A constant-strain wear test apparatus was built and used to test polyethyleneterephthalate. The results of the constant-strain wear tests were compared to results from a constant-strain fatigue test. The results of the wear test showed that the polymer did not fatigue below the surface where the distortion energy was calculated to be a maximum. Instead, the polymer fatigued in an interfacial region at the surface due to stress concentrations resulting from surface irregularities, inhomogeneities, and localized adhesive forces. The adhesive forces were seen to increase as the number of cycles of the test increased eventually becoming the dominant frictional force.