When tested in tension, a cross‐linked epoxy resin can be made to exhibit shear yielding. A modified von Mises criterion, τ = τ0 − μP 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 × 105 ergs/cm2. With 10 pph small particles, the value increases to 3.32 × 105 and with 10 pph of large particles, to 15.48 × 105 ergs/cm2.

Tensile yield, flow, and fracture mechanisms in styrene A (SA), a styrene‐acrylonitrile copolymer rubber‐modified styrene A (RMSA) have been studied and characterized into two basic types: tensile crazing and shear banding. Each can be represented by a master curve and equation over ranges of temperature and deformation rates. Each exhibits distinctive activation energy and volume values. Tensile crazing predominates in unoriented SA below 70°C, and in RMSA. Shear yielding predominates in oriented SA.

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Previous work established that tensile stress cycling of fibrous glass reinforced resin composites degrades their macroscopic mechanical properties by inducing microcracks in the cross-linked glassy polymer matrix. While the greatest damage occurs in the first stress cycle, it continues to accumulate during subsequent loadings, and composite modulus reductions of 20 to 25 percent after 100 to 200 cycles are common. Detailed studies of unrein-forced matrices indicate that much of the work required to drive a crack is consumed by molecular cold drawing and partial orientation in the glassy material at the crack tip. Further, this work can be increased substantially by suspending small particles of certain elastomers throughout the cross-linked matrix material; increases in the fracture surface work term of an order of magnitude, or more, are possible. When such toughened epoxies or polyesters are reinforced with fibrous glass and then stress cycled, the internal microcracking previously observed in the composite material is eliminated. The degree of toughening conferred by the elastomer particles is related to their average size, the distribution of sizes, their composition, the composition of the matrix, the volume fraction of elastomer present, and the adhesion between particles and matrix. Recent work has revealed that other, additional factors are important. These latter include the initial molecular weight of the elastomer, the nature of elastomeric copolymers which can be used, the conditions of plane stress or plane strain under which toughness measurements are made, the detailed nature of the catalyst used to harden the glassy matrix, and the morphological features of the particles. All of these factors will be described and their significance discussed.