Applications of guided wave propagation on waveguides with irregular cross-section
Creators: name: Fan, Zhang
Publisher: name: Imperial College London
Year: 2010

description: Guided waves are interesting for Non-destructive Testing (NDT) since they offer the potential for rapid inspections of a large variety of structures. Analytical methods are well known for predicting properties of guided waves such as mode shapes and dispersion curves on regular geometries, e.g. plain plates or cylindrical structures. However these methods cannot be used to study guided wave propagation in waveguides having irregular cross-sectional geometries, such as railway lines, T-shape beams or stiffened plates. This thesis applies and develops a Semi-Analytical Finite Element (SAFE) method, which uses finite elements to represent the cross-section of the waveguide and a harmonic description along the propagation direction, to investigate the modal properties of structures with irregular cross-section. Two attractive applications have been investigated with the SAFE method, and the results are encouraging. The first application relates to fluid characterization. Guided torsional waves in a bar with a non-circular cross-section have been exploited by previous researchers to measure the density of fluids. However, due to the complexity of the wave behavior in the non-circular cross-sectional shape, the previous theory can only provide an approximate prediction; thus the accuracy of the measurement has been compromised. The SAFE method is developed to model accurately the propagation velocity and leakage of guided waves along an immersed waveguide with arbitrary non-circular cross-section. An accurate inverse model is then provided to measure the density of the fluid by measuring the change of the torsional wave speed. The model also enables the optimization of the dipstick sensor by changing the material of the dipstick and the geometry of the cross-section. Experimental results obtained with a rectangular bar in a range of fluids show very good agreement with the theoretical predictions. The second application relates to the inspection of large areas of complex structures. An experimental observation on a large welded plate found that the weld can concentrate and guide the energy of a guided wave traveling along the direction of the weld. This is attractive for NDE since it offers the potential to quickly inspect for defects such as cracking or corrosion along long lengths of welds. The SAFE method is applied to provide a modal study of the elastic waves which are guided by the welded joint in a plate. This brings understanding to the compression wave which was previously observed in the experiment. However, during the study, a shear weld-guided mode, which is non-leaky and almost non-dispersive has also been discovered. Its characteristics are particularly attractive for NDT, so this is a significant new finding. The properties for both the compression and the shear mode are discussed and compared, and the physical reason for the energy trapping phenomena is explained. Experiments have been undertaken to validate the existence of the shear weld-guided mode and the accuracy of the FE model, showing very good results. The sensitivity of compression and shear weld-guided modes to different types of defects close to the weld is investigated, by both finite element simulations and experiments. Due to similar reasons for energy trapping, the feature guiding phenomena also exists in a wide range of geometries. This thesis finally discusses feature guided waves on lap joints, stiffened plates and interconnected heat exchanger tube plates, and their potential applications.