The weldability of thermoplastic composites enables their increased use in the aerospace industry due to cost-efficiency and high-volume manufacturing. This research investigates how damage in induction and conduction welded single-lap shear joints can be quantified experimentally. 2D and 3D Digital Image Correlation (DIC) cameras are used to measure the displacement fields of single-lap shear joints in quasi-static tests. Additional cameras take images at regular and high-speed frame rates. Calibrating the DIC strain fields with crack growth images allows introducing a simple failure criterion. If a threshold strain is exceeded, it is assumed that a crack has formed. Hence, both crack tip location and crack length are known. 3D DIC images are analyzed to track the crack front. The 2D DIC images grant a detailed view at the longitudinal and shear strain distributions in the weld line. High-speed images provide further insight into the fracture process.