Issue 31

J. Xavier et alii, Frattura ed Integrità Strutturale, 31 (2015) 13-22; DOI: 10.3221/IGF-ESIS.31.02 20 curves (Tab. 2). It is interesting to notice that Iu  (Tab. 3, 4) is of the same order of magnitude as the radial tensile strength (7.93 MPa) measured from tensile tests on P. pinaster (with density of 0.7 g/cm 3 ) in [22]. The mean cohesive law in mode I for P. pinaster was determined from the mean values of the parameters governing the logistic function, as shown in Figs. 4d (CBBM) and 5d (Irwin-Kies). For comparison purposes, the exponential (Eq. 7) cohesive law is also plotted in Figs. 4d (CBBM) and 5d (Irwin-Kies), determined from mean values of the fitting parameters. Qualitatively both laws have a similar behaviour as summarised in Tab. 3 and 4. Figure 5 : DCB test processed by Irwin-Kies equation: (a) characteristic R –curves I I ( ) G w  ; (b) least-square regression with the logistic function; (c) cohesive laws in mode I I I ( ) w   ; (d) comparison between logistic and exponential mean cohesive laws. Specimens 1 A 2 A p I,0 w I u w I c w Iu  Logistic Exponential (N/mm) (N/mm) (-) (mm) (mm) (mm) (MPa) (MPa) 1 0.016 0.35 2.45 0.026 0.018 0.12 9.19 9.16 2 0.011 0.29 2.05 0.017 0.010 0.12 10.5 9.90 3 0.016 0.18 3.09 0.020 0.016 0.08 7.14 5.49 4 0.005 0.28 2.43 0.027 0.019 0.13 7.31 6.37 5 0.016 0.30 2.21 0.027 0.017 0.15 7.16 6.52 6 0.004 0.29 1.81 0.016 0.008 0.12 11.3 11.2 7 0.006 0.21 2.40 0.015 0.010 0.08 9.44 8.32 8 0.013 0.31 1.99 0.032 0.019 0.19 6.02 5.71 9 0.019 0.35 2.81 0.037 0.029 0.16 7.04 5.60 10 0.004 0.25 2.24 0.024 0.015 0.13 7.30 6.61 Mean 0.011 0.28 2.35 0.024 0.016 0.13 8.24 7.48 Coefficient of variation C.V (%) 51.5 19.5 16.2 30.1 36.4 25.2 21.1 26.9 Table 3 : Parameters of the logistic function ( 1 A , 2 A , p and I,0 w ), and characteristic values of maximum stress ( Iu  ) and relative displacements ( I u w , I c w , see Fig. 2) determined by CBBM.