Issue 31

J. Xavier et alii, Frattura ed Integrità Strutturale, 31 (2015) 13-22; DOI: 10.3221/IGF-ESIS.31.02

 (Tab. 3, 4) is of the same order of magnitude as the radial tensile

curves (Tab. 2). It is interesting to notice that Iu

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.

) G w  ; (b) least-square regression with the

Figure 5 : DCB test processed by Irwin-Kies equation: (a) characteristic R –curves I I (

( ) w   ; (d) comparison between logistic and exponential mean cohesive laws.

logistic function; (c) cohesive laws in mode I I

I



Iu

p

w

A

A

w

w

Specimens

Logistic (MPa)

Exponential

I,0

I u

I c

1

2

(N/mm) (N/mm)

(-)

(mm) 0.026 0.017 0.020 0.027 0.027 0.016 0.015 0.032 0.037 0.024 0.024

(mm) 0.018 0.010 0.016 0.019 0.017 0.008 0.010 0.019 0.029 0.015 0.016

(mm) 0.12 0.12 0.08 0.13 0.15 0.12 0.08 0.19 0.16 0.13 0.13

(MPa)

1 2 3 4 5 6 7 8 9

0.016 0.011 0.016 0.005 0.016 0.004 0.006 0.013 0.019 0.004 0.011

0.35 0.29 0.18 0.28 0.30 0.29 0.21 0.31 0.35 0.25 0.28

2.45 2.05 3.09 2.43 2.21 1.81 2.40 1.99 2.81 2.24 2.35

9.19 10.5 7.14 7.31 7.16 11.3 9.44 6.02 7.04 7.30 8.24

9.16 9.90 5.49 6.37 6.52 11.2 8.32 5.71 5.60 6.61 7.48

10

Mean

Coefficient of variation C.V (%)

51.5

19.5

16.2

30.1

36.4

25.2

21.1

26.9

 ) and relative

Table 3 : Parameters of the logistic function ( 1 A , 2

A , p and

I,0 w ), and characteristic values of maximum stress ( Iu

w , I c

w , see Fig. 2) determined by CBBM.

displacements ( I u

20