Integrated experimental–numerical investigation of strain partitioning and damage initiation in a low-carbon lath martensitic steel

Lath martensite is a key constituent of advanced high strength steels (AHSS). Its deformation behavior is intrinsically related to its hierarchical structure composed of packets, blocks and laths exhibiting specific topological and orientation relationships. To improve the ductility of lath martensite, it is therefore important not only to clarify the deformation mechanisms but to also gain a better understanding of the damage initiation mechanisms and their connections with local micro-mechanical features. To this end, an integrated experimental–numerical investigation of a low-carbon lath martensitic steel combining large field-of-view high-resolution digital image correlation (HR-DIC) analyses and crystal plasticity (CP) simulations with different modeling assumptions is proposed. The global and local plastic strain field distributions are first comparatively and quantitatively analyzed to clarify the predictive capabilities and limitations of the CP models within the continuum mechanics approximation. The results confirm that plasticity is predominantly carried out by interface sliding so that a model discriminating bulk and boundary elements with respect to their initial in-lath and out-of-lath slip strengths can reasonably reproduce the plastic deformation in terms of spatial distribution and intensity. Severe strain localization is observed along block boundaries with high in-lath Schmid factor, at packet interfaces but also at prior austenite grain boundaries and probably at an annealing twin boundary. The relationship between the strain localization sites and local micro-mechanical fields, in particular the stress triaxiality is discussed.