Deformation and Fracture of a Particulate MMC Under Nonradial Combined Loadings

Nonradial, elastic-plastic cyclic loading paths have been applied to a 17% SiC particulate/2124 aluminum alloy metal matrix composite at room temperature. By combining axial loading with torsion these cycles involve stepped loading and reversals through zero with successively increasing peak stresses through to failure. Strain data taken from a three-element rosette strain gage, bonded to the outer diameter of the tubular test piece, are processed to enable an examination of the axial and shear surface strain response to the loading paths. The component stress-strain plots and the total strain paths are given for the elastic-plastic deformation preceding fracture in this composite. The behavior is complex although it permits an assessment of the existence of a yield surface and the appropriateness of the concepts of the normality rule, isotropic and kinematic hardening. A qualitative judgment is offered in the form of rotation and distortion accompanying a translating yield surface. It is shown how a rigid translation can admit reversed softening and ratchet strain. The particulate arrangement offers greater resistance to tensile plasticity so that strain ratcheting occurs predominantly under compressive stress. This alloy offers no preference to shear flow from forward and reversed torsion other than the usual appearance of a Bauschinger effect within the metal matrix when one torsional mode follows the other. However, cyclic torsional ratcheting appears with simultaneous axial tension. The rotation and distortion features are implied from an examination of the strain paths. Their gradients are a guide to the direction of a plastic strain increment vector lying normal to the yield surface. Further plots reveal the degree to which the principal axes of stress rotate and deviate from the principal strain axes. This composite is light and strong but the SiC particulates sacrifice the available tensile ductility of the matrix alloy. Under cyclic loading the likelihood of an early brittle tensile failure appears to be offset by the prior compressive flow that occurs under a comparable stress magnitude. The compressive residual strain can lessen the severity of a subsequent tensile strain, thus contributing to survival when cycling to combined stress levels of ±500 MPa in torsion and ±200 MPa in tension/compression.