This paper introduces a new test, method, named internal-notched flexure (INF) test, that is designed to measure the critical energy release rate of fibre-reinforced polymers for delamination growth in shear mode (mode II). The INF test generates stable delamination growth, with a monotonic increase of load and displacement in a nearly linear fashion. Values of the mode II delamination toughness were deduced using experimental compliance fitting method. Good repeatability of the results was obtained. Compared with the end-notched flexure (ENF) test using the same material, the INF test yielded higher delamination resistance, possibly due to the bridging fibres found between fracture surfaces of the INF test specimens. 1. Introduction The use of a starting defect to quantify materials fracture resistance was firstly used in Griffith’s approach [1] for the measurement of the energy required to form fracture surfaces. The concept has since been adopted in many test methods to characterize toughness of a variety of materials. For fibre-reinforced polymers (FRP), this concept has successfully led to the development of double-cantilever beam (DCB) test that is now a standard method for measuring FRP’s resistance to delamination in an opening mode (mode I). Many similar approaches have been attempted to develop a standard for the measurement of FRP’s resistance to delamination in an in-plane shear mode (mode II), but with less success. Among the methods proposed, end-notched flexure (ENF) test [2], stabilized end-notched flexure (SENF) test [3], end-loaded split (ELS) test [4], and 4-point end-notched flexure (4ENF) test [5] are the most promising candidates for the standard, and rigorous efforts have been made to derive the corresponding expressions of energy release rate for delamination growth. Using simple beam theory, with the assumption of stress-free fracture surfaces, the expressions of critical energy release rate ( ) and beam flexure compliance ( ) are as follows. For the ENF and SENF tests [2] for the ELS test [4] for the 4ENF test [5] where is the critical load for the delamination growth, specimen width, half specimen thickness, half span length, flexure modulus, and in the expression for the distance between the two central loading pins. With the assumption of constant during the delamination growth, the schematic load-displacement curves generated by the 4 delamination tests are presented in Figure 1 in which the delamination growth commences where the initial slope of the loading curve is reduced. In principle, all tests are capable of
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