Cleavage forces

The three principal forces to which adhesive bonds are subjected are a shear force in which one adherend is forced past the other, peeling in which at least one of the adherends is flexible enough to be bent away from the adhesive bond, and cleavage force. The cleavage force is very similar to the peeling force, but the former applies when the adherends are nondeformable and the latter when the adherends are deformable. Appropriate mechanical testing of these forces are used. Fracture mechanics tests are also typically used for structural adhesives. 

Another important factor with regard to adherends is the uniformity and tolerances allowed in the joint fit-up. Improper tolerances on the geometry of the joint can lead to adhesive-starved areas or areas where the bond is so thick that unwanted peel and cleavage forces can easily develop. 

It is relevant in this context that, in work on the cleavage force of crystalline Si, Matthai and March [49] have used a coordination-dependent force field due to Tersoff [50] to study related bond-breaking1 in a condensed phase. Tersoff s force field was fitted to available density-functional calculations, With such a force law, Matthai and March [49] concluded that the Si bond is comparable in... 

The design of bonded joints must take into account the nature and magnitude of the likely operating stresses and strains, and should be contrived so that unacceptable peel or cleavage forces are not... 

Figure 1.1 Peel and cleavage forces induced by shock loads, a A bonded object striking a rigid surface experiences powerful and distorting shock waves, b Depending on a variety of factors this distortion may cleave the adhesive apart, particularly if the adhesive is brittle.

Figure 2.4 Peel or cleavage forces may be suppressed by the use of local mechanical restraints, such as rivets, bolts or spot welds.

Clearly, a joint should be designed to avoid peel. However, if peel and cleavage forces must be borne, then some means of distributing the load must be found, for example by restraining movement at the end of a flat, flexible joint with a rivet or spot-weld (Figure 2.4). If such remedies are not practicable, then complete re-design may be required. 

Design the joint to minimise peel or cleavage forces, and consider the possibility that these may arise as a result of accidental damage. 

This is of great practical importance since it is peel and cleavage forces induced by accidental impact that usually destroy joints (see also Section 2.3.3). 

Figure 2.33 The strapped lap joint, a Simple and cheap joint but poor if there are any peel and cleavage forces generated by distortion, b Simple and cheap joint but it requires access to both sides of a structure. It can give a very robust performance, c An even more effective form but one which can involve considerable fabrication costs if metal machining is required and extrusions cannot be employed (see Figure 2.35).

Variants of the strapped lap joint and the related double strapped lap joint are illustrated in Figure 2.33. The single strap joint is suitable only for light loads, since it is clearly prone to premature failure if subject to peel and cleavage forces. 

Figure 2.37 Various forms of the butt joint. Unless at least one of the adherends is highly compliant, assemblies a and b will fail readily as even minor misalignment will induce very high peel and cleavage forces which will rupture the joint (see Figure 2.29c). However, a reduction in height and an increase in diameter (c) will stabilise the joint.

Screws, rivets or welding are useful in holding components together while the adhesive cures and, where peel or cleavage forces cannot be avoided, they can be used to counteract such loads. Although the adhesive effect may well be secondary, it will still make a major contribution to the overall stiffness of the structure. 

Note that the cleavage force P required to propagate the debond decreases continuously as the debond propagates. If, on the other hand, a constant deflection 8 is imposed, then the energy available for debonding is given by... 

For closely fitted components, significant strength is achieved even with nonhardening liquids. If an adhesive is used, the hardening process strengthens the joint by adding resistance to shear and cleavage forces. 

According to the ISO Standard, the average cleavage force can be calculated from the time history by considering the data between 25% and 90% of the time interval from the beginning to the end of the curve. The energy is given by the area under the curve between the same end points multiplied by the test speed. The reason for this assumption is to discard the data... 

Maximise shear forces and minimise peel and cleavage forces (Figure 7.1). 

In the main, these separate applications require very different characteristics from their respective adhesives. Structural adhesives literally stick extremely well and must cope with occasional, generally accidental, peel and cleavage forces. As will be seen, there are several types of adhesive suitable for such applications. 

A ile it is important to understand how adhesives function it is equally important to understand why and how they fail. One of the primary reasons has been the brittleness of the traditional, high performance adhesive. In order to achieve very high shear strengths — and therefore the capacity to sustain high loads — it has been necessary to produce hard and brittle adhesives. Regrettably, such materials are particularly prone to damage when a bonded structure is shock loaded. The momentary distortion of the substrate induces powerful peel and cleavage forces which a brittle adhesive may not be able to resist... 

Fig. 7.12. The lap joint and its variants (a) simple lap (b) tapered lap, (c) rebated lap, (d) strapped lap (e/f) double strapped and tapered lap (g) stepped, strapped lap. These various forms of the simple lap joint have been developed in order to reduce peel and cleavage forces and to maximise performance some lend themselves particularly well to fabrication in composite but some, as is obvious, could be very expensive to fabricate in metal.

This latter, very practical, application depends upon the combination of a relatively narrow shank with an extended base. This arrangement ensures that the loaded shank will break before the cleavage forces generated in its bonded base can rise to a level where rupture of either the laminating resin or the adhesive will occur. 

Combination joints are also used from time to time when composites, or other plastics, are involved. These employ either screws or rivets to reduce, or eliminate, peel and cleavage forces (Fig. 7.19). 

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