Issue 46

A. Akhavan-Safar et alii, Frattura ed Integrità Strutturale, 46 (2018) 266-274; DOI: 10.3221/IGF-ESIS.46.24 273 loaded in shear. The tensile properties are highly influenced by the voids while the shear conditions are less sensitive to bubbles. Accordingly, it is concluded that the optimum amount of cork to obtain the best fracture properties of the specimens depends on the loading conditions. C ONCLUSION he effects of cork volume fraction on the strength of SLJs with different overlap lengths were studied. SEM was employed to analyse the fracture surfaces of bulk specimens and SLJs. A SEM analysis of the fracture surface of bulk specimens showed that increasing the volume fraction of micro cork causes considerable cleavage failure in the fracture surface of the specimens which results in higher energy absorption and higher toughness of the adhesives containing 1vol% of micro particles. On the other hand, the results of the tested SLJs showed that the static strength of SLJs increase with cork volume fraction up to 5vol%. According to the results, 5vol% of cork particles increased the failure loads of the tested joints with 25 mm overlap length up to 70%. It was shown that the cork particles act as a crack stopper and consequently stabilize the crack propagation and prevent the development of large and unstable cracks. This leads to more energy absorption which increases the fracture loads of the specimens. On the other hand, higher content of cork increases the possibility of agglomeration of the particles, acting as a source of stress concentration, with a higher possibility of delamination between the matrix and the micro corks and also causing the formation of a very thin adhesive layer between adjacent particles. Accordingly, there is an optimum amount of cork where the best adhesive fracture toughness and joint strength are reached. The results of the bulk and the SLJ tests also showed that the optimum amount of cork may vary depending on the loading conditions. A CKNOWLEDGEMENTS inancial support by Foundation for Science and Technology (PTDC/EME-TME/098752/2008 and SFRH / BD / 88173 / 2012) are greatly acknowledged. R EFERENCES [1] Adams, R.D., (2005). Adhesive bonding: science, technology and applications , Elsevier. [2] Banea, M. D. da Silva L.F.M. Campilho and R. D. Sato, C. (2014). Smart adhesive joints: an overview of recent developments, J. Adhes. 90(1), pp. 16-40. [3] Huang, Y., Kinloch, A., Hunston, D. and Riew, C. K. (1993). Mechanisms of toughening thermoset resins. American Chemical Society, Washington, DC (United States). [4] Barbosa, A., da Silva, L.F.M., Banea, M. and Öchsner, A. (2016). Methods to increase the toughness of structural adhesives with micro particles: an overview with focus on cork particles, Materialwissenschaft und Werkstofftechnik. 47(4), pp. 307-325. [5] Cardwell, B. and Yee, A. (1998). Toughening of epoxies through thermoplastic crack bridging, J. Mater. Sci. 33(22), pp. 5473-5484. [6] Silva, M., Marques, E. and Silva, L.F.M. (2016). Behaviour under Impact of Mixed Adhesive Joints for the Automotive Industry, Latin American J. Solids Struct., 13(5), pp. 835-853. [7] Ramos, V.D., da Costa, H.M., Soares, V.L. and Nascimento, R.S. (2005). Modification of epoxy resin: a comparison of different types of elastomer, Polymer Testing. 24(3), pp. 387-394. [8] Barbosa, A., da Silva, L.F.M., Öchsner, A., Abenojar, J. and del Real, J. (2012). Influence of the size and amount of cork particles on the impact toughness of a structural adhesive, J. Adhes. 88(4-6), pp. 452-470. [9] Barbosa, A., da Silva, L.F.M. and Öchsner, A. (2015). Hygrothermal aging of an adhesive reinforced with microparticles of cork, J. Adhes. Sci. Technol. 29(16), pp. 1714-1732. [10]Gil, L. (2009). Cork composites: a review, Materials. 2(3), pp. 776-789. [11]Mano, J.F. (2002). The viscoelastic properties of cork, J. Mater. Sci. 37(2), pp. 257-263. T F