Plastic flow in a discontinuously reinforced aluminum composite under combined loads.

摘要

Discontinuously reinforced aluminum (DRA) has become a viable structural material system for applications in the automotive, aerospace, aeronautics, and recreation marketplaces. While the majority of material models are based on results obtained from uniaxial stress experiments, many structural components are subjected to multiaxial stress states in service. This requires a better understanding of plastic behavior of DRA under combined loads.; The effect that various loads have on a 6092/SiC/17.5p-T6 particulate reinforced aluminum composite is determined. Cyclic tensile straining to increasingly higher amplitudes indicates a modulus reduction of 16% prior to fracture, strongly suggesting accumulation of internal damage, but no change in the elastic Poisson's ratio is observed. In contract, cyclic compressive loading results in no observable change in modulus, but an increase of 12% in the elastic Poisson's ratio. Cyclic shear loading leads to a small shear modulus reduction of approximately 6%.; In addition to the mechanical response from tensile, compressive, and shear loading, initial and subsequent yield loci in the axial-shear stress plane are constructed using axial-torsional loading of a thin-walled tube. Conventional methods for constructing yield loci rely on the assumption that nonlinear strains are permanent strains, which is not always the case. A DRA composite has been observed to violate this assumption. When yield loci of the DRA composite are determined by multiple yield probes of a single specimen using a 40 x 10-6 equivalent offset strain definition of yielding, the total strain was found to have a nonlinear strain component that was not permanent as well as one that was permanent. A new experimental technique that measures the permanent strain rather than the offset strain has been implemented for yield surface determination. The initial yield locus in the axial-shear stress plane has an eccentricity in the compressive stress direction that is known as a strength differential. The strength-differential was measured to be 55% and is believed to be associated with thermal residual stresses from quenching. Subsequent yield loci were constructed after shear prestraining, tensile prestraining, compressive prestraining and non-proportional loads. Material hardening due to multiaxial stress states can be described by tracking evolution of the subsequent yield surfaces and hardening of the DRA composite is observed to be primarily kinematic.; Based on experimental observations, a pressure sensitive plasticity model is developed for the DRA composite, where a non-associative flow rule and a nonlinear kinematic hardening law with multiple back stress variables are incorporated. The constitutive model is implemented by using the backward Euler method and the return mapping algorithm as an ABAQUS user subroutine (UMAT). A closed form of the consistent tangent tensor is systematically derived. Material parameters are characterized from uniaxial tension and uniaxial compression. For the monotonic tensile, compressive and shear loading, the model results are in excellent agreement with the experimental results, while for the non-proportional loading, the stress predictions essentially match the test data.