Joint design peel load

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. 

Although adhesives offer a number of advantages, they suffer from a number of limitations, listed in Table V, which must also be considered when assessing the suitability of an adhesive for a particular application. Most structural adhesives are strong in shear and tensile loading, but weak when peel or cleavage stresses are present. Joint design is therefore necessary to eliminate, as far as possible, these undesirable stresses. 

The successful and cost-effective application of adhesive bonding technology is crucially dependent on correct joint design. The adhesive joint must be adequately dimensioned for the forces to transmit. Large static loads should be avoided wherever possible - especially where the joint is exposed to higher temperatures. In such cases, joints should be optimised to provide additional support. Similar measures should be taken to counteract peeling stresses (Fig. 24). Some examples are illustrated in the chapter Technical Characteristics in Volume 1. 

Fig. 36. Design considerations for bonded joints subjected to primary peel loads.

Whole of bond area is used to resist the applied load. This is the most common general type of adhesive joint, and the most efficient Practical joint designs must avoid the tendency of adherends to distort and peel. 

Figure 6.2 Schematic diagram of some good and poor joint designs. The good designs attempt to distribute the imposed loads within the adhesive layer as a combination of shear and compressive stresses avoiding tensile, cleavage and peel stresses as far as possible.

The essence of the effective use of the joints that have been described is to design for bonding and not simply to substitute it for other means of joining. Care should therefore be taken that stress concentrations be avoided and the loads be carried over as large an area as possible. Peel loads are the greatest enemy of the designer of bonded joints. Wherever possible, the adhesive should be loaded in shear so that peel and cleavage stresses are avoided. 

If the adherends are rigid and a moment or offset load is applied to this joint then the adhesive will be subjected to quite a severe cleavage or peel load (Figure 5.20) and as such the butt joint is generally regarded as a poor joint design. 

Clearly, a structural joint should be designed to avoid peel loads. If peel and cleavage forces must be borne, then some other means of supporting the joint must be found. If this is not practical, then a complete redesign may well be required. 

In contrast with the co-cured single lap joint, tensile load bearing capacity of the co-cured double lap joint decreases as the stacking angle of the composite adherend increases. Sint e the co-cured double lap joint has symmetrical configuration, it is important to consider the out-ofplane shear stress rather than the out-of-plane peel stress in designing the joint. 

In many RPs, the most convenient joint will be a bonded type (possibly produced during the lay-up process), and the geometry is therefore very influential. The weakest joints are those where failure is limited by inter-laminar failure of the adherend, or peel of the adhesive. Next strongest are those where the load is limited by the shear strength of the adhesive. The strongest designs will fail outside the joint area, at a load equivalent to the strength of the adherend. 

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