Guided wave testing

Guided wave testing (GWT) is a non-destructive evaluation method. The methodemploys acoustic waves that propagate along an elongatedstructure while guided by its boundaries. This allows thewaves to travel a long distance with little loss in energy. Nowadays, GWT is widely used to inspect and screen manyengineering structures, particularly for the inspectionof metallic pipelines around the world. Insome cases, hundreds of meters can be inspected from a singlelocation. There are also some applications for inspectingrail tracks, rods and metal plate structures. Guided wave testing (GWT) is a non-destructive evaluation method. The methodemploys acoustic waves that propagate along an elongatedstructure while guided by its boundaries. This allows thewaves to travel a long distance with little loss in energy. Nowadays, GWT is widely used to inspect and screen manyengineering structures, particularly for the inspectionof metallic pipelines around the world. Insome cases, hundreds of meters can be inspected from a singlelocation. There are also some applications for inspectingrail tracks, rods and metal plate structures. Although guided wave testing is also commonly known as guided waveultrasonic testing (GWUT) or ultrasonic guided waves (UGWs) or long range ultrasonic testing (LRUT),it is fundamentally very different fromconventional ultrasonic testing. The frequency used in the inspection depends on the thickness of the structure, but guided wavetesting typically uses ultrasonic frequencies in the range of 10 kHz to several MHz.Higher frequencies can be used in some cases, but detection range is significantly reduced. In addition, the underlying physics of guided waves is morecomplex than bulk waves. Much of the theoretical background hasbeen addressed in a separate article. In thisarticle, the practical aspect of GWT will be discussed. The study of guided waves propagating in a structure can betraced back to as early as the 1920s, mainly inspired by the fieldof seismology. Since then, there has been an increased effort onthe analytical study of guided wave propagation in cylindricalstructures. It was only in the early 1990s that guided wave testing wasconsidered as a practical method for the non-destructive testing of engineeringstructures. Today, GWT is applied as an integrated healthmonitoring program in the oil, gas and chemical industries. Unlike conventional ultrasonics, there are an infinite number ofguided wave modes that exist for a pipe geometry, and they can begenerally grouped into three families, namely the torsional,longitudinal and flexural modes. The acoustic properties of thesewave modes are a function of the pipe geometry, the material and thefrequency. Predicting theseproperties of the wave modes often relies on heavy mathematical modeling which are typicallypresented in graphical plots called dispersioncurves.