Micromechanical Characterization of Ductile Damage in Sheet Metal

Abstract

Triggered by the recent popularity of advanced high strength steels (AHSS) and aluminium alloys for weight-reduction in automotive components, industrial interest in deformation-induced ductile damage in sheet metal is increasing in the last decades. Severe deformation during forming or service triggers different damage micro-mechanisms in the multi-phase microstructures of these materials, leading often to unpredicted failures. These failures can be avoided by (a) the optimization of metal microstructures to be less susceptible to damage-induced failures, which requires experimental characterization of damage micro-mechanisms or (b) the incorporation of continuum damagemodels in forming simulations to design forming operations within safe deformation limits, which requires experimental quantification of damage accumulation. However, both strategies are hampered by the limitations of the currently available experimental diagnostics. Therefore, the aim of this work is to develop new experimental methodologies that allow for (i) characterization of damagemicro-mechanisms and (ii) accurate quantification of damage accumulation, with a focus on industry-relevant sheet metal. As a starting point, the influence of damage evolution on localization and fracture is investigated by deforming two steels of different microstructure in different strain paths. The results revealed that for microstructures with many damage mechanisms (e.g. AHSS), damage accumulation significantly affects both necking and fracture limits, verifying the strong need for thorough characterization of damage micromechanisms in different strain-paths. The analysis of these mechanisms requires the development of a miniaturized testing setup that could fit within a scanning electron microscope (SEM) to track deformation-induced microstructure evolution in real time. To this end, a miniaturized Marciniak test setup is designed, built and tested, which allows the real-time, multi-axial testing of industrial sheet metal to the point of fracture within a SEM. A major benefit of the in-situ analysis with miniaturized equipment is the possibility of obtaining evolution of local strain distribution at the microstructure level, as demonstrated in a case study that clarifies the mechanical influence of the morphology and properties of microstructural banding in steels. The effect of band continuity and hardness are elucidated, yielding a clear detrimental influence especially for hard bands with a continuous morphology. Finally, an improved experimental methodology is developed to analyse 3D features of ductile deformation, with minimum specimen preparation artifacts. For the damage quantification problem a wide variety of experimental methodologies have been proposed in the literature, without a thorough evaluation with respect to measurement accuracy, precision, practicality, etc. To determine the most suitable damage quantification strategy for continuum damage models, damage morphology-based damage quantification methodologies (the volume fraction methodology, area fraction methodology, or density measurement methodology) and material property-based damage quantification methodologies (the indentationbased methodology, modified indentation methodology and micropillar compression methodology) are comparatively analyzed. The obtained results clearly indicated that methodologies that quantify ductile damage through its morphology have limited accuracy and probe a narrow damage spectrum, revealing the need for accurate material-property based damage quantification techniques. The indentation based methodology is a widely used example of such methodologies, however, a numerical-experimental analysis revealed that it cannot be used for this purpose, as the damage-induced degradation of both hardness and modulus is masked by other deformation-induced microstructural mechanisms (e.g. grain shape change, texture development, etc.). To this end, two original mechanical property-based damage quantification methodologies are proposed in this work. A new indentation-based methodology is developed and evaluated, that eliminates the influence of the microstructural heterogeneity to properly capture the damageinduced degradation of indentation hardness and modulus. And finally, an elastic compression-based methodology is developed where the elastic damage parameter is obtained through the deformation-induced degradation of the compression modulus of electro-discharge machined micropillars. The results from these two methodologies clearly indicate that methodologies that quantify ductile damage through its influence on mechanical property (e.g. hardness, modulus) have significantly higher accuracy, and therefore more suitable for identifying damage parameters for continuum damage models.