Characterizing Rubber's Fatigue Design Envelope

For many applications, endurance under cyclic loading is an important and challenging design requirement.  Ensuring adequate endurance requires knowledge of the limits of the material in the space of likely operating conditions, ie a design envelope.  Although defining the design envelope for a given material can be laborious, it can also be rewarding, and there are at least a few published examples of such curves that have been developed for certain rubbers (the Haigh and Cadwell diagrams are examples).  We have devised an efficient approach for characterizing rubber’s fatigue design envelope.  It is based on measurements of the fatigue crack growth rate law under both relaxing and nonrelaxing conditions.  The measurement procedures are executed using the pure shear specimen, and they employ novel strain-ramping techniques that increase test reliability.  After measuring the material’s fatigue rate laws, we then numerically integrate them to produce a contour map showing how the fatigue life depends on duty cycle parameters such as strain amplitude, mean strain, and minimum strain.