Influence of initial crack width in mode i fracture tests on timber and adhesive timber bonds

The evaluation of crack initiation and propagation of timber and timber bonds is a key factor in assessing damage resistance in various engineered timber products. The initial crack width of a sample in fracture tests contributes not only to stable crack propagation but also impacts the crack initiation and maximum load. This paper presents an experimental quantification of fracture energy release rates in timber and timber adhesive bonds subjected to Mode-I double cantilever beam (DCB) tests with different pre-crack geometry to establish the optimal pre-crack width of the test samples. A sensitivity study using digital image correlation (DIC) has been performed to ensure a precise and objective evaluation of crack propagation. Additionally, crack resistance curves for both timber and timber adhesive bonds have been calculated and compared. From the analysis of experimental results, maximum fracture energy release rates of a pure bond delamination are achieved from the minimum possible width of the initial crack. Based on obtained test results, 0.1 mm thin polyethylene duct tape was used in the initial crack zone to maintain a minimal gap between timber joints. However, for DCB tests on pure timber, 0.5 mm is the lowest achievable crack width which was found to yield more stable fracture propagation compared to the test samples with 1 mm wide crack widths.