ABSTRACT of the Doctoral Thesis

Computer Based Micromechanical Investigations of the Thermomechanical Behavior of Metal Matrix Composites

by Helmut J. Böhm (1991)

Metal matrix composites (MMCs), which have served mainly as high performance materials for aerospace applications, are currently a field of intensive research interest. The present study aims to improve the understanding of these materials by investigating the inelastic thermomechanical behavior of unidirectional boron fiber reinforced aluminum composites by micromechanics-based numerical modeling. The description of the microstress fields in the matrix is particularly emphasized.

An overview of selected analytical and numerical methods for describing the overall behavior of these materials is followed by an introduction into some properties of the investigated boron-aluminum system.

The assumptions and idealizations underlying the present micromechanical approach, which is based on a unit cell corresponding to a periodic hexagonal arrangement of fibers, are critically reviewed. Special emphasis is focused at discussing the influence of the selected microgeometry and the chosen constitutive material models on the computed microstress fields and the resulting overall behavior.

The approach is used for modeling the response of unidirectional MMCs to axial, transversal, and thermal loads. The predictions are compared to experimental data and to the results of analytical mean field approximations, good agreement being obtained. The validity of mean field descriptions is shown to be related to the microstress gradients in the matrix.

Residual stress states resulting from production-related cooldown procedures, from undercooling, and from mechanical loading are computed. Their influence on the overall behavior of the composite is investigated and shown to be of considerable importance to the initial response under axial and thermal loading.

In addition, the numerical micromechanics model is employed for describing the plastification behavior of unidirectional metal matrix composites under applied hydrostatic pressure.

The behavior of these materials under cyclic mechanical and thermal loads is investigated. Shakedown limits are computed, which are in good agreement with experimental results. Shakedown and plastic cycling behavior are demonstrated under mechanical and thermal loading. Finally, cyclic creep of unidirectional metal matrix composites is modeled.

revised 951214 (hjb)