The relationship between fracture surface roughness and fatigue load parameters

A procedure was sought for estimating fatigue loading information from roughness characteristics of fracture surfaces. Topographs of fracture surfaces produced in compact tension specimens of a titanium alloy in load-shedding and monotonically increasing ΔK tests were analyzed with a fast Fourier transform. The resulting elevation power spectrum density (EPSD) curve was correlated with the stress intensity range, ΔK, and the stress ratio, R. A plot of (EPSD)1/2 vs ΔK showed three distinct regions, possibly reflecting the competition between surface roughening and surface flattening from partial crack closure as ΔK increased. The plot further indicated the effect of the stress ratio. Although the influence of crack closure complicated the relationship among (EPSD)1/2, ΔK, and R, the EPSD curves and (EPSD)1/2 values systematically changed as a function of ΔK and R. These systematic changes may serve as a base for extracting fatigue loading information from roughness characteristics of fracture surfaces.