| The current research focused on the process of fretting fatigue crack nucleation and growth in two high strength structural steels: PH 13-8 Mo stainless steel and quenched and tempered 4340 steel. Carefully controlled fretting fatigue experiments were conducted to observe the influence of relative slip amplitude, contact size, and fatigue stress amplitude on the nature of the fretting fatigue cracking process and life of these two materials. Both materials experienced significant reductions in fatigue strength due to fretting, with PH 13-8 Mo stainless steel exhibiting a greater susceptibility to fretting than 4340 steel for the experimental conditions used. Cracks nucleated within the first 200 cycles of fretting fatigue, corresponding to less than 0.2% of the total fretting fatigue life, for both materials. In addition, significant damage was observed after 5,000 to 10,000 cycles of fretting fatigue. A significant refinement in grain size, characterized by a non-etch region and considerably higher hardness than the bulk material, was observed in PH 13-8 Mo stainless steel but not in 4340 steel. Possible reasons for this difference are presented and discussed. A fracture mechanics approach was implemented to estimate fatigue lives, which were compared to experimental fretting fatigue results. Cases of uniform cyclic stress and a linearly varying near surface cyclic stress field, which was used to simulate the presence of fretting stresses, were employed. Results indicate that such simplistic fracture mechanics methods can accurately characterize the fretting fatigue life at higher stress amplitudes where crack propagation would be expected to dominate the total fatigue life. Determination of the fretting fatigue behavior at stress amplitudes near the fretting fatigue limit is less amenable to the fracture mechanics methods used. Reasons for this are explored and possible methods to better determine fretting fatigue endurance limit behavior are discussed. |
