Annular butt joints

As stated in Section 6.4.2, an annular butt joint, or napkin-ring, specimen tested in shear minimizes the variation of shear stress in the adhesive and has been used by many workers to assess the shear strength and shear stress versus strain behaviour of adhesives [14,17,111-115] and is also listed for this purpose as ASTM E 229 (see Table 6.1). The independence of the measured strength upon specimen geometry has been substantiated by Bryant and Dukes [112] and Foulkes et al. [110] and the shear stress distribution calculated using linear elasticity theory is ... 

Adams et al (1978b) studied solid and annular butt joints loaded in torsion and tension, and examined the effects of adherend flexibility in the spew fillet. They used an eight-node parabolic isoparametric element which gives a good estimation of the stresses in regions of high stress gradient. Their results were confined to the linearly elastic behaviour of aluminium adherends and an epoxy based adhesive which... 

The shear stress distributions for an annular butt joint with and without a spew fillet are shown in Fig. 65. As with the solid butt joint,... 

Fig. 65. Shear stress distributions for annular butt joints in torsion (from...

In practice, it is probable that some stress relaxation will occur to reduce the high tensile stresses near the adherend corner, although high-strength structural adhesives often exhibit only limited non-linear behaviour in tension. Figure 68 shows the failure surface of an annular butt joint which failed in tension. The adhesive was the epoxy-based BSL-308A (manufactured by Ciba-Geigy (UK) Ltd) and the... 

In the classic D-M-E version, the manifold has large-diameter (24-50 mm) drilled flow channels in which cartridge heaters in a tubular body are located. The melt thus flows through an annular channel between 4 and 9 mm wide (see Chapter 1). Because of the heating pipe in the centre of the channel, the channels cannot cross over as in externally-heated manifolds, but must be connected adjacently. Torpedoes must also be connected with a butt joint to the flow channels. This creates certain restrictions as regards the mould cavity arrangement, and it is not easy to achieve a natural flow balance (see Figure 4.91). 

Butt joints for testing adhesives in tension are also usually designed with a circular cross-section to facilitate manufacture and to maintain symmetry. In this case, it would appear to make little difference whether they are annular (Fig. 63(a)) or solid (Fig. 63(b)). In a butt joint subjected to a tensile load, the adhesive is restrained in the radial and circumferential directions by the adherends. In the absence of this restraint, the adhesive would tend to contract radially with respect to the adherends because of its much lower modulus. The presence of the adherends has the effect of inducing radial and circumferential stresses in the adhesive, so increasing the stiffness of the joint. The simplest analysis makes the assumption that the radial and circumferential strains in the adherend and the adhesive are zero, in which case the radial and circumferential stresses are given by ... 

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