Mixed-mode fracture experiments on quartz/epoxy and sapphire/epoxy interfaces

This work describes the development of a new biaxial loading device for investigating mixed-mode fracture at bimaterial interfaces. The new device made use of piezoelectric actuators and specially arranged flexures to provide independent displacements normal and tangential to the interface. Capacitive probes and special washer load cells were used for measuring the displacements and reactive loads, respectively. A closed loop circuit was formed with a PC to control the applied displacements to within 10 nm. Experiments with quartz/epoxy/aluminum and sapphire/epoxy/aluminum sandwich specimens with cracks growing between the quartz/sapphire and the epoxy found that the intrinsic toughness of these interfaces was lower than the value for a glass/epoxy interface. Crack opening interferometry measurements having a resolution of 30 nm revealed the presence of a cohesive zone whose size was about 0.5 mum for both interfaces.; Finite element analysis using a rate dependent plasticity constitutive law for the epoxy and traction-separation laws for the Interphases were used to model the fracture process. The measured crack opening displacements were used to calibrate the model. The cohesive zone size was determined to be 0.5 mum for the quartz/epoxy and 0.35 mum for the sapphire/epoxy interfaces. It was found that the combined shielding effects do not contribute significantly to the mode 1 interfacial toughness, which represents the apparent intrinsic toughness. However, for higher mode-mixes shielding was responsible for the most significant part of the toughness. Angle resolved X-Ray spectroscopy of sapphire fracture surfaces revealed that up to 43% of the surface was covered with epoxy, and that a significant part of the intrinsic toughness came from the epoxy cleavage. The remaining portion came from the work done during the formation of highly localized ridges and peaks.; Experiments with sapphire substrates coated with self-assembled monolayers were performed to vary adhesive interactions with the epoxy. It was found that the presence of a bromine terminal group in the SAM, in a proportion as low as 10%, increased the intrinsic toughness of the interface by a factor of 4.5.