Assessment of flow stress and plastic strain by spectrum analysis

The relation between the angular distribution of the reflected-plus-scattered light intensity (scattered field) from a metallic surface and the flow stress, plastic strain the material has experienced is experimentally and theoretically investigated. A scattered field, which is obtained by illuminating a specimen surface using a laser beam, carries surface-feature-related information. Experimental evidence suggests that surface correlation length of a material decreases in proportion to the flow stress and plastic strain that the material experiences. A theoretical derivation based on Huygens-Fresnel principle, Fraunhofer approximation, and Wiener-Khintchine theorem shows that the correlation length may be obtained by performing a Fourier transform to the scattered field from the surface. This leads to the development of a noncontact, nondestructive, and remote technique for measuring flow stress and plastic strain.