Joint width bond area

As bond area is the product of width (diameter) and length, the strength of the joint will only increase significantly with width once the joint length exceeds some 10 mm. Once again, estimates of joint strength can only be very approximate until appropriate mathematical models and computer software become freely available. 

If we divide the stress at failure by the area of bond, we have a joint strength of 25 MPa. By doubling the bond area we might expect a failure stress of around 16 kN. This turns out to be true if we double the bond width, but if we double the overlap, a different picture is seen (Fig. 6). 

The bonding area for the windshield is calculated by multiplying the overall length of the perimeter joint (7 m) by its width (15 mm) ... 

Two other design considerations are pertinent to generalized width versus depth guidelines for reliable simple sealant butt joints. At very low depths, there is usually insufficient bond area of the sealant to the substrates to have reliable adhesion. On the other hand, at very large width-to-depth ratios, buckling of the sealant can occur when the joint contracts. This can cause the sealant to be extruded from the joint where abrasion of the sealant can occur, drastically reducing the sealant life. Extruded sealant can also be a safety hazard in some instances. 

Alternatively, bonded steel or fibre reinforced rods can be inserted from the sides of members in the joint area, in the direction perpendicular to the grain, thus stitching the timber fibres together and avoiding splitting (Fig. 9). Whenever possible, long rods should be driven all the way across the timber member width, or shorter ones should be driven from both sides. 

In adhesively bonded joints copper wires with a diameter of 0.34 mm were used in between the laminates to control the adhesive layer thickness. The copper wires were placed in areas to be discarded. Adhesive fillets with a 45° angle were formed at the joint ends. The adherend ends were not shaped. Test specimens with 25 mm width were cut from the bonded laminates. Typically five specimens from each batch were tested. 

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