ATOMIC-FORCE MICROSCOPY AND MODELING OF FATIGUE-CRACK INITIATION IN METALS

We have obtained the first atomic force microscope (AFM) images of cel l like boundaries and slip band emergence of fatigued metals surfaces (titanium and high strength-low alloy steel). The AFM has been used to obtain quantitative information about slip spacings and slip height d isplacements. Using these measurements we have determined the fraction of plastic strain that contributes to surface displacement upset. Val ues of 1.6% for Ti and 0.9% for HSLA resulted for completely reversed plastic strain cycling. We introduce a model for fatigue crack initiat ion based on slip band spacings, slip height displacement, and cumulat ive plastic strain. To first order these compare well with experimenta l initiation data obtained by other investigators. The relative ease o f sample preparation for AFM, along with its outstanding height resolu tion, offers a higher degree of simplicity and precision in fatigue cr ack initiation studies compared to more conventional techniques.