Cleavage loading

A common example of a proof test is to apply a cleavage load to a bonded honeycomb sandwich by placing an instrument between the face and core and applying a predetermined force perpendicular to the core. If there is no bond disruption due to this test, it is supposed that the product will meet all its service requirements. Other common proof tests used with sealants are leak-testing with a mobile and easily detected gas such as helium or by the application of hydrostatic pressures. 

Peel and cleavage loadings should always be avoided. When their presence can not be avoided, their effects should be minimised. Peel loads are mainly produced by out-of-plane loads acting on a thin adherend or by the eccentricity of the in-plane loads. When the adherend is thick and stiff, the out-of-plane loads typically produce cleavage loads. The critical areas in the joint with respect to peel and cleavage loadings are the ends of the overlap. 

Cleavage loading (Fig. Ic) will concentrate stress at one side of the joint. The bond area will have to be increased to withstand this uneven loading. 

Designs deliberately incorporating flexible rubbery inserts between two stiff adherends are well known. For example, cyanoacrylate adhesives are successfully used to bond spectacle lenses to frames through an intermediate rubber layer. Without the rubber to dissipate peel and cleavage loads, the joint between the lens and the metal frame would be readily over-stressed, resulting in premature failure. 

Figure 2.39 highlights correct and incorrect methods of designing or assembling a variety of bonded structures. The key lesson in every case is to load the joint with a combination of compression and shear forces and to avoid destructive peel and cleavage loads. 

Figure 10.6 The crack only gets as far as the rubber particle and the stress is dissipated. The crack then has to reform, only to meet another crack arrester . The resulting adhesive bond line is therefore considerably more resistant to peel and cleavage loads than a conventional cyanoacrylate...

Japanese researchers [18] described a stress analysis of butt joints of steel to aluminium in which joints were assembled with epoxy adhesives and subjected to cleavage loads. They found that the normal and shear stresses were maximised at the edge of the interface on the load application side between the substrates and the adhesive bond. However, both stresses were greater at the edge of the interface between the higher-modulus substrate (steel) and the bond. 

Increasing the bonded area will usually decrease the overall load on the joint and therefore improve the reliability and durability as there is a lower overall stress on the joint. The adhesive will always be stronger in compressive, tensile and shear loads than in peel or cleavage loads and typically the peel load of an adhesive will be less than 5% of the nominal shear strength so ensuring that the bonded joint cannot be subjected to peel or cleavage loads will improve the overall integrity of the joint. 

Tensile loads supposedly normal to the plane of a joint can be highly destructive if they induce peel and cleavage loads. Therefore such loads must be avoided if at all possible. Indeed, every effort should be made to design a joint in such a way that it carries a compressive element within its load pattern, for in this manner the destructive peeling mechanism is suppressed. 

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