Fatigue effects in the wear of polymers
A wear equation has been developed using the concept of repetitive loading of asperities on the mating surfaces in a sliding situation. It postulates elastic deformation in the contact zone and asperities with spherical tips. The tensile principal stress component in the contact zone is considered responsible for the initiation and propagation of fatigue cracks. The wear equation is expressed in terms of the sliding parameters, surface topographical parameters, modulus of elasticity and fatigue properties of the weaker material. The equation is consistent with experimental data for the case of poly(methyl methacrylate), poly(vinyl chloride) and high density polyethylene pins sliding against a steel disk;Scanning electron microscopy revealed that the worn surfaces of poly(methyl methacrylate) and high density polyethylene pins were covered with arced ripples stretching across the transverse direction. In the case of poly(vinyl chloride) there were signs of considerable plastic deformation on the worn surface. The fracture surfaces of notched fatigue samples of poly(methyl methacrylate) exhibited well-defined striation markings which were obscure, ill-defined and discontinuous in the case of high density polyethylene and poly(vinyl chloride). The final stage of fatigue fracture in the latter two materials was accompanied with plastic deformation. Topographical analysis of the sliding surfaces was performed using a data acquisition system. The arithmetic average and root-mean-square surface roughness, slope and radius of curvature of asperities, standard deviation and distribution of profile ordinates and slopes, radii of curvatures and heights of asperities were computed using a FORTRAN IV program. It was found that the asperity heights and ordinate heights followed a Gaussian distribution, whereas the asperity slopes remained unchanged during the steady state wear as was also the case with the average interface temperature.