Joris Vanherzeele

A method for crack sizing using Laser Doppler Vibrometer measurements of Surface Acoustic Waves

The goal of non-destructive testing (NDT) is to determine the position and size of structural defects, in order to measure the quality and evaluate the safety of building materials. Most NDT techniques are rather complex, however, requiring specialized knowledge. In this article, we introduce an experimental method for crack detection that uses Surface Acoustic Waves (SAWs) and optical measurements. The method is tested on a steel beam engraved with slots of known depth. A simple model to determine the cracks size is also proposed. At the end of the article, we describe a possible application: fatigue crack sizing on a damaged slat track. This technique represents a first step toward a better understanding of the crack growth, especially in its early stages (preferably when the cracks can still be repaired) and when it is possible to assume a linear propagation of the crack front. The ultimate goal of this research program is to develop a useful method of monitoring aircraft components during fatigue testing.

Fourier fringe processing by use of an interpolated Fourier-transform technique

Recently a powerful Fourier transform technique was introduced that was able to extract the phase from only one image. However, because the method is based on the two-dimensional Fourier transform, it inherently suffers from leakage effects. A novel procedure is proposed that does not exhibit this distortion. The procedure uses localized information and estimates both the unknown frequencies and the phases of the fringe pattern (using an interpolated fast Fourier transform method). This allows us to demodulate the fringe pattern without any distortion. The proposed technique is validated on both computer simulations and the profile measurements of a tube.

Absorption measurement of acoustic materials using a scanning laser Doppler vibrometer

In this article a method is proposed to estimate the normal incidence reflection ratio and absorption coefficient of acoustical materials using measurements in a transparent tube excited with a loudspeaker and terminated with the material under investigation. The waveforms are measured at different locations in the tube using a scanning laser Doppler vibrometer. Because the measurement probe (i.e., the laser beam) does not interfere with the wave in the tube, narrow tubes can be used. This means that—in contrast to the standardized wide tube tests using microphones—the proposed experiment could be used for high frequencies (in the paper an 8 mm tube was used, resulting in a 25 kHz upper frequency limit). It is shown based on theoretically known scenarios (i.e., an open tube and a rigid termination) that the absorption coefficient can be obtained with an error of about three percent. In addition, the absorption coefficient of two commonly used absorption materials—glass fiber wool and carpet—were determine...

Design of multi‐input interleaved multisine excitation signals for ultrasonic testing

Multisines are periodic signals consisting of harmonically‐related sine waves. By optimizing the phases of the different sine waves, a periodic signal is obtained with a small crest factor resulting in high signal‐to‐noise ratios. One disadvantage of multisines when several ultrasonic actuators are used simultaneously is that, in general, it is very difficult to distinguish from a measured ultrasonic signal the contribution of the different actuators. This is easier when pulses are used and when they do not overlap. In this contribution, a new approach for periodic continuous wave signals will be presented to separate the contribution coming from the different actuators (senders) at every receiver. The proposed approach is based on the use of multi‐input interleaved multisines. Interleaved multisines contain energy at different spectral lines allowing an easy separation by means of a discrete Fourier transform. The separation becomes more complex when non‐linear effects are present, but even then it is po...

Acoustic source identification using a generalized regressive discrete Fourier series for tomographic reconstruction

When measuring three‐dimensional phenomena such as acoustic fields using an interferometric technique, one is prone to measure different angles of view to obtain a full three‐dimensional representation of the phenomenon under investigation. This is due to the fact that an interferometric technique measures a line integral across the optical path. To obtain the full three‐dimensional view, the different angles of view are passed through a tomographic algorithm. The most widely used tomographic method is filtered back projection. However, this process suffers from a series of drawbacks, the most important one being the fact that substantial truncation errors occur in the back projection step. In this article, a method is devised to eliminate these errors, based on a parametric frequency domain approach called generalized regressive discrete Fourier series (GRDFS). The method will be applied to laser doppler vibrometer measurements on a loudspeaker. This source will be rotated to obtain continuous angle views of the acoustic field, hence eliminating the tedious process of rotating the measurement set‐up. By demodulation of a measured signal, it is possible to determine the position of the loudspeaker in space. The results obtained with the GRDFS will be compared to the classical filtered back projection method.