Small crack growth and fatigue life predictions for high-strength aluminium alloys: Part I - Experimental and fracture mechanics analysis

The small crack effect was investigated in two high-strength aluminium allo ys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical inv estigations were conducted to study crack initiation and growth of small cr acks. In the experimental program, fatigue tests, small crack and large cra ck rests were conducted under constant amplitude and Mini-TWIST spectrum lo ading conditions. A pronounced small crack effect was observed in both mate rials, especially for the negative stress ratios. For all loading condition s, most of the fatigue life of the SENT specimens was shown to be crack pro pagation from initial material defects or from the cladding layer. In the a nalysis program, three-dimensional finite element and weight function metho ds were used to determine stress intensity factors and to develop SIF equat ions for surface and corner cracks at the notch in the SENT specimens. A pl asticity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions. Predicted crack-growth ra tes and fatigue lives agreed well with experiments. A total fatigue life pr ediction method for the aluminium alloys was developed and demonstrated usi ng the crack-closure model.