Issue 43

D. Gentile, Frattura ed Integrità Strutturale, 43 (2018) 155-170; DOI: 10.3221/IGF-ESIS.43.12 159 Figure 3: ENF geometry configuration and dimensions This configuration has been found to produce shear loading at the crack tip without introducing excessive friction between the crack surfaces, [10, 11]. From the classical beam theory, the following expression for the GII can be derived, [12]:   2 2 3 3 9 2 (2 3 ) II P Ca G w L a (11) where the specimen compliance C is given by:   3 3 3 1 2 3 8 f L a C E wh (12) The stability of the crack growth may be estimated by the sign of the first derivative with respect to the crack advance,  a . Eqn. (11) and (12) for fixed load lead to:    2 2 3 1 9 8 II f G aP a E w h (13) while for fixed displacements,            2 2 3 2 3 3 1 9 9 1 8 2 3 II f G a a a E w h C L a (14) The first condition is always defined positive indicating that this configuration is always unstable. On the contrary the second condition is negative (i.e. stable crack growth) only for   3 0.7 3 L a L (15) Since in most of the cases a is usually close to L/2, this configuration always leads to unstable crack growth. Consequently, very few experimental data points (theoretically just one) are expected to be measured on a single sample. The estimation of the fracture toughness requires a record of the load displacement response. In the case of ductile matrix where nonlinearities in the load vs displacement curve may occur, the G II at the onset non-linearity, visual stable crack extension and maximum load can be determined as illustrated schematically in Fig. 4a.