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 applied stresses along the plane of a fatigue crack in a metal matrix composite. The specimens were studied in two conditions: one was as heat treated, and the other was plastically deformed in tension before fatigue cracking. Neutron diffraction was used to measure the three principal strains as a function of position along the crack growth direction, ahead of and in the wake of the crack, and these measurements were used to calculate the stress variation along the crack line. An Eshelby based model was used to separate the individual components of the stress. It was found that the thermal misfit stress between matrix and reinforcement was changed significantly by the plastic deformation and was effectively 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.