EFFECT OF THERMAL RESIDUAL-STRESSES ON FATIGUE-CRACK OPENING AND PROPAGATION BEHAVIOR IN AN AL SICP METAL-MATRIX COMPOSITE/

The effects of a thermal residual stress field on fatigue crack growth in a silicon carbide particle reinforced aluminum alloy have been mea sured. Stress fields were introduced into plates of material by means of a quench from a solution heat-treatment temperature. Measurements u sing neutron diffraction have shown that this introduces an approximat ely parabolic stress field into the plates, varying from compressive a t the surfaces to tensile in the center. Long fatigue cracks were grow n in specimens cut from as-quenched plates and in specimens which were given a stress-relieving overaging heat treatment prior to testing. C rack closure levels for these cracks were determined as a function of the position of the crack tip in the residual stress field, and these are shown to differ between as-quenched and stress-relieved samples. B y monitoring the compliance of the specimens during fatigue cycling, t he degree to which the residual stresses close the crack has been eval uated.