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Testing for adhesion

Looking at the best methods for modelling lap shear forces

In high performance applications, the adhesive bonding of components is crucial to product performance. Engineers will carefully design the joint configuration and overlap of a part to ensure maximum spread of load thus preventing stress concentration at one point. Testing the adhesion of plastic and composite joins is always tricky, especially when trying to compare laboratory based data with real life scenarios. A good model relies on the understanding of the forces that the joint will experience in use – whether it will be subjected to peel forces, shear forces or a mixture of both.

However, in the laboratory environment the testing regimes that can be used are limited. Lap shear testing is the most common test method in order to test for shear forces and although relatively easy to perform care must be taken in order to generate meaningful results. If the thickness of the plastic or composite substrate is not the same as in the product then the stress experienced at the bonded interface will not be the same. The thickness of the samples is generally governed by the maximum open width of the testing jaws and this may be difficult to overcome.

The choice of bonding the test samples as either a single lap shear or as a double lap shear assembly is however under operator control. While single lap shear samples are easier to prepare due to only needing to control one bondline they are unsuitable for the testing of high modulus materials like composites. This is because when under tension a single lap shear specimen will start to bend.

In a plastic sample the elastic nature of the material means this effect is not as severe but in high modulus materials this bending distorts the sample and introduces high areas of stress at either end of the bonded joint. These stress areas induce a peeling regime which will lower the “lap shear” force obtained. By moving to a double lap shear specimen the distortion is minimised and consequently a value closer to the true shear strength will be found.