Issue 16

G. Pesquet et alii, Frattura ed Integrità Strutturale, 16 (2011) 18-27; DOI: 10.3221/IGF-ESIS.16.02 19 expand throughout the joint on the action of heat, they may dismantle the joint. A low fraction of TEMs does not allow the dismantling of the joint and a too high fraction reduces the bond strength. Thus, 5 to 15 % [3] of TEMs in terms of weight fraction is generally considered for this use. It was even shown that the shear strength of a common lap joint increased thanks to microcapsules [1]. Obviously, resin and particles must match to permit the dismantling. Theoretically, matching means here that the resin must be in its rubbery plateau when particles expand to allow its expansion. The microcapsules cannot transmit more than 1 or 2 MPa to the resin [2]. This means that the temperature of expansion of the particles must be slightly higher than the glass transition temperature ( T g ) of the resin. Thus, the joint can dismantle by itself or with a low force. If it is not the case, i.e. the T g of the resin is higher than the temperature of expansion of the TEMs, the joint cannot be dismantled. However, one can distinguish at least two cases. In the first, the T g of the resin is much higher than the temperature of expansion of the TEMs and the strength of the adhesive does not permit the expansion of the particles. In the second case, the T g of the resin is slightly higher than the temperature of expansion of the TEMs. In this last case, the stress increase due to the expansion of the microcapsules throughout the resin might create a kind of hydrostatic pressure and will affect its properties and its mechanical behaviour. An increase of hydrostatic stress leads to an increase of elastic limit and failure load [4]. The expansion of the particle would also possibly cause a crack barrier which would increase the fracture toughness of the adhesive. The propagation of cracks brings energy dissipation, hence local heating, especially in dynamic conditions such as in a fatigue test. This local heating may lead to the expansion of the TEMs and thus to a local healing by creation of hydrostatic pressure of crack barrier. In other words, a self-healing or self-toughening of the modified adhesive would occur. The obvious capacity of living beings to heal themselves was transposed recently to polymers [5] and polymer composites [6, 7]. It means that a secondary function of healing is embedded into the adhesive and the reaction of healing is triggered by the damage of the material. So it permits counteracting the service degradation whilst still achieving the primary structural requirement. The healing must lead to a local restoration of mechanical properties. The concept of a self-healing material already exists using numerous ways such as bleeding capsules [5] or hollow fibres [8]. It relies on the microencapsulation of a monomer spreading into cracks when ruptured. The polymerisation starts with the contact of the monomer with a dispersed particulate catalyst dispersed in the resin. Thus, repair self occurs when a crack initiates. This concept of self-healing might also occur in the case of TEMs. Note that if heat must be provided to enhance the mechanical properties of the composite TEMs-resin, then an artificial healing is taking place and not a self-healing. These ideas need to be confirmed or invalidated by experiments, which was the aim of the present investigation. Assuming that TEMs may modify the fracture toughness by increasing the resistance to crack propagation, one has to choose a proper test to quantify this change. Microcapsules expansion is wanted, and it would be even better if the test itself could lead to the expansion of the microcapsules. An idea is to use mechanical work in the material in order that it generates heat by itself, as fatigue does. Therefore, self-heating by fatigue was considered. Nevertheless, this local generated heat has to be well contained and not transfer to the full specimen. An adhesive joint between metallic adherends such as the double cantilever beam (DCB) test defined in standard ASTM D3433 was not considered because of the high conductivity of metals. Composite adherends, which are less conductive, could have been used but to avoid interaction of adherends in a first analysis, a single-edge-notch bending (SENB) test (ASTM D5045) using a bulk specimen was selected. The test might not bring enough heat to expand the particles. Therefore, a solution to heat the joint must be defined and validated. Moreover, the concentration of TEMs in the resin affects directly the results. Several concentrations and different heating methods were tested. To exhibit differences brought by microcapsules, three types of tests were carried out. E XPERIMENTAL DETAILS Materials he diameter of these microcapsules ranges mainly from 10 to 20  m with an average of 15  m and the shell thickness from 3 to 4  m at room temperature. Data from the manufacturer (Matsumoto Yushi-Seiyaku Co., Japan) is given in Tab. 1. These particles start to expand at 60ºC. Epoxy resins are the most common structural adhesives. The following points were taken into account in the selection of the epoxy resins. The cure temperature must be lower than the temperature expansion of particles. The T g of the adhesive should be around the temperature of expansion of particles to maximise the effect of TEMs on adhesive. In light of these facts, the adhesives presented in Tab. 2 were selected. The tensile stress-strain curves for the three used adhesives are presented Fig. 1. These curves were determined on bulk dogbone samples. These curves show that adhesive AV138M is T