Issue 31

J. Xavier et alii, Frattura ed Integrità Strutturale, 31 (2015) 13-22; DOI: 10.3221/IGF-ESIS.31.02 14 I NTRODUCTION ood is a hierarchical, anisotropic and heterogeneous composite material formed by trees. Recently, green composites based on lignocellulosic fibres and forest-based resources have attracted increasing interest in both research and market [1, 2]. Moreover, in a policy of sustainability, wood and wood products are increasingly used nowadays, for instance, in structural and semi-structural applications [3]. However, for a better and efficient utilisation of wood material, several issues must be further investigated. One fundamental aspect concerns the fracture mechanical behaviour of wood. Relatively extensive fracture process zones (FPZ) are observed in wood due to fibre bridging and micro-cracking ahead of the crack tip [4]. However, the microstructural mechanisms in wood fracture are usually confined to a region of reduced thickness [5]. Therefore, at the macroscopic scale, the wood behaviour in the FPZ can be conveniently described through a phenomenological constitutive cohesive law [6, 7]. In order to obtain the cohesive law, one approach consists in minimising an objective function quantifying the difference between numerical and experimental load-displacement ( P   ) curves by inverse analysis, assuming a given shape of the softening law. This approach, however, is semi-empirical and does not guarantee the uniqueness of the solution. Notwithstanding, it has been shown that the inverse identification of cohesive laws provide good agreement between experimental and numerical finite element simulations [7, 8]. Instead, a direct method for evaluating the cohesive law can be proposed based on independent determination of strain energy release rate and crack tip opening displacement (CTOD) [9]. The advantages of this approach are: (i) the shape of the cohesive law does not need to be assumed a priori; (ii) the cohesive law is determined based on local measurements. In this work, a direct identification of the cohesive law in mode I of P. pinaster was investigated by coupling the double cantilever beam (DCB) test with digital image correlation (DIC). Specimens oriented in the radial-longitudinal (RL) propagation system were used. The strain energy release rate in mode I ( I G ) was explicitly determined from the P   curve by means of the compliance-based beam method (CBBM). This data reduction scheme is based on the concept of equivalent elastic crack length ( eq a ) and, therefore, does not require the measurement of the crack length during test. An independent evaluation of CTOD in mode I ( I w ) was determined from displacement fields at the initial crack tip. The direct differentiation of the I I G w  curve and the reconstruction of the I I w   cohesive law, by means of least-squares regression using a continuous approximation function, were addressed. The proposed procedure was also validated by finite element simulations including cohesive zone modelling. Figure 1 : Schematic representation of the DCB test ( 2 h = 20 mm, 1 L = 300 mm, L = 280 mm, B = 20 mm and 0 a = 100 mm). D ATA REDUCTION he DCB test is schematically shown in Fig. 1. The specimen is a 1 2 L h B   mm 3 rectangular beam. The resistance curve ( R –curve) can then be determined from the Irwin-Kies equation 2 I d 2 d P C G B a  (1) W T

RkJQdWJsaXNoZXIy MjM0NDE=