Hand using sensor to conduct nondestructive testing on a surface, a machine with a screen is in the background.

Ultrasonic Testing: A Versatile Method for NDT

Discover the advantages of the ultrasonic testing (UT) method, understand the basic principles of UT, and explore the variety of techniques for applying this highly adaptable method in nondestructive testing across industries.

What Is Ultrasonic Testing and How is it Used in NDT?

Ultrasonic testing is an NDT method that uses high-frequency sound waves to detect and measure discontinuities in industrial components. Similar to naval sonar and medical ultrasound, industrial UT operates on the principle of sending sound waves into a material and analyzing the returned echoes to gather information about the internal structure of the test part.

The beauty of this method is that it can be applied through a variety of techniques, making it useful in many industries and environments.

A skilled NDT specialist uses field experience and knowledge to apply the right technique for the job—and can communicate findings through a variety of reporting formats.

The primary objectives of UT in industrial applications include:

  • Measuring Thickness: UT is commonly used to measure the thickness of a part by comparing the ultrasonic echoes from the front surface to the back surface.

  • Detecting Internal Defects: By detecting the returned signal when a discontinuity interacts with the ultrasonic wave, UT helps identify and measure internal defects.

  • Stress Measurement: UT can be used for stress measurement, such as bolting stress measurement and residual stress measurement, which are unique applications of this method.

Watch this video to see how Jesse Groom, a Level II NDT technician, uses phased array UT to create a 3D model that clearly demonstrates the severity of a discontinuity.

Advantages and Limitations of Ultrasonic Testing in NDT

UT is a versatile method applicable in many industries, including manufacturing, aerospace, oil and gas, and more. Its ability to perform volumetric inspections and detect flaws that are not visible on the surface makes it invaluable for ensuring the integrity and safety of equipment and structures.

Advantages of Ultrasonic Testing

  • Volumetric Inspection: Unlike surface-based methods like liquid penetrant testing (PT) and  magnetic particle testing (MT), UT can detect internal discontinuities.

  • Safety: Unlike radiographic testing (RT), the ultrasonic waves used in UT are harmless to personnel in the testing area, eliminating radiation safety concerns.

  • Sensitivity: UT is highly sensitive to defects critical to structural integrity, particularly planar discontinuities oriented parallel to the test surface.

  • Versatility: UT can detect surface, near-surface, and subsurface discontinuities, making it suitable for a wide range of applications.

  • Portability: UT equipment is portable, allowing for inspections in various environments and locations.

  • Speed: UT provides rapid inspection results, supporting time-sensitive projects.

  • Discontinuity Evaluation: Advanced UT techniques can help inspectors determine the size and severity of discontinuities to support informed decision making.

Limitations of Ultrasonic Testing

  • Complex Geometries: Inspecting parts with irregular shapes or complex geometries can be challenging with UT.

  • Surface Preparation: UT generally requires good surface contact between the transducer and the material, calling for careful surface preparation.

  • Requires Couplant: A gel or liquid medium is required to provide an interface between the transducer and the material, which can be a logistical challenge in some situations.

  • Material Properties: UT may be less effective with materials that scatter or absorb sound such as concrete or stainless-steel castings or nonelastic materials such as rubber and soft plastics.

  • Discontinuity Orientation: Certain orientations of discontinuities may pose detection challenges and require measurement with additional methods.

  • Training and Experience: UT requires a high level of expertise. Technicians need to be familiar with physics and confident in their ability to analyze data and interpret results. This makes UT training and certification essential.

The results from UT are an important source of information for decision making when it comes to equipment repair and replacement planning. Because a variety of advanced UT techniques are in use today, up-to-date personnel qualification and certification is critical for NDT practitioners.”

Huidong Gao
UT Level III

How Ultrasonic Testing Works: Basic Principles

In the UT method, ultra-high-frequency sound waves are introduced into the part being inspected. Technicians place an ultrasound transducer (or probe) on the material, typically with a layer of liquid or gel called couplant to facilitate sound wave transmission into the part.

The transducer converts electrical impulses into sound waves, then converts returning sound back into electrical impulses that can be displayed visually on a screen.

When sound waves encounter a reflector—a material with different density and acoustic velocity—they bounce back to the transducer. By analyzing the time and amplitude of these echoes, skilled operators can determine the distance to the reflector and identify the type of discontinuity, such as slag, porosity, or cracks in a weld.

NDT practitioners leverage a comprehensive understanding of various wave types, transducer types, and display options, enabling the UT method to be flexible across many kinds of requirements, environments, and applications.

Sound Wave Interaction with Materials

The sound waves used in UT for industrial applications are beyond the range of human hearing, often exceeding 1 MHz to ensure precise measurements. When these sound waves penetrate a material, they interact with any internal discontinuities—such as cracks, porosity, or inclusions—and reflect back to the transducer.

UT operates on the principles of sound reflection, refraction, and penetration to accurately locate and identify discontinuities within the material.

Reflection

Similar to light reflecting off a mirror, sound waves reflect when they hit a material boundary or internal discontinuity.

Refraction

Comparable to a straw appearing bent in water, sound waves change direction when not perpendicular to the surface, described by the mathematical formula known as Snell’s law.

Penetration

Like light passing through glass, sound waves can penetrate materials, transferring energy into the second medium.

Wave Modes in UT

By using different types of transducers, NDT practitioners can create ultrasonic sound waves that propagate through materials in different modes, each with unique characteristics and applications.

Longitudinal Waves (L-mode): Particle motion occurs in the same direction as wave propagation. These waves have the highest velocity, longest wavelength, and can travel through solids, liquids, and gases.

Shear Waves (T-mode): Particle motion is perpendicular to wave propagation. Shear waves travel only through solids and are highly sensitive, making them ideal for inspecting welds.

Surface Waves (Rayleigh Waves): These waves have elliptical particle motion and are confined to the material's surface. They are effective for inspecting surface discontinuities.

Plate Waves (Lamb Waves): Similar to surface waves but used for thin materials and bonded composites. They are used in pitch-catch techniques for inspecting over long distances or specific surface issues.

A diagram showing two types of ultrasonic waves: longitudinal and shear waves. The illustration depicts a test object with two transducers, one directly emitting longitudinal waves and another angled with a plastic wedge to emit shear waves. Arrows indicate the direction of particle motion for both wave types, with longitudinal waves moving vertically and shear waves moving at an angle.

UT Testing Modes

Ultrasonic testing relies on directing soundwaves with transducers—devices made of piezoelectric materials that convert electrical impulses to sound waves. With an ultrasonic instrument to initiate electrical impulses, the UT technician uses transducers to direct sound waves through the material in one of two modes.

Pulse-Echo Mode

  • A single transducer sends and receives ultrasonic waves.

  • Discontinuities are identified by the amplitude and location of the echo received back from the material.

Through-Transmission Mode

  • Uses two transducers: one to send and one to receive the ultrasonic signal.

  • Discontinuities are detected by a partial or total loss of the received signal.

A technician performing an ultrasonic testing inspection on a large welded pipe. The technician is using a handheld transducer connected to a portable ultrasonic testing device with a screen displaying data. The setting appears to be outdoors, with industrial structures visible in the blurred background.

UT Data Display

UT systems display data in various formats to aid in analysis.

A-scan

Displays data on an X-Y grid, showing the time taken for sound waves to travel and return, and the energy received.

B-scan

Displays data in a sliced plane a profile view that enables inspectors to determine the approximate dimensions of discontinuities.

C-scan

Offers a top-down view, layering data collected over time to associate with specific locations.

A technician performing ultrasonic testing on a large welded pipe outdoors. The technician, wearing a blue jacket, is using a handheld transducer connected to a portable ultrasonic testing device. The background shows a clear blue sky with light clouds.

Ultrasonic Testing Techniques Used in NDT

Ultrasonic testing (UT) employs a variety of techniques to detect and measure discontinuities in materials. These techniques can be categorized based on how the ultrasonic waves are introduced into the material and the specific modes and advanced methods used to enhance flaw detection and measurement.

Techniques can be applied in three ways.

Contact Testing

  • Straight beam testing uses a transducer directly placed on the test material to send and receive ultrasonic waves.

  • Angle beam testing uses a transducer with a wedge to introduce sound waves at an angle, commonly used for inspecting welds.

  • Surface wave testing employs a transducer to generate surface waves for inspecting surface flaws.

Immersion Testing

  • Conducted in a water tank with waterproof transducers.

  • Used in industries such as manufacturing to perform noncontact inspections of large metal and composite parts.

Air-Coupled Testing

  • Uses air as the medium for ultrasonic wave transmission.

  • Ideal for inspecting composite structures where contact or immersion techniques are not feasible, although lower frequencies are used due to attenuation losses.

Advanced UT Techniques

Phased Array Ultrasonic Testing (PAUT)

  • Employs multiple elements in a transducer to form and focus the beam of an ultrasonic wave.

  • Provides the ability to record data and display a discontinuity image in three dimensions, increasing the reliability of inspections.

Time of Flight Diffraction (TOFD)

  • Uses two transducers, one to transmit and one to receive.

  • Measures tip-diffracted signals, providing high sensitivity and accuracy for discontinuity sizing.

  • Produces a two-dimensional image, so results may require verification with another technique to verify discontinuity location.

A technician wearing protective gloves is adjusting a time of flight diffraction (TOFD) ultrasonic testing device attached to a rusty metal surface. The device includes multiple sensors and cables for performing detailed inspections of the material.

A technician conducts the time of flight diffraction (TOFD) technique on a metal surface.

Full Matrix Capture (FMC) + Total Focusing Method (TFM)

  • Captures data from multiple elements and performs synthetic focusing in post-processing.

  • Allows flexibility in selection of a focal point, providing a highly representative image of discontinuity size and location.

Electromagnetic Acoustic Transducer (EMAT)

  • A noncontact, couplant-free technique using interacting magnetic fields to induce ultrasonic waves into the test object.

  • Suitable for high-temperature and high-speed inspections, though it requires special instrumentation and a larger-than-usual transducer probe.

Guided Wave Applications

  • Ultrasonic waves can detect discontinuities at the material surface, away from the location of the transducer.

  • Similar to fiber-optic guided wave inspection but provides the advantage of enabling inspection in remote and non-accessible areas, such as buried pipelines.

  • Complexity in signal patterns requires specialized data-analysis training.

  • Multiple variables in guided wave interaction with a discontinuity increase the margin of error, making it essential to verify results with additional techniques for accurate sizing.

A guided wave testing device is attached around a rusty pipeline outdoors. The device features multiple sensors and cables wrapped around the pipe to inspect for defects or corrosion. The surrounding area is grassy with exposed soil and vegetation.

Guided wave (GW) testing device in action, inspecting a rusty pipeline for defects and corrosion in an outdoor environment.

Add Ultrasonic Testing Certification to Your Qualifications

ASNT certifications enable you to become a qualified Level II or Level III in UT.

What Certification Is Right for Me?

Application of Ultrasonic Testing in NDT Across Industries

Ultrasonic testing (UT) is used in the testing of nearly all solid materials, from fine-grained aluminum, steels, and alloys to composites and plastics. Its ability to detect and measure internal discontinuities as well as perform precise thickness measurements makes it invaluable for ensuring the integrity and safety of critical components and structures.

These characteristics make it a versatile nondestructive testing (NDT) method that’s widely used across a range of industries.

Energy

In the energy industry, UT is widely used in oil, gas, and petrochemical sectors for assessing corrosion damage in pressure equipment and piping; detecting and measuring cracks in in-service equipment to prevent failures; and inspecting welds during the fabrication of new structures to ensure they meet safety and quality standards. UT is crucial in the power generation sector for the inspection of pressure equipment, ensuring the integrity and safety of boilers, turbines, and other critical components.

Learn More
"A composite image showcasing various energy sources: solar panels in the foreground, oil pump jacks in the middle ground, and wind turbines and a power plant in the background. The scene illustrates the diversity of energy production methods at sunset.

Aerospace

NDT inspectors in the aerospace industry use UT to detect internal defects in equipment such as landing gear and to check critical components of aircraft engines to ensure they meet stringent safety standards.

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A technician performing maintenance or inspection work on the landing gear of a large commercial airplane inside an aircraft hangar. The scene is illuminated with a blue tint, highlighting the aircraft's engines and the structural details of the hangar.

Transportation

NDT inspectors in transportation rely on UT to ensure structural soundness of everything from rail tracks to axles, roads, and ships.

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A modern high-speed train moving swiftly through a train station at sunset. The motion blur effect emphasizes the train's speed, with vibrant colors in the sky and station lights creating a dynamic and futuristic atmosphere.

Manufacturing

UT is essential in metal manufacturing for checking thickness and detecting discontinuities during the production of metal products, including plates, tubes, pipes, rods, and forged components. It ensures the quality and reliability of these materials before they are used in further manufacturing or construction processes.

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A modern manufacturing facility with robotic arms working on an automated assembly line. The scene is well-lit with blue overhead lighting, showcasing advanced machinery and precision engineering in a clean, industrial environment.

Infrastructure

In infrastructure and construction, NDT practitioners use UT for performing quality control of components such as steel beams and concrete structures and inspecting welds to ensure they meet required safety and quality standards.

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A large infrastructure project featuring a highway under construction. Several cranes are positioned along the unfinished sections of the elevated roadway and bridge. The scene is set on a clear, sunny day with blue skies and some scattered clouds.

Example: UT in the Real World

In petrochemical plants, NDT practitioners use UT daily to measure the thickness of pressure vessel walls and piping, looking for signs of corrosion and wear. In these plants, cracks can be produced by routine service and maintenance of systems, and UT is essential for detecting those cracks. Advanced UT techniques are used widely for new weld inspection and quality control.

NDT activities such as these are required by the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel codes (BPVC Section VIII) and the American Petroleum Institute (API) code for pressure vessel inspection (API 510) and code for piping inspection (API 570). Because the codes are often adopted at the state level and treated similar to a law, employers in oil, gas, and petrochemical sectors must ensure that UT technicians are specially qualified to perform these inspections.

A technician performing ultrasonic testing on a large industrial pipe. The technician is wearing protective gear, including gloves and a helmet, and is using an ultrasonic device to inspect the pipe for defects or irregularities.

Deeper Learning About Ultrasonic Testing

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Ultrasonic Testing (UT) Courses

Advance your skills and knowledge with courses and webinars on ultrasonic testing.

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Programmed Instruction Series: Introduction to NDT

A comprehensive self-study resource for Level I and II candidates covering 16 NDT methods. Includes theory, principles, applications, quizzes, and an online interactive training program.

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Nondestructive Testing Handbook, Vol. 7: Ultrasonic Testing (UT), 3rd ed.

A practical guide on UT for Level II and III inspectors, featuring industry-specific applications, material characterization, digital processing, and metric units.

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