Published on 01-Nov-2023

Ultrasonic Testing-Principle, Advantages and Disadvantages

Ultrasonic Testing-Principle, Advantages and Disadvantages

Did you know that the ultrasonic testing principle has applications beyond the medical field? Indeed, its significance extends to various industries.

Ultrasonic testing inspection is a widely used NDT method that plays a crucial role in various industries, such as manufacturing, construction, aerospace, and oil and gas, focusing on the ultrasonic flaw detector and transducers, and delving into the procedure to give readers a clear understanding of its significance and applications.

What is Ultrasonic Testing?

The ultrasonic testing principle serves as a Non-destructive Testing procedure extensively employed in contemporary industries. There are many ultrasonic testing advantages that are highly beneficial approach for detecting material discontinuities.

Just like in healthcare and underwater navigation, ultrasonic testing plays a vital role in ultrasonic testing testing equipment, ultrasonic sound test, ultrasonic metal testing, ultrasonic inspection, and detecting anomalies. In healthcare, gynecologists employ it to check for malformations in pregnant women, while on ships, it detects items beneath the water's surface.

To inspect materials and identify faults or characterize the material, high-frequency noises, commonly known as ultrasonic waves, are sent through the object or material. In typical ultrasonic tests, short ultrasonic pulse waves with center frequencies ranging from 0.1 to 15 MHz, occasionally up to 50 MHz, are delivered into materials for characterization and internal fault identification.

For instance, ultrasonic thickness measurement is a frequently encountered application that checks the thickness of the test subject, such as monitoring pipeline corrosion.

Why is the Ultrasonic Test Important?

The ultrasonic test detects material discontinuities by using high-frequency sound waves that propagate through the material. There are several advantages of ultrasonic testing, that help with the sound portion.

As it encounters a material with a different acoustical impedance, some of the sounds will reflect back. The sending device detects this reflected sound and displays it on the screen as an indication.

By knowing the sound's velocity through the material and the time of transit, the technician can calculate the distance to the indicated material with a change in acoustic impedance.

To conduct Ultrasonic Testing, a technician introduces sound waves into the object through a piezoelectric crystal transducer. This crystal's role is to convert electrical current into sound waves.

The two most common types of sound waves used in this process are compression waves, also known as longitudinal or straight beam waves, and shear waves, also known as transverse or angle beam waves.

The ultrasonic testing principle is of utmost importance as it finds extensive applications. One such example is ultrasonic thickness measurement, which is used to determine the thickness of an object, such as in assessing piping corrosion.

Being a non-destructive testing technology, ultrasonic testing excels at detecting errors and flaws without causing any material damage using various approaches like ultrasonic sound test and ultrasonic inspection. 

By regularly employing the ultrasonic testing principle to check for defects or corrosion growth, potential issues can be avoided. Early detection of damage can prevent machine parts, components, or entire structures from failing. Industries such as construction, aerospace, medical, automotive, metallurgy, and manufacturing widely embrace ultrasonic testing. 

Ultrasonic Testing Principle

A sound wave is sent into a test substance by a probe, as depicted on the left. There are two signs, one from the probe's first pulse and the other from the back wall echo.

As illustrated in the right-hand figure: A fault produces the third indication while also lowering the amplitude of the back wall indicator. The ratio D/Ep determines the depth of the flaw.

Ultrasonic Testing is based on a process known as ultrasonography, which is basically the transmission and reflection of sound waves. Ultrasonic waves are sent through the substance to be evaluated and then reflected back into a receiver in this procedure. This receiver then processes the signals, which creates a three-dimensional representation of the materials, allowing any flaws to be spotted.

The ultrasonic waveform is received in one of two ways. Reflection and attenuation techniques are the two types of ultrasonic testing.

  • Reflection Method

In the reflection method, the transducer accomplishes both the sending and receiving of pulsed waves (pulse-echo mode), as the "sound" is reflected back to the instrument. Reflected ultrasound is generated at interfaces, such as the object's back wall or a flaw within the object.

The diagnostic machine displays these data as a signal, with an amplitude representing the reflection's strength and distance representing the reflection's arrival time.

  • Attenuation Method

In the attenuation method, a transmitter delivers ultrasound through one surface while a separate receiver measures the quantity of sound reaching another surface after passing through the medium.

Imperfections or other factors in the area between the transmitter and receiver cause less sound to be transmitted, indicating their presence. The use of a coupling medium improves the process efficiency by reducing ultrasonic wave energy losses due to the separation between the surfaces.

Also Read, What is Pipeline Isometric Drawings

What is the Ultrasonic Testing Procedure?

Ultrasonic testing uses high-frequency sound energy to perform tests and measurements. It is employed to detect flaws and characterize materials using all major ultrasonic testing equipment.

A conventional ultrasonic testing and detection system consists of three functional components: the transducer, receiver, and display devices.

The receiver generates high-voltage electrical pulses, while a pulser drives a transducer to generate high-frequency ultrasonic energy. Sound energy propagates through the materials in the waveform. Calibration is achieved using an ultrasonic calibration block.

When using an Electromagnetic Acoustic Transducer (EMAT) for ultrasonic testing, couplant is not required. In the event of a discontinuity, some energy reflects from the flawed surface, which may result from cracks in the wave path.

The transducer converts the reflected wave signal into an electrical signal, displaying it on a screen. The procedure involves several essential steps to ensure accurate and reliable results. Here are the various steps.

1. Surface Preparation

Before conducting the NDT UT, the test surface must be adequately prepared. It involves cleaning the surface to remove any dirt, oil, or debris that could interfere with the inspection.

A clean surface ensures better contact between the transducer and the test material, improving the accuracy of the results.

2. Calibration

Calibrating the UT equipment is a crucial step to ensuring accurate defect detection. During calibration, the instrument is set to specific parameters, such as sound velocity and gain, using reference samples with known defects or features. This process establishes a baseline for the inspection, allowing the operator to interpret the results accurately.

3. Coupling Media

For contact UT, a coupling medium is used to enhance the transmission of ultrasonic waves between the transducer and the test material.

Common coupling media include water, oil, or gel, depending on the application and material being inspected. The coupling medium eliminates air gaps that could hinder sound wave transmission, ensuring effective inspection.

4. Transducer Placement

The transducer, which emits and receives ultrasonic waves, is placed on the test surface with the help of the coupling medium. Proper transducer placement is crucial for obtaining reliable data.

For contact UT, the transducer is placed directly on the surface, while for immersion UT, the test material is immersed in a liquid couplant.

5. Scanning Techniques

Scanning involves moving the transducer over the test surface in a systematic pattern to cover the entire area of interest. The operator may use different scanning techniques based on the object's geometry and the type of flaw being sought. Common scanning techniques include:

  • Straight Beam Scanning

In straight-beam scanning, the transducer emits sound waves perpendicular to the test surface. This technique is used to detect defects oriented parallel to the surface, such as cracks or delaminations.

  • Angle Beam Scanning

Angle beam scanning involves tilting the transducer at a specific angle to the surface. This technique is used to detect defects at different depths within the material, making it suitable for inspecting welds and other components with oblique defects.

6. Data Collection and Analysis

During scanning, the ultrasonic flaw detector captures the data in the form of A-scans, which display the amplitude of the reflected waves over time.

The operator analyses the A-scans to identify any indications of defects or irregularities within the test material. Proper analysis and interpretation are crucial for making informed decisions regarding the material's integrity.

7. Reporting

After completing the inspection, a detailed report is generated, documenting the inspection results, any detected flaws, and their locations within the test material. This report serves as a valuable reference for future evaluations and maintenance decisions.

Also Read, Acceptance Criteria for Liquid Penetrant Testing

Applications of Ultrasonic Testing

1. Ultrasonic Testing of Welds

Ultrasonic testing's ability to pinpoint the exact location of a weld discontinuity is one of its most helpful features. This form of testing necessitates a high level of operator training and competence and the development and implementation of appropriate testing protocols.

This procedure can be performed on ferrous and nonferrous materials. It is typically appropriate for testing thicker parts accessible from only one side and can often reveal finer lines or plainer faults that radiographic testing may not be able to detect.

2. The Role of the Ultrasonic Flaw Detector

The Ultrasonic Flaw Detector is the heart of the UT equipment. It acts as a control unit, generating, sending, and receiving ultrasonic waves during the inspection process.

The flaw detector provides the necessary functionality to adjust parameters, interpret results, and display the data in a user-friendly manner.

  • Components of an Ultrasonic Flaw Detector

The Ultrasonic Flaw Detector typically consists of the following components.

  • Pulser-Receivers

Pulser-receivers are essential components used in ultrasonic flaw detection equipment. Ultrasonic testing is a Non-destructive Testing method that uses high-frequency sound waves to inspect materials for defects or irregularities. These defects could include cracks, voids, inclusions, or other structural issues that may affect the integrity of the material.

  • Display Screen

The display screen shows the ultrasonic waveform, A-scan, and other relevant data captured during the inspection. It allows the operator to interpret and analyse the results effectively.

  • Controls and Keypad

The controls and keypad enable the operator to adjust various parameters such as gain, frequency, and time settings. These settings play a crucial role in tailoring the inspection according to the material and test requirements.

3. Transducers: Contact and Immersion

Transducers are devices that convert electrical energy into ultrasonic waves and vice versa. There are primarily two types of transducers used in UT: contact and immersion transducers.

  • Contact Transducers

Contact transducers have a flat, smooth surface that requires direct contact with the test material. They are ideal for inspecting solid materials and structures with regular surfaces.

  • Immersion Transducers

Immersion transducers, as the name suggests, are placed in a liquid couplant that acts as a medium for transmitting ultrasonic waves into the test material. They are often used for inspecting large or irregularly shaped objects and offer a better signal-to-noise ratio.

Advantages and Disadvantages of Ultrasonic Testing

Advantages of Ultrasonic Testing

Ultrasonic testing offers numerous advantages in the manufacturing environment. Here are some of the advantages of ultrasonic testing.

  • It can be used to create images using automated systems, as it is highly portable.

  • It has excellent sensitivity, allowing it to detect very minute defects.

  • It is sensitive to both surface and subsurface continuity.

  • When using the pulse-echo method, only one side access is necessary.

  • It poses no threat to individuals near the testing facility and does not affect nearby equipment or materials.

  • It can be used for flaw detection and thickness measurements.

  • Ultrasonic inspection's penetration depth outperforms other NDT approaches for fault detection and quantification.

  • Ultrasonic testing requires minimal preparation.

  • It accurately determines the position of the reflector and estimates the sizes and shapes of materials.

  • Results are immediate, allowing for on-the-spot evaluation.

  • It can estimate the structure of alloys containing components with varying acoustic characteristics.

Disadvantages of Ultrasonic Testing

While ultrasonic testing has several advantages, it also comes with a few disadvantages as well. Here are a few disadvantages. 

  • Inspection of small, rough, extremely thin, and irregularly shaped materials is difficult.

  • ultrasound testing requires an accessible surface, and the dead zone in ultrasonic testing may lead to overlooking near-surface defects in thin steel plates.

  • Ultrasonic testing is challenging to evaluate with cast iron and other coarse materials due to limited sound transmission and considerable signal noise.

  • Ultrasonic testing can fail to detect linear faults oriented parallel to the sound beam, requiring more training and competence than other non-destructive procedures.

  • Reference standards are required for equipment calibration and fault characterization.

  • A coupling medium is frequently needed to enhance the passage of sound energy into the test specimen.

  • Ultrasonic testing is relatively more expensive than other techniques.


What is an Ultrasonic transducer?

Any device that transfers one form of energy into another is a transducer. Electrical energy is converted into mechanical vibrations (sound waves) using an ultrasonic transducer.

Sound waves are converted back into electrical energy. They are typically compact, hand-held assemblies that come in various frequencies and styles to meet specific testing requirements.

What is an Ultrasonic thickness gauge?

An ultrasonic thickness gauge is a device that emits sound pulses in a test piece and measures the time interval between them with extreme precision.

The gauge, which has been programmed with the sound velocity in the test material, uses that information along with the measured time interval to compute thickness using the simple formula [distance] = [velocity] multiplied by [time].

How accurate is ultrasonic thickness gauging?

Commercial ultrasonic gauges may attain an accuracy of +/- 0.001 mm in ideal conditions, with an accuracy of +/- 0.025 mm or greater in most common engineering materials.

The homogeneity of sound velocity in the test material, the degree of sound scattering or absorption, the surface quality, and the precision and care with which the instrument has been calibrated for the application at hand are all factors that affect accuracy.

Who uses ultrasonic gauges?

The assessment of remaining wall thickness in corroded pipes and tanks is a common use for ultrasonic gauges. The measurement can be completed quickly and easily without requiring access to the interior or emptying the pipe or tank.

The thickness of moulded plastic bottles and similar containers, turbine blades and other precision machined or cast parts, small diameter medical tubing, rubber tyres, conveyor belts, fibreglass boat hulls, and even contact lenses are all examples of vital applications.

What is an ultrasonic flaw detector?

Sound waves flowing through a material will reflect off imperfections like cracks and holes in predictable ways. An ultrasonic flaw detector is a device that generates and processes ultrasonic signals to produce a waveform display that a trained operator can use to locate hidden faults in a test piece.

The operator recognises a good part's characteristic reflection pattern and then searches for changes in that pattern that could signal faults.

What kind of flaws can you find with a flaw detector?

Ultrasonic defect detectors can detect and analyse a wide range of cracks, voids, disbonds, inclusions, and other issues that compromise structural integrity.

The size of the minor, detectable imperfection in a specific application is determined by the type of material being evaluated and the type of flaw being considered.

Who uses ultrasonic flaw detectors?

Ultrasonic flaw detectors are prominently used in critical safety and quality-related applications involving structural welds, steel beams, forgings, pipelines, and tanks; aircraft engines and frames; automobile frames; railroad rails; power turbines; and other heavy machinery; ship hulls; castings; and a variety of other critical applications.

What are the other types of instruments available?

Ultrasonic imaging systems use sound waves to scan the internal structure of a part, producing highly detailed images similar to X-rays. Cross-sectional images are created using phased array technology, initially developed for medical diagnostic imaging.

The aerospace industry and metalworking suppliers employ a major part of the ultrasonic testing principle to check for hidden faults in both raw materials and final items. Signal analyzers and ultrasonic pulses and receivers are used in several materials research applications.


Ultrasonic testing principle is a powerful and widely adopted non-destructive testing technique that provides valuable insights into the internal structure of materials. The UT equipment, with its flaw detector and transducers, plays a central role in this process, allowing for accurate and reliable inspections.

By following a well-defined procedure, professionals can detect flaws and ensure the integrity of critical components across various industries. For more details head to OnestopNDT.

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Application Notes