Optical correlation evaluation of surface deformation due to fatigue

Coherent optical correlation techniques were applied to the detection and evaluation of microstructural surface alterations during cyclic stressing of Fe-3Si. Correlation data were obtained by measuring the light intensity in the central portion of the optical correlation function, at various cyclic intervals, using a holographic filter made with the wavefront reflected by the specimen in its original uncycled, preload condition. The changes in correlation intensity were supplemented by measurements of the spatial frequency spectrum of the reflected beam, and changes in both sets of measurements were correlated with optical microscopy and surface interferometry, and also with electron replica microscopy, of the surface topography. The results are discussed in terms of cumulative fatigue damage and ultimate fatigue crack initiation.