Fatigue is an important consideration in the design of parts for cyclic dynamic applications. Fatigue and cut growth resulting from cyclic stress-strain are related. When testing for fatigue resistance for a specific application it is important to test at a strain energy experienced by the part in actual service. Strain energy is a function of both the modulus of the material and the strain cycle the material sees. Also important with polyurethanes is to test at various levels of stoichiometry (polymer to curative ratio) because we have seen dramatic improvements in flex fatigue resistance by this chemical adjustment. This is another reason why teamwork between the design engineer, the product engineer, and our chemists is so important to problem solving.