Static and dynamic brittle fracture
The project examined the static and dynamic fracture mechanics of brittle materials. Destructive testing was performed on brittle, elastic, isotropic and homogeneous epoxy resin specimens made of Araldite CT-200 with Hardener HT-907. Three types of specimen were investigated, namely the three point bend (3PB) beam, the compact mixed-mode (CMM) specimen and the pressure tube. The 3PB and CMM specimens contained both narrow notches and real cracks. The pressure tubes included semi-circular notches. The real cracks were obtained by controlled fatiguing. The research involved the evaluation of the static mode-I and mode-II real and apparent critical stress intensity factors. The fracture surfaces and the phenomenon of crack branching were studied. The dynamic mode-I stress intensity factor was obtained at the inception of crack instability and also at branching. The concept of the existence of a unique relationship between the dynamic stress intensity factor and the instantaneous crack velocity was addressed. The possibility of modelling cracks in structural components by using cast shim notches in epoxy resin was discussed. The modelling of the static behaviour was proposed to be accurate and relatively easy. The dynamic behaviour would be approximately modelled; therefore suggestions on how to improve the dynamic modelling of propagating cracks were recommended, paying particular attention to the branching process and the instantaneous crack velocity. In addition to the experimental work, finite element analysis was conducted for the 3PB and CMM specimens containing narrow notches. It was shown that the specific geometry and loading conditions were unimportant and that the loading was conveniently characterised by the stress intensity factors for an equivalent crack. A method was devised which provided a relatively cheap and efficient means of determining stress concentration factors for what might appear to be complex geometries and loading conditions.