A GENERALIZED-MODEL FOR THE YIELD BEHAVIOR OF EPOXY NETWORKS IN MULTIAXIAL STRESS STATES

Several yield criteria for glassy polymers are reviewed, and their lim itations in predicting the effects of stress state, strain rate, test temperature, and molecular architecture are noted. These criteria are then generalized, so that a working model can be developed for predict ing the yield response of glassy networks subjected to a multiaxial st ate of stress. To form the model, we summarize the phenomenological yi eld and fracture response of amine cured epoxies. In stress states ran ging from uniaxial compression to biaxial tension, the yield response of these glassy networks follows a modified von Mises criterion (tau(g amma)(oct) = tau(yo)(oct) - mu sigma(m)), when tested at a constant te mperature and octahedral shear strain rate, (gamma) over dot (oct). Fu rthermore, changes in (gamma) over dot (oct) and molecular weight betw een crosslinks, M-c, affect tau(yo)(oct) only, and mu remains unchange d. This was shown to be true for a broad range of M-c (380 to 1790 g/m ol). Additional results are included to illustrate the effects of temp erature and strain rate on yield response. These results show that the yield behavior of epoxy resins is best described by a thermally activ ated process, similar to an Eyring type process. Finally, we extend th e model to include intrinsic properties of the resin (e.g., M-c, phi, and T-g) and compare the model's predictions with experimental results . (C) 1997 John Wiley & Sons, Inc.