Issue 48

J.P.S.M.B. Ribeiro et alii, Frattura ed Integrità Strutturale, 48 (2019) 332-347; DOI: 10.3221/IGF-ESIS.48.32 342 Experimental strength Fig. 10 provides the experimental P m of the SLJ (a) and DLJ (b) bonded with the three adhesives as a function of L O . For both SLJ and DLJ, the behaviour highly differs depending on the adhesive. Here, the values of E and G xy have a major influence on the stress distributions and, thus, on P m . Actually, Adams [4] proved that a smaller adhesive stiffness is linked to more uniform stresses along L O . In view of this, the joints with the Araldite ® AV138 should have higher peak stresses. It is also known that  y and  xy peak stresses in the overlap increase with L O [41], turning P m in joints with large L O highly dependent on the plasticization ability while, in joints with short L O , P m is mostly dependent on the adhesive strength. Moreover, joints with a brittle adhesive should fail when the adhesive’s strength is attained. Joints with ductile adhesives undergo plasticity at the bond edges at the same time that the inner bond becomes increasingly loaded, which is usually linked to an increase in P m [42]. a) b) c) Figure 9 : Idealised fracture envelopes and experimental G I /G II data points for each of the adhesives: (a) Araldite® AV138, (b) Araldite® 2015 and (c) Sikaforce® 7752. In view of this, the SLJ results of Fig. 10 (a) reveal that the strong and brittle Araldite ® AV138 shows higher P m for L O =12.5 mm compared to the less strong but ductile Araldite ® 2015 (difference of 2.5%). The increase of peak stresses for higher L O [41] inhibits large P m improvements with L O for the joints with the Araldite ® AV138. As a result, the Araldite ® 2015 progressively performs better than the Araldite ® AV138 by increasing L O , due to its ductility. The difference between these two adhesives attains 62.5% for L O =50 mm. The Sikaforce ® 7752 is the less strong adhesive but, on the other hand, it is highly ductile, enough for the adhesive layer to fracture under generalized plasticization conditions up to large L O . Thus, for small L O this adhesive is not competitive due to failure being governed by the strength rather than the toughness. The P m offset, for L O =12.5 mm, is 33.1% to the Araldite ® AV138 and 31.4% to the Araldite ® 2015. On the other hand, due to its marked ductility, P m increases nearly linearly with L O . For L O =50 mm, P m is proximal to that of the Araldite ® 2015 (difference of only 5.3%), while it is higher than the Araldite ® AV138 by 54.0%. For the DLJ (Fig. 10 b) and L O =12.5 mm, the Araldite ® AV138 is also close to the Araldite ® 2015 ( P m of the Araldite ® AV138 only excels by 1.0%). However, the P m vs. L O plots gradually deviate with the increase of L O due to the less marked P m increase for the Araldite ® AV138 (the approximate differences attained 30%). Apart from this, the plastic deformation initiated at P  16 kN in the inner adherend, while tensile net failure of the same adherend occurred at P  24 kN. This occurrence was responsible for the disruption of the P m vs. L O curves of both the Araldite ® 2015 and Sikaforce ® 7752, and it also affected the behaviour of the joints bonded with the Araldite ® AV138, even for small adherend plasticization. The