Because of their extraordinary cracking resistance’ medium-density polyethylenes (MDPEs) do not fail in a brittle manner when tested with conventional fracture toughness procedures, but brittle fractures and J1c values can be obtained for these materials by utilizing fatigue loading. However, because brittle fracture in polyethylene is a result of low stresses (energy) applied over long periods of time, and since J1c neglects time, J1c is incapable of differentiating MDPEs on the basis of their fracture resistance. Thus, the power to fracture method, which incorporates both energy and time, has been developed. During increments of crack growth, the product of the potential energy and the number of elapsed cycles is used to calculate the power to fracture. Within limits that assure a similar failure mechanism, the power to fracture for a particular resin is constant despite varied fatigue conditions. The power to fracture is capable of differentiating between resins on the basis of their brittle cracking resistance.