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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.
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.
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
Refraction
Penetration
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.
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.
UT Data Display
UT systems display data in various formats to aid in analysis.
A-scan
B-scan
C-scan
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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.
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 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.
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.
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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 and composite components to ensure they meet stringent safety standards.
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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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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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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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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.
Deeper Learning About Ultrasonic Testing
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