Fracture testing of Brazilian disk sandwich specimens

The reliability of a joint subjected to mechanical and thermal loads during processing and service constitutes a major technical problem. Joints contain flaws. The observed strength of a joint depends upon the location and size of the flaws, as well as the crack path through the joint. The aim of this investigation is to measure the fracture toughness of a crack in an adhesive joint, as well as to observe its path through the bond. Sandwich Brazilian disk specimens made of two aluminum adherends joined by a thin layer of epoxy are employed in the testing. A thin paraffin notch is located within the adhesive layer. Numerical analyses are carried out to calibrate the specimens. It is seen that asymptotic expressions for the energy release rate and phase angle of a sandwich specimen with a thin layer are inappropriate in this investigation. A wide range of mixed modes is attained with this specimen. Values of the critical energy release rate G(c) are obtained as a function of the phase angle psi. A fracture criterion is compared to the results. During testing, all cracks divert from within the layer and grow toward and into the interface. Comparison of crack path direction is made to two theories.