Axially loaded butt or poker-chip joints

3 Axially loaded butt or poker-chip joints 

This type of joint geometry is shown schematically in Fig. 6.6 and it might be supposed that the tensile stress, o-z, like the applied normal strain, z, is the same everywhere in the adhesive layer. However, this situation would only arise if, under the action of the normal applied stress, o-o, neither the substrates nor the adhesive tended to deform laterally or did so by the same amount, i.e. if the ratio of Poisson s ratio, i/, to the Young s or tensile modulus, , were the same for both materials. This condition is seldom, if ever, fulfllled so that the tendency for lateral (Poisson s) deformation will be unequal in the two materials. Usually, the ratio i/ E is higher for the adhesive. The tendency for lateral strain in the adhesive will therefore be far greater than in the substrates but, assuming the interface remains intact, the regions of adhesive near the 

Alwar and Nagaraja [22] and Adams et al. [10] have used an elastic finite-element method to analyse the stress distribution in butt joints loaded in tension and a typical stress distribution is shown in Fig. 6.7 for an epoxy adhesive bonding aluminium alloy substrate. The bonded area comprises two different regions. 

Firstly, in the central region of the adhesive layer, the tensile stresses are uniform and the shear stress, Xrz, is zero. The radial, o-r, and the circumferential, (Te, stresses in the adhesive layer were found to be essentially the same as those predicted by Kuenzi and Stevens [23]. These workers used an analytical approach and assumed that the radial and circumferential strains in the adhesive layer were equal to the Poisson s ratio strain in the substrates. Thus  

in the central region of the joint, the rubbery adhesive is subjected to a hydrostatic tensile stress, a feature which has been recognized for many years [24-28] and which has important consequences, as discussed below. 

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Despite the fact that adhesive joints are rarely designed to be loaded directly in tensile mode, tensile tests are eommon for evaluating adhesives. The axially-loaded butt (or poker chip ) joint geometry, as recommended by ASTM D897(52), is depicted in Fig. 4.11. 

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