One of the emerging welding techniques for thermoplastic composites is ultrasonic welding. Ultrasonic spot welding has the potential to be composite counterpart of the widely used, highly automatized spot welding technique used to join metals. State of the art research on ultrasonic welding is focusing on the welding process itself in search of the process parameters that allow consistent manufacturing of high quality welds. To complement this effort, the research of this thesis looks ahead at assessing the durability of welded products, which is the next step after assuring a good weld was created. This research aims to support the study of damage progression in ultrasonically welded components subjected to repeated loading, by providing a method of comparison between the observed surface deformation of joined components under lab experiment conditions and expected deformations predicted for the experiment with numerical modelling techniques.

This thesis proposes the interpretation of surface strain fields, measured through digital image correlation, and the interpretation of the increase in compliance of a welded joint as fatigue damage progresses, recorded by the test machine as force-displacement relationships, as indirect methods of measuring the state of damage accumulated within the ultrasonic weld spots. The interpretation of measured surface strain fields and of recorded load-displacement data is done by comparison to a collection of finite element simulations corresponding to ultrasonic weld spots containing various degrees of damage. This research aims to present a method of strain field interpretation that is generalizable to the analysis of lap shear specimens containing multiple ultrasonic weld spots.