On the experimental determination of the $$\mathcal {J}$$ J -curve of quasi-brittle composite materials

Four different methods to experimentally determine the $$\mathcal {J}$$ -curve of quasi-brittle materials are analysed and discussed in this work. The first two methods measure the integral of the cohesive law, $$\mathcal {J}(\omega )$$ , from an initial notch. However, the correct definition of the notch geometry is of critical importance for an accurate identification of the cohesive law. The other two methods measure $$\mathcal {J}(\omega )$$ when the crack is propagating with a fully-developed cohesive zone. In this case, the cohesive law is obtained by determining the crack opening displacement along the fracture process zone without requiring information about the geometry of the initial notch. The four methods are discussed highlighting the corresponding advantages, limitations and required experimental results. Then, the results of the four methods are compared and validated by considering the experimental results of the Compact Tension test of a quasi-isotropic carbon fibre composite laminate. Finally, some recommendations are given on which of the four methods is the most appropriate to characterise the material $$\mathcal {J}(\omega )$$ law based on the available measuring techniques.