Crack Opening Stress Measurements of Surface Cracks in 7075-T6 al Alloy PlateSpecimens Through Electron Fractography

摘要

Crack opening stress measurements were carried out on 9.6 mm thick specimens of7075-T6 aluminum 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 level (MPa) (Sop) is deduced from the striation pattern. The variation of Sop along the crack periphery and during crack extension was measured. Close to the front surface, Sop is higher due to plane stress condition. At a large crack depth a strong influence of plasticity in the ligament of Sop was found, i.e., Sop decreases as the crack extends to breakthrough. The agreement of (da/dN = effective stress intensity range) curves between surface cracks and through cracks data was only found for a limited range of crack depth of the elliptical cracks.