Issue 48

C. E. Cruz Gonzalez et alii, Frattura ed Integrità Strutturale, 48 (2019) 530-544; DOI: 10.3221/IGF-ESIS.48.51 541 highest ductility reached by the MP55420 adhesive lap joint (see red inset in Fig. 3) and the lowest fatigue loading at 10 6 cycles (Fig. 7) compared to that of other adhesives, clearly confirmed the described fatigue fracture. Fig. 10 showed the fatigue fractured surfaces of Betamate 120 adhesive at fillet zone near steel side from single-lap shear specimens with overlap length of 50 mm for 70 % of applied maximum load, which exhibited fatigue fracture area with less rough surface containing several nearly rounded particles and micro-dimples (Fig. 10a). At bigger amplification, a nearly flat surface could be seen in Fig. 10b), and a clearly flat surface containing well resolved striations and small particles is observed in Fig. 10c). Besides, it was observed several small semi-rounded metallic particles within fatigue fractured surfaces linked to the composition of the adhesive with aluminum as filler. The Betamate 120 adhesive used in fatigue specimens reached a fatigue behavior that was linked to the moderated elastic deformation and Young modulus, limited tensile strength and the highest fatigue loading at 10 6 cycles (Fig. 6) compared to that of DC ‒ 80 and MP55420 adhesives. Therefore, single-lap shear specimens with Betamate 120 adhesive reached the best dynamic behavior under fatigue. Figure 10: SEM fractographs of fatigue fractured single-lap shear specimens with overlap length of 50 mm containing Betamate 120 adhesive at fillet zone: a) less rough fracture with several particles, b) nearly flat surface, and c) resolved fatigue striations. For the fatigue fractured surfaces at the DC ‒ 80 adhesive at fillet area near steel side from single-lap shear specimens with overlap length of 50 mm and 70 % of applied maximum load (Fig. 11), it’s clearly observed a brittle failure [25, 26] with interesting features, i.e., flat surfaces of variable extension (Fig. 11a), well-defined flat quasi-cleavage facets containing notable river patterns in the direction of fatigue crack propagation (Fig. 11b), which were observed at major magnification in Fig. 11c). Moreover, the fatigue fractured specimens joined by DC ‒ 80 adhesive exhibited notable facet fragile failure that was confirmed by the shape of stress-strain plot with straight line from the origin to the point of fracture [25], completely elastic strain behaviour without plastic deformation, the highest tensile strength and abruptly fracture, as can be seen in the curve of Fig. 3.