Abstract
Interlaminar fracture mechanics is now a recognised tool for the characterisation of laminated fibre reinforced plastic matrix composites. There are national and international standard test methods for the determination of the mode I strain energy release rate for unidirectional laminates, where the delamination is constrained to move in the fibre direction. In real components, made up of multidirectional plies, there is no reason to suppose that the location of a delamination and its direction of growth would be constrained in this way. It is much more likely that any delamination will occur between plies of different orientation. It may be that unidirectional values of fracture toughness are sufficient to rank the fracture resistance of different materials; but, if fracture mechanics is to be used to predict delamination growth in multidirectional components, then the fracture toughness as a function of fibre orientation in the delaminating plies needs to be determined. This turns out to be not straightforward, as the fracture event for multidirectional interfaces is generally complex. This paper reports on some preliminary investigations into the measurement of fracture toughness values for multidirectional interfaces, where additional side delaminations have been introduced into the double cantilever beam specimen in an attempt to constrain the delamination growth to the interface of interest. This strategy works well with some materials but not all. The question is why not all?
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