Prestressed fracture specimen for delamination testing of composites

The prestressed end-notched flexure fracture specimen is developed in the present work, which combines the traditional double-cantilever beam and the end-notched flexure specimens in a very simple way. The most important features of the new beam-like specimen are that it is able to provide any combination of the modes I and II strain energy release rates and it may be performed by using a simple three-point bending fixture. The mode-I part of the strain energy release rate is fixed by inserting a steel roller, which causes a fixed crack opening displacement. The mode-II part of the energy release rate is provided by the external load. A simple closed-form solution using beam theory is developed for the energy release rates of the new configuration. The applicability and the limitations of the novel configuration are demonstrated using unidirectional glass/polyester composite specimens. If only propagation onset is involved then the prestressed end-notched flexure specimen can be used to obtain the fracture criterion of transparent composite materials in a very simple way.