How does ultrasonic non-destructive testing work?
Fundamentally, the concept behind ultrasonic non-destructive testing (NDT) is quite simple. We generate a sound, send it out, and listen for the echo. By listening to the echo, we can learn about the objects the sound bounced off of. Sonar, as used by bats or vessels at sea, is one form of this. Ultrasonic NDT for structural analysis or medical imaging follows a similar concept, but usually at much higher frequencies – typically in the MHz range. Such high frequencies produce acoustic waves with very small wavelengths, which allows you to resolve very small structures or defects; these can be cracks, fouling layers on a filtration membrane, damage to the meniscus in your knee, or corrosion in a pipe.
The sound waves are generated by an ultrasonic transducer, the core of which is a piezoelectric element. Piezoelectric elements are interesting materials because when you apply a voltage to them, they physically expand and contract. So, if you put a voltage pulse across the piezoelectric element, it will expand and contract, displacing the material next to it and sending out a wave.
Conversely, when an external wave impacts the piezoelectric element, a voltage is generated and can be measured. This allows the element to function both as a generator and as a detector of sound.
If the piezoelectric element is used to send out a pulse, the pulse will reflect off of interfaces with other materials, and the echo may return to the transducer. Upon returning, the echo itself (a pressure wave) imposes a mechanical stress on the element, generating a terminal voltage. This simple configuration is called an A-Scan. A COMSOL simulation is shown in Figure 3, where a pressure pulse is generated by a transducer; it travels through a plastic block (called a delay line), through an aluminium wall and into water. This might represent, for example, a reactor system for an industrial chemical process.
Recording returning echoes is a technique that is often used for calculating the distance of objects below the surface, such as for measuring the wall thickness here. If you know the speed of sound in the material and the return time of the echo, it’s easy to calculate the distance.
The reason such echoes occur is because of differences in a material property called the acoustic impedance. In an isotropic material, this material property is simply equal to the product of the density and the speed of sound. At locations where there is an interface and a difference in the acoustic impedance, some portion of the wave will be transmitted, and some will be reflected. The part that is reflected hits the ultrasonic transducer and is recorded as a voltage pulse.
When an acoustic wave is incident on a surface at an angle, things can get a bit complicated, with reflections, transmissions, and the transformation of waves from longitudinal waves to shear waves. That’s a topic for another day.
The voltage vs. time shown in Figure 3 is a representation of Ultrasonic Time-Domain Reflectometry at a single point (A-scan). Expanding upon this concept, you can characterize wall thickness (or any other buried features) along a path, by simply moving the transducer left/right (B-scan), or over a surface by moving the transducer left/right and up/down (C-scan). The surfaces, of course, don’t have to be flat; many transducers can be outfitted with delay lines that have a curved surface, allowing fitting to pipes of different diameters, for example. The refraction due to the curved surface is then accounted for in signal processing.
Scans can be carried out by physically moving a single-element transducer, or alternatively, with a single sensor head containing an array of transducers mounted at a fixed location. By correctly timing when each element in the array triggers a pulse, the series of waves emitted will constructively and destructively interfere, forming a beam in a specified direction. This is called beam steering and is performed by phased-array transducers; it is powerful technique and allows the user a large degree of flexibility in performing maintenance surveys. Indeed, most medical imaging systems are built on a form of this.