PLASTIC ZONE RADIUS OF CROSS-LINKED BISMALEIMIDE RESINS

There appear to be very few literature estimates of the size or the sh ape of the plastic zone in polybismaleimides, whether calculated or ob served experimentally. Bismaleimides are characteristically much more brittle on a macroscopic scale than other thermosetting resins and mig ht be expected to have exceptionally small plastic zone radii. In this paper, experimental investigations are described, which conclude that the plastic zone must be much smaller in bismaleimides than in epoxie s, even after rubber toughening, and before the network is fully devel oped by postcure. No direct observation of the zone radius could be ma de, but model calculations were carried out using Irwin, Dugdale, Tres ca and von Mises expressions. The estimated zone radii declined with c ure time, and became more or less negligible for all except the highes t rubber concentrations after postcure. Plane stress models gave highe r radii than plane strain ones. The very small calculated plastic zone s of bismaleimides were consistent with the low observed ratio of tens ile to compressive strength. This ratio implies marked resistance to y ielding compared with that to crack propagation, and suggests a morpho logy consisting of extensive regions of continuous, rigid crosslinked structures, with minimum scope for shear.