Separation of measured fatigue crack stress fields in a metal matrix composite material

Neutron diffraction has been used to measure the strain profile around fatigue cracks in Al/SiC_p metal matrix composite specimens which had been previously heat treated to set up different initial levels of residual stress. Uncracked samples were used to determine the residual strain variations present in the samples prior to growth of the cracks. A specially-designed four point loading rig was used to enable the samples to be studied in situ on the neutron diffractometer, both in the unloaded and loaded conditions: i.e. with the crack closed (no applied load) and open (loaded to the same K_max experienced during crack growth). A modelling technique based on the Eshelby method was used to separate the measured stresses into their contributing components: the macroscopic stress field which would exist in the material if it were homogeneous; the elastic mismatch stress in each phase arising from the mismatch in elastic properties of the two components; and the thermal misfit stress arising from the difference in coefficient of thermal expansion between the matrix and the reinforcement. The effects of the passage of the fatigue crack on the pre-existing residual stress field, and of the change in stress distribution when the opening load is applied to the crack, are discussed in the light of the results.