Joint design shear

Load bearing capabiUties are dependent upon the adherend, joint design, rate of loading, and temperature. Values given represent the type of adherends normally used at room temperature. Lap shear values approximate those obtainable from an overlap of 3.2 cm. ... 

Adherend stresses in weldbonded joints are lower and more uniform than those for comparable spot welded joints. This provides increased in-plane tensile shear and/or compressive buckling load-carrying ability for a given joint design. The presence of the spot weld provides enhanced out-of-plane load-carrying capability compared to adhesive bonding only. 

ASTM D 1144 provides a recommended practice for determining the rate of bond strength development for either tensile or lap shear specimens. However, peel and can-teliever tests can also be used effectively. Measured bond strength values of partially cured test specimens are compared with those of a reference (i.e., fully cured adhesive joint) to assess the extent of cure. This method may suit some applications, but it is limited in accuracy because it does not directly measure the degree of cure in the adhesive, and the effect on the joint design and substrates may override the effect of cure development. 

Polysulphide sealants fulfil the requirements of expansion joints in concrete structures such as bridges, subways and high-rise buildings where a high level of joint movements occur. They are suitable for compression movement, extension movement and shear movement in butt joints as well as overlap joints. The only joint in which a sealant can accommodate more than +50% movement is in lateral shear in an overlap joint. In all other joint designs the sealant cannot take more than 25% move-... 

In selecting an adhesive system, one must calculate the strength required. If, for example, you wish to design an adhesive-bonded lifting device for a 50 lb (23 kg) machine component, you can use an adhesive with a 100 psi (0.689 MPa) shear strength and make the bond area 0.5 in (3.2 cm ). You must use this type of joint-strength analysis in all adhesive-joint designs [Ref. 4, pp. 171-172]. 

Following the above quoted design rule, a lap joint s shear strength is typically defined as the force at failure divided by the area of the overlap ... 

A future trend in composite bolted joint design is the use of more advanced continuum damage mechanics approaches to model joint failure, so that non-linear shear behaviour and gradual stiffness degradation can be implemented. An important element of joint failure is delamination, and many efforts are being undertaken to implement delamination in finite element models. However, for bolted joints analysis this poses many problems due to the large number of delamination interfaces in thick laminates. 

Wherever possible, joints should be designed so that loads applied in service act on the joint as shear stresses rather than as tensile stresses. This means that lap joints are preferred to butt joints. Fig. 12.16. The recommended length of overlap on lap joints is between three and four times the thickness of the thinnest component in the assembly. 

The choice of joint design will depend on the nature of the structure being created. As already indicated, joint strength is higher under shear loading and so it is desirable to choose a joint geometry to take... 

The excellent peel strength properties of silicones are more important in joint designs than the tensile or lap-shear properties. Some examples of peel and lap-shear strengths with silicones are presented in Table 5.9. 

In the bonded joint design the most basic problems are the unavoidable shear stress concentrations and the inherent eccentricity of the forces causing peel stresses both in the adhesive and in the adherends. At the ends of the overlap both the peel and shear stresses reach their maximum values, resulting in reduced load-bearing capacity of the joint, see Figure 5.28. 

Types of Stress. To effectively design joints for adhesive bonding, it is necessary to understand the types of stress that are common to bonded structures. Four basic loading stresses are common to adhesive joints tensile, shear, cleavage, and peel. Any combination of these stresses, illustrated in Fig. 7.13, may be encountered in an adhesive application. 

Lap joints are the most commonly used adhesive joint, because they are simple to make, are applicable to thin adherends, and stress the adhesive to be stressed in shear. However, the simple lap joint causes the adhesive to be stressed in shear. In this design, the adherends are offset, and the shear forces are not in-line, as was illustrated in Figure 7.15. This factor results in cleavage stress at the ends of the joint, which seriously impairs its efficiency. Modifications of lap-joint design (Figure 7.17) include ... 

For the best possible performance, joints should be specifically designed for adhesive bonding. In a few cases only can an adhesive be used on a joint not specifically designed for adhesives - mainly cylindrical joints. Bond stresses, materials, type of adhesive, surface preparation, methods of application and production requirements can then all be considered in relation to each other at the outset. The designer should consider especially the effect of shear, tension, cleavage and peel stresses upon the joint (Fig. 1) (see Joint design strength and fracture perspectives). 

In adhesive bonding, shear is a major type of stress when one substrate is forced to move parallel and relative to the other substrate. The entire bonded area is efficiently used when joints are stressed in shear. Thus tensile-shear overlap design is a common joint design used in adhesive bonding. (See adhesives tests. Fig. A.6.)... 

For the KFUPM specimen designed with relatively high reinforcement ratio p = 0.01 (J-Bl-18), experimental results showed that the specimen collapsed due to failure of joint under shear, as the joint collapse load was lower than the flexural capacity of the beam (27.6 % lower). That was confirmed from the combined mechanistic/experimental computations and also further corroborated from DIANA results. 

When the design shear load on an adhesive joint makes overloading or fatigue failure likely (see pp. 6 22 Fatigue), alternative means of transmitting load from one component to the other must be examined. 

Wherever possible, joints should be designed so that loads applied in service act on the joint as shear stresses rather than as tensile stresses. 

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