Double-scarf joints

Figure 7.15 shows several types of joints for rubber under tension. The horizontal white lines are equidistant when the joints are unstressed. It is obvious that the scarf joint is least subject to stress concentration with materials of equal modulus, and the double scarf joint is the best for materials of unequal modulus. These designs offer the best resistance to peel and, all other factors being equal, represent the best choices. ... 

Figure 50 shows the adhesive shear stress distributions for a doublescarf joint and a double butt-strap joint and compares them with ordinary single- and double-lap joints. The double butt-strap joint gives the lowest stress concentration at its middle, but is then identical to the ordinary, parallel double-lap joint at the other end. For this reason, it is suggested that the straps should be bevelled or scarfed so that the stress concentration might approach that of the double-scarf joint. 

If we now allow for non-linear adhesive behaviour, the high adhesive stress concentrations predicted by the linear elastic analysis will be relieved to some extent. Figure 54 shows the predicted spread of the yield zone of adhesive at the tension end of a double-lap joint as the load is increased. As would be expected, plastic flow begins near the adherend corner and the load corresponds to a joint efficiency of 21%. Each subsequent load increment represents an increase in joint efficiency of 4 4%. When elastic perfectly-plastic behaviour is assumed for the adhesive, a maximum strain criterion for failure seems appropriate. In Fig. 55 the joint efficiency is plotted against the maximum principal strain in the adhesive at each end of a double-lap joint. Assuming a failure strain for the adhesive of 5%, the analysis predicts a joint efficiency of 31% for a double-lap joint compared with 16% predicted by the linear elastic analysis. Similarly, the non-linear analysis predicts an efficiency of 39% for the double-scarf joint compared with 20% predicted by the linear elastic analysis. Although the predicted efficiencies are almost doubled by allowing for non-linear behaviour in the adhesive, failure in the adhesive is still predicted to be more probable than failure in the adherends (Table 5). 

Although the adhesive fails in tension at the end of the joint, most of the load is transferred in the overlap region. It is, therefore, instructive to examine the effect of non-linear behaviour on the adhesive shear stress distribution. The shear stress distributions corresponding to maximum adhesive strains of 10% and 20% in a double-lap joint are shown in Fig. 56(a). The shear stress distributions are only modified significantly at high values of maximum adhesive strain, by which time the joint is likely to have failed. The adherend cTx stress concentrations are reduced, but not significantly, by plastic flow in the adhesive. For a double-scarf joint, the elastic shear stress distributions are similarly modified by yielding of the adhesive (see Fig. 56(b)). 

Fig. 4.14. Some adhesive joint fracture mechanics specimens, (a) Tapered double cantilever beam (TDCB). (b) Thick double cantilever beam (DCB). (c) Thin double cantilever beam or wedge cleavage specimen, (d) Independently loaded mixed-mode specimen (ILMMS). (e) Scarf joint.

The procedure for the symmetrical double taper scarf joint is identical to that for the single taper scarf joint case except that the required bond length is half the bond length of the single-taper joint. 

Step-lap joints share features in common with both double lap joints and scarf joints. The scarf joint represents the mathematical limiting case of a step-lap joint with an infinite number of steps, (see reference 5.30). 

A wide variety of adhesively bonded joint configurations are available to the designer based on the application and scope of use [7]. Commonly, joint configurations that have been analyzed in the literature are single-lap joints, double-lap joints, scarf joints, and step joints (Figure 1). 

Figure 31. The single-strap joint (a), the double-strap joint (b), the tapered-strap joint (c), and the tapered-strap scarf joint (d).

Although the joggle lap joint allows shear stresses to be in the same plane as the bonded parts, it does not combine tensile and shear stresses in line with adherends. The double-butt lap joint, the recessed and landed-scarf joint, and the tongue-and-groove butt joint combine both types of stresses in line and, therefore, are recom-mended ... 

Tapered butt joints and double scarf-lap joints also permit bonding and provide maximum tensile and shear strength. Overlap joints are recommended for bonding cylindrical parts ... 

Flexible plastics and elastomers. Thin or flexible polymeric substrates may be joined using a simple or modified lap joint. The double strap joint is best, but it is also the most time consuming to fabricate. The strap material should be made out of the same material as the parts to be joined or at least have approximately equivalent strength, flexibility, and thickness. The adhesive should have the same degree of flexibility as the adherends. If the sections to be bonded are relatively thick, a scarf joint is acceptable. The length of the scarf should be at least four times the thickness sometimes, larger scarfs may be needed. 

Figure 26.19 The joggle lap, inverted scarf, and double strap joints.

Fig. 56. Adhesive shear stress distributions in CFRP-CFRP joints (a) Double-lap joint (b) scarf joint (from Adams and Peppiatt, 1977a).

The effect of scarfing the adherends (Fig. 60) is to unbalance the stress concentrations to such an extent that they are much higher at the compression end than at the tension end of the joint. Although the maximum shear stress is about 7% less than in a double-lap joint, the adhesive fillets of the scarf joint are much smaller, giving a 9% higher stress concentration (Table 6). 

Composite repair configurations (a) singie strap joint (b) double strap Joint (c) scarf joint (d) step joint... 

Composites may be joined by bonding, bolting or both. Scarf, stepped lap, supported single-lap joints are the most common, but single-lap or double-lap types are used (Shear tests). 

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