A fracture mechanics approach to interior fatigue crack growth in the very high cycle regime

• The sizes of ODAs and fish-eye areas decrease with increasing stress. The stress intensity factors remain constant.
• In repeated two-step tests, the cumulative damage decreases with inclusion size.
• Fish-eye growth is smaller than long crack growth and higher than that of ODAs.
• ODA morphology formation is explained by repeated fracture surface contact.

Growth rates of optically dark areas (ODA) and fish-eyes (FE) were quantified in kHz-ultrasonic fatigue tests on SUJ2 and 17-4PH steels at constant and repeated two-step amplitudes. Sizes of ODAs and FEs depended on the stress intensity factor (SIF) range, and interior fatigue crack growth rates (FCGR) were slower than those of “long” cracks in air, suggesting vacuum as ODA growth environment. Repeated two-step tests on SUJ2 steel served to form beach marks so that, a quantification of ODA sizes, interior FCGRs and SIFs became possible. Additional FCGR measurements of long cracks in vacuum and comparable fracture morphologies allowed estimating the growth rates of ODAs and FEs in 17-4PH steel.