The fatigue properties of cross-plied boron 6061 aluminium

The mechanical properties of 0/90° and 0/90/+-45° boron-aluminium laminates have been examined at room temperature, 200 and 300° C. The increase in test temperature resulted in decreases in both the tensile and fatigue properties. Relatively constant critical stress intensity values were obtained from centre-notched static tensile specimens, indicating high notch sensitivity. Smaller values were obtained from specimens tested at elevated temperatures.The tension-compression (R=–1) fatigue behaviour of unnotched specimens cut from either material indicated 10⁷ cycle endurance limits of about 255 MPa (37×10³psi) and endurance ratios of 0.3 to 0.4. Microscopic examination of failed specimens detected the presence of off-axis fibre splitting and multiple cracks that were distributed throughout the matrix. Fractographic observations indicated that the fatigue crack had propagated completely through the matrix before final failure of the fibres occurred.Notched specimens failed by fracture at the fatigue extended notch at approximately the same values of the critical stress intensity that had been determined statically. However, fatigue lives obtained from centre-notched specimens subjected to identical net section stresses indicated that the rate of propagation of the premachined notch was faster for specimens containing longer crack sizes only for crack length: specimen width ratios, 2a/w, up to 0.4; thereafter, larger ratios yielded slower crack propagation rates.