Issue 30

P.N.B. Reis et alii, Frattura ed Integrità Strutturale, 30 (2014) 431-437; DOI: 10.3221/IGF-ESIS.30.52 434 ) 2.b.(a 3.P. IG Δ δ   (1) where P is the load, δ is the load contact point displacement, b is the specimen width, a is the delamination length and Δ is the modulus of a correction factor. This factor is obtained experimentally by generating a least squares plot of the cube root of the compliance C against the delamination length a (Δ is the value for C 1/3 equal to zero). It is possible to observe an initial stage, which is characterized by an increasing of the interlaminar fracture toughness (G I ) with the crack length. After reaching a maximum value, or very close to it, is almost constant independently of crack length. This phenomenon agrees with Fig. 4, where the load increases with the increase of the displacement and, after a peak load, unstable crack propagation can be observed promoting an oscillatory behaviour of G I . a) b) Figure 4 : Typical load-displacement curves for: a) DCB tests, b) ENF tests. G IC values for the different initial crack lengths are presented in Tab. 1. According to the ASTM D 5528-01 Standard [16], three definitions for an initiation value of G IC can be used. In the present study the G IC values were determined using the load and deflection measured at the point at which the compliance has increased by 5 % or the load has reached a maximum value (5 %/max) [16]. However, the intersection of the load-deflection curve occurs after the maximum load point for all tests performed. Therefore, the maximum load was used to calculate G IC values according to the standard. Figure 5 : G I versus crack length.