Fatigue Testing and Thermal-Mechanical Treatment Effects on Aluminum-Boron Composites

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Abstract
Fatigue strength data are presented in the form of constant lifetime curves over a short ratio range, principally for a tension/tension mode with a low minimum stress and a maximum stress closely approaching the full tensile strength of the material. A brief description of the complex stress distribution within the matrix and its consequences are given, and the effects of thermal/or mechanical treatments or both are discussed in terms of the stress state modifications and the effects on longitudinal tensile strength. Regions of high triaxial stress within the matrix results in a higher than predicted Young's modulus, with a fracture stress which approaches and occasionally exceeds the rule of mixtures at strain levels less than the strain capability of the filaments. Under these circumstances, it was suspected that the strength of the composite was limited by the early fracture initiation in the matrix. Thus, composite material strength should be optimized by decreasing the initial or residual matrix stress level to the point where fracture is initiated simultaneously in the matrix and the filament. Stress modifications and resultant strength improvements were accomplished by selective thermal and mechanical treatments to the composities after fabrication.

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