Local constraint near fatigue cracks in alloys and particulate composites

An experimental method with high spatial resolution has been developed for determining material constraint on the surface of specimens. The method uses strains computed from stereoimaging measurements of displacements. A derivation of constraint in terms of strain is given and compared to the usual stress definition. Constraint was found to be high near fatigue crack tips growing at low Delta K. Monotonic loading of a crack tip that formed shear bands relaxed constraint in the regions of high deformation, as expected. Constraint levels in particulate reinforced metal matrix composites depend on proximity of the crack tip to particles. In general, particulate composites show higher levels of constraint than the matrix alloy for the same K level, which partially explains why the composites have lower ductility and fracture toughness.