Effect of sample thickness on local contact behavior in a flat-on-flat fretting fatigue apparatus

Thin plate samples of Ti–6Al–4V contacted on both sides with pads of the same material were used to simulate contact conditions in real structures subjected to fretting fatigue. Laboratory tests on specimens of varying thicknesses were used to determine the stresses that correspond to a fatigue life of 107 cycles using a step-loading procedure. For the specific apparatus used in this study, changes in thickness produced changes in the ratio of shear load to clamping load for a specific fretting pad geometry. Specimen thicknesses of 1, 2, and 4 mm, and stress ratios of R=0.1 and 0.5 were investigated for two different contact pad lengths. Fatigue limit stresses in the specimen were found to be relatively insensitive to the average clamping or shear stress. Finite element analyses of the test geometry were used to provide details of the stress distribution in the contact region for the flat-on-flat geometry with blending radius. Results show that stress and displacement fields for a variety of test conditions corresponding to a fatigue life of 107 cycles vary widely and do not provide any clear indication of the existence of a simple parameter equivalent to a uniaxial fatigue limit stress. The stress and displacement fields are also shown to be very sensitive to the coefficient of friction used in the analysis.