AbstractรNeutron diraction has been used to measure the strain proยฎle around fatigue cracks in Al/SiC p metal matrix composite specimens which had been previously heat treated to set up dierent 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 diractometer, 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 ยฎeld 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 misยฎt stress arising from the dierence in coecient of thermal expansion between the matrix and the reinforcement. The eects of the passage of the fatigue crack on the pre-existing residual stress ยฎeld, and of the change in stress distribution when the opening load is applied to the crack, are discussed in the light of the results.