Characterization of thin structures under in-plane biaxial loading regimes using non-destructive testing methods

The aim of this dissertation is to develop a robust experimental protocol for characterising thin metallic and composite structures under multiaxial loadings. Mechanical testing is crucial for predicting the material behaviour under complex loading conditions. Thin structures often experience buckling in compressed zones during shear, compromising their integrity and reducing service life. To address this issue, stabilisation adapters were designed and validated for the Modified Arcan Fixture (MAF) to suppress shear buckling and enable for accurate material characterisation. Their effectiveness was further validated using Finite Element (FE)-based stereocorrelation (SC). Finite Element Model Updating (FEMU) was employed to calibrate the friction coefficient between the adapters and the sample surface. The measured displacements were applied as Dirichlet boundary conditions in the numerical model, and their influence on the stress triaxiality fields was evaluated, demonstrating their critical role in the numerical model accuracy. Limited sample buckling could be simulated accurately. Furthermore, trustworthy material parameters were calibrated by including the stabilisation adapters in the numerical model. Moreover, FEMU was employed to evaluate two sample geometries and revealed that the sample geometry played a key role in activating specific material parameters. The upgraded MAF setup was then applied to Fibre Reinforced Polymers (FRPs), which are widely used in demanding applications due to their high strength-to-weight ratio and customizable mechanical properties. To characterise FRPs, an FE- based hybrid multiview correlation method was used, integrating infrared and visible light imaging to quantify strain localisation and temperature variations in samples subjected to three different loading regimes, namely, tension, simple shear, and their combination at a 45° angle. Multiaxial mechanical testing revealed variations in strain and temperature distributions for different loading regimes. Additionally, a multi-modal correlation method was developed, incorporating visible light, infrared, and X-ray imaging within a unified FE framework. This approach enabled for detailed surface strain and temperature field measurements correlated with 3D bulk strain fields measured via Digital Volume Correlation (DVC), allowing for the identification of multiple damage mechanisms based on surface and bulk kinematics as well as correlation residuals.