Ultrasonic methods of Non-Destructive Testing (NDT), use beams of mechanical waves (vibrations) of short wavelength and high-frequency, transmitted from a small probe and detected by the same or other probes. Such mechanical waves can travel large distances in fine-grain metal, in the form of a divergent wave with progressive attenuation.
The frequency is in the range 0.1 to 20 MHz and the wavelength in the range 1 to 10 mm. The velocity depends on the material and is in the range 1000-6000 m/s.
Ultrasconic techniques detects internal, hidden discontinuities that may be deep below the surface. Transducers and coupling wedges are available to generate waves of several types, including longitudinal, shear and surface waves. Applications range from thickness measurements of thin steel plate to internal testing of large turbine rotors.
Most non-porous, resilient materials used for structural purposes (steel, aluminium, titanium, magnesium and ceramics) can be penetrated. Even large cross-sections can be tested successfully for minute discontinuities.
Time-of-Flight Diffraction – TOFD – is an ultrasonic technique which measures the time of flight of a pulse as it travels from a transmitting probe to a receiving probe. Divergent beams are used and it is necessary to scan the TOFD probe pair over the flaw for the technique to function correctly.
What distinguishes the technique from a standard pitch-catch configuration is the D which stands for diffraction. The technique relies on the detection of the diffracted sound wave which is generated from both the top and bottom edges of a planar defect.
The time of arrival of the diffracted signals from the flaw tips is measured with respect to the probe firing time. Time measurement can be done to great accuracy. The combination of this accuracy and the fact that the scattering source is the flaw tip forms the basis of a highly accurate sizing technique.
Phased array is the name given to a special type of ultrasonic probe.
An array is a group of transmitters, receivers or transmitter/receivers, generally called array elements. When used as a transmitter, firing the elements at different times can lead to interference between the sound waves produced by each individual element. This interference can be both constructive (waves add together) and destructive (waves cancel).
It is this interference which gives the array probe its main advantage – the ability to change beam shape and angle depending upon the timing at which elements are fired. When an array is used as a receiver, the difference between the times at which a pulse arrives at each array element contains information about the location of the pulse source.