Study of Back-Stress and Void Damage in Dual Phase Steels Subjected to In-Plane Shear
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The objective of this thesis is two-fold: to study the relationship between microstructural parameters and the work hardening and back-stress that develops in dual-phase (DP) steels using an in-plane shear test, and to use tools such as micro-computed X-ray tomography and scanning electron micrography (SEM) to examine the potential effect of damage on back-stress at higher pre-strains. Five microstructural variants were prepared using various thermo-mechanical processing methods and inter-critical (IC) annealing. These variants were then subjected to uniaxial tension until failure to measure work-hardening behaviour, and to forward-reverse in-plane shear tests to measure the back-stresses at varying shear pre-strains. The high-volume fraction of martensite in the DP steel microstructures meant that all variants exhibited continuous yielding in both uniaxial tension and in in-plane shear. Microstructures with uniform distributions of martensite particles exhibited the most uniform elongation, while higher strengths were observed when a mixture of large and small martensite particles was present. Similarly, the highest back-stress was observed in microstructures with mixtures of large and small martensite particles. In all microstructures, back-stress initially increased with increasing shear pre-strain. At higher shear pre-strains the rate of increase decreased until a plateau was reached. Historically, this saturation can be attributed to void damage in the microstructure, the annihilation of GNDs around the hard martensite particles, and plastic deformation of the martensite. When examined with micro-CT imaging, no voids could be resolved in the deformed samples of the microstructures; hence, SEM imaging was conducted. While some voids were observed in all the microstructures, they were too few and too small to fully account for the saturation of the back-stress at high pre-strains. Furthermore, plastic deformation of martensite particles was not observed.