The effect of grain size on local deformation near a void-like stress concentration

The deformation-induced nucleation, growth, distortion, and coalescence of voids have been the subject of numerous experimental, theoretical, and computational studies. However, a vast majority of prior work does not consider the role of the local microstructure on void behavior. The present work considers an isolated cylindrical hole undergoing far-field tensile deformation in cartridge brass (70% Cu, 30% Zn). A polycrystal plasticity model was employed to examine the role of grain-size-to-void-size ratios of 0.14, 1, and 7. These numerical simulations clearly demonstrated that when the grain size is comparable to or larger than the void, inhomogeneous deformation of the microstructure can distort and even overwhelm the role of the void in concentrating plastic strain. To confirm and further elucidate these effects, deformation of brass tensile bars with microscale cylindrical holes was performed in situ in a Scanning Electron Microscope (SEM). This approach permitted Electron Backscattered Diffraction (EBSD) measurements of the evolution of local intragrain misorientation and full-field Digital Image Correlation (DIC) measurements of the evolution of intragranular strain fields. As expected, when the hole diameter decreased in relation to the grain size, the effects of local microstructure became increasingly important. In an extreme case, the strain localization due to the hole was completely confined within a single grain. In light of these results, existing continuum-based methodologies to represent ductile fracture, such as the Gurson-Tvergaard-Needleman approach and shear-modified variants, may need to be further modified to include these prominent microstructural effects. Such microstructurally-sensitive representations provide one pathway towards stochastic/statistical models for ductile tearing.

► A study of the effects of grain structure on the mechanics of stress concentrations. ► Microstructure causes significant departures from classical continuum behavior. ► Polycrystalline anisotropy creates significant distortions in local strain fields. ► These findings have broad implications in the field of micromechanics of metals. ► Particular emphasis is placed on interpreting void mechanics during ductile tearing.