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

Neutron diffraction has been used to measure the strain profile around fati gue cracks in Al/SiCp metal matrix composite specimens which had been previ ously heat treated to set up different initial levels of residual stress. U ncracked samples were used to determine the residual strain variations pres ent 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-ma x experienced during crack growth). A modelling technique based on the Eshe lby method was used to separate the measured stresses into their contributi ng components: the macroscopic stress field which would exist in the materi al if it were homogeneous; the elastic mismatch stress in each phase arisin g from the mismatch in elastic properties of the two components; and the th ermal misfit stress arising from the difference in coefficient of thermal e xpansion between the matrix and the reinforcement. The effects of the passa ge of the fatigue crack on the pre-existing residual stress field, and of t he change in stress distribution when the opening load is applied to the cr ack, are discussed in the light of the results. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.