DETERMINATION BY NEUTRON-DIFFRACTION OF EFFECT OF PLASTICITY ON CRACK-TIP STRAINS IN A METAL-MATRIX COMPOSITE

Neutron strain scanning has been used to determine residual and applie d stresses along the plane of a fatigue crack in a metal matrix compos ite. The specimens were studied in two conditions: one was as heat tre ated, and the other was plastically deformed in tension before fatigue cracking. Neutron diffraction was used to measure the three prinical strains as a function of position along the crack growth direction, ah ead of and in the wake of the crack, and these measurements were used to calculate the stress variation along the crack line. An Eshelby bas ed model was used to separate the individual components of the stress. It was found that the thermal misfit stress between matrix and reinfo rcement was changed significantly by the plastic deformation and was e ffectively reduced to near zero in both phases. This changes the crack tip strains in the material, decreasing the strains in the matrix and increasing those in the reinforcement. Possible implications of this for fatigue crack propagation are discussed. MST/4042