In thermoplastic composite joints with a circular ultrasonic spot weld, the damage growth is located at the interface between the joined components. This means that any damage in these joints is invisible from the outside. This experimental study compares three different in-situ methods to measure the damage growth indirectly. The motivation lies in the potential benefits of using multi-spot welded joints for increased damage tolerance and the need to prove the damage arresting and damage progression behaviour for certification. The current study focused on measuring the damage in single-spot welded Single Lap Shear (SLS) joints during fatigue test with the global specimen compliance, the local out-of-plane displacement and the local strain on the surface. Digital Image Correlation (DIC) measurements were used to obtain the local quantities. The results showed that the local quantities are better suited to obtain detailed information on the damage state. The local surface strain showed more distinct features that facilitate the recognition of damage locations. The benefits and challenges of all methods are discussed as well as the difference in the level of detail, generalization and the prior knowledge necessary to obtain the damage state.

Ultrasonic spot welding is a joining technique for thermoplastic composites with great potential regarding processing speed and cost. To investigate the damage tolerance and possible inherent damage arresting behavior of multi-spot welded joints, a technique is necessary to measure damage growth in the joints under cyclic loading. Visual inspection is not possible because the damage is not located on the outside surface and conventional techniques such as C-scan are not practical during a fatigue test because the specimen would have to be removed from the setup. This paper details a methodology for quantifying damage growth rates in singlespot welded joints using surface strain measurements made by Digital Image Correlation. This represents the first step towards developing a methodology for quantifying damage progression behavior in complex multi-spot welded joints.