Small‐crack growth and fatigue life predictions for high‐strength aluminium alloys. Part II: crack closure and fatigue analyses

Abstract

Small‐crack effects were investigated in two high‐strength aluminium alloys: 7075‐T6 bare and LC9cs clad aluminium alloys. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue and small‐crack tests were conducted on single‐edge‐notch tension (SENT) specimens and large‐crack tests were conducted on middle‐crack tension specimens under constant‐amplitude and Mini‐TWIST spectrum loading. A pronounced small‐crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three‐dimensional finite‐element and weight‐function methods were used to determine stress intensity factors, and to develop equations for surface and corner cracks at the notch in the SENT specimen. (Part I was on the experimental and fracture mechanics analyses and was published in Fatigue Fract. Engng Mater. Struct. 21, 1289–1306, 1998.) This part focuses on a crack closure and fatigue analysis of the data presented in Part I. A plasticity‐induced crack‐closure model was used to correlate large‐crack growth rate data to develop the baseline effective stress intensity factor range (ΔKeff ) against rate relations for each material, ignoring the large‐crack threshold. The model was then used with the ΔKeff rate relation and the stress intensity factors for surface or corner cracks to make fatigue life predictions. The initial defect sizes chosen in the fatigue analyses were similar to those that initiated failure in the specimens. Predicted small‐crack growth rates and fatigue lives agreed well with experiments.