Modelling of guided ultrasonic waves in aircraft wiring

3. Experimental work

In order to validate the findings of the numerical modelling carried out, it is important to carry out real physical experimentation trials. This section of the paper will look at the effect of exciting longitudinal wave mode inbare wire and insulated wire as well as looking at the optimum parameters with respect to the frequency and the number of cycles for the excited signal.

3.1 Initial experimentation

The work is carried out to look at the effect of propagating longitudinal wave-modes in bare and insulated wire. The bare wire is made from copper, and it has been excited longitudinally (see Figure 2) with a frequency of150 kHz. Figure 3 shows the propagation of the longitudinal wave at 150 kHz. A pulse echo technique is used to detect the reflected signals from the excited wave mode. The longitudinal wave is decaying as it propagates. Thisdecay is the result of the dispersion effect and attenuation. According to this experimental condition, longitudinal waves can travel up to 8 meters in bare wire. A second test has been carried out for an insulated wire at anexcitation frequency of 20 kHz. The reason for choosing this frequency is its high energy. As can be seen from Figure 4, the wave in the insulated wire is highly attenuative and dispersive. According to this experimentalcondition, the longitudinal wave has travelled 4 meters in insulated wire. The longitudinal wave travels in bare wire further than in insulated wire. This is due to the presence of the soft material in the insulation, which results insignal attenuation and dispersion. These findings agree with the modelling and the literature. However, there is a need to find the optimum experimental parameters with respect to the excitation frequency and number of cycles.

3.2 Optimum excitation parameters

There is a need to find the optimum frequency range and number of cycles as this will have a significant effect on the nature of the excited wave, propagation distance, attenuation and dispersion. Therefore, a full study andanalysis is needed to determine the parameters which satisfy these two factors. An initial experimental trial has been carried out to look into the effect of the number of cycles at different frequencies. A range of frequencies from 20kHz up to 100 kHz with an increment of 10 kHz between each test was applied to the insulated wire.

Figure 5 shows the number of cycles (vertical axis) versus time (horizontal axis) at the excitation frequency of 60 kHz. From the figure, it shows that 5 cycles are a reasonable compromise number of cycles for thetone-burst signal to be constructed and propagated. Furthermore, the frequency range is also sufficient to propagate guided waves. The lower the frequency is the higher energy the wave has. In contrast, the high frequency is, the lowerenergy the wave has, hence, higher attenuation. This attenuation is due to the presence of the insulation. At this frequency, the effect of dispersion of the output signal does not exhibit itself.

3.3 The effect of insulation in an insulated wire

To study the effect of insulation on propagating guided waves, as well as to confirm the findings in section 2, verification was required to confirm that there is only one wave mode present at the low frequency region. A number ofexperimental works were carried out for different lengths of insulation. The experiment started with a 1 meter insulated wire, the insulation was then gradually removed every 5 cm. This is shown in Figure 6, where L representsthe length of the removed insulation. Throughout the experiment at 40 kHz, 5 cycles, tone burst signal was used. The results are shown in pitch catch form in Figure 7, where the x-axis shows the time of arrival for the signal,while the (y-axis) represents the insulation length. The results show that there is only one wave mode present at this structure. However, the presence of the insulation has decreased the inspection range. In addition, insulationimposes the effect of dispersion as well as increases the presence of attenuation of the wave.

3.4 Correlation between experimental and modelling results

In order to validate the numerical modelling and the experimental results, the received signal in both the modelling and the experimental results were correlated against each other. The correlation is done on 20 kHz and 100 kHz onan insulated wire with the presence of a defect. The amplitude of the reflected signals were normalised with respect to the 1st reflection of the wire end. Figure 8 shows the correlation of the signals, there is a large degreeof correlation between the modelling and the experimental results. However, there is an amplitude difference between the modelling and the experimental results. In addition, there is a velocity difference by 1.8% and 2.5% for the twofrequencies, 20 kHz and 100 kHz respectively. These differences are due to the attenuation existing in the experimental conditions which is not included in the model, as well as the difference in the insulation loss representation inthe model. The model can be improved by including attenuation and damping, and a better description of the defect.

4. Discussion and conclusion

This paper has shown the ability of guided wave to travel in insulated wire structures from a single point of access. However, this propagation depends on different parameters, which include: the number of cycles of the tone burstsignal, the form of excitation, the excitation frequency, the stiffness of the conductor material and the insulation.

This paper has focused on exciting insulated wires at low frequency with an axi-symmetric excitation. This excitation processed a longitudinal wave mode, type L (0, 1). This is a compression wave mode where the propagation takesplace in both materials (conductive and insulation). The compression wave mode becomes a Rayleigh wave mode in which most of the displacement takes place in the insulation as the frequency increases. L (0, 1) has proved its ability todetect defects within the insulated wire structure. This wave mode has the limitation of being dispersive in nature and attenuative when an insulated material (soft material) is present.

Finally, the experimentation and modelling results have shown a large degree of agreement with few variations. These variations are in part due to the electronic noise generated by the equipment used in the experimentation and alsobecause the attenuation and damping have not been taken into account in the model.

Improvements to the results can be made by conducting further work and integrating those factors into the model.

References

1. M.Papelis, 2005, Project proposal, 'Intelligent structure condition monitoring system for the aerospace industry', TWI, UK
2. E.Tucholski, 2004, 'Non-destructive evaluation of aromatic polyimide insulated aircraft and spacecraft wiring', World Conference Non-Destructive Testing, www.ndt.net/article/wcndt2004/html/htmltxt/119_tucholski.htm
3. C.Kim, N,Johnson, 'Detection of intermittent faults in aircraft electrical wire by utilising power line communication' www.agingaircraftconference.org/all_files/12/12a/33_doc.pdf
4. R.Anastasi, E. Madaras, 2001 Ultrasonic guided waves for aging wire insulation assessment, NASA centre: Langley Research Centre.
5. M.Sadok, M.Jaidane, M Walz,2006,'Joint time frequancy domain reflectometry and stationarity index for wire diagnostics in aircraft' 9th Joint FAA/DoD/NASA Conference on aging Aircraft, Atlanta.
6. E.Doorn, D.Tolani, 2006, 'Radical extension of time domain reflectometry for detection and location of flows in aircraft wiring systems', 9th Joint FAA/DoD/NASA Conference on Aging Aircraft, Atlanta
7. C.Furse, Y.Chung, M.Nielsen, G.Mabey, R.Woodward. 'Frequency domain reflectometery for on-board testing of aging aircraft wiring, IEEE transactions on electromagnetic compatibility', Vol.45, No.2
8. Y.Chung, C furse, J Pruitt, 2005, 'Application of phase detection frequency domain reflectometry for locating faults in an F-18 flight control harness' IEEE transactions on electromagnetic compatibilities Vol.45, No.2.
9. R. Anastasi, E.Madaras, 1999, 'Pulse compression techniques for laser generated ultrasound', IEEE Ultrasonic Symposium, Proceedings, Vol.1, pp: 813-817.
10. E.Madara, R.Anastasi, 2001 'Investigation the use of ultrasound for evaluating aging wiring insulation', 5th Joint NASA.FAA.DoD conference on Aging Aircraft, NASA LaRD and U.S. Army Research Lab
11. E.Madaras, R.Anastasi, 2002. 'Evaluating thermally damaged polyimide insulated wiring (MIL-W-81381) with Ultrasound', NASA centre: Langley Research Centre.
12. R.Anastasi, E.Madaras, Investigation the use of ultrasonic guided waves for aging wire insulation assessment, Vol. Issue:Site Visited 01.03.2008 techreports.larc.nasa.gov/ltrs/PDF/2002/mtg/NASA-2002-7spiende-rfa.pdf
13. R.Anastasi, 2004, 'Aging wire insulation assessment by phase spectrum examination', SPIE's 8th international Symposium on NDE for Health Monitoring and Diagnostics, Vol.9, No.8.