Issue 43

D. Gentile, Frattura ed Integrità Strutturale, 43 (2018) 155-170; DOI: 10.3221/IGF-ESIS.43.12 162 performances were comparable and in the acceptance range. The nominal normal separation stress for the tested glues was 160 Kg/cm 2 approximately. Based on the experimentalist confidence, the best performances have been obtained with a bi- component glue from PATTEX. The glue hardening time is 15 min. approximately but the full load carrying capabilities are obtained after 24h curing time at room temperature. The sample external surface has been previously polished using sand paper in order to increase the surface roughness for an optimal bonding without damaging the material. For each sample the planarity of the hinge wings has been checked before bonding. In order to ensure a uniform glue layer thickness and to avoid relative sliding during curing, a uniform pressure has been applied by means of calibrated weights. This procedure resulted in a two stages gluing for both plane hinge wings carefully avoiding positioning errors. Figure 6: Detail of the gripping fixture showing the different length of the hinge wings. Specimen preparation In order to perform the measure of the crack extension during the delamination process it is necessary to draw a scale along the expected crack path. The fact that the material under investigation is made of carbon fiber makes very difficult to visualize the current crack length and to trace visible markers. The sample thickness has been initially covered with water- based liquid paper. Successively a graduated ruler with a minimum step of 1.0 mm has been traced along the crack path. This operation has been performed paying maximum attention in order to minimize possible errors verifying the measure with a caliper at different locations. In Fig. 7, a detail of a prepared sample is given for reference. Figure 7: Detail of the length scale for crack advance measurements, step: 1mm. Three point bend apparatus ENF test requires appropriate three points bend fixture to be performed. For the present investigation, an adaptable span fixture has been built on purpose. Particular attention has been paid to the dimensions and mass of the fixture in order to avoid any possible influence on the measure of the G II of the fixture compliance. The fixture, depicted in Fig. 8, has been realized according to the ASTM standard. E XPERIMENTAL RESULTS DCB specimens he specimen has been grouped according to the type of the ply. For each test has been recorded the applied load vs imposed displacement. The measured crack advance as a function of the imposed displacement is also collected. These values are used to calculate the corresponding G values. If more than three data point are available, the G T