Stress ratio effects on crack opening loads and crack growth rates in aluminum alloy 2024

The effects of stress ratio (R) and crack-opening behavior on fatigue crack growth rates (da/dN) for Aluminum Alloy (AA) 2024-T3 were investigated using constant-ΔK testing, closure measurements, and fractography. Fatigue crack growth rates were obtained for a range of ΔK and stress ratios. Results show that constant ΔK fatigue crack growth for R ranging from near 0 to near 1 is divided into three regions. In Region I, at low R, da/dN increases with increasing R. In Region II, at intermediate R, fatigue crack growth rates are relatively independent of R. In Region III, at high R, further increases in da/dN are observed with increasing R. Near-crack-tip surface closure measurements, using a new noncontact displacement method, show that Region I transient crack growth rates are a result of crack closure. Increasing fatigue surface microvoid density, coinciding with increasing da/dN for Region III fatigue crack growth, suggests that a K_max-dependent damage mechanism is responsible for the transient fatigue crack growth rates characteristic of Region III. For AA 2024-T3, the onset of Region III correlates with a critical value of K_max (13 MPa m^1/2). Region II fatigue crack growth is unaffected by both closure and K_max effects. In this intrinsic region, ΔK = ΔK_eff Rates from this region are used to test the validity of closure measurements for Region I fatigue crack growth. Poor correlation of da/dN and ΔK_eff occurs when ΔK_eff is calculated from the fully open load obtained by the reduced strain technique. Good correlation of da/dN and ΔK_eff are found when ΔK_eff is calculated from local opening loads approximately 0.1 mm behind the crack-tip.