On the relevance of the equivalent crack length in the determination of the ERR in modes I and II

• Determination of pure mode ERRs obtained with equivalent and visual crack lengths. • Energy release rates can be determined during the test even in cases of large FPZ. • The use of the equivalent crack length is necessary in cases where FPZ is significant. • Monte Carlo analysis shows stable results of ERR with respect to geometric and elastic uncertainties. The equivalent crack length in a fracture test is determined by the compliance, being known the elastic properties of the specimen. In this study, asymmetric double cantilever beam in pure mode I and asymmetric end notched flexure in pure mode II are analysed numerically. In a first step, three bi-material cases of small fracture process zone are analyzed to compare the crack length obtained by an analytical approach with the input value of the numerical model. Then, an aluminium-composite joint with a significant fracture process zone has been analysed. The energy release rate has been determined by two values of the crack length: the equivalent crack length and the visual crack length. The values obtained with the equivalent crack length agree with numerical values determined by the J -integral. A sensitivity analysis is carried out by the Monte Carlo method to elucidate the effect of uncertainties on the values of the equivalent crack length and on the energy release rates. Finally, the sensitivity analysis is done assuming that the value of the crack length is fixed, varying only the parameters concerning the energy release rates. The coefficients of variation obtained in this second case are slightly greater than in the first case.