Issue 48

F.A.L. Viana et alii, Frattura ed Integrità Strutturale, 48 (2019) 286-303; DOI: 10.3221/IGF-ESIS.48.29 298 Thus, stresses for DLJ have an improved efficiency over SLJ and, as a result, P m should over-double P m of the SLJ for the same L O . a) b) Figure 13 :  y stress distributions at the adhesive mid-thickness as a function of LO for the SLJ (a) and DLJ (b) . a) b) Figure 14 :  xy stress distributions at the adhesive mid-thickness as a function of LO for the SLJ (a) and DLJ (b) . Fig. 14 (a) shows the obtained  xy stress profiles for the SLJ, in which τ xy stresses peak at the overlap edges, and present smaller values at the overlap inner region [36, 37]. This is caused by the adherends’ differential deformation along the overlap. Indeed, the adherends are increasingly loaded from their free overlap edge towards the other overlap edge. τ xy peak stresses increase with L O mainly due to the growing variations of adherend longitudinal strains as L O becomes larger [16]. This is detrimental for the joint behaviour especially when using brittle adhesives [38]. Actually, the overall strength is affected by the smaller allowable plastic deformation that the adhesive can endure, and the strength improvement with L O should be reduced. While brittle adhesives fail upon the attainment of the adhesive strength at the overlap ends, the ductile ones allow plasticization, putting the adhesive layer under load when the adhesive at the overlap ends starts to yield [39]. Therefore, ductile adhesives have a significant P m improvement with L O . The  xy stress distributions for the SLJ depicted in Fig. 14 (b) show a higher peak at the outer overlap end than the opposed one due to the higher loads carried by the middle adherend. Comparing with the SLJ design,  xy stresses diminish at the inner overlap end due to the reduced variation between the exterior and interior adherends’ longitudinal deformations, thus reducing the differential straining effect. Moreover,  xy stresses for the DLJ at the both overlap ends are considerably smaller in normalized magnitude, which will benefit P m and, probably, more than the double of the SLJ strength should be attained [40]. Additionally, and similarly to the SLJ, P m should not increase proportionally with L O , because  y peel and  xy peak stresses at the overlap edges increase with L O . Experimental joint strengths Fig. 15 depicts the P m average experimental values and respective deviation for the SLJ (a) and DLJ (b) bonded with the three adhesives. At first glance, P m highly depends on the used adhesive. Regarding the SLJ evaluation (Fig. 15 a), the P m increase with L O for the adhesive Araldite ® AV138 is not so pronounced as for the others adhesives. For L O =12.5 mm, P m is very similar for the two Araldite ® adhesives (≈5.3 kN), and the Sikaforce ® 7752 is lower than the AV138 by 33.1%. The -4 0 4 8 12 0 0.2 0.4 0.6 0.8 1  y /  avg x / L O 12.5 mm 25 mm 37.5 mm 50 mm -4 0 4 8 12 0 0.2 0.4 0.6 0.8 1  y /  avg x / L O 12.5 mm 25 mm 37.5 mm 50 mm 0 2 4 6 8 0 0.2 0.4 0.6 0.8 1  xy /  avg x / L O 12.5 mm 25 mm 37.5 mm 50 mm 0 2 4 6 8 0 0.2 0.4 0.6 0.8 1  xy /  avg x / L O 12.5 mm 25 mm 37.5 mm 50 mm