The BTT1 transition resulting from a shift from
predominantly unstable to stable crack propagation is
associated with a change in crack propagation
mechanism from microvoid coalescence to shear
flow (see Fig. 26). The individual crack-growthplanes
(fracture parables) are separated by clearly
visible shear-lips. Crack growth via shear flow is
typical for many tough or semibrittle amorphous
polymers, such as polycarbonate [85].