A computer model for fatigue crack growth from rough surfaces

The effects of roughness characteristics of machined surfaces on fatigue life were determined using a series of computer simulations. The random surfaces were generated with a wide variation of asperity heights, distance between asperities, asperity radii, and initial crack sizes to account for the presence of intrinsic defects. The growth rates of multiple cracks were determined as a function of stress amplitude and crack length accounting for local stress fields from the asperities, crack closure effects, and crack interaction. The fatigue life was governed by the number of cycles for the first crack to reach a critical length. The simulations were repeatedly conducted to test the effects of the average asperity height, the standard deviation of the asperity heights, the length of the sample, and the material constants. Results showed that fatigue life was a strong function of average asperity heights when the average height exceeded 0.1 μm. The role of the standard deviation of the asperity heights was evaluated, and when its magnitude approached the average asperity height considerable scatter in fatigue lives became evident. The crack growth constants were systematically varied in the simulations, with suitable normalization of results. Finally, the model was applied to ground and rough milled 4340 steel with satisfactory predictions of fatigue lives.