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ABSTRACT: Discontinuously reinforced aluminum (DRA) is currently used where design considerations include specific stiffness, tailorable coefficient of thermal expansion, or wear resistance. Plastic deformation plays a role in failures due to low cycle fatigue or simple ductile overload. DRA is known to exhibit pressure dependent yielding. Plastic deformation in metals is widely regarded to be incompressible, or very nearly so. A continuum plasticity model is developed that includes a DruckerPrager pressure dependent yield function, plastic incompressibility via a nonassociative PrandtlReuss flow rule, and a generalized ArmstrongFrederick kinematic hardening law. The model is implemented using a return mapping algorithm with backward Euler integration for stability and the Newton method to determine the plastic multiplier. Material parameters are characterized from uniaxial tension and uniaxial compression experimental results. Model predictions are compared to experimental results for a nonproportional compressionshear load path. The tangent stiffness tensor is nonsymmetric because the flow rule is not associated with the yield function, which means that the commonly used algorithms that require symmetric matrices cannot be used with this material model. Model correlations with tension and compression loadings are excellent. Model predictions of shear and nonproportional compressionshear loadings are reasonably good. The nonassociative flow rule could not be validated by comparison of the plastic strain rate direction with the yield function and the flow potential due to scatter in the experimental results. The model is capable of predicting the material response obtained in the experiments, but additional validation is necessary for the condition of high hydrostatic pressure. No preview · Article · Apr 2007 · Journal of Engineering Materials and Technology

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ABSTRACT: Conventional methods for constructing yield loci rely on the assumption that nonlinear strains are permanent strains, which
is not always the case. A nickelbase alloy, SiC fiberreinforced titanium, an aluminum alloy, and particlereinforced aluminum
have been observed to violate this assumption. We present a method for constructing yield loci using a proof strain criterion
for the permanent strain that relies on cyclic, proportional, probes of the yield surface. Two criteria are implemented: one
for stress reversal and one for yielding. The method is demonstrated by the construction of initial and subsequent yield loci
in the axialshear stress plane using thinwalled tubular specimens. Results are presented for 6061T6 aluminum as well as
for 6092/SiC/17.5pT6, which is 6092 aluminum reinforced with 17.5 volume percent silicon carbide particulate. The centers
of the initial yield loci for the composite are eccentric to the origin of the stress plane most likely because of the residual
stresses induced during processing. Material hardening due to multiaxial stress states can be described by tracking evolution
of the subsequent yield surfaces and here hardening of the particulate composite was primarily kinematic No preview · Article · Feb 2004 · Experimental Mechanics

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ABSTRACT: The effect that various loads have on a 6092/SiC/17.5pT6 participate reinforced aluminum composite is determined. In addition to the mechanical response from tensile, compressive, and shear loading, yield loci in the axialshear stress plane are constructed using axialtorsional loading of a thinwalled tube. Yield loci are determined by multiple yield probes of a single specimen using a 40 × 106 equivalent offset strain definition of yielding. Cyclic tensile straining to increasingly higher amplitudes indicated a modulus reduction of 16% prior to fracture, strongly suggesting accumulation of internal damage, but no change in the elastic Poisson's ratio was observed. Cyclic compressive loading resulted in no observable change in modulus. Cyclic shear loading led to a minimal shear modulus reduction of approximately 6%. The initial yield locus in the axialshear stress plane had an eccentricity in the compressive stress direction that is known as a strength differential. The strengthdifferential was measured to be 55% and is believed to be associated with thermal residual stresses from heat treatment. After shear prestraining subsequent yield loci were constructed. Hardening was observed to be primarily kinematic. No preview · Article · Jan 2003

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ABSTRACT: The mechanical properties of ductile iron can be improved by ausforming, that is, applying work during austempering. The resulting yield strength and ductility are comparable to those of SAE 4140 steel, while the density is approximately 10 percent less. The viability of manufacturing components by casting a preform, austenitizing it, quenching it to the austempering temperature, forging it, austempering it, and finally, quenching it to the net shape is investigated by simulating the forging operation with finite element analysis. The preform geometry and die set geometry are determined such that the forging operation imparts a reasonably uniform equivalent plastic strain of 20 percent to the workpiece and the prescribed final component geometry is obtained. Forging of two components of varying geometric complexity is simulated using a commercial software package. The results indicate that the geometry of the final part is reasonably close to the goal and that the equivalent plastic strain distribution is reasonably uniformover 80 percent of the material was plastically deformed 1525 percent. The design of the preform and die sets appears to be an excellent application for an optimization algorithm. No preview · Article · Aug 2001 · Journal of Manufacturing Science and Engineering