The dangers of single-lap shear testing in understanding polymer composite welded joints

Single-lap shear (SLS) joints are straightforward to manufacture. This makes them especially attractive for testing polymer composite welded joints. Owing to local heating, which is characteristic of composite welding processes, the production of more geometrically intricate joints (such as double-lap or scarfed joints) or bigger joints (such as end-notched flexure or double cantilever beam) typically entails significant complexity in the design of the welding process. Testing of SLS joints is also uncomplicated and, even though, owing to mixed-mode loading and uneven stress distribution, it does not provide design values, it is widely acknowledged as a valuable tool for comparison. Even so, comparing different aspects of composite welded joints through their corresponding SLS strength values alone can be deceptive. This paper shows that comparison of different welding processes, adherend materials, process parameters or different types of joining techniques through SLS testing is only meaningful when strength values are combined with knowledge on other aspects of the joints such as joint mesostructure, failure modes and joint mechanics. This article is part of a discussion meeting issue 'A cracking approach to inventing new tough materials: fracture stranger than friction'.