Experimental aspects of Mode I and Mode II fracture toughness testing of fibre-reinforced polymer-matrix composites

Abstract Research on fracture mechanics has yielded a number of different test set-ups for determining the critical fracture toughness of fibre-reinforced polymer-matrix (FRP) composite materials under tensile/opening (Mode I), in-plane shear (Mode II), and anti-plane shear/torsion (Mode III) loads, as well as for mixed mode loads I/II. Test procedures for Mode I and Mode II critical fracture toughness testing of unidirectional FRP-composites are currently being evaluated for international standardisation. This paper discusses recent developments with the emphasis on experimental aspects of fracture toughness testing of FRP-composites in Mode I and Mode II. Experimental determination of the critical fracture toughness of FRP-composites involves a number of issues. Most activities to date have aimed at reducing in-laboratory and inter-laboratory variation to “reasonable” bounds (typical coefficient of variation 10–20%) when testing identical material. Specifically, the following problems will be addressed and discussed: (1) the type of starter crack or delamination starter, (2) the definition of delamination initiation from an existing crack, (3) the distribution and number of data points used in the analysis, (4) the accuracy of the experimental data, and (5) validation criteria for test method and data. Beyond the specific technical issues mentioned above, there are more general issues that relate to the applicability of the test procedures. So far, some of these issues have received little attention. A crucial point is the definition of the scope of the procedure, but differences in interpretation and realisation of the procedures by the operator are also important. From the point of view of the users of FRP-composites, general experimental aspects, such as, e.g., the simplicity of the test procedure and the availability of test set-ups, as well as the time needed for test preparation, performance, and analysis (which translate directly into cost), and the applicability of the procedures to engineering materials will gain in importance, once the technical test problems have been resolved.