Joint width adhesive shear strengths

Tensile shear strength (adhesive strength), in the sense of this standard, is defined as the maximum force Fmax at the break of the bonded joint in relation to adherend surface A. The adherend surface A results from the test piece width h (25 mm) and the overlap length lu (12.5 mm) ... 

For a given adhesive and adherend, the strength of a joint stressed in shear depends primarily on the width and depth of the overlap and... 

This is because the joint starts to break at the stress peak at the end of the overlap where the adhesion or cohesive strength of the adhesive is exceeded. By increasing the width of the joint, the shear stress distribution is not changed and so the failure load of lap joints increases in the same proportion as the joint width increases (Figure 5.6). 

As with other adhesives peel strengths are considerably lower than direct tensile or tensile shear values so joint design is important. A peel strength of about 4 Newtons per millimetre joint width is the best that can be achieved to date with a toughened... 

Modem adhesives usually exhibit a degree of plasticity before failure. Typically, we might find 20% shear strain at a shear stress, t, of 40 MPa. One simple predictive tool is to say that the absolute maximum strength for a lap joint is when the whole of the adhesive layer is at the shear yield strength. Thus, the maximum tensile load P which could be carried by a lap Joint of width b and length / is given by... 

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