Fracture toughness measurement without force data – Application to high rate DCB on CFRP

The measurement of the fracture toughness of fiber reinforced composites at high rates of loading is still, despite years of research, not well established. This can be related to challenges in applying appropriate high rate loading on the specimen, accurately measuring the load, and in-situ determination of the crack length. In this work these challenges are addressed by using a direct wedge-on-specimen type loading of a double cantilever beam (DCB) specimen, high resolution optical deformation tracking, and a beam theory based analysis of the specimen deflection and crack length. This approach results in symmetric mode I opening of the crack and a robust analytical determination of the fracture toughness without the need to measure the external forces acting on the specimen nor to visually estimate the crack length. Tests carried out on carbon fiber reinforced epoxy composite at quasi-static and high rates (relative velocity up to 15 m/s) show the validity of the approach.