TWI Industrial Member Report Summary 886/2007
By R Sanderson
Long range ultrasonic testing has opened up the possibility of plant and pipe inspection with minimum intrusion on the normal operation of the plant. Key aspects in the development of an effective guided wave inspection system are (i) a high signal to noise ratio (ii) defect sensitivity (iii) the length of propagation with minimum attenuation. Teletest® is a long range ultrasonic testing system for pipes that uses guided waves (with typical test frequencies of around 70kHz) to screen a length of steel pipe for corrosion defects from a single test location. Modelling and signal processing techniques have been developed at TWI that enable understanding and therefore control of these key aspects (Sanderson, 2003a).
Currently guided wave technology has been demonstrated successfully as a screening procedure. However, tests have shown that by phasing an array of exciters around a (ferritic) steel pipe, energy can be focused into specific regions (Sun et al, 2002). Because the energy is concentrated in one region, the sensitivity to defects potentially increases, making sizing and more exact positioning of corrosion a possibility. However, understanding of how to produce a focused beam was limited.
If the guided wave inspection technique can be transformed from a predominantly screening procedure to a more exact screening and sizing inspection technique, the cost and time required for inspection will be greatly reduced. In addition it may be possible to control the direction of the focused beam and 'steer' the beam around bends in pipes. This would open up the applicability of the inspection technique to currently inaccessible regions of pipework.
The current focusing work (Sun et al 2002) in the literature does not take into account the dispersive characteristics of the wave modes and therefore the range of applicability is limited. TWI therefore wishes to develop its own expertise in focusing guided waves from both a theoretical and experimental stand point.
- Find an effective technique for focusing guided waves into one region of steel pipe.
- Determine the applicability of the focusing technique for a range of pipe geometries.
- Assess the potential for sizing corrosion damage using techniques for modelling the focusing of guided waves.