Strain path dependence in ferritic steel polycrystals

There is evidence to show that subjecting steel sheets to non-proportional biaxial strains can result in higher limit strains or conversely, premature failure depending on the strain path followed. It is therefore imperative to understand the concept of non-proportionality of strain, its relationship with texture and industrially important effects such as material localization and consequent failure in forming processes for high strength steels used in auto components. The role of texture, hardening and non-proportionality of strain was investigated followed by the consequent effects on ductility under a range of non-proportional strain paths. It was seen that most significant increases in ductility were achievable under non-proportional uniaxial straining as opposed to biaxial strain straining. A methodology was developed to evaluate peak broadening due to lattice spacing distributions in polycrystals by accounting for the contribution of elastic strains and geometrically necessary dislocations (GNDs). This study showed that whilst elastic strains were important, peak broadening is typically dominated by the contribution of GNDs. This methodology to calculate lattice spacing was then adopted to create a link to experimental x-ray diffraction lattice spacing measurements. It was shown that differing lattice spacing distributions were achieved by following differing strain paths to an identical strain state. Further, biaxial pre-strain had more influence on the subsequent deformation history compared to uniaxial pre-strain. Experiments showed that higher ductility is achievable under a uniaxial pre-strain as opposed to a biaxial pre-strain. Hence, the effects of pronounced strain re-distribution seen in the calculated response of the latter compared to the former was attributed to account for the differences in ductility seen.Open Acces