Prediction of fatigue life in relation to surface finish parameters

One can safely claim that not one single surface roughness parameter, in terms of completely characterizing a surface, can be exhaustive. This study is both an experimental and theoretical study of characterizing surface finish in relation to fatigue; and the role of this finish in fatigue initiation. A total of forty-two samples, comprising three different groups that exhibited different surface finish properties, were fatigued using constant amplitude loading. The S-N curves for these groups were established. A correlation between surface finish parameters and fatigue initiation was established. The parameters of importance were expected to be the deepest scratch and the rms valley curvature (sharpness) of the measured surfaces. The product of both parameters showed a reasonable degree of correlation with fatigue initiation at different stress levels. This product became a basis of establishing the functional relationships between surface finish parameters and fatigue failure. A number of specimens were prepared having a single scratch that varied in depth from 0.004 inches to 0.02 inches. The scratches had a fixed radius of curvature of 0.003 inches. The specimens were fatigued and the resulting S-N curves were grouped into one spectrum. The single scratch spectrum utilizing the deepest scratch and sharpness was used as a basis to predict fatigue failure. First-passage criteria were used to predict the deepest scratch in longer sampling lengths. The product of the predicted deepest scratches and curvature were used with the single scratch spectra to predict fatigue failure. The predicted life was within the 95% confidence bands of the S-N curves generated from fatiguing the specimens