Fatigue life predictions in small volume components

Over the years there has been a gradual recognition that the one to one correspondence between crack advance per cycle and fatigue striation spacing broke down at low ΔK. To resolve whether this might apply to small volumes, 250 μm diameter wires of a Co-Ni based superalloy were evaluated in tension-tension fatigue. Fatigue striations on the order of 20 nm per cycle were resolved at 100,000-200,000X magnifications at ΔK = 4 MPa-m^1/2 and R ≥ 0.5. From fatigue crack propagation tests a Pads Law exponent of two was found and a threshold dependent growth law allowing for zero growth as ΔK approached threshold was developed. Using standard integration procedures, it is shown that the fatigue life of notched specimens is divided into regimes where initiation is no greater than about half of the total life. Additionally, recent experiments on damage accumulation by repeated surface slip have allowed initiation models applicable to both low and high cycle fatigue regimes. These initiation and propagation models are shown to be equally important in the high cycle fatigue regime for both the superalloy of this investigation and normalized cast mild steel data extracted from the literature. It is proposed that fatigue life partitioning into initiation and growth stages is most satisfactory for the inclusion of atomistic and microstructural variables of importance to small volumes.