Mode II fracture toughness evaluation using four point bend, end notched flexure test

A four point bend, end notched flexure test is described for determining the mode II fracture toughness of laminated composites. The test consists of a unidirectional laminate with a midplane delamination loaded in four point bending. The test configuration is analysed by beam theory and by finite element analysis. When the delamination front is in between the inner loading rollers the energy release rate, expressed in terms of the applied load, is independent of delamination length. In addition, delamination growth is shown to be stable for a displacement controlled test. The advantages of this test over conventional mode II tests are the reduction of frictional effects and the ability to perform an experimental compliance calibration as the delamination advances. Further, an R-curve characterisation may be conducted. This results in a reduced time to conduct tests to generate mode II data compared with the tests currently used by industry.