Piezoelectric Transducer Monitoring of Structural Vibration

TWI Limited

Paper published in Condition Monitoring 2005, 18-21 July 2005, Cambridge, UK.

Abstract

Commercial systems for long-range inspection based on Lamb waves are limited by attenuation due to beam spread and scattering. The PiezoDiagnostic (PD) concept uses piezoelectricity to generate and detect structural vibration thereby reducing the testing frequency and overcoming the limitations of attenuation, leading to much longer testing ranges. The overall aim of the EC project within which this research was conducted was to validate this technology on pipes and tanks. This paper describes work conducted at TWI to investigate the application of PD technology to the monitoring of storage tank walls for structural change representative of corrosion. The work successfully demonstrated the feasibility of using microscopic structural vibrations, generated and detected using piezoelectric transducers, to detect simulated corrosion damage in the walls of a small tank under laboratory conditions. Simulated structural changes to the tank wall could be detected by a noticeable change in the modal properties. The results also indicate the possibility of damage location.

1. Background

The work described in this report was conducted at TWI and jointly funded by the TWI corporate research programme (CRP), and partly by the EC through a 5 ^th framework project called 'Piezodiagnostics'. The project consortium had nine partner companies from the UK, France, Poland and Spain. Piezodiagnostics was part of the EC GROWTH programme, its duration was 3 years and it lasted between 2002 and 2005 (visit www.cordis.lu/ for details).

2. Introduction

The Piezodiagnostics project was dedicated to the development of a novel concept. The principle is that by testing at frequencies that excite structural vibrations, scattering can be reduced leading to much longer testing ranges than achievable with Lamb waves. The novelty of the Piezodiagnostic approach was to excite with piezoelectric rather than electromechanical devices, thereby generating microscopic vibrations. The project consortium has named this concept 'PD' (PiezoDiagnostic) technology. Such low amplitude vibrations require much less energy to induce in the structure, are intrinsically safe and represent a practical approach to the application of structural dynamic testing to condition monitoring large structures. Lamb waves, which exist at frequencies of tens to hundreds of kilohertz in steel, are increasingly being used for long range monitoring. Although commercial systems exist for pipelines none exist for storage tank inspection owing to attenuation due to beam spread and scattering.

This experiment seeks to determine whether by employing microscopic structural vibrations PD technology offers a practical solution that can be implemented on storage tank walls. Furthermore it seeks to determine whether the sensitivity obtained by using such vibrations for modal testing is sufficient to allow the detection of structural change comparable with typical corrosion. Although of great importance, tests on the tank in various states of fill and tests on the tank floor are not included in the present discussion. Both applications are of great relevance in the monitoring of tanks for corrosion and will be the focus of future development of this method.

3. Aims

1. To investigate by experimentation the possibility of obtaining modal properties from the wall of a model storage tank by generating and detecting microscopic structural vibrations with piezoelectric transducers (PD technology).
2. To assess the potential of PD technology for detecting structural modifications typical of localized corrosion in storage tanks.

4. Experimental approach

The main approach was to obtain the modal response of the tank wall by conducting a stepped frequency scan and comparing the fingerprint generated under different configurations (locations of transducers) and in different tank states (location of simulated corrosion). In a stepped frequency scan the excitation frequency is changed in steps of a fixed size. At each frequency, a steady state sinusoidal structural response is established and measured simultaneously at a number of locations on the structure. At a mode the structural vibrations reinforce (constructive interference) and a large response is measured. The frequency response function (FRF) was used for fingerprint analysis. It is defined as:

FRF = Response / Force ............(1)

5. Equipment and apparatus

5.1 Transducers

The force was measured by a sensor located in an impedance head. The response was measured by five sensors; one of these sensors was a Bruel and Kjaer type 8001 impedance head. The other four sensors were prototype devices provided by the project partner CEGELEC CNDT of France. The response of these sensors was proportional to local deformation. A conventional FRF is derived from displacements, velocities or accelerations and therefore these sensors do not directly measure the FRF. However, it is reasonable to assume that some consistent relationship exists between radial displacement and local distortion in the region of the sensor. The sensors, together with the impedance head, each generated an independent transfer-FRF (sensor response divided by force measured at impedance head at different location). The basic structure of the CEGELEC sensor is shown in Fig.1(a). It is attached with a layer of fast-acting HBM X60 epoxy (a very hard adhesive). The actuators supplied by CEDRAT of France are amplified piezo-actuators (APA). The basic structure is shown in Fig.1(b). The actuator works by pulling its two poles together in response to an applied voltage within the range -20V to 150V, generated by the conditioning unit from an order signal between -1V and 7.5V. The voltage causes the bar of piezo-ceramic material between the poles to extend and contract axially.

5.2 Model tank and added masses

The model tank was designed and built at TWI. It was a 4m diameter tank with a bottom plate comprising two 6mm thick semi-circular plates of steel. The plates were butt-welded together and a 1 metre high shell consisting of three sections was fillet welded to the bottom plate. The three sections were themselves butt-welded together. The bottom plate was trimmed to leave a rim of 100mm. The tank was mounted on sand to represent the foundation material.

In this experiment added mass rather than metal loss represented structural change due to corrosion. Loss of section due to corrosion primarily reduces the local mass and stiffness causing changes in the FRF. An equivalent increase in mass is expected to cause local stiffness increase and comparable changes in the FRF. This approach is also more convenient as it is reversible. There were 2 attached masses and these are illustrated in Fig.2(a) and Fig.2(b). One was a 1kg circular steel disc of 2.5 mm (half wall) thickness and 26 cm diameter. This was attached to the wall with Araldite rapid adhesive. The other was a 0.89kg steel block attached with a G-clamp.

5.3 Testing apparatus

The tank wall was excited with the 'active-tendon' configuration. Each end of the actuator is connected to an adapter that interfaces it with a Dyneema cable (made by Eurocord BV). This material was chosen because it was lightweight and therefore the effects of self-weight would be negligible. The cable is attached to the tank wall with a clamp and a light tension (~100N) was applied. Fig.2(c) shows the actuator suspended in the centre of the tank and Fig.2(d) shows the actuator's wall mounting. The two positions of the impedance heads allowed verification that the force produced by the actuator was undistorted by the cable. Fig.2(e) illustrates the different testing configurations and tank states. In this figure 'W' indicates a wall mounting and the numbers indicate transducer locations. 'Active tendon' was used for excitation of the tank wall. In 'active tendon' there were two possible locations of the cable. In AT1 the cable was connected to location 1, whereas in AT2 it was connected to location 2. Both locations were across diameters of the tank and were at right angles to the each other. The results for AT1 are omitted for brevity, they were generally similar to AT2.

6. Experimental procedure

To test the PD concept on tank walls, stepped frequency scans were conducted using piezoelectric actuators and sensors. The experiments are summarized in Table 1. In five of the experiments (experiment 1 to 5) scans were performed between 1Hz and 200Hz at a frequency resolution of 0.1Hz, both unmodified and in various states of modification. Each scan lasted approximately 1hour. Two follow-up stepped frequency experiments were performed in the range 85Hz to 115Hz with a step size of 1Hz: five repeat experiments with the tank in the unmodified state (experiment 6); and eight repeat experiments with the smaller mass clamped at d' (experiment 7, Fig.2).

Table 1. Summary of tank wall experiments with actuator in configuration AT2.

7. Results and discussion

7.1 Repeatability of stepped frequency scans

Experiments 1 and 5 were repeat scans at 0.1Hz step size of the tank wall with no modification. Examples of the FRFs for sensors 2 and 4 are overlaid in Fig.3(a) and Fig.4(a) respectively. Note that in these figures the 100Hz to 200Hz range is stacked above the 1Hz to 100Hz range with an arbitrary offset. The repeat scans were taken two weeks apart yet all the scans indicated good qualitative agreement. There did not appear to be any difference in repeatability between the upper and lower frequency ranges. The closest agreement appears to be in the frequencies of the modal peaks. The amplitudes of the peaks are noticeably more variable, although not excessively so. An exception to this is the prominent that mode appears at 97Hz in the FRF of sensor 2 for one experiment but not the other. This is discussed below in relation to results from the 1Hz-resolution scans.

At 1Hz resolution the scans appear smoother than the scans at 0.1Hz but no important information is lost (see Fig.5). This means that future applications of PD technology to tanks could be completed 10 times faster (e.g. 200Hz range could be scanned in 6 minutes instead of 1 hour). Fig.5(a) shows the low resolution scans of the unmodified tank. These show good agreement with the scans taken at 0.1Hz resolution 120 days earlier. The repeatability, illustrated by the proximity of the overlaid scans, is good for these scans which were all taken on the same day. However, the repeatability is noticeably poorer for two modes either side of a prominent mode at 103Hz at sensor 2. These modes exhibit frequency variability that is practically absent in other modes. One of the modes corresponds with the mode that appeared on only one of the repeat scans taken at 0.1Hz resolution (see Fig.3, 97Hz). The different repeatability of these modes suggest that they may be physically different to the other modes. However, further experimentation is required to establish with certainty the nature of these modes. The significance of the observed repeatability will depend on how it compares with the sensitivity of the method to added masses. This is the subject of the next section.

7.2 Sensitivity to added masses

Experiment 2, experiment 3 and experiment 4 were scans of the tank wall with modification 'W(a)', 'W(b)' and 'W(d)' respectively. Examples of the FRFs for these experiments are overlaid in Fig.3(b) (sensor 2) and Fig.4(b) (sensor 4) respectively. Note that in these figures the 100Hz to 200Hz range is again stacked above the 1Hz to 100Hz range with an arbitrary offset. Note also that all three scans were taken over a period of one week. The first point to comment on is that, below 100Hz, there is very little difference between the scans for the modified and unmodified tank (with the single exception of the 90Hz mode at sensor 2 when the tank had modification W(a)). In other words there is generally no sensitivity to structural change below 100Hz. However, between 100Hz and 200Hz there is clearly more difference between the modified and unmodified states, i.e. the higher frequency range has better sensitivity to structural change. The most important differences are discussed below.

Between 196Hz and 200Hz there were dramatic indications on the spectra of the tank modified with 'W(b)' and 'W(d)' on all sensors but particularly sensor 2. A prominent growth in modal amplitude, and a slight frequency shift, occurred at sensor 2 (wall states W(b) and W(d)). Similar indications appeared on the spectra of sensor 1 and sensor 3 (not shown for brevity). Sensor 4 showed slight sensitivity to modification B in this region of the spectrum. In some cases, most notably sensor 2, the change in amplitude is very pronounced. The modal amplitudes for sensor 2 in this region of the spectrum are 12 times higher for states W(b) and W(d) than for the unmodified state.

Inspection of the 100Hz to 200Hz frequency band reveals that, while the spectra of the tank wall in states W(b) and W(d) are similar to each other, that of state W(a) differs noticeably from both of them. In fact the spectrum of W(a) is very similar to that of the unmodified tank, suggesting that there was less sensitivity to this modification than for states W(b) and W(d). There is no obvious explanation for this since all modifications are in the same position relative to the actuator cable attachment points (see Fig.2(e)). However state W(a) does show some notable indications. For example, between the modes at approximately 83Hz and 103Hz on sensor 2 there is a subtle difference between the spectra of the unmodified tank and the tank with the modification 'W(a)' (see Fig.3). Apparently a pair of modal peaks detected on sensor 2 at approximately 90 Hz on the modified tank have slightly shifted frequency and widened (the second of these peaks is interrupted at the break between the two frequency ranges). Indications also appear at about 110Hz in the sensor 2 spectrum of the tank with modifications 'W(b)' and 'W(d)' (see Fig.3). Despite being less dramatic than the modal amplitude increases between 196Hz and 200Hz, these spectral shifts are, nonetheless, consistent in that all of the tank modifications produced a change in this region of the FRF.

The lower resolution (1Hz) scans between 85Hz and 115Hz were conducted to confirm this sensitivity to structural change (using a smaller 0.89kg attached mass) in this region of the spectrum. These scans cannot be directly compared with the 0.1Hz scans because the location and method of attachment of the mass was different. However, the weight and location were similar in magnitude and therefore the damage sensitivity observed with the larger mass would be expected to be comparable with that obtained with the smaller mass. Fig.5(b) shows that the FRFs of sensor 1, sensor 3 and sensor 4 are most prominently changed by the presence of the added mass. The most obvious changes are in the modal amplitudes, which appeared to be related to the torque applied to the clamp attaching the mass to the tank wall. The greater the applied torque, the greater were the modal amplitudes. The effect on sensor 2 was much less prominent. This may have been influenced by its close proximity to the actuator attachment point suggesting that damage sensitivity is reduced as the sensor is brought closer to the actuator attachment point. A less obvious but equally important feature is the slight reduction in the frequency of the mode at 115Hz. Sensor 3 on Fig.5 shows this mode is downshifted by about 1Hz by the added mass. Although only a small shift it nevertheless has significance because of the good frequency repeatability observed at this mode. A similar frequency shift occurs on sensor 1 at the 115Hz mode and at the 93Hz mode, both of which are downshifted by approximately 1 Hz. In fact it does appear that other modes are downshifted slightly, although some by less than 1Hz. Finally, the 1Hz scans revealed two spectral peaks that occur at 86Hz on sensor 1 and sensor 3 of the modified tank. These were observed on only one of the eight repeat scans. This peak might therefore be considered anomalous.

7.3 Summary

The method of using piezoelectricity to generate and detect the modal response (so called PD technology) of a tank wall has been shown to produce repeatable and consistent FRFs, in the modal frequencies in particular. The sensitivity to structural change appeared to be frequency dependent. The upper 100Hz to 200Hz frequency range was more sensitive than the lower 1Hz to 100Hz range. In most cases, changes in the state of the tank wall were represented by differences the spectral amplitude (most notably amplitude increases of more than an order of magnitude in some cases). However, the sensitivity was dependent on attached mass location and it was found that the test was more sensitive the nearer the structural modification was to the top of the tank. In this position modal frequency shifts were also detected in spectra of the modified tank wall. Despite the design symmetry, different circumferential locations of the added mass produced different modal properties suggesting that it may, in future, be possible to infer the location of damage from a more detailed analysis of the spectra.

Conclusion

It has been shown to be possible to reliably obtain the modal properties of a model storage tank using piezoelectric transducers (PD technology). PD technology has detected structural modifications on the wall of a model storage tank comparable with that of localised corrosion, demonstrating the potential of this method for corrosion monitoring on full sized tanks.

Acknowledgements

The author would like to thank TWI industrial members and the European Commission for providing the funding for this research. Also staff at TWI for their support.

References

1. EC project G1RD-2001-00659 (PIEZODIAGNOSTICS) Project report (23/03/04). Technical appendices.

2. Ewins D J: 'Model testing: Theory and practice'. Publ: Research Studies Press Ltd, Taunton, Somerset, England, 1984.