Yield and fracture behaviour of cross-linked epoxies

The yield stress and fracture energies of a series of cross-linked epoxy resins were studied in order to correlate the macroscopic mechanical properties with the polymer microstructure. Five networks with varying cross-link densities were synthesized by reacting a homologous series of epoxy resins with stoichiometric quantities of m-phenylenediamine. For all the networks, the yield stress decreased with increasing temperatures in accordance with the predictions of the Eyring theory of viscosity. At constant temperatures, the yield stress decreased with increasing molecular weight between cross-links. The fracture studies revealed two distinct types of crack propagation behaviour above and below approximately 0 °C. Below 0 °C the cracks propagated in a stable and continuous manner, while the crack propagation behaviour changed to an unstable "stick-slip" mode as the test temperature was increased above 0 °C. For unstable crack growth, the fracture energies for crack initiation increased with increasing temperature, while the fracture energies for crack arrest were, within the limits of experimental error, independent of temperature. The crack arrest fracture energies were similar in magnitude to the fracture energies for stable crack propagation. An empirical power-law type correlation was observed between the glassy arrest fracture energies and the average molecular weight between cross-links. Micrographs of specimens which failed by the unstable, "stick-slip" mode revealed characteristic plastic deformation zones which highlighted the positions of crack initiation and arrest along the crack path. The deformation zone widths were observed to increase with increasing test temperatures, providing evidence of greater localized plastic deformations and higher fracture initiation energies at higher temperatures.