In the last Part 4, the indivisible relationship between stick-slip motion, abrasion pattern formation and wear of rubber is shown experimentally. However, many questions have been left unanswered concerning the abrasion pattern formation. For example, why is the periodic abrasion patter formed always on the surface of the rubber through the whole wear process? Why does a root (crack) of ridge of the abrasion pattern go ahead along in the sliding direction of slider? In addition, why is the crack deepened only slightly (almost 15 degree) to the surface of the rubber?
Here the author discusses about the questions mentioned above from dynamics and fracture mechanics points of view. In the formation of initial microcrack, the geometrical condition shaped between a tip of the slider and a wall of the rubber induced in the front of the slider at a stick stage of stick-slip motion produces the rotary force for the microcrack, which works to change the propagation direction of the microcrack to a lower angle to the rubber surface. In the propagation of abrasion pattern, the rotatory force generated when the slider passes over the ridge bends and opens the root of the ridge, resulting in the steady propagation of the ridge with a low angle to the rubber surface.