# Torsion Test vs. Other Methods to Obtain the Shear Strength of Elastic-Plastic Adhesives
## 3 Results and Discussion
### Comparison of Different Tests  
It was also remarkable to see that the SLO in compression used in this work (Figure 2, designed to constrain and avoid plane bending) gave similar results as those obtained by both A4 PB and correctly used torsion.  
This similarity could be explained because in the present work a peculiar version of SLO under compression (Figure 2) [20] (derived from ASTM D905-08 [7]) was adopted, which did not suffer from the common problems affecting a lap joint under compression, such as non uniform stress-strain distribution with peaks in the ends of the overlap, and the presence of peel stress due to the possible non-alignment of the adherends and their bending deformability. The two joined adherends were relatively thick and short, thus their bending was minimized. Moreover, the fixture kept the specimen aligned during the test by means of a roller supporting its back face; therefore, a nearly uniform relative sliding of the adherends was obtained during this test.  
This condition was similar and even more favourable to the case of the thick adherend shear test ASTM D5656 in tension, which aimed to achieve a condition of nearly pure shear [24].  
Thus, by testing an elastic-plastic adhesive able to \"smooth\" the stresses, and by using a carefully designed SLO, as the one described in this paper, the ultimate load was obtainedunder a nearly uniform shear stress distribution that was equal to the elastic limit of the adhesive, i.e., its shear strength.  
In the case of the asymmetric 4-point bending (A4 PB), the elastic shear stress distribution in the joint section was parabolic, exhibiting at its mid-height the peak value equal to 1.5 of its mean value. With an elastic-plastic adhesive, plasticity progressively extended under an increasing load from mid-height to the joined section edges and the ultimate load was, again, attained under a uniform shear stress distribution, equal to the elastic limit of the adhesive, i.e., the shear strength.  
Thus, in the final situation, being similar for both tests (this SLO and A4 PB), it is not surprising that the results were nearly equal.  
In general terms, it is usually found that interfacial failures lead to lower strength values than cohesive ones, as the potential of the adhesive is not completely exploited. However, in this case the strength of the interface was most likely close to the intrinsic strength of the adhesive, leading to nearly equivalent results in terms of shear strength.  
As already discussed above, it was remarkable that the experimental torsion curves, in the case of interfacial or cohesive failure of the adhesive, showed a very different behaviour, which could be explained in terms of partial or full exploitation of the adhesive plasticity, respectively.  
Most of all, the plastic failure must start and propagate in the joined region, in a cohesive mode. Results from failures that occurred in interfacial (or mixed cohesive-interfacial as well) were suspicious, as in such a case the stress distribution was not homogenized by plasticity. Samples with failure starting and/or propagating in the substrates should not be considered when calculating the shear strength; in these cases, results might be defined \"torsion resistance\" of the joined sample, and the joint strength might be higher than the substrate strength.  
Table 1 summarizes results obtained by SLO, A4B, and torsion (THG only) on Hysoljoined samples: all failures were interfacial, except those for surface-engineered Si3N4, which were cohesive, as discussed in [19].  
The results in Table 1 were calculated considering the maximum of each test curve for SLO and A4 PB; in the case of torsion (THG), the reported shear strength results were obtained by using Equations (2)-(4) of the model.