Table of Contents ICAC'98

Embedded optical fibre sensors for the permanent monitoring of filament wound pressure vessels Joris Degrieck1,2 and Wim De Waele1 1 Department of Mechanical Construction and Production, University of Ghent 2 WTCM, Belgian Research Centre for the Metalworking Industry St.-Pietersnieuwstraat 41, B9000 Gent, Belgium Corresponding Author Contact: Wim De Waele Email: Wim.DeWaele@rug.ac.be

I. Abstract

Worldwide a lot of research effort is carried out concerning new inspection techniques for the in-service monitoring of composite structures. This paper focuses on the use of optical fibre sensors. The sensor part of the optical fibre is a Bragg-grating. This is a periodic perturbation in the refractive index of the fibre core. When broadband light is coupled into the optical fibre sensor, a reflection peak will be obtained centered around a wavelength called Bragg-wavelength. The Bragg-wavelength depends on the refractive index and the period of the grating, which both change due to mechanical and thermal strain applied to the sensor. Optical fibres with Bragg-sensor have been embedded into filament wound pressure vessels.

A test setup has been implemented to demonstrate the ability of remote sensing. This is of great importance for the applicability of Bragg-sensors to mechanical and civil structures. The controlling computer and the test set-up are connected to an optical spectrum-analyser by means of independent optical links of 200m. In this way the relative positions of the different parts of the test set-up are irrelevant. The vessels have been subjected to static loading cycles as well as to slowly varying dynamic loadings. The measured shifts in Bragg-wavelength show excellent linear agreement with the pressure applied to the vessel. The combination of this measuring method and the test set-up is an essential step towards a continuous remote monitoring system for composite structures. It is also an important part in the total design of "smart" structures.

II. Introduction

There is an important international research work going on in the domain of non destructive evaluation techniques. And quite right, because there is a great necessity for techniques that are able to evaluate mechanical or civil constructions during their fabrication, installation and lifetime. This can be the detection of possible damage, the measurement of mechanical quantities, the measurement of pressures, temperatures, corrosion.
Today one can find many constructions made of fibre reinforced plastics that are in service since many years. Many of these undergo a gradual degradation and some are already in an advanced stage of deterioration. This can effect the safety of persons and installations. Therefore one wants to know the condition of a construction at every time and if possible correlate this with the remaining lifetime. This is even more true for constructions made of fibre reinforced plastics because of the lack of experience with the long term behaviour of this materials, notwithstanding the many constructions that yet have been built.

At this moment the observation of constructions under high load is mainly based on regular (but expensive and extensive) inspections. The constructions must therefore be put out of service during a certain period, causing serious financial implications for the user. This explains why permanent monitoring techniques arouse interest. The number of regular inspections can be reduced or even become totally unnecessary. Only at the time that some aberrant behaviour is recorded a more thorough inspection should be done. A monitoring technique could also be used to detect temporary or permanent overloads, inadmissible vibrations, abnormal temperatures, damage of the construction. A major advantage of monitoring is that in function of the measured load, fatigue cycles, overload, an estimation of the remaining lifetime of the construction can be made. The feedback from the recorded loads and deformations of (part of) the construction in real conditions can also lead to usable information in design or even the development or adaptation of standards and rules.

A monitoring technique is certainly eligible for mechanical and civil structures of which the integrity is of primordial importance. Examples of these are aeroplanes, pipelines, pressure vessels, chemical installations, bridges, dams, machine part.

In this paper some experiments on the use of optical fibre sensors for the permanent monitoring of pressure vessels and containers are reported. Pressure vessels find wide application in many different domains, such as transport, chemical industry, processing industry, food industry. These vessels work under high pressures, often under alternating stresses and ambient conditions (temperature, moisture. Therefore the aspect of safety is of major importance in the sector of pressure vessels. Because of the inadequate knowledge of the long-term behaviour of filament wound pressure vessels, high safety factors are applied in design rules, and there is often no consensus about the acceptability of certain damage patterns. The possibility of monitoring the condition of an in-service pressure vessel, should certainly elucidate these matters.

III. Optical fibre sensors

Table 1 gives a summary of the major advantages and disadvantages of the use of optical fibres in relation to more common monitoring techniques, such as electrical resistance strain gauges, accoustic emission^1-3.


Table 1: Advantages and disadvantages of optical fibre sensors.
Advantages	Disadvantages
high sensitivity for multiple quantities (temperature, strain) possibility of measuring at multiple points with one optical fibre insensitive to corrosion ideally suited to be embedded in composite structures does not affect the mechanical properties of the material in which it is embedded withstands high temperatures very small and light can have a sensor function and be signal carrier (optical transmission) at the same time insensitive to electromagnetic interference geometrical versatile	because of the high sensitivity, the measurement of one quantity can be influenced by other quantities can't be repaired because of the brittleness one has to be very cautious when handling these optical fibres complex techniques often have to be used for the treatment of the signal high cost

The major efforts that take place in the development of optical fibre (sensor) technology will certainly have as effect that a number of the disadvantages cited above will disappear. There exist a lot of different optical fibre sensors, all suited for specific purposes (strain sensing, pressure sensing, chemical sensing, vibration sensing. The sensor studied in this project is a Bragg sensor that is ideally suited for strain measurements. A Bragg grating is a periodic perturbation in the refractive index of the fibre core. The realisation of a Bragg grating in an optical fibre is based on the photosensitivity of the Germanium doped fibre core. This means that the refractive index of the fibre core can be permanently changed by side illumination with ultraviolet light⁴. When light with a sufficiently broad spectrum is coupled into the optical fibre, a narrow band around a central peak wavelength will be reflected by the Bragg sensor. A typical reflected spectrum is shown in figure 1.


Fig 1. Measured and filtered spectrum of a Bragg sensor

This central peak wavelength, also called Bragg wavelength, is given by the Bragg condition:



Herein n_eff is the effective index of refraction and the period of the grating. When strain is applied to the sensor part of the optical fibre, the peak wavelength will change due to two reasons. Primary the period of the grating will change according to the applied strain, and secondly the effective index of refraction will change due to internal stresses. The strain can be caused by mechanical loads or by changes in temperature. By measuring the shift in Bragg wavelength, one can easily determine the applied strain according to formula 2:



Herein L is the length of the sensor and P is the optical strain coefficient which has a typical value of 0,22 for axial strain⁵.One major advantage of a Bragg sensor is that the information is encoded directly into the wavelength, which is an absolute parameter. This means that the output does not depend on the total light level, nor on losses in the connecting fibres and couplers, nor on the power of the light source. The wavelength encoded nature of the sensor output also facilitates wavelength division multiplexing⁶, by assigning each sensor to a different portion of the available source spectrum. The measurement of the optical signals is done with an optical spectrum analyser (OSA). The OSA has a white light source which is used to couple light into the optical fibre. The signal reflected from the Bragg sensor is coupled into the detection unit of the OSA via a 50/50-coupler.

This is shown in figure 2. The used light source has a relatively low light power and is liable to noise. Because of the coupler, splices and connectors the light power will also be attenuated when reaching the detector. This means that the influence of noise can be of importance and thus the reflected spectrum won't be of great quality. But for the measurements only the shift of the peak wavelength is of importance, thus a spectrum with a clear distinct peak wavelength is of importance. The measured spectra can also be mathematically filtered to reduce the influence of noise. There is obviously an important influence of noise on the recorded spectrum of figure 1.

As stated above this is inherent to the use of the white light source of the optical spectrum analyser (OSA). This means that the wavelength at which the maximum reflection occurs, will not necessarily be the wanted Bragg wavelength. Therefore the recorded spectra are mathematically filtered before extracting the peak wavelength. From this filtered spectra the two wavelengths at which half of the maximum reflection is recorded are determined and the mean value is defined as Bragg wavelength. One test also has been performed with a tunable laser source, which has more light power. With this source there is no longer influence of noise on the recorded spectra and thus the Bragg wavelength can easily be determined without the need of filtering the spectrum.

IV. Test set-up

Fig 2. Test set-up

Fig 3. Pressure vessel with embedded optical fibre sensor

A schematic overview of the test set-up is shown in figure 2.

The optical spectrum analyser is placed in an optical room at the department of Information Technology (INTEC) of the University of Ghent. The experiments on the pressure vessels are carried out in a test room at the department of Applied Mechanics. This means that an optical link of approximately 200m had to be established between these two rooms. The controlling computer is also placed in the test room and is connected to the OSA via a second optical link of approximately 200m.

This remote character of the tests makes this sensor technique even more interesting for practical use. This makes it possible to monitor and control various composite structures from a central dispatching unit via optical data transmission lines. The pressure medium used in these tests is compressed air. But for safety reasons the compressed air is applied to a water buffer of 40 litres which is connected to the pressure vessel. An electronic pressure transducer is used to measure the real applied pressure.

Specific software based on LabView was developed for the control of the valves on the pressure line, as well as for processing of the signals from the pressure transducer and the OSA. The pressure vessels were fabricated from prestressed glass fibres drenched with epoxy, and wound on a mould made of soluble plaster. The numerical controlled winding machine was also designed at the department of Applied Mechanics. The optical fibre sensor is positioned in the outer layer parallel to the reinforcing fibres. An example of a vessel can be seen on figure 3.

V. Experimental results

The selected experiments represent possible in-service situations of a pressure vessel, eg. the filling and emptying of a vessel, a temporary overload, slowly varying pressure cycles. All tests were conducted at a constant ambient temperature. For each type of test conducted, one example is shown on the following figures. Figures 4a, 5a and 6a show the applied pressure (left y-axis) and the measured wavelength (right y-axis) against time, while figures 4b, 5b and 6b show the relation between applied pressure and measured wavelength. In the legend lpeak is the peak wavelength and lBragg is the Bragg wavelength of the smoothed spectrum as described above. In each of the conducted tests the shift in the reflected wavelength shows the same variation in time as the applied pressure.

This is illustrated in figures 4a, 5a and 6a. To examine the relationship between Bragg wavelength and measured pressure a linear regression based on the method of the least squares has been done. The results are shown on the figures 4b, 5b and 6b. To evaluate this linear relation between the two quantities the regression coefficient is calculated for each regression and is stated on the figures. It can be concluded that there is an overall excellent linear relation between applied pressure and Bragg wavelength.

Fig 4a. Load cycle type 1	Fig 4b.Wavelength vs. pressure for load cycle type 1	Fig 5a. Load cycle type 2
Fig 5b. Wavelength vs. pressure for load cycle type 2	Fig 6a. Load cycle type 3	Fig 6b. Wavelength vs. pressure for load cycle type 3

VI. Concluding remarks and future perspectives

From the results it can be seen that the use of Bragg sensors is a very promising NDE technique for the use on composite materials and structures. It is clear that the proposed technique can be used for the remote monitoring of a composite structure. For an in-service structure it is possible to perform the control via optical data transmission on a remote place that can serve as central dispatching unit for the monitoring of several structures. Because of the very promising results followed on from this project, the authors plan an extension of the current research on filament wound pressure vessels and a broadening towards other fibre reinforced composite materials and structures.

Substantial parts of this work were conducted with the financial support of IWT (RD/95/08-01), IWONL (CIB234) and DGTRE (nr. 3216). Their help is gratefully acknowledged by the authors. The authors also wish to thank professor Roel Baets from the department INTEC for the informative discussions and hints.

References

1. Green R.E., "Existing technologies for condition monitoring of construction materials and bridges", Fiber optic sensors for construction materials and bridges, Technomic Publishing, 1998, pp 17-28.
2. Udd E., "Fiber optic smart structures", Proceedings of the IEEE, vol. 84, no. 1, jan. 1996.
3. Michie W.C., Culshaw B., Uttamchandani D., "Optical fibre techniques for structural monitoring in composites", Agard Meeting Lindau, oct. 1992.
4. Bennion I., Williams J.A.R., Zhang L., Sugden K., "UV-written in-fibre Bragg gratings", Optical and Quantum Electronics, 1996, pp 93-135.
5. Davis M.A., Bellemore D.G., Kersey A.D., "Distributed fiber Bragg grating strain sensing in reinforced concrete structural components", Cement and concrete composites, vol. 19, no. 1, 1997, pp. 45-57.
6. Kersey A.D., "Interrogation and multiplexing techniques for fiber Bragg grating strain-sensors".