Assessment of the Picture-Frame Test for Element-Level Validation of Conduction-Welded Thermoplastic Composite Joints

Abstract

This thesis investigates whether the picture-frame test can be used as an element-level validation method for conduction-welded thermoplastic composite joints. Pristine and defect-containing panels were tested under free-buckling and anti-buckling conditions, with their response assessed using force–displacement data, Digital Image Correlation, strain gauges, visual inspection, and ultrasonic C-scans. The experiments were compared with finite element models using Cohesive Zone Modelling and the Virtual Crack Closure Technique, based on independently obtained interface fracture properties. The results show that the picture-frame test can generate interface failure at panel scale, while also revealing the influence of buckling, weld geometry, manufacturing variability, and competing laminate damage on the structural response. The numerical analyses reproduce selected experimentally observed deformation and damage mechanisms and provide useful insight into the transfer of coupon-derived fracture properties to a larger structural scale. The picture-frame test can therefore be used as an element-level validation method, but its reliability is conditional rather than universal.