As an alternative method, wave mechanics may be used to analyze the boundary constrains and then describe the rule of wave propagation as possible wave modes in the plate. In this way, it is considered a guided wave, which may be as a simple to analyze as a single mode of propagation.
Propagation in Waveguide.
By definition, guided wave is a general term used to describe the wave propagation phenomenon where the guidance of a boundary plays a very important role. The structure in which a guided wave may propagate is called a waveguides. Many everyday structures are natural waveguides, for example:
- Surface of a billet, casting or forged structure
- Plate or thin sheet
- Bar or rod
- Tube or pipe
Therefore, several types of guided waves are used in nondestructive testing, namely:
- Lamb waves in plates, classified as either symmetric or asymmetric based on their displacement fields.
- Shear horizontal (SH) guided waves having particle motion only in the direction that is perpendicular to the wave propagation direction and parallel to the surface in a plate
- Wave modes in tubular structures in either the axial (longitudinal, torsional or flexural mode) or circumferential (lamb-type and SH-type) directions.
Dispersion as Main principle:
The key to guided wave analysis is to understand dispersion.
Dispersion menas the wave velocity is a function of frequency. Although this appears to be in conflict with the understanding of the constant velocities of bulk waves, it is a very important feature of guided waves analysis.
Because of boundary constrains, many guided wave modes may exist in a plate. Each mode is represented with a curve showing the relationship between wave velocity and frequency. The set of curves of possible guided wave modes plotted together is called a dispersion curve plot. Dispersion curves for a given material can be plotted to show the relationship of velocity to frequency and test-object thickness. Here, velocity can be either phase velocity or group velocity:
- Phase velocity: the speed at which a specific point or phase on the wave propagates through a test object
- Group velocity: the velocity measured form the propagation of the entire wave package
Advantages of guided wave. The main advantages of GW as an NDT method include the capability to propagate sound waves over a com probably long range with not need for scanning, as all data are acquired from a single probe position. in addition, GW may be used to inspect areas not easily accessible with other methods as well as thin structures with improved sensitivity over conventional UT.