Fatigue performance has many aspects and dependencies that may be measured, but the most basic question is whether or not a crack can grow. Crack growth can occur if enough energy is provided to cleave polymer chains that cross the crack plane, and it cannot occur otherwise, irrespective of how many cycles might be applied. We have developed an instrument -the Intrinsic Strength Tester - to measure this minimum energy requirement for crack growth. The instrument uses a highly sharpened blade to cut material under controlled conditions, while simultaneously measuring and recording all of the information needed to account fully for the energy balance on the crack.