Inversion and Finite Element Analysis of Mechanical Properties ofPure Titanium Gradient Material by Ultrasonic Surface Rolling Processing

Ultrasonic surface rolling processing was conducted on the pure titanium TA2, and the microstructure and residual stress distribution on cross section were studied. The load-indentation depth curves at different distances from surface were measured by nanoindentaion tests, and then the stress-strain curves were obtained by inversion. With the stress-strain relationship as the material property, the load-indentation depth curves were simulated by the finite element method, and were compared with the test curves to verify the inversion method. The influence of the initial yield stress and strain hardening exponent on the load-indentation depth curves was investigated. The results show that a gradient structure with gradually increasing grain size was formed in the sample surface layer, and the residual compressive stress increased and then decreased with increasing distance from surface. The load-indentation depth simulation curves were basically consistent with the test curves, and the relative errors of the maximum indentation depth were within 8%, indicating the inversion method was reliable. With increasing initial yield stress and strain hardening exponent, the loading curvature of the load-indentaion depth curves increased, the plastic work to total work ratio decreased, and the variation of the initial stiffness was not obvious.