Generalized Initial Yield Surfaces for Unidirectional Composites

Abstract

A numerical method is described for determination of generalized initial yield surfaces of unidirectional metal matrix composites under arbitrary external loads and uniform temperature changes. The method leads to the representation of the surface in a three-dimensional system of generalized stress coordinates which, respectively, coincide with the direction of the normal composite stress in the fiber direction, and with the two principal directions of the composite stresses acting in the transverse plane. The initial yield surface of the composite is an irregular ellipsoid with its longest axis inclined toward the hydrostatic stress axis. A thermomechanical analogy is used to show that as a result of a uniform temperature change, the yield surface experiences a rigid-body translation in the direction of the hydrostatic axis in the stress space. The initial yield behavior of a B-Al composite is described in detail. It is shown that microplastic yielding can take place in the composite under relatively small magnitudes of external loads, and hydrostatic stress, or as a result of moderate temperature changes.

Fatigue of metal matrix composites

“…The results indicate relatively high endurance limits at 107 cycles, which depend on the cyclic loading conditions. Specifically, in the case of unidirectional specimens the magnitude of the 107 cycle endurance limit depends on the stress amplitude and is almost independent of the mean stress [9].…”

“…A simple analysis of the stress distribution in cyclicly loaded unidirectional metal matrix composites reveals that in annealed or as-fabricated B-A# specimens the matrix yields and deforms plastically at relatively low applied stresses, but may resume the elastic response during subsequent cyclic loading [9,10]. In any case, the response of the matrix to sustained cyclic loading depends on the stress amplitude and is independent of the mean stress.…”

“…The possible relationship between fatigue and shakedown in metal matrix composites was first suggested by Dvorak and Tarn [9] and related to then available experimental data, obtained primarily for unidirectional 6061 Ae-B materials. In the present paper, the relationship is examined theoretically and experimentally for both unidirectional and laminated 6061-0 Ae-B composites.…”

Mechanisms of Fatigue Damage in Boron/Aluminum Composites

“…These and other related results [21][22][23] describe a procedure for the determination of initial yield surfaces for composite laminates, the translation of these surfaces in the load space during plastic loading, and their relation to shakedown envelopes. The analytically predicted magnitudes of ASsh/Y are listed in Table 2.…”

Forming of Metal Matrix Composites

“…This theory was applied to composite laminates by Bahei-El-Din and Dvorak (1979) and Dvorak and Bahei-El-Din (1979). Also, finite-element calculations to obtain the yield surface were conducted by Dvorak et al (1974). These techniques, however, deal mainly with laminate problems in plane stress, and are not appropriate for calculating stresses under large deformation conditions.…”