The need to predict the mechanical behaviour of polymers is motivated by the increasing use of these materials in structural applications. In order to consider safety in the design of engineering components, the question of a criterion for rapid crack propagation has been investigated in a medium density polyethylene (MDPE) used to extrude pipes for gas distribution. Work on the strain rate-temperature equivalence ((epsilon) over dot<-->T) was performed to study the yield stress, sigma(s), as a function of (epsilon) over dot and T. A shifting procedure enabled a behavioural master curve, sigma(s) versus 1/(a(T)(epsilon) over dot to be drawn, where a(T) is the shift factor. Both cracked-ring and Charpy specimens were tested to characterize the fracture toughness K-IC (T) of the MDPE. The values of the K-IC (T) based on linear elastic fracture mechanics are superposed to form a fracture toughness master curve : K-IC versus 1/(a(T)(epsilon) over dot).