Noncontact hardness assessment of carbon steel with nonlinear Rayleigh waves induced by grating laser

Hardness is a crucial mechanical property to characterize the metal performance in service, and its nondestructive assessment is undoubtedly a challenging work. The objective of this paper is to realize a noncontact assessment of the heat-treated medium carbon steel by using grating laser-induced nonlinear Rayleigh waves. The generation and suppression mechanism of the instrument nonlinearity introduced by a grating laser source is explained theoretically. A different nonlinear parameter is adopted and its relevant feasibility is discussed. Ten heat-treated 45 steel samples underwent NDE using grating laser ultrasonics and the destructive testing using a Rockwell hardness tester in a successive manner. A logarithmic function is utilized to fit the correlation between the Rockwell A hardness and nonlinear parameter with a goodness of fit of 0.9102. The reasonableness of experimental fitting outcomes is discussed in the following, which confirms the potential application prospect.