As the characteristic scale of products and production processes decreases, the plasticity phenomena observed start to deviate from those evidenced at the macroscale. The current research aims at investigating this gap using a lower-order gradient enhanced approach both using phenomenological continuum level as well as crystal plasticity models. In the phenomenological approach, a physically based hardening model relates the flow stress to the density of dislocations where it is assumed that the sources of immobile dislocations are both statistically stored (SSDs) as well as geometrically necessary dislocations (GNDs). In the crystal plasticity model, the evolution of the critical resolved shear stress is also defined based on the total number of dislocations. The GNDs are similarly incorporated in the hardening based on projecting the plastic strain gradients through the Burgers tensor on slip systems. A rate-independent formulation is considered that eliminates any artificial inhomogeneous hardening behavior due to numerical stabilization. The behavior of both models is compared in simulations focusing on the effect of structurally imposed gradients versus the inherent gradients arising in crystal plasticity simulations.

Weight reduction and fuel consumption play an important role on material selection in automotive industry. In this respect, ferritic-martensitic dual phase steels are gaining popularity thanks to their versatile combination of strength and formability. In this study, we investigate evolution of damage and active damage mechanisms in a commercial DP600 steel. Interrupted tensile tests are conducted in both rolling (RD) and transverse directions (TD). Subsequently, damage mechanisms and void evolution is characterized by cross-sectional SEM micrographs. The results reveal that, in both RD and TD, damage occurs by three different damage mechanisms. Namely, void formation due to inclusions, cracking of martensite islands and decohesion between ferrite and martensite. From these damage mechanisms, void formation due to large inclusions occur in the early stages of deformation, whereas the other two are both active throughout the complete stretching. The most commonly observed damage mechanism was martensite cracks and seem to be the primary reason of failure. In addition, void evolution studies clearly show that damaged area as well as number of voids increase more rapidly in RD than TD. Furthermore, in both directions, damage concentrates at the mid plane of the specimens, leading to an inhomogeneous distribution of voids in the thickness direction.

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The intent of this work is to numerically investigate the effect of second phase morphology on damage evolution characteristics of dual-phase (DP) steels. A strain gradient enhanced crystal plasticity framework is used in order to capture the deformation heterogeneity caused by lattice orientations and microstructural size effects. The investigation is focused on two different martensite distributions (banded and random) that are relevant for industrial applications. The effects of martensite morphology are compared by artificially generated 2D plane strain microstructures with initial void content. The Representative volume elements (RVEs) are subjected to tensile deformation imposed by periodic boundary conditions. Evolution of voids are analyzed individually as well as a whole and characterized with respect to average axial strain. It is found that during stretching voids exhibit varying evolution characteristics due to generation of inhomogeneous strain fields within the structure. The behavior of individual voids shows that the stress-state surrounding the void is different from the imposed far field macroscopic stress-state. The voids at the ferrite martensite interface and in ferrite grains of the randomly distributed martensite grow more than in the banded structure. On the other hand, voids formed by martensite cracking growth shows an opposite trend.

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