Joint test geometries compared

Table 2.5 summarises typical values of these bulk mechanical properties at around 20 °C for a range of epoxy adhesives. Of particular note is the low strength and stiffness of the epoxy polyamides and polysulphides as compared to those with aliphatic polyamine hardeners. For the design of bonded assemblies, joint tests are often used to determine the relevant mechanical properties. It must be remembered, however, that the results will be highly dependent on the specimen geometry and testing conditions and... 

Secondly, the same rubber-toughened epoxy adhesive as was used for the T-peel tests [45] discussed in Section 3.3 has also been studied [51,57] using a LEFM test specimen, i.e. the standard tapered-double cantilever-beam specimen [58]. At the same rate of test and for the same locus of joint failure, a value of Gc of 2750 100 J/m was determined using the LEFM test, compared with a value 2900 400 J/m from the T-peel tests. Thus, here we have completely different test geometries giving the same value of Gc. So, again, a cross-check indicates the robustness of the above analytical approach for modelling the peel test. 

It is emphasized that the failure load of any adhesive joint is influenced by the joint geometry, rate of loading and test conditions. Therefore, lap shear tests should be used only for comparative purposes in adhesive assessment, selection and control. 

Only one scarf Joint geometry was tested. It had the maximum allowable scarf angle and therefore the Joint was not very effective. However, it provided a maximum loading density of 282 N/mm, compared to 218 N/mm in the single-lap joint, while the bond lengths of the scarf joint and the single-lap joint were 14 and 25 mm, respectively. 

As described in Section II of this chapter, there are many types of peel tests available to characterize adhesives. These tests are important because peel stresses arise in the loading of many joint geometries, such as lap joints. Peel tests are severe because they constitute a test of the adhesive in its weakest stress mode. However, the peel test is a comparative test for adhesives and is dependent on many parameters. These parameters, such as peel speed, peel angle, bond thickness, and temperature, must be held constant to obtain valid results. The stress analysis of peeling is highly complicated because of these variable dependencies. 

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