The scattering of the first antisymmetric Lamb wave mode A0 at obstacles in plate-like structures is studied in this dissertation. The propagation in an isotropic, homogeneous plate, the scattering at a circular hole, and the scattering at a hole with a defect are investigated experimentally and theoretically. Guided flexural waves have the advantage of propagating over large distances in plates, thus allowing the fast and efficient detection of defects in large structures. This method holds promise for the nondestructive testing of aircraft. Airplane fuselage and wings often consist of aluminum face sheets, connected with fasteners or containing holes, which are sources of stress concentration and crack formation at their boundaries. When the guided wave hits such a discontinuity, a typical scattered displacement field is obtained. A change in the scattered field indicates the development of a fatigue crack, and thus the growth of such cracks can be monitored.

As a model system to gain a well-founded understanding of the interaction of the flexural wave with an obstacle in the plate, the case of a through hole with a notch at an arbitrary angle is studied. In the experiments, the A0 mode is excited selectively by means of a piezoelectric transducer with a well-defined time signal. The used frequency range is below the cut-off frequencies of the higher wave modes in the plate. The scattered field is measured on a grid around the hole with a heterodyne laser-interferometer. Using fast Fourier transformation, the amplitude and phase values of the scattered field are extracted from the measured time series. The introduction of a small imperfection, like a notch, at the boundary of the cavity changes the measured scattered field significantly. The first antisymmetric Lamb wave mode A0 physically represents a flexural wave propagating along the structure. It can be described well using approximate theories. Therefore no three-dimensional theory needs to be implemented, and a fast calculation is achieved. Different approximate analytical approaches to calculate the wave propagation and the scattering at a circular hole, employing classical plate theory, Mindlin’s theory, and an asymptotic expansion of the threedimensional theory are compared. Good agreement between the experimental data and the analytical solutions is found for the extent of validity of the different models.

The influence of a defect like a crack or a notch at the hole boundary on the scattered field is modelled numerically implementing a finite difference scheme. Discretizing Mindlin’s equations of motion on a staggered, Cartesian grid, the transient wave propagation is calculated by explicit time integration. The stressfree boundary conditions at the hole and a notch are implemented on a Cartesian approximation of the boundaries. This way a stable and fast numerical calculation of the scattered field around the hole and notch is achieved. Good agreement with the analytical calculation and the measurements for the propagation and the scattering at an undamaged hole is found. The numerical calculations agree well with the measurements for a notch or a crack at the hole boundaries. Accurate descriptions of the influence of a defect on the scattered field can be made. The detectability of a defect is studied numerically for a parameter variation, and the predictions are compared to the experiments.

The method is applied experimentally to a variety of specimen, proving its usefulness for nondestructive testing purposes. In aluminum plates well-defined geometries like a notch at different angles relative to the propagation direction of the incident wave, and a line of holes symbolizing the multiple scattering at a line of rivets are studied. Broadband excitation and measurements at only a few points are investigated to achieve a fast defect detection. Fatigue cracks at holes in tensile specimens are studied in collaboration with an industrial partner as a realistic problem. The cracks are initiated and propagated by cyclic tensile loading of the test specimen in a servo-hydraulic material testing machine. An on-line monitoring of the crack length during the crack propagation is implemented and found to give repeatable results. The minimum detectable crack length is evaluated and problems like crack closure are studied.

Thorough theoretical and experimental know-how on the interaction of flexural waves with obstacles in plate-like structures is gained. Accurate predictions on the detectability of fatigue cracks at fastener holes, an important problem in aerospace industry, can be made. The practical applicability of the method is shown.