Issue 16

G. Pesquet et alii, Frattura ed Integrità Strutturale, 16 (2011) 18-27; DOI: 10.3221/IGF-ESIS.16.02 25 The TEMs expanded did not increase the fracture toughness of modified adhesives but the simple presence of TEMs did. TEMs are filled with liquid hydrocarbon and are, therefore, probably poorly compressible as rubber particles are. In addition, their size and the mass fractions used are comparable to those of rubber particles. Therefore, the dominant toughening mechanism might be similar to the one that occurs with adhesives modified with rubber particles. The TEMs as a second phase may trigger inelastic processes at a lower macroscopic stress than the stress required to trigger inelastic processes in single-phase adhesives [11]. Therefore, the adhesive is able to absorb the energy of deformation through plastic shear deformation between the particles [12-13]. Tensile test Fig. 11 presents stress-strain curves for neat and modified Araldite 2015 adhesive. The TEMs decrease the tensile strength and increase the failure strain. The energy under the tensile curve was computed (toughness modulus) and is presented in Tab. 4 along with other values taken from the tensile curves such as the Young’s modulus, the tensile strength and the failure strain. The tensile curves also show that the TEMS increase the toughness and confirm the results obtained with the bending tests. The response of the modified adhesives shows a drop in stress after the proportional limit after which the stress starts to increase again. The same effect was obtained with rubber particles by Imanaka et al [14]. Figure 11 : Tensile stress-strain curves for adhesive 2015. Araldite 2015 0 wt% 10 wt% 20 wt% E (MPa) 1658 1565 1677 Modulus of toughness (mJ/mm 3 ) 0.81 1.14 1.27 σ max (MPa) 26.9 21.8 19.8 ε max 5.4 % 7.7 % 9.1 % Table 4 : Results from the tensile tests on adhesive 2015. F INITE ELEMENT ANALYSIS finite element (FE) analysis of the bending test was carried out. The experimental values of the toughness were the input in the FE analysis and the corresponding numerical load-displacement curves were compared to the experimental ones. Details of the numerical simulation The geometry represented in Fig. 2 was considered. The simulation was conducted assuming linear elastic fracture mechanics (LEFM) and the strain energy release rate was computed using the Virtual crack closure technique (VCCT). The crack was propagated when the strain energy release rate at the crack tip was above the critical strain energy release A