| Micromechanics-based thermoviscoplasticity theories based on overstress are developed for metal matrix composites. Three micromechanics models, the vanishing fiber diameter model, the Mori-Tanaka method, and the bimodal model, are used in conjunction with the orthotropic thermoviscoplasticity theory based on overstress to study the thermomechanical behavior of metal matrix composites.; Orthotropic thermoviscoplasticity theory based on overstress is a unified theory and does not use yield surfaces and loading/unloading conditions. All material constants can depend on current temperature. It is capable of modeling creep, relaxation, rate sensitivity, and cyclic loading.; Numerical experiments using the vanishing fiber diameter model and the orthotropic thermoviscoplasticity theory based on overstress are performed to study the effects of residual stresses, volume fraction, creep, thermal cycling, and ratchetting on the behavior of metal matrix composites. Numerical results for uniaxial and biaxial proportional loading cases of the rate-dependent and the rate-independent vanishing fiber diameter composite model are compared. In addition path-dependent phenomenon is shown for the rate-dependent vanishing fiber diameter composite model under biaxial nonproportional loading cases.; To characterize fiber-dominated and matrix-dominated behavior of the metal matrix composite systems numerical experiments using thermoviscoplastic composite models (the vanishing fiber diameter model and the Mori-Tanaka method) are performed for boron/aluminum and graphite/aluminum under thermomechanical loading in different directions. The results for both models are compared and the bimodal assumption (fiber-dominated and matrix-dominated modes) is found to be appropriate for boron/aluminum.; For matrix-dominated stress state the bimodal viscoplasticity theory based on overstress is proposed based on the local orthotropic invariant approach. Numerical simulations of uniaxial (transverse or longitudinal shear direction) and biaxial (transverse and longitudinal shear direction) cases are performed and are compared with the experimental data. The results are discussed.; For the purpose of modeling the matrix-dominated low-cycle fatigue damage of metal matrix composites under multiaxial creep-fatigue interaction, an incremental multiaxial life prediction law for metal matrix is proposed. This model consists of the three-dimensional, cyclic neutral thermoviscoplasticity theory based on overstress combined with a multiaxial damage accumulation law to compute the life-time or cycles to crack initiation. The method is intended for application to high temperature low-cycle fatigue with and without hold times and for triangular and trapezoidal waveforms when creep-fatigue interaction takes place. The damage accumulation law correlates fatigue life of 304 stainless steel at 1000{dollar}spcirc{dollar}F under biaxial tension-torsion cycling with and without hold times. Application to metal matrix composites is possible for matrix low-cycle fatigue failure. At the present time sufficient metal matrix composite low-cycle fatigue data do not appear to be available. |
