Issue 49

F.J.P. Moreira et alii, Frattura ed Integrità Strutturale, 49 (2019) 435-449; DOI: 10.3221/IGF-ESIS.49.42 444 adhesive in the T-joint configuration. The damage evolution is more gradual for the other adhesives, due to their inherent ductility, especially for the Sikaforce ® 7752. For the Araldite ® 2015, despite this modification in the failure process, it is clear that a damaged portion of adhesive of non-negligible dimensions is present which, in this particular loading case, can prove to have a positive effect on P m . This ductility effect is even more visible in the Sikaforce ® 7752. Inclusively, for t P2 =4 mm, a portion of adhesive in the vicinity of x / L O =1 also undergoes damage. For all adhesives, a modification of the SDEG and a clear tendency are found depending on t P2 . Irrespectively of the adhesive, the damage tends to extend further with the increase of t P2 . At P m , the portion of adhesive under damage (SDEG between 0 and 1) for the Araldite ® AV138 is 1.8; 2.6; 9.5 and 31.3% for t P2 =1, 2, 3 and 4 mm, respectively. By the same sequence, these percentile values increase to 6.3; 9.5; 13.0 and 19.1% for the Araldite ® 2015 and further to 20.9; 33.9; 39.1 and 40.0% for the Sikaforce ® 7752. This evolution between adhesives is clearly due to the increasing ductility of the adhesives in the mentioned order, depicted in Tab. 1 through the values of G IC and G IIC . Moreover,  y and  xy peak stresses reduce from the Araldite ® AV138 to the Araldite ® 2015, and even further to the Sikaforce ® 7752, due to the differences in elastic stiffness (Tab. 1), which helps in spreading the damage across the adhesive layer. Higher damage zones should help in attaining higher P m in this particular joint configuration, which traditionally concentrates the load in a small portion of the adhesive. On the other hand, irrespectively of the adhesive, its damaged portion always increases with t P2 , which can be explained by the corresponding reduction of L-part deformations due to a load scenario that gradually shifts from peeling to cleavage as t P2 increases. This shift should also bring a higher joint efficiency, because the adhesive region resisting pull-out increases. Experimental strength The experimentally obtained P m for the T-joints bonded with the three adhesives are presented in this Section. Fig. 7 reports the average P m vs. t P2 curves, including the standard deviation of the experiments. Figure 7: Experimental P m vs. t P2 curves for the three adhesives. It can be found that P m always increases, and by a large amount, with t P2 , irrespectively of the adhesive. The two Araldite ® adhesives show an increasing growth with t P2 , while the Sikaforce ® 7752 has a marked linear evolution of the P m vs. t P2 curve. The percentile P m increase for the T-joints with t P2 =2, 3 and 4 mm, over the T-joint with t P2 =1 mm, was respectively of 75.0%, 173.8% and 419.3% (Araldite ® AV138), 81.2%, 197.8% and 403.7% (Araldite ® 2015) and 110.9%, 227.1% and 358.6% (Sikaforce ® 7752). This marked P m improvement with t P2 is mainly due to the  y stress levelling effect that is visible in Fig. 5 (a) near x / L O =0, which corresponds to the stress initiation site. Subsequently, this also reflects in a more widespread damage span (i.e., 0<SDEG<1) with the increase of t P2 , as it is visible in Fig. 6 for the three adhesives, although more clearly for the Araldite ® 2015 and Sikaforce ® 7752. Between adhesives, the Sikaforce ® 7752 clearly outperforms the other two adhesives, despite being the less strong amongst the three adhesives (Tab. 1). The P m improvement of the T-joints bonded with this adhesive, over those bonded with the Araldite ® AV138, ranges between 213.8% ( t P2 =4 mm) and 328.0% ( t P2 =2 mm). On the other hand, compared against the joints bonded with the Araldite ® 2015, the P m improvement varies between 78.5% ( t P2 =4 mm) and 128.1% ( t P2 =2 mm). It is also visible in Fig. 7 that P m for the joints bonded with the Araldite ® 2015 are higher than those with the Araldite ® AV138. In fact, depending on t P2 , P m may almost double that obtained with the Araldite ® AV138 adhesive, although this adhesive has higher strengths. The improvement is minimum for t P2 =4 mm (78.4%) and maximum for t P2 =3 mm (97.2%). These results show that, in a test geometry that is mainly loaded in peel, and which 0 1 2 3 4 5 6 0 1 2 3 4 P m [kN] t P2 [mm] AV138 2015 7752