CRACK OPENING STRESS MEASUREMENTS OF SURFACE CRACKS IN 7075-T6 ALUMINUM-ALLOY PLATE SPECIMEN THROUGH ELECTRON FRACTOGRAPHY

Crack opening stress measurements were carried out on 9.6 mm thick specimens of 7075-T6 aluminium alloy with semi-elliptical surface cracks. Measurements were made through a fractographic technique based on a load sequence which can produce distinct striation characteristics on the fracture surface. The crack opening stress S(op) is deduced from the striation pattern. The variation of S(op) along the crack periphery and during crack extension was measured. Close to the front surface S(op) is higher due to plane stress condition. At a large crack depth a strong influence of plasticity in the ligament on S(op) was found, i.e. S(op) decreases as the crack extends to breakthrough. The agreement of da/dN = DELTA-K(eff) curves between surface cracks and through cracks data was only found for a limited range of crack depth of the elliptical cracks.