Effect of crack opening velocity on fracture behavior of hyperelastic semi-structural adhesive joints subjected to mode I loading

Abstract

Rubber-like adhesive joints provide substantial advantages compared to epoxy-based adhesive joints regarding their damping properties, fatigue resistance and energy consumption under impact. Polyurethane-based adhesives with high modulus are even used in structural applications like car body production. The work to be presented describes the dependency of mechanical mode I failure of a hyperelastic semi-structural adhesive on loading rate. Double cantilever beam (DCB) specimens were manufactured with an adhesive layer thickness of 3 mm. In a first test setup, driving velocity of the testing machine was controlled on optically measured crack opening velocity, varying over several orders of magnitude within external setpoint generation. The tests were driven until crack propagation took place and the position of optical measurement was not valid anymore. The J-integral according to Rice (1968) was calculated directly from measured force acting on the DCB specimen and rotation angle of force introduction points, using an analytical approach of Anthony and Paris (1988). In a second test setup, driving velocity of the testing machine was varied over several orders and tests were driven until complete failure of the adhesive joint. Pictures recorded from specimen's edge were used to analyze the rate dependency of crack propagation and fracture behavior.The observed correlation between J and current crack opening velocity showed that fracture energy was significantly and cohesive strength slightly increasing under higher crack opening velocities, while cohesive stiffness remained constant. In a certain range of loading rates, discontinuous crack growth with stick-slip crack propagation was observed. Below and above this range, crack propagation rate was tending to be more stable.