Fracture mechanics of adhesion test specimens using finite element analysis

Adhesion tests have been developed to quantify the bond strength between an elastomer and rigid substrate (ASTM D 429 1993). However, the mechanics of how an interface crack would initiate and propagate using these adhesion samples have not been fully quantified. The purpose of this work was to determine the strain energy release rate (also know as tearing energy in the rubber literature) for rubber cylinders with flat and conical end pieces having a crack at the rubber-to-metal bond. The geometry used corresponds to the ASTM D 429 method A and method C.; The focus of the work was to use finite element analysis (FEA) as a tool to determine the tearing energy as a function of crack length. Once the tearing energy versus crack length relationship is known, the fatigue life can be predicted. Cracks in rubber are formed naturally as defects in the material or by internal void formation by cavitation. Therefore, the crack configurations considered were an internal penny shaped crack and an external ring crack for a bonded rubber cylinder. Both cracks were considered to be at the bond and propagate along the metal-to-rubber interface in a uniform manner. These crack patterns have been experimentally verified. The tearing energy from FEA was found to vary as a function of crack length. Therefore, a theory has been developed for bonded rubber cylinders based on linear elasticity, which describes the tearing energy as a function of crack length behavior for short and long cracks with excellent agreement with the parametric FEA studies. The peak tearing energy as a function of crack length was found to be related to the height of the specimen. An empirical approach was also used to develop the entire tearing energy as a function of crack length curve with excellent agreement to the FEA results. The linear analysis was compared to nonlinear analysis at 10% strain and the tearing energy was within 16%.; The conical adhesion specimen was analyzed by varying the cone angle, thickness of elastomer between cone apex, and location of crack initiation using linear and nonlinear FEA. The results show that as the cone angle becomes more obtuse, the tearing energy increases, which has been experimentally verified.; The penny and ring-shaped cracks in bonded rubber cylinders have been experimentally verified by introducing a 1.27 mm (0.05″) flaw and fatigue testing at 10% nominal tension strain. The fatigue life was predicted by using the analytical models and the fatigue crack propagation data (FCP) and compared to the actual fatigue test of the bonded cylinders. The results show that the analytical models can be used to predict the fatigue life.