The brittle-tough transition of rapid crack propagation in pressurised polyethylene pipes is examined. The transition is correlated with the presence of shear lips. Two new tests are described and then used to examine the mechanism of sheer lip formation. The mechanism is found to be largely governed by the post-yield drawing behaviour of the polyethylene and is sensitive to both rate and temperature. The degree of drawing before failure is discussed from a micro-structural viewpoint and is shown to depend, at least in part, on lamella size and hence processing conditions.