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.