Atomic force microscopy and modeling of fatigue crack initiation in metals

We have obtained the first atomic force microscope (AFM) images of cell 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 displacements. Using these measurements we have determined the fraction of plastic strain that contributes to surface displacement upset. Values of 1.6% for Ti and 0.9% for HSLA resulted for completely reversed plastic strain cycling. We introduce a model for fatigue crack initiation based on slip band spacings, slip height displacement, and cumulative plastic strain. To first order these compare well with experimental initiation data obtained by other investigators. The relative ease of sample preparation for AFM, along with its outstanding height resolution, offers a higher degree of simplicity and precision in fatigue crack initiation studies compared to more conventional techniques.