STUDY ON MECHANISM OF MICROSCOPIC FAILURE OF ROUGH SURFACE UNDER REPETITIVE CONTACT STRESS

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At first as a theoretical study, a method of calculation of 3-dimensional stress fields near the asperity contacts which whould be caused in macroscopically 2-dimensional geometry bodies having surface roughness, was developed in this study. This method was applied to calculate stress intensity factors of cracks by body force method in rolling contacts of tough surfaces, taking into consideration of existence of sticking and slipping conditions for the sake of frictional resistance between crack-faces when compressional forces remain. And. it was shown that the friction between crack-faces decrease stress intensity factors by the sticking of crack-faces and a decrease in friction by oil-penetration into the crack accelerates the propagation of crack. The stress intensity factor, which was calculated taking into consideration of surface roughness, becomes a enough value for the propagation of micro-crack and explains the fact that pittings apt to be caused on the surface moving with lower speed, but not to be caused on the surface moving with higher speed. In expermental studies, repetitive compression tests, two-roller tests, fretting tests were conducted. In repetitive compression tests, a roller surface was repetitively compressed to a flat surface after a small angle rotation while surfaces was separated. and numerous cracks in the flat surface were observed within a hundred thousand cycles, under a comparable load condition with pittings limits in rolling contact. By a successive observation of pitting cracks in rolling-sliding contact, it was elucidated that initial cracks of pitting made their appearance in the direction corresponding to the roughness lay direction of mating roller. A clear relationship between the largeness of asperity contact regions and the scale of failure was observed in all those experiments. These investigators wish to study further so that predictions by theoretical study will be confirmed by experiments.