Designing for adhesives

For the best possible performance, joints should be specifically designed for adhesive bonding. Follow this principle and much better joints will be achieved than if bonding is adopted as a substitute for welding in a joint designed for that purpose. Bond stresses, materials, type of adhesive, surface preparations, method 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. Bonded joints perform best under conditions of tension (pure), compression or shear loading; less well under cleavage; and relatively poorly under peel loading. The loading conditions are shown in Fig. 29.1.

Designing a joint to take pure tensile or compressive stresses is normally impracticable with sheet materials, so all joints in sheet materials should be designed so that the main loading is in shear. Joints between massive parts perform well in tension or compression loading, provided this is uniform - a side load may set up excessive cleavage stresses in a tension-loaded bond. (Fig. 29.1(d)). Cleavage loading will concentrate stress at one side of the joint. Bond area may have to be increased to withstand this load so the joint will not prove so economical in terms of material and/or adhesives as joints designed for shear and tension stresses. Peel strength is usually the weakest property of a joint. A wide joint will be necessary to withstand peel stresses, plus the use of an adhesive with high peel strength.

For an adhesive to be used, a joint must allow the easy application of the adhesive, must allow for the t

Fig. 29.1 Loading conditions. (a) Tension (b) Compression (c) Shear (d) Cleavage (e) Peel

Fig. 29.1 Loading conditions. (a) Tension (b) Compression (c) Shear (d) Cleavage (e) Peel

adhesive to cure fully, and must be designed to give uniform stress. Even in a simple face-to-face joint it must be possible to apply adhesive to one surface and for it to remain there until the two parts are brought together and after that until curing takes place.

These requirements highlight the need for a choice of thin, thick or thixotropic adhesives. Design details which may also be significant include removal of sharp edges and substitution of a bevel or radius.

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