b. SEM analysis
The fracture surfaces of the dogbone tensile specimens unmodified and modified with TEPs were examined using a SEM analysis. As expected, for the TEPs-modified specimens (Fig. 11b and c) it can be seen that the surface is more irregular indicating a higher deformation, i.e. the material is more ductile (as the yield strength is lowered, the fracture is more ductile in nature). The particles dispersion and sizes were also examined (see also Fig. 2). As can be seen in Fig. 11b and c the TEPs-modified adhesive samples showed a uniform dispersion of particles.
2. DCB tests
a. DCB failure mode
After the tests, the failure modes of the specimens were evaluated visually. As can be observed in Fig. 12, the failure in the DCB specimens was cohesive for all cases. By examining the fracture surface morphology of the DCB specimens, it can be seen that it reveals a multi-scale fracture mechanism and that topographic marks were developed on the failure surfaces. The formation of these marks might be explained by the presence of secondary microcracks ahead of the main crack, which grow and eventually link up with the main crack. Due to the presence of large damage zones, the secondary cracks do not always nucleate in the plane of the main crack. This is specific of modern toughened adhesive joints where fracture occurs by the development and propagation of a damage zone, rather than a single sharp crack [25].
b. Effect of wt% TEPs on the fracture toughness
Representative experimental load (P)-displacement ( δ ) curves of the DCB specimens as a function of wt% TEPs are presented in Fig. 13. It can be seen that the elastic stiffness was almost the same for all TEPs concentrations and that the maximum load increased by addition of TEPs. Also, an increase in displacement can be observed for all the wt% TEPs concentrations.
The critical fracture energy in mode I, G Ic , was evaluated using the Compliance-Based Beam Method (CBBM). This method was developed by de Moura et al. [26,27] and is based on the crack equivalent concept, depending only on the specimen's compliance during the test. G Ic can be obtained by the following expression:
\(G_{IC}\ =\ \frac{6P^2}{b^2h}\left(\frac{2a_{eq}^2}{h^2E_f}+\frac{1}{5G}\right)\)