Double butt-strap joint

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. 

Fig. 80. Double-lap joints (a) to ASTMD 3165-73 (b) stresses in a doublelap joint (c) double butt-strap joint (dimensions in mm).

Another variation of the lap test is the true double lap illustrated in Fig. 80(b). Even in this case, as was shown in Chapter 2, we still have differential straining of the adherends and the loads are still non-colinear internally, even though the external forces balance. Such joints tend to fail at T where the adhesive is in transverse tension rather than at C where it is in compression. The double butt-strap joint shown in Fig. 80(c) is a form of double-lap joint and again it fails at T. Sometimes, if the gap between the two tension members is small, most of the load is carried directly across the stiff, butt faces. Premature failure ensues here but the joint is still able to carry a substantial load in excess of this. 

A typical laminated joint is a butt joint on to which doublers are laminated, the result being a double-strap joint. The following features distinguish this laminated joint from the corresponding adhesively bonded double-strap joint ... 

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