Issue 43

D. Gentile, Frattura ed Integrità Strutturale, 43 (2018) 155-170; DOI: 10.3221/IGF-ESIS.43.12 158  2 2/3 1 3 2 Ic P C G A bh (8) Here, the coefficient A 1 is determined as the intersection of the best fitting line of the experimental data given in terms of C 1/3 versus a/h with the vertical axis. However, the G values estimated with Eqn (1), (6), have to be corrected again in order to account for the large displacement and the progressive reduction of the bending arm due to rotation around the hinge. In this case, the rotation to be applied is given as:   0 I I G F G (9) where 0 I G is the value obtained with one of the relationships given above and F is the correction factor defined as                  2 2 3 3 1 10 2 t F a a (10) where t is the distance from the delamination crack mid-plane and the rotation center of the hinge. Finally, a fourth method for the estimation of G is given by the evaluation of the area below the curve applied load and opening displacement measure for two crack lengths. In Fig. 2 the two limit cases, relative to the imposed load and displacement condition, are given.  P c  a a da   P c P a a da  Figure 2: Theoretical definition of the strain energy release rate for a crack advance equal to da. Even though this approach is not described in the prescription is a valuable and effective method for evaluating G in those cases where only few data points can be measured from a test. Since this method is based on the theoretical definition of G and does not requires the knowledge of other constants of material properties, is not limited by any assumptions and allows one to compensate the effect of eventual non-linearities in the load-displacement response of the specimen as well as the influence of the compliance of the measurement chain. According to this, it is suggested that, when possible, even though the ASTM D5528 prescriptions indicates to use alternatively one of the three methodologies (Modified Beam Theory, MBT, Compliance Calibration Method, CC, e Modified Compliance Calibration Method, MCC), a comparison with the values obtained with the area below the curve is also performed. The experimental data measured on the DCB tests performed are detailed reported in the following section. For each sample, the measured load, applied displacement and crack advance are given and the G values are calculated with three procedures indicated above, when enough data are available. Samples showing geometrical oddness or where extensive fiber bridging was observed are highlighted and eventually excluded from statistical considerations. Preliminary considerations and fundamental relationships for laminate fracture resistance under mode II The same notched beam geometry configuration used for the DCB can be used in order to load the crack under pure mode II (sliding). In this case, the sample is loaded in three points bending conditions.