Ultrasonic Testing: What You Need to Know

When the need arises to inspect, test, and evaluate a material, component, or assembly without compromising its serviceability, non-destructive testing (NDT) in the best approach.

Non-destructive testing is a set of inspection methods and procedures used to interrogate parts with minimal intervention and with the aim of preserving their condition.

Ultrasonic testing (UT) is one of the most widely used branches of NDT that uses high-frequency sound to reveal hidden flaws and measure thickness without harming components.

In this article, we will explore ultrasonic testing, its history, mechanics, advantages, and limitations. Keep reading to learn more…
 

What is ultrasonic testing?

At its simplest, ultrasonic testing is a fast, flexible non-destructive evaluation method that uses high-frequency sound waves to examine materials for internal discontinuities, measure dimensions and, in some cases, infer material properties. 

The technique operates on the same physical principle as sonar and medical ultrasound. A transducer sends a short burst of ultrasound into a test object and records the returning echoes. Variations in the returned signal (timing, amplitude, and frequency content) expose the presence, location and, to an extent, the nature of anomalies.

UT spans a wide frequency range, usually between 0.1 MHz and 15 MHz, though specialised laboratory or microstructural studies can extend up to 50 MHz or more. Lower frequencies penetrate further but resolve less detail, while higher frequencies resolve smaller features but attenuate more rapidly.

So what makes UT a preferred evaluation method? Its primary industrial objectives are threefold:

• Detection - UT locates internal defects such as cracks, voids, delaminations, and inclusions. 
• Thickness measurement - UT provides reliable thickness readings through pulse-echo time-of-flight measurements, a routine requirement in corrosion monitoring and fabrication control. 
• Specialised measurements - adaptations of ultrasonic methods can assess residual stress, measure bolt preload and characterise layered structures.

UT is inherently volumetric, inspecting the interior rather than only surfaces. It typically requires access to one surface and a means to transmit the ultrasonic wave effectively into the material, such as a liquid or gel couplant, although non-contact methods also exist. 

Success depends on selecting the right wave mode, frequency and technique for the material and geometry in question; proficiency, therefore, combines equipment knowledge with practical experience.
 

A history of ultrasonic technology

The history of ultrasonic testing is one of curiosity and iteration.

The earliest seeds were sown by developments in underwater acoustics. Sonar systems, developed and refined for detecting objects beneath the sea, showed that sound could reveal hidden structure across a medium that was otherwise opaque to sight. 

This insight led researchers and inventors to ask whether the same principle could be applied to solids and biological tissues.

During the late 1920s and early 1930s, a number of investigators published experimental work and patents exploring ultrasonic interrogation of solids. 

In 1929, Soviet physicist M. Sokolov reported experiments studying ultrasonic methods for detecting metal inclusions and boundaries, while other experimenters explored using paired transducers to transmit and receive through a solid specimen. In 1931, Mulhauser obtained a patent that described detecting flaws using two separated transducers. 

These early efforts established two core ideas: 

• High-frequency sound propagates through solids, that discontinuities scatter or reflect energy.
• Measuring those responses could provide useful information about unseen features.

A crucial advance was the shift from continuous-wave approaches to pulsed operation. Continuous-wave methods can detect changes in transmission but are limited in locating depth. 

Pulsed systems, by contrast, send a short broadband burst and measure the time taken for echoes to return. Time-of-flight information immediately provides a depth coordinate, opening the door to practical thickness measurement and internal flaw sizing.

In the 1940s American researchers formalised pulse-echo techniques for solids. Firestone and Simons produced significant work that led to patents and practical devices. Firestone’s 1942 patent described transmitting high-frequency vibrations into a workpiece and determining the time interval of reflected vibrations to locate flaws. 

The earliest instruments presented their signals in A-mode, a simple trace showing amplitude versus time on an oscilloscope. This was transformational. A “blip” on the screen could be correlated directly with the presence and depth of a reflector inside the material.

The introduction of B-mode extended capability by rendering echoes as a two-dimensional, grey-scale image. A-mode remained invaluable for simple thickness checks and rapid field work, while B-mode brought improved visualisation for more complex geometries and, eventually, for rudimentary imaging of defects.

As instruments improved through the 1960s and early 1970s, the sensitivity of UT increased to the point where ever-smaller flaws could be detected.  But a surge in rejection rates and an impractical intolerance for minute, non-critical anomalies presented a problem.

The rise and maturation of fracture mechanics offered a solution. By predicting whether a discontinuity of a given size would remain stable under expected loads, engineers could shift from the uncompromising “no defect” stance to a “damage tolerant” design philosophy. 

This approach accepted existing flaws, provided they are characterised accurately and remain below critical dimensions for the expected loading conditions.

Implementing damage tolerance required quantitative information. That requirement fuelled the development of quantitative non-destructive evaluation (QNDE), an engineering and research discipline that blends signal-processing methods, calibration protocols, fracture mechanics, and reliability assessment. 

QNDE elevated ultrasonic practice. Waveform analysis, beam modelling, and calibration standards became as important as the hardware itself.
 

How does ultrasonic testing work?

Ultrasonic testing relies on a few physical principles, but applying them properly in the workshop or the plant requires practical judgement and an understanding of how waves behave in real materials.
 

Transducers and piezoelectric materials 

At the core of any ultrasonic instrument is the transducer, within which sits a thin crystal or ceramic element with piezoelectric (the ability to generate an electric charge when mechanical stress is applied) properties.

When an electrical pulse is applied, the element changes shape and produces a short burst of mechanical vibration. When a returning mechanical vibration strikes the element, it generates an electrical signal. 

Common materials include natural quartz and engineered piezoceramics such as lead zirconate titanate (PZT). Transducer design (element thickness, backing material, matching layers and housing) determines the centre frequency, bandwidth and sensitivity, and therefore the trade-off between resolution and penetration.
 

Couplant and energy transfer 

Sound cannot jump efficiently from a transducer into a solid if an air film intervenes. 

The acoustic impedance mismatch causes reflection and severe signal loss. A couplant (a light oil, glycerol-based gel or water) fills the microscopic gaps between probe face and part surface to allow efficient transmission. 

The choice of couplant matters. Viscosity, wetting behaviour, and temperature stability affect signal fidelity and inspector convenience. For periodic or remote inspections, water squirter systems or immersion tanks are used to maintain consistent coupling, although non-contact alternatives exist for cases where couplants are undesirable.
 

Pulse-echo timing and flaw localisation 

Most UT relies on time-of-flight information. A short broadband pulse is sent into the part, where the instrument measures the time elapsed until an echo returns. 

Knowing the wave speed in the material allows conversion of that time to a distance. For example, the distance to a reflector and back is given by the product of half the time-of-flight and the known sound speed.

Accurate depth sizing requires correct velocity data, which can vary with alloy composition, heat treatment and temperature. Hence, good practice includes measuring the longitudinal and shear wave speeds in representative test pieces or using calibration blocks traceable to standards.

 

Reading an A-scan 

An A-scan displays received signal amplitude as a function of time.