Issue 16

G. Pesquet et alii, Frattura ed Integrità Strutturale, 16 (2011) 18-27; DOI : 10.3221/IGF-ESIS.16.02 18 The use of thermally expandable microcapsules for increasing the toughness and heal structural adhesives Guillaume Pesquet ENSIETA, 2 rue François Verny, 29806 Brest, France Lucas F. M. da Silva Departamento de Engenharia Mecânica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal; lucas@fe.up.pt Chiaki Sato Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan A BSTRACT . In this research, the effect of thermally expandable microcapsules (TEMs) on mode I fracture toughness of structural adhesives were investigated. The single-edge-notch bending (SENB) test was used. Firstly, a standard toughness test was performed on adhesives with microcapsules. Secondly, since TEMs start their expansion at approximately 60ºC, the next specimens were fatigue tested expecting a local heating in the notch leading to the desired expansion before being statically loaded for fracture toughness determination. Thirdly, a manual local heating at 90ºC was applied in the notch before the fracture static test. The experimental results were successfully cross-checked through a numerical analysis using the virtual crack closure technique (VCCT) based on linear elastic fracture mechanics (LEFM). The major conclusion is that fracture toughness of the modified adhesives increased as the mass fraction of the TEMs increased. K EYWORDS . Epoxy/epoxides; Fracture mechanics; Finite element stress analysis; Thermal analysis; Thermally expandable microcapsules (TEMs). I NTRODUCTION hermally expandable microcapsules, thermally expansive particles, thermo-expandable microsphere (TEMs), or a combination of these words, are all particles made up of a thermoplastic shell filled with liquid hydrocarbon. On the action of heat, the shell softens and the liquid hydrocarbon boils. The hydrocarbon gas works as a blowing agent because the shell expands as the inner pressure increases. The growth in volume can be from 50 to 100 times. This capability brings new and quite unique possibilities for engineers. Because of its high sensitivity to temperature, storage at over 40ºC is not advised [1]. The use of TEMs in adhesives has allowed industry to have easily dismantlable joints. The simple heating of the joint over 100ºC leads to an easy separation of the bonded materials. The adhesive expansion may be up to 400% according to the study of Nishiyama and Sato [2]. This technique is promising because it does not need much time to make this kind of joint and greatly facilitates the dismantling. TEMs are mixed with the resin before the manufacture of the joint. Since they T