Issue 31

J. Xavier et alii, Frattura ed Integrità Strutturale, 31 (2015) 13-22; DOI: 10.3221/IGF-ESIS.31.02 17 Isoparametric 8-node planar solid finite elements were used for the bulk material, whilst 6-node cohesive elements were applied along the FPZ, which in this case was confined to a line at mid-height of the specimen (Fig. 2). Wood was modelled as an orthotropic linear elastic material with properties summarised in Tab. 1 [7, 15-17]. E XPERIMENTAL WORK pecimens for the DCB test were cut from a single P. pinaster tree. Specimens were selected in the mature region within the stem at a given location to minimise material variability. Twelve specimens were prepared with axes oriented along the RL propagation system. The nominal dimensions of the DCB specimens were those already used in the numerical simulation (Fig. 1). The crack was generated by using a band saw of 1 mm thickness followed by an impacted blade to guarantee a sharp initial crack surface. The fracture tests were performed in an INSTRON 1125 universal testing machine, with a controlled cross- head displacement rate of 3 mm/min. The load was measured by means of a 5 kN load cell, setting the gain at 50 N/V. Specimens were tested after stabilisation at laboratory conditions of 60-65% relative humidity and temperature of 20-25 ºC. The ARAMIS DIC-2D system was used in this work [18-20]. The speckled pattern required in the DIC method was painted over the region of interest by means of an airbrush to guarantee suitable granulometry, contrast and isotropy at the scale of magnification. For DIC analyses, a subset size of 15×15 pixel 2 (0.270×0.270 mm 2 ) and a subset step of 13×13 pixel 2 (0.234×0.234 mm 2 ) were selected for enhancing spatial resolution [19]. A resolution in displacement of 1–2×10 −2 pixel (0.18–0.36 μm) was obtained. For measuring CTOD I ( ) w , the coordinates of the initial crack tip were firstly identified in the reference image. The I w was then determined by post-processing the displacement of subsets chosen upper and lower the crack tip during the test (Eq. 8). The base distance between the two subsets was 0.468 mm Figure 3 : DCB test: (a) curves and macroscopic visualization of the crack propagation; (b) R –curve obtained by CBBM; (c) R –curve obtained by Irwin-Kies equation S