/xewfe7.
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碩士<br>國立臺灣藝術大學<br>版畫藝術研究所<br>100<br>his creative discussion "Edge of the situation ─ The “backstreet” notes" content is based on my National Taiwan University of Arts in the "Backstreet" life experience and observation as a starting point, to create works of art and creative description.
In the course of my growth, for living in the crude environments temporarily and moving to different places many times, they become a memory deep in my mind. After I came to Taipei for study, seeing “the backstreet” due to the influences of the land requisition by government, land recovery by school in late 2007 and urban renewal of Fu-Zhou District in 2009, the neighbors’ change in mood, then they faced the problems and the solution; Lots of things that I seem to have met before, they aroused the author’s regret in the past that the writer couldn’t catch up. So, the writer decided to keep the memory of "the backstreet" on doing works, tend to connecting the impression among the society, by the notice of the countryside realized that the city, and fulfill the author’s imaging.
The thesis is divided into five parts. The first chapter of the paper is introduction, it shows that the motivation, purpose, methodology and scope of the research, and explain the words “edge”, “situation” and “backstreet” in the definition of terms. The second chapter is the foundation of academic theory, and it emphasizes urban planning, discusses the condition of the role in the urban city; then it focuses on the sociology of art, explains the politics contents on art working in different periods and the artists how to use “art” to concern our society. The third chapter is the author’s invented ideas. The author do the analysis and own thoughts from the data which the writer observed and collected, and they change into the footstone of this working arts; however, it expounds the past own experiences to get the origin of existence feeling, and through the body to be a media to get the way to expand the art into place. The fourth chapter is invented analysis. It claims that the series content of creation, skills and materials, then announce the ideas of individual artworks. The fifth chapter is conclusion, for these invented experiences in personal creative process, and describe the possible of the develop in the future.
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"Why do women opt for backstreet abortions?: a sociological study." Thesis, 2008. http://hdl.handle.net/10210/1322.
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M.A.<br>In 1996, the South African Government implemented the Choice on Termination (CTOP) Act to curb maternal mortality. This study emerged after an observation by experts in the field that there were a growing number of women who were admitted in antenatal care units with incomplete abortions, commonly known as miscarriage. These experts observed that some of these women, on closer observation, had signs of either scared wombs or had foreign objects in their uteri. This was understandable in provinces where there were fewer health facilities offering TOP serves. In Gauteng, however, it was not understandable because this province has the most designated facilities in the country. It was, therefore, necessary to investigate why women had unwanted pregnancies and, more so, why they opted for back-street abortion services instead of accessing the many available legal services. It was discovered that most of these women had low socio-economic status, did not use contraceptives and did not know about the CTOP Act. The few who did know about the Act, did not use the legal abortion options because of the fear of being judged by health care workers. In addition, some of these women were turned away from health facilities because the list of women waiting for these services, was too long. Involved in assisting these women to terminate their pregnancies illegally were mothers, sisters, boyfriends/partners, traditional healers, pharmacists, nurses and doctors. It is suggested that intense education on the CTOP Act should be rolled out to the whole community; contraceptive services should be improved; health education with the emphasis on reproductive health and services should be expanded to private doctors and midwives.<br>Dr. Ria Smit Prof. J.M. Uys
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Akinde, Elizabeth Nkugbo. "Knowledge of adolescents on abortion in Lagos University Teaching Hospital complex." Diss., 2009. http://hdl.handle.net/10500/3780.
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The study sought to assess the knowledge of adolescents regarding abortion in a selected area in Lagos Nigeria. The researcher used a non-experimental, exploratory, descriptive research design for the study. One hundred adolescents participated in the study.
The study found that many adolescents will not admit to having had an abortion. Moreover, the respondents gave different meanings for abortion, had inadequate knowledge of abortion and sexual and reproductive health. Cultural taboos and religious beliefs have a great impact on adolescents’ sexual behaviour. Most adolescents would not access abortions services because they regard it as killing an innocent baby.
Efforts should be strengthened to make contraceptives and family life education available and accessible to the adolescents.<br>Health Studies<br>Thesis (M.A. (Health Studies))
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Mkhize, Bonginkosi Alloys. "The Termination of Pregnancy Act of 1996 : a theological ethical evaluation of abortion on demand." Diss., 2000. http://hdl.handle.net/10500/17465.
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This dissertation deals with a theological-ethical evaluation of the Termination of
Pregnancy Act of 1996 on the area of abortion on demand. It aims at empowering
women and also solving the problem of backstreet abortion.
Chapter one gives a brief introduction to the Termination of Pregnancy Act of 1996.
Chapter two gives a historical background of abortion and the factors tbat eventually
led to the Termination of Pregnancy Act of 1996.
Chapter three focuses mainly on the teachings of the Roman Catholic Church on
abortion. Issues relating to the value of human life are discussed in this chapter.
Is~~es relating to the Termination of Pregnancy Act of 1996 and their theologicalethical
in.Jplications are discussed in this chapter, i.e. chapter four.
Empowering of women, sex education, instilling good moral values to the youth and
also changing the pastoral attitude of churches towards sexuality can help to alleviate
the problem of unwanted pregnancy.<br>Philosophy, Practical & Systematic Theology<br>M. Th. (Theological Ethics)
Books on the topic "Backstress"
More sourcesBook chapters on the topic "Backstress"
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Brahme, Abhijit, Raja K. Mishra, and Kaan Inal. "A New Crystal Plasticity Based Constitutive Model Incorporating Backstress for FCC Polycrystals." In ICAA13: 13th International Conference on Aluminum Alloys. John Wiley & Sons, Inc., 2012. http://dx.doi.org/10.1002/9781118495292.ch127.
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Aktaa, Jarir, and Bernd Schinke. "A Model for Damage and Lifetime Prediction Taking into Account the Backstress." In Low Cycle Fatigue and Elasto-Plastic Behaviour of Materials—3. Springer Netherlands, 1992. http://dx.doi.org/10.1007/978-94-011-2860-5_113.
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Brahme, Abhijit, Raja K. Mishra, and Kaan Inal. "A New Crystal Plasticity Based Constitutive Model Incorporating Backstress for FCC Polycrystals." In ICAA13 Pittsburgh. Springer International Publishing, 2012. http://dx.doi.org/10.1007/978-3-319-48761-8_127.
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Sargeant, Dane, Md Zahidul, Rishabh Sharma, Marko Kenezevic, David T. Fullwood, and Michael P. Miles. "Modeling of Springback Behavior in AA6016-T4 Sheet via an Elastoplastic Self-consistent Model Incorporating Backstress." In Light Metals 2022. Springer International Publishing, 2022. http://dx.doi.org/10.1007/978-3-030-92529-1_37.
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Sargeant, Dane, Md Zahidul Sarkar, Rishabh Sharma, Marko Knezevic, David Fullwood, and Michael Miles. "Modeling the Effect of Backstress on Springback Predictions in AA 6016-T4 as a Function of Pre-strain." In Lecture Notes in Mechanical Engineering. Springer Nature Switzerland, 2023. http://dx.doi.org/10.1007/978-3-031-40920-2_70.
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Sieglohr, Ulrike. "From ‘Backstreet Marilyn’ to New Marlene: The Rise to International Fame as a Star Directed by Fassbinder, 1975–82." In Hanna Schygulla. British Film Institute, 2014. http://dx.doi.org/10.1007/978-1-84457-828-3_3.
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"The Backstreets." In The Backstreets. Columbia University Press, 2022. http://dx.doi.org/10.7312/turs20290-002.
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Sperber, Daniel. "Roads and Backstreets." In The City in Roman Palestine. Oxford University Press, 1999. http://dx.doi.org/10.1093/oso/9780195098822.003.0011.
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Let us now look at the roads and side streets in the Roman Palestinian town. The literary evidence about the width of such streets is somewhat problematic. In the baraita in B. Baba Batra 99ab we read as follows: . . .A private path is four cubits wide, a path from one town to another is eight cubits, a public path 16 cubits, and a path to the cities of refuge 32 cubits wide. . . . Likewise, M. Baba Kama .5, in the name of Rabbi Eliezer (late first century C.E.), tells us that a standard public path is 16 cubits wide. If we assume the cubit equals approximately 70 cm, we arrive at the following approximate road widths: . . .private path 2.80 m (=8.5 ft.). . . . . .from one town to another 5.60 m (=17 ft.). . . . . .public path 11.20 m (=34 ft.). . . . . .to cities of refuge 22.40 m (=68 ft.). . . This pattern does not correspond to the standard Roman road measurement. Most major Roman roads were about 16 ft. wide (10.5 cubits) and rarely more than 21.5 ft. wide (14 cubits). The narrower streets (angipontus or semitae) had to be at least 9.57 ft. (2.9 m) wide (a little more than 4 cubits) to allow for projecting balconies. The great trunk roads through Gaul or Italy or along the Euphrates frontier in Syria might be 24 ft. wide (16 cubits). Apparently, some roads were even broader than this, since the Pergamene law states that the minimum width of a main country road must be 30 ft. and that of a byroad 12 ft. Krauss noted these discrepancies, writing that “ordinary Roman stratae were about 5 m wide, making the Rabbinic stratae some 3 m broader, and we do not know wherefore there was this great difference between them.” He adds that in the “Palestinian town of Petra there are remains of the Roman road, which is only 2.8 m wide, and must therefore be considered as a via secundaria, but we cannot determine what is its equivalence in Rabbinic parlance.”
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"Contents." In The Backstreets. Columbia University Press, 2022. http://dx.doi.org/10.7312/turs20290-toc.
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"Introduction." In The Backstreets. Columbia University Press, 2022. http://dx.doi.org/10.7312/turs20290-001.
Full textConference papers on the topic "Backstress"
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Green, Richard, Jonathan Douglas, Andrew Moffat, and Brent Scaletta. "A Constitutive Model for Predicting Modified Creep Strain Rates due to Cyclic Loading of Gas Turbine Components." In ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/gt2019-91521.
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Abstract The traditional approach to the design of industrial Gas Turbines considers base load operation. This assumption is no longer applicable as owner operators require more operational flexibility and increased availability and reliability. Flexibility in operation manifests as increased cyclic loading and variations in on and off load dwell periods and thermal loads. These complex loading profiles inevitably lead to damage from both creep and fatigue, and interaction of these two damage mechanisms over the duration of the service interval. These interactions can result in higher average creep rates and more damage than expected. Robust, path dependent modeling approaches are required to better understand the effects of flexible operation on material response and subsequent damage. Moreover, a unified approach to creep-fatigue is significantly more effective at capturing this behavior. There are several types of interactions that can drive additional damage. These include relatively well understood mechanisms, such as the effect of plasticity on primary creep and the effect of creep dwells on cyclic material properties. Other interactions that are less well understood include interruptions to the load during creep dwells and the effects of off-load periods on the overall creep rate. This paper considers a constitutive approach to predict the modified creep rate due to load interruptions and off-load dwells using a backstress model. The backstress model is included in the calculation of inelastic strain rate equations, using a Chaboche type formulation. The model has been fitted to conventional material test data for typical superalloys used in gas turbine applications. To validate the approach, forward creep tests were conducted with varying interruptions to the load during the creep dwell period. These tests show a reduction in creep life and an increase in overall creep rate, when compared with the results for a constant stress and temperature condition. Previous work, presented by the authors [1], outlined a hypothesis that attributes the increase in overall creep rate to the influence of a recovery potential stress. This paper presents the subsequent work which demonstrates that the recovery potential stress can be defined by the difference between the applied stress and the backstress. It is shown that the dwell period between reload cycles is critical for calculating the recovery potential and overall creep rate.
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Moffat, Andrew, Richard Green, Calum Ferguson, and Brent Scaletta. "Development of a Constitutive Backstress Model for the Prediction of Creep and Stress Relaxation in Gas Turbine Materials." In ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2020. http://dx.doi.org/10.1115/gt2020-16191.
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Abstract There is a drive towards a broader range of fuels in industrial gas turbines, with higher levels of sulphur and potentially hydrogen. Due to these harsher environments, there is also a drive for corrosion resistant alloys and coatings. A number of key corrosion resistant superalloys, which are being employed to cope with these evolving conditions, exhibit primary creep. It is therefore imperative that fundamental material models, such as those for creep deformation, are developed to ensure they can accurately predict the material response to evolving operating conditions. The requirements for a creep model are complex. The model must be able to: predict forward creep deformation in regions dominated by primary loads (such as pressure); predict stress relaxation in regions dominated by secondary loads (such as differential thermal expansion); predict the effects of different creep hardening mechanisms. It is also clear that there is an interaction between fatigue and creep. With flexible operation, this interaction can be significant and should be catered for in lifing methods. A model that has the potential to account for the effect of plasticity on creep, and creep on plasticity is therefore desirable. In previous work the authors presented the concept for a backstress model to predict creep strain rates in superalloys. This model was fitted to a limited dataset at a single temperature. The approach was validated using simple creep-dwell tests at the same temperature. This paper expands on the previous work in several ways: 1) The creep model has been fitted over a wide range of temperatures. Including the effect of temperature in complex creep models presents a number of difficulties in model fitting and these are explored. 2) The model was fitted to constant load (forward creep) and constant strain (stress relaxation) tests since any creep model should be able to predict both forms of creep deformation. However, these are often considered separately due to the difficulty of fitting models to two different datasets. 3) The creep deformation model was validated on stress change tests to ensure the creep deformation response can cope with changes in response variables. 4) The approach was validated using creep-fatigue tests to show that the creep deformation model, in addition to our established fatigue models, can predict damage in materials under complex loading.
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Lapczyk, Ireneusz, and Juan A. Hurtado. "A Viscoelastic-Elastoplastic Finite Strain Framework for Modeling Polymers." In ASME 2014 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/imece2014-36831.
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In this paper we present a new constitutive framework, the Parallel Rheological Framework (PRF), for modeling polymers that has been recently developed by the authors and implemented in the commercial finite element software Abaqus [1]. The framework is based on parallel finite-strain viscoelastic and elastoplastic networks. For each viscoelastic network a multiplicative split of the deformation gradient into elastic and viscous components is assumed. The evolution of the viscous component of the deformation gradient is governed by a flow rule obtained assuming the existence of a creep potential. The flow rule is expressed as a function of stress invariants and internal variables, and different evolution laws for the internal variables are allowed within the framework of the model. Similar to the viscoelastic networks, the deformation gradient in the elastoplastic network is decomposed into elastic and plastic components. The yield surface is defined assuming combined isotropic/kinematic hardening. The yield surface is a function of a scalar internal variable that describes isotropic hardening, and a tensorial internal variable (backstress) that describes the shift of the yield surface in the stress space. The evolution of the scalar variable is governed by associated flow rule, while the evolution of backstresses is determined by the Armstrong-Frederick law [2], which is extended to finite-strain deformations. Finally, stress softening is introduced into an elastoplastic network using a modified version of Ogden and Roxbourgh’s pseudo-elasticity model [3]. This paper presents an outline of the framework, including two recent enhancements: a new creep model (the power law model) and combined isotropic/kinematic hardening plasticity model. The framework is then applied to analyze numerically the uniaxial loading/unloading behaviors of filled natural rubber and an EPDM polymer. The results obtained using finite element simulations show very good correlation with experimental data.
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Islam, Nazrul, and Tasnim Hassan. "Uniaxial Fatigue, Creep and Ratcheting Response Simulations of Alloy 617 Using Damage Coupled Viscoplastic Model." In ASME 2018 Pressure Vessels and Piping Conference. American Society of Mechanical Engineers, 2018. http://dx.doi.org/10.1115/pvp2018-84756.
Full textAbstract:
A damage coupled unified constitutive model (UCM-CDM) is developed in this study to predict uniaxial fatigue, fatigue-creep, ratcheting and creep responses of Alloy 617. The experimental data used for validating the UCM-CDM included these responses for different strain rates and strain ranges for temperatures 760–1000°C. Rate dependent modeling features like Norton’s power law, static recovery, and isotropic damage evolution law are incorporated in an existing UCM for improving simulations of short-term stress relaxation and long-term creep responses. A backstress threshold modeling feature is incorporated in the UCM-CDM for improving ratcheting prediction for a wide range of mean and amplitude stresses. Simulations of the creep responses, including the primary, secondary and tertiary creep responses, in addition to the fatigue, fatigue-creep and ratcheting responses using the UCM with one set of model parameter are examined.
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Gilman, Tim, Bill Weitze, Jürgen Rudolph, Adrian Willuweit, and Arturs Kalnins. "Using Nonlinear Kinematic Hardening Material Models for Elastic-Plastic Ratcheting Analysis." In ASME 2015 Pressure Vessels and Piping Conference. American Society of Mechanical Engineers, 2015. http://dx.doi.org/10.1115/pvp2015-45674.
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Applicable design codes for power plant components and pressure vessels demand for a design check against progressive plastic deformation. In the simplest case, this demand is satisfied by compliance with shakedown rules in connection with elastic analyses. The possible non-compliance implicates the requirement of ratcheting analyses on elastic-plastic basis. In this case, criteria are specified on maximum allowable accumulated growth strain without clear guidance on what material models for cyclic plasticity are to be used. This is a considerable gap and a challenge for the practicing CAE (Computer Aided Engineering) engineer. As a follow-up to two independent previous papers PVP2013-98150 ASME [1] and PVP2014-28772 [2] it is the aim of this paper to close this gap by giving further detailed recommendation on the appropriate application of the nonlinear kinematic material model of Chaboche on an engineering scale and based on implementations already available within commercial finite element codes such as ANSYS® and ABAQUS®. Consistency of temperature-dependent runs in ANSYS® and ABAQUS® is to be checked. All three papers together constitute a comprehensive guideline for elasto-plastic ratcheting analysis. The following issues are examined and/or referenced: • Application of monotonic or cyclic material data for ratcheting analysis based on the Chaboche material model • Discussion of using monotonic and cyclic data for assessment of the (non-stabilized) cyclic deformation behavior • Number of backstress terms to be applied for consistent ratcheting results • Consideration of the temperature dependency of the relevant material parameters • Consistency of temperature-dependent runs in ANSYS® and ABAQUS® • Identification of material parameters dependent on the number of backstress terms • Identification of material data for different types of material (carbon steel, austenitic stainless steel) including the appropriate determination of the elastic limit • Quantification of conservatism of simple elastic-perfectly plastic behavior • Application of engineering versus true stress-strain data • Visual checks of data input consistency • Appropriate type of allowable accumulated growth strain. This way, a more accurate inelastic analysis methodology for direct practical application to real world examples in the framework of the design code conforming elasto-plastic ratcheting check is proposed.
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Dewees, David J. "Application of Elastic-Plastic Design Data in the New ASME B&PV Code Section VIII Division 2." In ASME 2010 Pressure Vessels and Piping Division/K-PVP Conference. ASMEDC, 2010. http://dx.doi.org/10.1115/pvp2010-25641.
Full textAbstract:
The updating and re-writing of the ASME Boiler and Pressure Vessel Code, Section VIII Division 2 (2007) [1] has introduced several new and unique features. One of these features is the inclusion of specific materials data for use in elastic-plastic analysis of pressure vessel components. Both monotonic and cyclic stress strain curve models are provided, with supporting constants for a range of materials and temperatures. The elastic-perfectly plastic material model has been used in commercial Finite Element (FE) codes for many years to perform limit load and ratcheting analyses. The new material models and data of Section VIII Division 2 (S8D2) include strain hardening and are intended for use in deformation assessments, and for determining cyclic plastic strain ranges in fatigue evaluations. This paper presents one possible implementation of the Code models and data into a standard cyclic hardening model; the multiple backstress, nonlinear kinematic-hardening model of Chaboche, as implemented in the commercial Finite Element program Abaqus, versions 6.8 and later.