13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- The relationship between damage and shear localization in literature continues to remain ambiguous. Ductile fracture process generally includes three stages, namely void nucleation, void growth and void coalescence. A number of continuum mechanics models have been developed [11-12] to describe this sequence of events. However, the interplay between shear localization and void formation often depends on the materials studied and this effect is not captured in continuum mechanics (macro scale) models. There is very little quantitative data on the development of the complex sequence of damage events in materials such as the ones studied here to ascertain if the existing models can capture any differences in the sequence of events arising from the anisotropic nature of the deformation described above at the microscale. In IF Steel, limited void growth from aluminum oxide particles has been observed [13] prior to localization in uniaxial tension. It is concluded that damage does not play a role before (or upon) localization, but only beyond localization (Fig. 9 (a) and (b)) [13]. It is in contrast with the observations in other steel alloys [11]. In AA5754, void nucleation is observed only in the very final stage of the tensile deformation and the damage is very localized near the fracture surface [14-16]. This suggests that damage may be a consequence of the fracture process rather than a trigger that determines material ductility (Fig. 9 (c)). Further, it is observed that particle distribution as random particles or in stringers affects the final fracture process. For example, anisotropic distribution of stringers in continuous cast AA5754 sheets significantly reduces the fracture strains [17]. In AZ31, it is revealed by X-ray tomograpghy that microcracks formed at the later stage of diffuse necking (Fig. 9 (d)) contribute to the final fracture without transition into localized necking [18]. (c) (d) Fig. 9 Damage and fracture in (a) and (b) cross section of a fractured IF steel specimen [13] showing void growth within localized shear bands, (c) cross section of a fractured AA5754 tensile specimen showing no damage or voids just underneath the fracture surface and (d) fracture surface of an AZ31 alloy tensile specimen revealed by X-ray tomography showing microcracks underneath the fracture surface [18].
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