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Abstract When tested in tension, a cross‐linked epoxy resin can be made to exhibit shear yielding. A modified von Mises criterion, τ = τ 0 − μ P describes the yielding behavior of the same resin under a biaxial stress system, indicating that the flow of the material is pressure sensitive. Butadien‐acrylonitrile elastomer particles suspended in the cross‐linked epoxy matrix induce large local deformations when the composite material is stressed. Particles a few hundred Angstroms in diameter cause the glassy matrix to exhibit shear banding, and the macroscopic failure envelope of such a system follows a modified von Mises criterion similar to that of the matrix resin. It was found that the coefficient of internal friction, τ, and the activation energy for yielding are approximately the same for the two cases. With larger particles (5‐15,000 Å diam) the failure mode changes as shown by the macroscopic yield envelope and the associated activation energy. Electron micrographs of the fracture surfaces show microcavitation, similar to crazing around each particle; the deformed glassy polymer around each particle retracts upon heating the matrix above its Tg . The fracture surface work value of the unmodified matrix is 1.75 × 10 5 ergs/cm 2 . With 10 pph small particles, the value increases to 3.32 × 10 5 and with 10 pph of large particles, to 15.48 × 10 5 ergs/cm 2 .
Sultan et al. (Mon,) studied this question.