Fundamental mechanisms causing reduction in fretting fatigue strength by hydrogen (Effect of hydrogen on small crack initiation at the adhered spot)

The authors have reported a significant reduction in fretting fatigue strength of austenitic stainless steels due to hydrogen. One of the causes of the reduced fretting fatigue strength in hydrogen is adhesion between contacting surfaces and following formation of small cracks which emanate from the adhered spots. The objective of this study is to understand the effect of hydrogen on the initiation of the small cracks under fretting fatigue conditions. Since the adhesion between contacting surfaces during fretting in hydrogen is very localized, a small contact length was used in this test in order to facilitate understanding by avoiding such localization. The fretting fatigue test of an austenitic stainless steel SUS304 was performed in air and 0.13MPa hydrogen. In the fretting fatigue test, hydrogen participates in the initiation of the fretting fatigue crack. It can be presumed that high strain at the contact edge activates hydrogen assisted fracture in terms of dislocation mobility. Adhesion mimic test, in which a small contact area was welded, was also performed. As the result, the crack initiation limit evaluated by the maximum range of shear stress was significantly lower in hydrogen than in air. Hydrogen assists small crack initiation under fretting fatigue conditions. This is one of the possible causes of the significant reduction of fretting fatigue strength in hydrogen.