Effect of understress on fretting fatigue crack initiation of press-fitted axle

Axles are one of the most important components in railway vehicles with regard to safety, since a fail safe design is not available. The problems of fretting fatigue crack initiation in a press fitted axle have not been completely solved even though up-to date fatigue design methods are employed. The objective of the present study is to clarify the effect of understress on fretting fatigue crack initiation behavior in the press fitted axle. Most of the stress amplitude given to the axle in service is smaller than the fretting fatigue limit based on the stress to initiate cracks under a constant load σ_wf1. Rotating bending fatigue tests were performed using a 40 mm diameter press fitted axle assembly. Two step variable stresses consisting of σ_wf1 and half or one third of σ_wf1 were used in the experiment. Crack initiation life was defined as the number of cycles when a fretting fatigue crack, which is longer than 30 μm, was found using a metallurgical microscope. Fretting fatigue cracks were initiated even when the variable stress did not contain the stress above the fretting fatigue crack initiation limit. The crack initiation life varied from 4.0 × 10⁷ to 1.2 × 10⁸ depending on the stress frequency ratio n_L/n_H. The sum of the number of cycles of higher stress at crack initiation N_H was much smaller than the number of cycles to initiate cracks estimated from the modified Miner's rule. The value of the modified Miner's damage ranged from 0.013 to 0.185. To clarify the effect of variable amplitude on the fretting fatigue crack initiation, a comprehensive investigation related to relative slip, tangential force and fretting wear is necessary.