Stress Distribution in Lap Joints

Consider a lap joint with an overlap of 25 mm and a length for the purposes of stress calculation of only 25 mm. For the initial assessment two pieces of mild steel of thickness 1.5 mm are bonded together using an adhesive of modulus 0.4 GPa, an elastic limit of 19 MPa and a glue line thickness of 0.05 mm. When a high load of 3000 N is applied, the distribution of stress is calculated to be as follows (Fig. 11). 

A maximum stress of 13.6 MPa at the two ends of the overlap joint is well within the elastic limit of the selected adhesive. If we increase the load to 4000 N, the stress is accommodated as shown in Fig. 12. 

The increase in the maximum stress in the joint to 18 MPa is still acceptable and the stress carried by the central zone remains the same. A further incremental change pushes the stress level above the elastic limit (Fig. 13). 

If we assume that the load of 5000 N has to be accommodated, we can consider how to manipulate the assembly. The first consideration is to increase the overlap length from 25 mm to 75 mm without changing any other parameters (Fig. 14). 

Increasing the overlap length has virtually no benefit in reducing the maximum stress. The analysis shows that the stress in the central zone is reduced to zero. From an adhesive performance point of view, an area of low stress means that the joint will not suffer from creep under load. Reduction in joint overlap by 35 mm 

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As was the case for tensile specimens, the stress distribution in lap joints is intricately related to the details of the specimen geometry [2,6-16]. Such factors as amount of overlap, adhesive thickness, adherend thickness, relative stiffness (moduli) of the adhesive and adherend, and other factors critically influence the stress distribution. The maximum... 

Thamm, E, Stress distribution in lap joints with partially thinned adherends. J. Adhes., 1, 301-309(1976). 

Greenwood, L., Boag, T.G. and McLaren, A.S., Stress distributions in lap joints. In Adhesion Fundamentals and Practice. McLaren, London, 1969. 

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