ATENCIÓN Y SERVICIO AL CLIENTE

Since the potential for guided wave inspection of pipes was first realised, a number of researchers have been working on improving the technique. The technique started as a simple screening procedure for relatively large corrosion flaws. However, recent research suggests that it can be used to provide more detail about the flaws that are detected and be used to find smaller flaws.

Different transduction methods including generalised solutions for variable applied surface tractions have been investigated byDitri & Rose[1992],Shin & Rose

[1999] and Li & Rose [2001a]. The issue of attenuation into the surroundings has also received some attention by authors such asKwun et al. [2004] and Longet al.

[2003].

One advanced technique that has received a lot of interest is the idea of fo- cusing the sound energy on predetermined regions of the pipe. Some of the earliest work on this topic involved the idea of using both non-axisymmetric modes and axisymmetric modes to inspect pipes [Shin & Rose,1998]. The idea of applying dif-

ferent amplitudes and delays to individual transducers was investigated by authors such asLi & Rose[2002], Rose et al.[2003], Barshinger et al.[2002]. Hayashi used the semi-analytical finite element method (SAFEM) to simulate focusing [Hayashi

et al., 2003a,2005a]. Since then a number of authors investigated focusing further

for different situations such as pipe size [Li & Rose,2006], viscoelastic coatings [Luo

& Rose, 2007] and beyond welds [Zhang & Rose, 2006]. The idea of ‘total focus-

ing’, where the focusing algorithm is applied at every location, was investigated by

Velichko & Wilcox[2009b] for relatively high frequencies with the aim of detecting

relatively small flaws. The idea of synthetic focusing, where an image of the pipe is formed using post processing techniques was also investigated byHayashi & Murase

[2005] and later by Davies & Cawley[2009].

Another topic of recent interest has been the application of guided wave in- spection to other structures. Much work has been done on the inspection of plates by authors such asAlleyne[1991] and this work is still on-going. For example, some recent work by Fromme et al. [2006] has looked at detecting a range of different flaw types in a plate and Li & Rose [2001b] has studied guided wave propagation in plates with the application to inspection in the nuclear industry. Railway rails are also a potential candidate for guided wave inspection and have received quite a lot of recent interest. Gavric [1995] used semi-analytical finite element analy- sis to calculate dispersion curves in rail for relatively low frequencies (< 5kHz). Later, Sanderson & Smith [2002] published a 3D finite element analysis technique for calculating dispersion curves for prismatic structures of any cross section. The 3D method was used to calculate dispersion curves for rails at typical guided wave testing frequencies of up to 50kHz. Around the same timeRoseet al.[2002] carried out some experimental studies into the ability of guided waves to detect a saw cut in a rail, andWilcoxet al.[2002] published a 2D finite element analysis method for calculating approximate dispersion curves for structures of any cross section. Then,

Hayashiet al. [2003b] published a semi-analytical finite element method for disper-

sion curve calculation for structures of any cross section, giving a rod and a rail as an example andMaceet al.[2005] published a similar technique termed Waveguide FE. The inspection of rail was investigated further by others such asWilcox et al.

[2003] who used a combination of experiments and finite element analysis to study the reflections from a range of flaws in rails, and Hayashi et al. [2007] who devel- oped methods for detecting flaws in the bottom edge of the rail. Other structures have also been investigated such as grouted tendons and bolts [Beard,2002] and the technique has been applied to steam generator tubing [Rose et al.,1994a,b]. Kwun

in various structures including a square rod and a seven wire strand.

A number of authors such asPeiet al.[1996],Leonard & Hinders[2003] and

Leonard & Hinders[2005] have also investigated the use of higher frequencies and the

use of tomographic techniques to detect flaws through imaging sections of pipe over relatively short distances. The idea of using time reversed, experimentally measured signals as inputs to a numerical model has also been investigated by Leutenegger

& Dual[2004]. This required the use of a 3D laser vibrometer to accurately record

displacements on the surface of the pipe. Wavelets have also been investigated to either give an automatic flaw or no-flaw assessment [Tucker et al., 2003] or to increase the signal to noise ratio [Siqueiraet al.,2004;Mallett et al.,2007].

Finally, a useful textbook on the topic of ultrasonic waves including some information on guided waves was published byCheeke[2002] and Rose also published a number of useful review papers [Rose,2000,2002b,a;Roseet al.,2007] and a text book on ultrasonic waves [Rose,1999].

In summary, a number of concepts have been developed that improve the inspection of guided waves in pipes and techniques such as focusing have enhanced the ability of guided waves to detect smaller flaws. However, the challenge of deriving a sufficient level of quantitative information on detected flaws in pipes still remains.