Academic literature on the topic 'Ratio yield strength to ultimate tensile strength'

The effect of the chemical composition, austenitisation temperature and tempering temperature and time on the mechanical properties and on the ballistic performance of martensitic steel armour plates was studied. It was established in this study that the mechanical properties and the ballistic performance of martensitic steels can be optimised by controlling the chemical composition and the heat treatment parameters. However, it was observed that for a given chemical composition of the steel the heat treatment parameters to be applied to advanced ballistic performance armour plates were different from those required for higher mechanical properties. Such a contradiction rendered the relationship between mechanical properties and ballistic performance questionable. Systematic analysis of the microstructure and the fracture mechanism of some martensitic armour plate steels was carried out to explain the improved ballistic performance of steels whose mechanical properties were below that specificied for military and security applications. It was inferred from phase analysis and its quantification by X-ray diffraction, characterisation of the martensite using scanning electron microscopy, transmission electron microscopy and atomic force microscopy that the retained austenite located in the plate interfaces and on grain boundaries of the martensite was the main constituent resisting localised yielding during ballistic impact on thin steel plates. A part of the kinetic energy is transformed into adiabatic heat where a reaustenitisation of the plate martensite and the formation of new lath martensite was observed. Another part is used to elastically and plastically deform the ballistic impact affected region around the incidence point. Dislocation pile-ups at twinned plate interfaces suggest that the twin interfaces act as barriers to dislocation movement upon high velocity impact loading. The diameter of the affected regions, that determines the volume of the material deforming plastically upon impact, was found to vary as a function of the volume fraction of retained austenite in the martensitic steel. Upon impact, retained austenite transforms to martensite by Transformation Induced Plasticity, the “ TRIP ” effect. High volume fractions of retained austenite in the martensitic steel were found to yield low values of the ratio yield strength to ultimate tensile strength (YS/UTS) and a high resistance against localised yielding and, therefore, against ballistic perforation. A Ballistic Parameter was proposed for the prediction of ballistic performance using the volume fraction of retained austenite and the thickness of the armour plate as variables. Based on the martensite structure and the results of the ballistic testing of 13 armour plate steels a design methodology comprising new specifications was proposed for the manufacture of armour plates whose thicknesses may be thinner than 6mm.<br>Dissertation (MSc (Metallurgical Engineering))--University of Pretoria, 2007.<br>Materials Science and Metallurgical Engineering<br>unrestricted

Thinner walled (about 6 mm thickness) line pipe steels for smaller diameter pipelines tend to have a relatively high ratio of yield strength to ultimate tensile strength (YS/UTS) of 0.93 or higher. This study focused on the effect of the microstructures, prior deformation in the austenite, cooling rate, coiling simulation and the additions of some micro-alloying elements on the YS/UTS ratio of a currently produced Nb-Ti and some experimental Nb-Ti-Mo line pipe steels. The experimental research included the design of the chemical compositions for five experimental alloys, simulation of the controlled hot rolling process, the determination of the strain-free as well as the strain affected continuous cooling transformation (CCT) diagrams, phase identification and quantitative assessment of the microstructures by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the latter especially on shadowed carbon extraction replicas and, tensile tests etc. This study indicated that the transformed microstructures of the alloys were a mixture of acicular ferrite plus polygonal ferrite and the volume fraction of acicular ferrite varied from 46.3 to 55.4%. Molybdenum additions did not markedly affect the formation of acicular ferrite after hot rolling and rapid cooling. The microstructural details of the acicular ferrite were successfully revealed by TEM on shadowed extraction replicas. This technique was useful to distinguish the acicular ferrite from the polygonal ferrite through a more smooth surface relief after etching in 2% Nital of the little etched polygonal ferrite whereas the deeper etched acicular ferrite showed parallel sets of internal striations. This made it possible to measure the volume fraction of acicular ferrite in the mixed microstructures of acicular ferrite and polygonal ferrite. The continuous cooling transformation behaviors of two alloys with no molybdenum and with 0.22% Mo were constructed with no prior deformation as well as with prior deformation of the austenite. Molybdenum additions shifted the strain-free CCT diagram towards longer times and expanded the region in which acicular ferrite formed from a cooling rate range of 0.3 to 5 ºCs-1 (Mo-free) to 0.1 to 15 ºCs-1 (with 0.22% Mo). However, its effect was significantly overshadowed by prior deformation in the austenite. The strain affected CCT diagrams for both alloys appear to be similar. The prior deformation had a stronger effect on the CCT diagram than molybdenum additions and promoted acicular ferrite formation, whereas it suppressed the formation of bainite. The prior deformation had two effects in acicular ferrite formation: it promoted nucleation and suppressed its growth and, therefore, resulted in a finer overall grain size. The effect on the YS/UTS ratio at various cooling rates ranging from 1 to 34, 51, 54 or 60 ºCs-1 was investigated in three cases: (i) without prior deformation and coiling simulation, (ii) with no prior deformation but with coiling simulation at 575 and 600 ºC and, (iii) with prior deformation of 33% reduction in the austenite below the Tnr followed by coiling simulation at 575 ºC for 1 hour. It was determined that the YS/UTS ratio was a function of the microstructure and cooling rate in the case treatment (i)) without any coiling simulation and prior deformation. The coarse bainite or acicular ferrite, which was formed at high cooling rates, raised the YS/UTS ratio under conditions of no deformation prior to the transformation. The yield strength and ultimate tensile strength also increased with an increase in cooling rate. With coiling conditions (treatment (ii)), the ratio was not sensitive to the cooling rate or the microstructure for the reference Mo-free alloy #6 because the coiling allows recovery of dislocations, thereby decreasing the difference in dislocation density that had arisen between a low and a high cooling rate. The YS/UTS ratio ranged from 0.75 to 0.8 after a simulated coiling at 575 ºC and from 0.76 to 0.78 after a coiling simulation at 600 ºC. Prior deformation (treatment (iii)) in the austenite raised the ratio from 0.81 to 0.86. However, the YS/UTS ratio was not sensitive to cooling rate after coiling at 575 ºC for 1 hour in the cases with and without prior deformation in the austenite. Deformation with a 33% reduction below the Tnr prior to the transformation increased the yield strength more than the ultimate tensile strength, leading to a high YS/UTS ratio that ranged from 0.81 to 0.86. The prior deformation, therefore, had a stronger effect on the YS/UTS ratio than the microstructure. Towards cache optimization in finite automata<br>Thesis (PhD (Metallurgical Engineering))--University of Pretoria, 2007.<br>Materials Science and Metallurgical Engineering<br>PhD<br>unrestricted

Digital image correlation techniques (DICT), a non-contact deformation measuring technique based on gray value digital images, have become increasingly used over the last years. By using the DIC technique during a tensile test, the deformation behavior of different engineering material under an applied load can be determined and analyzed. Digital images, acquired from a tensile test, can be correlated by using DICT software and from that the local or global mechanical properties can be calculated. The local or global mechanical properties determination of a flat test specimens are based on the displacements or changes in a previous stochastic sprayed or natural pattern. The used material for this purpose is cast silicon (Si) based aluminium (Al) component, designated as AlSi7Mg0.3 (Anticorodal-78 dv). The hypoeutectic Al- Si alloy is widely applicable for engine constructions, vehicle and aerospace constructions, shipbuilding, electrical engineering and constructions for food industry. There are many microstructural parameters in a binary system Al- Si alloys, which the mechanical properties can be depended on, for instance phase distribution, Secondary Dendrite Arm Spacing (SDAS), morphology of Si particles (Roundness) and microscopic defects or pores. All these parameters can contribute to enhance the proper mechanical performance (e.g. Strength and ductility) in the Al-Si cast components.