Issue 46

A. Akhavan-Safar et alii, Frattura ed Integrità Strutturale, 46 (2018) 266-274; DOI: 10.3221/IGF-ESIS.46.24 269 Before manufacturing the joints, the substrate surfaces were firstly pre-treated using sand blast technique. To do this a brown fused alumina with average dimension of 710 µm at pressure of 6.5 bar was used to clean the substrate surfaces. After sand blasting, the surfaces were cleaned with acetone. Fig. (4) shows the dimensions of the manufactured SLJs. To cure the adhesive, the SLJs and the bulk plates were hot pressed at 14 bar for 15 minutes at 100°C. To investigate the effect of cork density on mechanical properties of the adhesive and on the strength of SLJs, adhesive joint samples containing different volume fractions of cork particles (including 1%, 2% and 5 vol%) were considered. Figure 4 : Dimensions of the tested joints. SLJ and bulk tests Tab. (2) gives the configurations of the joints tested. The tests were carried out under displacement control condition at 1 mm/min. End-tabs were fixed at the end of all the joints as shown in Fig. (4), to reduce the bending effect. At least three specimens for each configuration were manufactured and tested. Test parameters Cork volume fraction (%) 0, 1, 2 and 5 SLJ overlap length (mm) 25 and 50 Table 2 : Configurations of the tested joints. Fracture surface analysis A magnifying glass and SEM were used to perform a careful analysis of the fracture mechanisms and toughening phenomena. For this purpose, a magnifying glass (Zeiss/Germany) was used in conjunction with an image capture software, Leica LAS 4.3 (Leica Microsystems/Germany). SEM analyses were also performed to investigate the distribution of the cork particles and to analyze the fracture surface of the joints specimens. To achieve this, a JEOL JSM 6301F/Oxford INCA Energy 350/Gatan Alto 2500 microscope (Tokyo, Japan) were used, at CEMUP (University of Porto, Portugal). Samples were coated with an Au/Pd thin film, by sputtering, using the SPI Module Sputter Coater equipment, for 120 s and with a 15 mA current. R ESULTS AND DISCUSSIONS he volume fraction of particles dispersed in a structural adhesive matrix is usually a significant parameter in controlling the resulting properties of the toughening adhesive. Authors showed that toughness increases with an increasing volume of particles and eventually reaches a maximum value [4]. Fig. (5) shows typical load- displacement curves of bulk specimens with different amounts of corks. As shown in Fig. (5), bulks with 1 vol% micro particles give higher elongation at break. Fig. (6) shows the fracture surface of the tested bulk specimens for different cork volume fractions. Fig. (6) shows that the micro cork particles cause several cleavage failures, which result in a higher energy absorption and consequently higher toughness of the adhesive. According to Fig. (6), cleavage in the fracture surface of the bulk specimens with 1 vol% cork is higher than the adhesive with no-cork or 5 vol% micro particles. Using a higher fraction of cork particles cause formation of more voids and more air entrapment. Since the tensile strength of the bulk specimens is highly sensitive to the presence of voids, higher density of bubbles considerably decreases the static strength of the specimens. Formation of micro cracks around the voids and subsequently the void coalescence are the main reasons of the lower static strength of bulks with 5 vol% cork particles. These results are in agreement with the previously published data. In previous studies, T