TWI Industrial Member Report Summary 832/2005
By S M Williams
The work described in this report investigates the use of piezoelectric actuators and transducers to generate and measure structural vibrations in storage tanks for the purposes of detecting corrosion or some other structural change from the characteristics of the vibration. The work was part of the EC collaborative Piezodiagnostic Project, which has nine partner companies from the UK, France, Poland and Spain.
The Project aims to demonstrate the concept of using low frequency, low amplitude elastic waves for long-range monitoring of a variety of large industrial installations which may suffer structural degradation, particularly by corrosion. The detection of corrosion in large installations is generally difficult without full access, which is often not possible. Hence there is a requirement for remote condition monitoring to indicate the possibility of damage.
Lamb waves, which exist between tens and hundreds of kilohertz, are frequently used for long range structural monitoring of pipework. However, they are limited by attenuation due to beam spread and scattering. The success of Lamb waves for pipework inspection owes much to the ability to exploit its prismatic properties by generating non-divergent axisymmetric waves. Commercial systems based on this technology are now widely available.
The comparatively larger diameter typical of storage tanks render the generation of axisymmetric wave modes impracticable (a ring of transducer spanning the entire circumference would be necessary). Current developments of Lamb waves for the tank application are therefore limited to divergent signals. The loss of signal strength due to beam spread is a serious drawback and therefore there are presently no commercial Lamb wave monitoring systems for tanks.
The concept behind this project is that by generating structural vibrations with a longer wavelength using a lower actuating frequency, scattering can be reduced leading to much longer testing ranges. The approach used in this project differs from conventional dynamic testing because piezoelectric actuators rather than electromechanical actuators generate the vibrations. Consequently, the amplitude of vibrations generated and detected is much smaller. Such waves are easier to generate on large structures such as storage tanks.
Long wavelengths have the advantage that welds and other features do not affect them significantly. Furthermore, there is no attenuation due to beam spread as the technique effectively employs a standing wave. Structural dynamic testing involves the harmonic vibration of the whole structure at frequencies that are dependent on size and geometry. To date, the sensitivity of structural vibration based testing has been poor because of the comparatively high amplitudes involved. This research will establish whether lower amplitude vibrations give better sensitivity to structural change.
- To investigate by experimentation the possibility of generating and measuring low amplitude low frequency structural harmonic vibrations in a model storage tank using piezoelectric transducers.
- To assess the potential for condition monitoring of structural change in storage tanks using low amplitude low frequency structural harmonic vibrations.