Determination of cohesive laws in wood bonded joints under mode I loading using the DCB test

Abstract

The cohesive laws (CLs) have been investigated by means of direct and inverse methods concerning wood bonded joints under pure mode I. The experimental results were obtained by tests with double cantilever beam. The direct method is based on the differentiation of the relation between strain energy release rate and crack opening displacement at the crack tip. An equivalent crack method was used to evaluate the strain energy release rate in the course of the test without monitoring the crack length, which is difficult to observe exactly. The crack opening displacement was determined by postprocessing local displacements measured by digital image correlation. The inverse method requires a previous assumption of the CL shape, and as such, a trilinear law with bilinear softening relationship was selected. The cohesive parameters were identified by an optimization procedure involving a developed genetic algorithm. The idea is to minimize an objective function that quantifies the difference between the experimental and the numerical load-displacement curves resulting from the application of a given law. A validation procedure was performed based on a numerical analysis with finite elements. Both methods in focus provided good agreement with the experimental data. It was observed that CLs adopted by the inverse method are consistent with the ones obtained with the direct method.

Analysis of the metal adhesively bonded double cantilever beam specimen

“…The so-called inverse method provides an approximate traction-separation law by combining cohesive zone modelling with parameter optimization in order to achieve best fits to experimental load-displacement curves [46]. However, the importance of the portion of the load-displacement curve up to crack initiation and the associated 3D effects pose considerable challenges.…”

Measuring mode I cohesive law of wood bonded joints based on digital image correlation and fibre Bragg grating sensors

“…Although the widespread use of CZM, the experimental identification of the cohesive laws poses several difficulties. One way of tackling this problem is by an inverse method, in which a particular shape for the cohesive law is assumed and its constitutive parameters determined from global experimental tests using optimisation procedures [5][6][7]. However, this type of indirect identification method is impaired with an important limitation.…”

“…The cohesive law comes up through the differentiation of G versus COD relationship. This approach was recently applied to wood and composites bonded joints under both mode I [6,12,13] and mode II [14,15] loading. The opening displacement at the initial crack tip (CTOD) was evaluated by post-processing the crack tip displacements measured by means of digital image correlation (DIC) [16,17], whereas Resistance-curves (R-curves) were determined using the compliance-based beam method (CBBM) [5].…”

Initial studies of glued wood joints using FBG strain sensors

“…Although there have been great achievements in this topic, the characterization of bond strength of the adhesive at the adherent wood-wood interface remains an open problem due to the material heterogeneity at various scales derived from their natural origin. The final goal is to understand how fractures behave in these joints considering the overall structure, so more efficient and secure constructions can be built [1].…”