Early prediction of macroscale plastic strain localization in titanium from observation of mesoscale surface roughening

The numerical and experimental investigations have been performed to establish a correlation between local strains and mesoscale deformation-induced surface roughening in commercial purity titanium specimens. The evolution of surface profiles in selected regions of the specimens is examined in the course of uniaxial tension. Complementing the experimental studies, crystal plasticity finite-element simulations of deformation-induced surface roughening are performed for a model polycrystal. It has been shown that a series of out-of-plane surface displacements characterized by different amplitudes and frequencies are formed in the experimental and computational specimens under tension. Roughening intensifies on a larger scale when smaller length scale deformation mechanisms are exhausted. In order to quantify the multiscale surface patterns a dimensionless roughness parameter has been proposed. Unlike the traditional methods of roughness assessment, unfiltered profiles are analyzed to take into account the deformation mechanisms that are at work at different length scales. The dimensionless roughness parameter is shown to be sensitive to and well correlated with the local plastic strains. In the region of macroscopic necking, the mesoscale roughness begins to increase nonlinearly well ahead of the visible evidence of the macroscale strain localization, while in the other regions it ceases to increase at all. The results support the assumption that the plastic strain localization and fracture can be predicted early from estimations of mesoscale surface roughness.