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Noninvasive and simultaneous determination of thickness and sound velocity of layered structures with ultrasound

Subject Area Measurement Systems
Term from 2011 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 207003883
 
Final Report Year 2024

Final Report Abstract

Conventional ultrasound methods measure the time of flight (ToF) to determine the reflector distance (imaging method) if the speed of sound is known or similarly, the speed of sound (material characterisation), if the layer thickness is known. Simultaneous determination of layer thickness and sound velocity is preferable, for example for non-destructive determination of material parameters of unknown solid-state layered systems or to improve ultrasonic imaging. However, this is not possible in most cases, as reference reflectors or other transducers are required at defined distances. As part of the project, two non-invasive, non-destructive ultrasound methods were developed for the simultaneous determination of layer thicknesses and sound velocities using annular arrays. The functional efficiency test showed a deviation of less than 1% for singlelayered media. In the investigation of metal plates with a superimposed water layer (two-layered system), deviations of between 3% and 5% were achieved with both methods compared to the conventionally measured sound velocities and layer thicknesses, whereby the conventional measurement methods themselves have an uncertainty in this range. In each case, a geometric model was used to determine the delay times. This model does not adequately represent the sound propagation and therefore limits the achievable accuracy of the methods. Therefore, the main focus in the second project phase was on the development of an algorithm for calculating the impulse response of the individual elements of the annular array. The algorithm developed is based on exact, transient GREEN‘s functions for multi-layered structures and a spatial convolution for calculating the extended transducer. It is therefore very efficient without any loss of accuracy and, for the first time, enables fast calculation of the impulse response as a function of the respective layer structure. The signal for any excitation function can be calculated from the impulse response. By comparing simulated and measured echo signals, it is possible to iteratively determine sound velocities and layer thicknesses simultaneously. As the developed calculation algorithm, in contrast to fully numerical methods for calculating signals, can also be used with centre frequencies above 100 MHz, it has high potential for use in ultrasound microscopy for testing microelectronic components. For this reason, it is to be extended to more than two-layered structures and arbitrarily shaped transducers, taking multiple reflections into account.

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