AbstractThere appear to be very few literature estimates of the size or the shape of the plastic zone in polybismaleimides, whether calculated or observed experimentally. Bismaleimides are characteristically much more brittle on a macroscopic scale than other thermosetting resins and might 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 epoxies, even after rubber toughening, and before the network is fully developed by postcure. No direct observation of the zone radius could be made, but model calculations were carried out using Irwin, Dugdale, Tresca and von Mises expressions. The estimated zone radii declined with cure time, and became more or less negligible for all except the highest rubber concentrations after postcure. Plane stress models gave higher radii than plane strain ones. The very small calculated plastic zones of bismaleimides were consistent with the low observed ratio of tensile to compressive strength. This ratio implies marked resistance to yielding compared with that to crack propagation, and suggests a morphology consisting of exxtensive regions of continuous, rigid crosslinked structures, with minimum scope for shear.