Improvement of adhesive strength at a bi-material interface by adjusting the interface angles at the free edge

• Solved eigenproblem for singularity exponent to predict interface failure type.
• Performed fracture tests of joint with various interface angles at joint edge.
• Predictions of numerical solution match the observed failure type.
• Results indicate method for improving joint strength.
• Measured strength is size-independent for non-singular joint design.

In the bonding of dissimilar materials, there is usually a stress singularity at the edge surface whose effects cause failure at the surface, resulting in a premature failure of the joint. In this work, the corresponding generalized stress intensity factor (GSIF) for an aluminum–epoxy joint is studied to determine the angles for which the stress singularity at the edge disappears. Experimental results are gathered for three different edge angles: a “concave” angle of 37° and a butt joint with 90°, both associated with interface stress singularities, and a “convex” angle of 143° that from theory is singularity free. It is demonstrated that the joint strength can be greatly improved by avoiding stress singularities through the selection of the proper edge angle, still preserving the external surfaces of the cylindrical samples. Testing with a larger diameter rod joint for the convex angle geometry shows the same nominal fracture strength, indicating that the measured strength is a material property independent of size when the stress singularity is absent.