Issue 49

S.A.G. Pereira et alii, Frattura ed Integrità Strutturale, 49(2019) 412-428; DOI: 10.3221/IGF-ESIS.49.40 414 The values of the geometric factors I Y and II Y can be obtained from [11]. Q is the shear force on crack tip and can be obtained by II Y , P being the applied load. W is the specimen width, t is the specimen thickness and a the crack length. The relative length between the point of application of the load and the pre-crack is given by 0 / S W   . The 3-point eccentric bending load test also allows to test mixed mode I-II crack propagation. By varying the position of the crack in relation to the plane of application of the load (in the middle of the specimen), the measurement of the toughness in mode I ( K Ic ) or the analysis of mixed mode crack propagation are possible, Fig. 2. Figure 2 : Specimen for the eccentric 3-point bending test, SEN, [12]. Pure mode II loading is not conceivable with this set-up since the bending moments are zero only in the support points. Belli et al., [12], show that mode I and II stress intensity factors can be obtained by the following expressions: 0 π   I I K Y a   (3) 0 σ π   II II K Y a  (4) where: 3/2 1 I I a Y Y W          (5) 1/2 1 II II a Y Y W           (6) The values I Y  and II Y  are available for different ratios of / a W e / L W in [13]. The stress 0  can be calculated by replacing the maximum load obtained at the experiments in the following expression: 0 2 3 σ 2 max P S BW  (7) T-stress According to Williams, [14], the linear-elastic tangential stress at the crack tip region can be expressed in polar co-ordinates r and θ as:   2 2 1 cos 1 sin cos 2 2 2 rr I K T O r r                      (8)