Thickness mapping in aging structures suffering from corrosion is challenging. This is especially the case when the structure is only partially accessible. In plates the high order shear horizontal guided wave modes all have a cutoff frequency thickness product below which they cannot propagate. This property is attractive to estimate the minimum remnant thickness between a source and a sensor. In fact, by identifying the highest order mode detected by the sensor it is possible to estimate the minimum remnant thickness on the path between the source and the sensor. Finite element simulations were used to demonstrate the feasibility of this technique. In the proposed method, multiple modes are excited simultaneously by a multi-element source. The modes are then detected by a similar multi-element sensor. The modes are then identified using a two-dimensional Fourier transform across the elements of the sensor. The thickness resolution of this technique is inversely proportional to the excitation frequency. However, as the attenuation increases with frequency, a trade-off must be made between inspection distance and the desired thickness resolution. Initial experimental results show excellent agreement with the finite element simulations.

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