Unraveling the transformation of ductile damage mechanisms of void evolution and strain localization based on deformation heterogeneity

This study delves into the ductile damage mechanism via exploring the intrinsic nature of the transformation of void and strain localization induced damages. In tandem with this, tensile experiments were conducted by using ductile metals of pure titanium and austenite steel with different grain sizes (d¯) and sample thicknesses (t). Various damage behaviors were generated: the void damage (D_v) in which the damage is induced by voids and their evolution, the strain localization induced damage (D_l) in which damage is controlled by localized deformation, and their mixed mode. Examinations of damage characteristics show a transformation from D_l to D_v with the increase of t/d¯ or hardening. To identify the decisive factor behind the mechanism, crystal plasticity finite element simulations were performed, and the grain-level non-homogeneous deformation was carefully examined. The materials with severe inhomogeneous deformation were found to have fewer voids, while the ones with more uniform deformation possessed evident void growth. Deformation heterogeneity was thus identified as a pivotal factor for the D_l - D_v transformation. The change of micro-defect configuration of void and grain boundary (GB) with deformation heterogeneity was discovered to be the underlying cause. In the D_v case, the large number of strain localization zones penetrating within the bulk promotes void nucleation and growth from vacancies and dislocations. The D_l case, on the other hand, gets more new GBs but smaller number of voids during deformation. The larger continuous strain localization zone facilitates the dislocation-consuming process of GB formation, resulting in the difficulty of void formation. Additionally, a characteristic parameter representing the deformation heterogeneity degree was defined. A D_l - D_v paradigm, which involves the change of damage mode and micro-defect configuration with deformation heterogeneity, and the characteristic parameter was established. The paradigm was validated to have a wide applicability with its efficiency in interpreting extensive damage phenomena. These explorations are expected to add new insights into the understanding of damage mechanism and support the development of a unified damage prediction platform.