Air-coupled nondestructive testing has become feasible following recent improvements in air-coupled transducer design. However, the large acoustic impedance mismatch between air and solid materials does not allow normal incidence pulse-echo inspection. Nevertheless, air-coupled transducers can be used for the generation and detection of Lamb waves, the receiver being outside the field of the specular reflection. A finite element study of the generation of Lamb waves in plates from a finite air-coupled transducer, the interaction of these waves with defects, and their detection using an air-coupled receiver is described. These predictions are compared with experimental results obtained on a variety of specimens using a pair of 1-3 composite, air-coupled transducers. The use of an ideal collimated beam in the model, instead of using the real pressure field generated by the transducers, is demonstrated to have a negligible effect on the predicted Lamb waves. It is shown both theoretically and experimentally that the measured amplitude of the Lamb waves is very sensitive to the alignment of the transducers with respect to the test structure, misalignment of 0.6°r educing the amplitude by around 50%. The detection of notches of various depths in steel plates is investigated, and the sensitivity of the technique with different choices of incident mode and different positions of the receiver with respect to the defect is discussed. It is shown that in metal structures it is most satisfactory to use the a 0 mode since it has a large out-of-plane surface displacement and so is easy to excite.
The Semi-Analytical Finite Element (SAFE) method is becoming established as a convenient method to calculate the properties of waves which may propagate in a waveguide which has arbitrary cross-sectional shape but which is invariant in the propagation direction. A number of researchers have reported work relating to lossless elastic waves, and recently the solutions for nonpropagating waves in elastic guides and for complex waves in viscoelastic guides have been presented. This paper presents a further development, addressing the problem of attenuating waves in which the attenuation is caused by leakage from the waveguide into a surrounding material. This has broad relevance to many practical problems in which a waveguide is immersed in a fluid or embedded in a solid. The paper presents the principles of a procedure and then validates and illustrates its use on some examples. The procedure makes use of absorbing regions of material at the exterior bounds of the discretized domain.
The interaction of the low-order antisymmetric (a0) and symmetric (s0) Lamb waves with vertical cracks in aluminum plates is studied. Two types of slots are considered: (a) internal crack symmetrical with respect to the middle plane of the plate and (b) opening crack. The modal decomposition method is used to predict the reflection and transmission coefficients and also the through-thickness displacement fields on both sides of slots of various heights. The model assumes strip plates and cracks, thus considering two-dimensional plane strain conditions. However, mode conversion (a0 into s0 and vice versa) that occurs for single opening cracks is considered. The energy balance is always calculated from the reflection and transmission coefficients, in order to check the validity of the results. These coefficients together with the through-thickness displacement fields are also compared to those predicted using a finite element code widely used in the past for modeling Lamb mode diffraction problems. Experiments are also made for measuring the reflection and transmission coefficients for incident a0 or s0 lamb modes on opening cracks, and compared to the numerical predictions.
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