Environmental effects on fatigue crack development in HSLA steel and fine grain α-titanium

Surface slip upset and grain boundary displacement can be measured with considerable precision by AFM. For HSLA steel, measurement of surface damage by determination of the damage parameter, f, in conjunction with characterization of environmental effects on threshold criterion is sufficient to predict initiation. Plasticity at the grain boundaries is the dominant mechanism controlling initiation in HSLA; hydrogen embrittlement accelerates initiation in marine environments. For α-titanium, initiation behavior is apparently controlled by mechanical oxide rupture. Surface damage increases in aqueous chloride environments where repassivation is prevented and metal is effectively softened. As in the HSLA steel, the fraction of the cumulative plasticity emerging at the surface is critical in predicting initiation.