Modified lap joints

Various modifications have been suggested to both the basic single and double lap-shear joints in order to decrease their stress concentrations and so raise 

Number Description Fillet angle iraciurc luau per unit width (MN/m) Shear failure in the adhesive Interlaminar failure in composite Tensile failure in the adhesive 

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Flexible Plastics and Elastomers. Thin or flexible polymeric substrates may be joined using a simple or modified lap joint. The double strap joint is best but also the most time-consuming to fabricate. The strap material should be made out of the same material as the parts to be joined, or at least have approximately equivalent strength, flexibility, and thickness. The adhesive should have the same degree of flexibility as the adher-ends. 

Modified-lap joints such as the scarf, stepped-lap, or strap have the ability to lessen the stress concentrations in the joint. Typically, this occurs because of reductions in differential strains and in eccentricity of loading. 

Hill and (letin (2000) modified the surfaces of wood veneers with methacrylic anhydride (Figure 6.6a) and formed lap joints using either styrene or methyl methacrylate in the presence of the free radical initiator azoisobutyronitrile. Significant improvements in bond strength were found when compared to unmodified wood. 

Under the best of conditions, single lap joint samples do not fail in pure shear due to the tensile and peel forces present at the ends of the overlap. These non-shear forces are exacerbated when using thin gauge adherends. Because of this, the lap joint dimensions as well as the testing rate were modified from the ASTM D-1002 standard as a result of earlier work on thin gauge steel adherends. 

Water depresses the Tg of adhesives. This is worrying particularly for cold-curing epoxides with typical transitions when dry in the range 40°C-50°C, and underlines the need to select adhesives whose TgS do not drop substantially with water sorption. Several attempts have been made to relate the depression of Tg to current concepts of the glass transition(90, 91). The modulus and strength of the cured polymer matrix are also lowered by water-induced plasticisation(34, 41, 95, 101, 102), in a manner akin to the organic plasticisers often used to modify the mechanical properties of adhesives. Brewis et al.(94) showed that for one particular hydrophilic adhesive/aluminium shear lap joint system, the depression of Tg eould be used as a shift faetor to relate the strength/temperature eurve of dry joints to ones with saturated bondlines. 

Hart-Smith (references 5.25, 5.26, 5.30 and 5.31) has conducted extensive studies of bonded joints using the elastic—plastic model for the adhesive. He has covered the analysis of lap, strap, scarf and step-lap joints. He has modified the load eccentricity induced peel stress approach by using a modified bending stiffness. He has studied the effects of non-uniform adhesive thickness, adhesive non-uniform moisture absorbtion and defects in the bondline. He has also included thermal stresses in his models. 

Finite element analysis is useful for elastic or elastic-plastic analysis of single-lap joints. It can also be applied to other joint geometries such as double-lap and modified-lap types. 

Figure 7 Strengths of lap joints in aluminium alloy bonded with a modified epoxide adhesive on exposure to wet air at 50 C. O Aged at 50% r.h. and 100% rh. A aged at 100% r.h. for 5000 h, then at 50% r.h. for a further 5000 h. [30]. Crown Copyright.

Figure 6.18 Experimental fracture load, Ff, and stress, Tf, of single lap joints of modified phenolic/aluminium alloy as a function of overlap length, /a[51,69].

So far, we have only considered parallel-sided adherends in single- and double-lap joints. It has been shown that the mathematical treatment, whether it is by closed-form analytical methods or by finite-element techniques, is difficult if realistic results are to be obtained. For instance, it is essential to allow for adhesive and adherend plasticity if joint strength predictions are to be made. But, as shown in Fig. 5, there are several forms of the lap joint in which the adherends are not parallel-sided, constant-thickness sheets but can have a variety of forms. These deviants are an attempt to reduce the high stress and strain concentrations, which occur at the ends of the simple lap joint, by modifying the stiffness of the adherends. In these profiled joints, the load line direction must change and, in addition to the tensile stiffness, so the shear and bending stiffness of the adherends change. 

Although the adhesive fails in tension at the end of the joint, most of the load is transferred in the overlap region. It is, therefore, instructive to examine the effect of non-linear behaviour on the adhesive shear stress distribution. The shear stress distributions corresponding to maximum adhesive strains of 10% and 20% in a double-lap joint are shown in Fig. 56(a). The shear stress distributions are only modified significantly at high values of maximum adhesive strain, by which time the joint is likely to have failed. The adherend cTx stress concentrations are reduced, but not significantly, by plastic flow in the adhesive. For a double-scarf joint, the elastic shear stress distributions are similarly modified by yielding of the adhesive (see Fig. 56(b)). 

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