Modeling of Fatigue Crack Propagation During Sliding Wear of Polymers

The propagation of a crack initiating at the surface was analyzed to simulate the fatigue wear behavior of glassy polymer materials. A crack in a material half plane was assumed to propagate along a predefined path as a result of contact loading by a cylinder sliding on the polymer surface. The crack path consisted of a vertical straight-line segment and a declined straight line originating at a branch point on the vertical crack segment. The stress intensity factors $KI$ and $KII$ along the crack path were computed by using finite element methods, and their values utilized in the Paris law to determine crack propagation rates. Because this process simulates surface pitting, component fatigue life is assumed to be proportional to the time needed for the propagating declined crack to intersect a neighboring vertical crack, a condition known to lead to pitting. This fatigue life is estimated by integrating the Paris law. Numerical results show that the branch point where the declined crack path originates can effectively hinder crack propagation, and that the rate limiting step in fatigue is crack propagation along a small segment of the declined crack near the branch point. Some important factors that affect the reliability of numerically predicted fatigue life cycles are discussed. Experimental crack propagation paths and lifetimes are shown.