Know Each NDT Method's Differences

With many different methods of nondestructive testing (NDT), it’s important to know the difference between them and how each method works in an inspection.

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Roger Engelbart, a member of the American Society for Nondestructive Testing board of directors, says the primary methods are: eddy current testing (ET), magnetic particle testing (MT), penetrant testing (PT), radiography testing (RT), ultrasonic testing (UT), and visual testing (VT).

“VT is not included in the specification for training and certification in NDT but is an important process in aviation maintenance,” continued Engelbart.

Engelbart explains the difference between each NDT method:

Eddy Current

Eddy currents are electrical currents within a conductor that are induced by a nearby alternating magnetic field. To produce eddy currents and employ eddy current testing, an alternating electrical current (AC) is driven through a test coil. This current produces a magnetic field perpendicular to the current's flow. By causing this magnetic field to interact with a conductor, by placing the test coil and test piece in close proximity, eddy currents are induced in the secondary conductor.

“The eddy current forms its own magnetic field, of opposite direction to the primary field, which interferes with the primary field. The interaction between the two magnetic fields causes impedance changes in the primary coil. The impedance changes are displayed on a screen for interpretation by an experienced technician or engineer,” described Engelbart.

A surface crack is detected by a rapid change in instrument response as the eddy current probe moves across the crack. The rapid response change results from a localized disruption of the eddy current and its secondary magnetic field due to the crack. The disruption of the secondary magnetic field induces a momentary change in coil impedance. A change in coil impedance alters the current through the coil, which is indicated by a change in instrument response.

“Eddy current is impacted by the loss of conducting material and a reduced secondary magnetic field by such features as curvature, reduced thickness, and proximity to edges,” Engelbart said.

Magnetic Particle

Magnetic particle testing can locate surface and near surface discontinuities in a variety of ferromagnetic product forms. This is accomplished by establishing a magnetic field within a part using electrical current, then applying magnetic particles to the surface while observing the particles’ behavior. When a discontinuity such as a crack in ferromagnetic material is encountered by lines of force, they attempt to stay within the material. Since the lines of force never touch, they tend to go under or around discontinuities.

“However, it is only when a sufficiently strong magnetic field has been applied that the lines of force are increased in number until they are forced out into the air. The magnetic leakage field (bridge) that is formed in the air attracts the magnetic particles and the accumulation of these particles makes the discontinuity more readily visible,” Engelbart said.

Subsurface discontinuities may be detected if they are large enough and the magnetic field generated is strong enough to cause a flux leakage. The applied particles may be in the form of a colored dry powder or fluorescent particles in a liquid carrier.

Penetrant Testing

Liquid penetrant testing detects surface breaking indications such as pores and cracks and includes two types of dye: fluorescent and visible. Fluorescent dye is used almost exclusively for most aerospace applications, as visible dye has insufficient sensitivity. The penetrating liquid dye is applied to the surface of a part that has been thoroughly cleaned and dried; the time that is allowed for penetration is called dwell time. After the excess dye is removed from the surface, dye that was trapped in the cracks is drawn to the surface, usually by use of a developer, revealing the cracks.

“PT is the easiest and least expensive method to use. It is typically used on nonferrous metals but may be used on ferrous alloys as well. It requires a surface that is free of coatings, dirt, and contamination. If the surface has received any abrasive treatment, it must be treated with an acid etch to remove any smeared metal that would close off surface-breaking flaws,” he said.

Radiography

Radiography uses the principles of photon absorption as the basis for performing examinations. When x-rays are directed into an object, some of the photons interact with the particles of the matter and their energy can be absorbed or scattered. Other photons travel completely through the object without interacting with any of the material's particles. The number of photons transmitted through a material depends on the thickness, density and atomic number of the material, and the energy of the individual photons.

“The transmitted photons interact with radiographic film to produce a photo-like image of the object under examination. Radiography may also be performed with non-film imaging. Computed radiography (CR) uses a storage phosphor imaging plate in place of the film; digital radiography (DR) utilizes a digital detector array in place of the film or the phosphor plate. The availability of an electronic image allows some adjustments to be made to the quality of the radiographic image without repeating the exposure,” Engelbart said.

Radiography is used less often than other methods in aviation maintenance due to the need for strict radiation safety protocols when performing a portable on-aircraft examination, he noted.

“Radiography will not detect planar discontinuities such as delaminations in composites; it is useful for core damage in honeycomb structures. It can also be effective for crack detection, but the x-ray beam must be correctly aligned with the suspected crack location to obtain the best possible image,” Engelbart said.

Ultrasonic

“Ultrasonics operates by sending high-frequency sound (above 20 kHz) through a part. The method functions in two modes: through-transmission ultrasound (TTU) and pulse echo (PE). TTU requires access to two opposite faces of a structure, and that structure must have no designed air gaps between those accessible surfaces,” Engelbart said.

TTU works by sending a stream of ultrasonic pulses from a transmitting transducer located on one side of the structure, through the structure to be detected by a receiving transducer on the opposite side.  TTU shows the location of a discontinuity but provides no information on its depth. Single-sided, or PE ultrasonic examination, uses a single transducer positioned on one side of the structure to transmit and receive sound. Unlike TTU, PE will provide depth information for a discontinuity.

“Much of the use of ultrasonics in aviation maintenance is in the PE mode, as it can be accomplished with portable equipment. Ultrasound does not penetrate air and the method depends on a coupling medium such as water, oil, or a light gel between the transducer and the surface of the structure to be examined,” he explained.

Visual Examination

Visual examination is the most basic method of NDT and is capable of finding a number of discontinuity types in both composite and metallic structures. Engelbart notes the keys to a good visual examination are the following:

• Adequate Lighting
• Magnification
• Accessibility/Orientation
• Examiner visual acuity and patience

“The unaided eye is usually the first approach used in visual inspection; this is especially true for in-service damage assessments during which a complete walk-around of the aircraft is the initial procedure. General visual inspection utilizes such aids as magnifiers, mirrors, hand measurement tools, and borescopes. Magnifying instruments may be useful in distinguishing fine features; some magnifiers may have a measuring lens attached, while in other cases a supplemental scale might need to be placed in the field of view to assist in determination of an approximates size of a condition,” he continued.

Adequate lighting may be achieved by locally illuminating the region of interest with a supplemental light source. Uniform lighting is strongly recommended; however, circumstances may limit light sources to flashlights, or floodlights. Light angle and viewing angle may also improve evaluation of the inspection region. This is especially true for metallic structures where surface reflectivity may be a factor in the quality and accuracy of the visual assessment.

“For interior structures and areas with limited access, a remote visual imaging (RVI) system may be needed – borescope, fiberscope, or video image scope that provides the image to be viewed on a video monitor,” Engelbart said. 

When to Use Each

Unless there’s specific damage location that is already known, the process begins with a visual walk around of the aircraft, looking indications of potential damage, said Engelbart.

“The first indication is the condition of the paint – chipping, cracking, indentation due to impact, or discoloration due to overheating,” he said. “Any of these indications prompts a closer visual examination to determine if there is a likelihood of subsurface or internal damage.”

Below is a list of defect/damage types and circumstances for each method:

ET – surface and shallow subsurface cracks in metallic structures, and metal loss due to corrosion.

MT – cracks in landing gear components.

PT – surface breaking cracks in metal structures due to impact.

RT – fractures of internal structural members, damage to honeycomb core in sandwich structures.

UT – disbands due to failed adhesive bonding (metal or composite), and delaminations of composites due to impact.

Table Of Contents

Introduction

In a world where safety and quality are paramount, industries rely on cutting-edge methods to ensure the integrity of structures and materials without causing any damage.

Non-destructive Testing (NDT) plays a pivotal role in ensuring the structural integrity and safety of various materials and components.

Now we will delve into the top 7 NDT Methods extensively used in different industries.

Before delving into specific methods, it's crucial to understand why NDT is essential.

It allows NDT Professionals to inspect materials without causing damage, making it an indispensable tool in industries such as manufacturing, construction, and aerospace.

What are the Industries where Non-destructive Testing is Used?

NDT could be used in almost every industry type as Visual Inspections occur around every workplace.

For a better understanding, here are some industries listed below that use NDT:

• Aerospace Mining
• Mineral oil & gas
• Chemical
• Construction
• Automotive Industry
• Biomedical devices
• Primary metals
• Marine

Importance of Non-destructive Testing

These are the primary reasons why Non-destructive Testing is adopted by so many industries globally :

1. Efficient:

NDT Techniques are considerably quick and easy when it comes to evaluating any element that assures safety and delivers high-quality performance.

2. Safe:

Most of the NDT Techniques are innocuous to people.

The tests are held to ensure the safety of the product as well as the safety of the person who has to work on any machinery.

3. Accurate:

NDT techniques are proven to be accurate and reliable at the same time.

4. Economical:

NDT Methods are cost-effective as they don't cause any harm or destruction to the examined product, which leads to saving time, money, and resources.

5. Prevent Accidents:

NDT techniques empower peace of mind to the operators by giving them satisfaction about the functioning of the NDT Equipment in the right direction eventually preventing accidents in the future and determining measures that can be adapted for life extension.

Top 8 Most Widespread NDT Methods

There are various techniques used under NDT Methods, with each having its own types of tools, data, & training.

Few techniques enable detailed assessment of an element, and some of them simply allow for an exterior examination.

Given below are the seven most often used Non-destructive Testing Methods:

●   Liquid Penetrant Testing (LPT)

●   Eddy Current Testing (ET)

●   Visual Testing (VT)

●   Ultrasonic Testing (UT)

●   Radiography Testing (RT)

●   Acoustic Emission Testing (AE)

●  Magnetic Particle Testing (MPT)

Let's have a detailed study of these seven NDT Methods.

1. Liquid Penetrant Testing (LPT)

Liquid Penetrant Testing involves applying a liquid penetrant to the surface of a material to detect surface-breaking defects.

This section will guide readers through the step-by-step process of LPT.

LPT is particularly useful for inspecting non-porous materials. This part will outline scenarios where LPT is the preferred NDT Method.

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To ensure accurate results, adhering to best practices is crucial in Liquid Penetrant Testing. This subheading will provide tips and recommendations for professionals conducting LPT.

2. Eddy Current Testing(ET)

To determine the areas of the damage through this electromagnetic testing focuses on utilizing the ratios of the strength of eddy currents, i.e. electrical currents in a magnetic field that surrounds the element.

3. Visual Testing(VT)

Visual Testing is the most straightforward NDT Method, involving a visual examination of the material or component. This section will provide an overview of VT principles.

Visual Testing is versatile and finds applications in various industries. This part will discuss the breadth of scenarios where VT is employed for inspections.

Despite its simplicity, VT has its challenges. This subheading will explore common challenges faced during visual inspections and how to overcome them.

4. Ultrasonic Testing(UT)

Ultrasonic Testing utilizes high-frequency sound waves to detect flaws in a material. This section will explain the science behind UT and how it is applied in practice.

From Weld Inspections To Thickness Measurements, UT finds applications in various fields. This part will discuss the specific scenarios where UT is most effective.

While Ultrasonic Testing is highly versatile, it's essential to understand its advantages and limitations. This subheading will provide a balanced view of what UT can and cannot achieve.

5. Radiographic Testing(RT)

Radiographic Testing involves the use of X-rays or gamma rays to inspect the internal structure of materials.

This section will provide an in-depth explanation of the RT process. RT is widely used in the aerospace and oil industries.

This part will shed light on how Radiographic Testing contributes to ensuring the quality and safety of components in these sectors.

Given the nature of radiation involved, safety is paramount in RT. This subheading will outline the necessary safety measures to be implemented during an RT inspection.

6. Acoustic Emission Testing(AE)

Acoustic Emission Testing involves monitoring acoustic emissions from a material under stress. This section will elaborate on the principles and processes of AET.

AET is particularly useful in specific scenarios. This part will explore the niche applications where AET outshines other NDT Methods.

The world of NDT is continually evolving. This subheading will discuss potential future trends in AET and how they might shape the industry.

7. Eddy Current Testing (ECT)

Eddy Current Testing uses electromagnetic induction to detect defects. This section will explain the scientific principles behind Eddy Current Testing.

ECT is prevalent in industries such as aerospace and automotive. This part will delve into specific applications within these sectors.

While Eddy Current Testing is powerful, it's crucial to understand its pros and cons.

This subheading will provide a balanced view of what Eddy Current Testing excels at and where it might have limitations.

8. Magnetic Particle Testing (MPT)

Magnetic Particle Testing is based on the principle that magnetic fields are disturbed by the presence of defects. This section will elucidate the fundamental concepts of MPT.

MPT is commonly used in the automotive and construction industries. This part will explore specific applications and scenarios where MPT is most effective.

While Magnetic Particle Testing is effective, certain factors need to be considered for optimal results. This subheading will provide insights into these considerations.

Non-destructive Testing Instruments

NDT includes many NDT Types of Equipment such as noise detectors, thickness gauges & eddy current instruments etc.

Some of the devices are handheld, and others can be mounted in a cabinet or on a rack.

We will discuss some of the instruments that are used in Non-destructive Testing.

1. Eddy Current Instruments

This instrument is used to provoke eddy currents in the conducive materials.

These instruments help in detecting drawbacks, defining the density, Inspecting Welds etc.

Eddy's current instrument has an analog/digital meter and a magnetic probe which is either straight or angled.

2. Acoustic Emission Instruments

These instruments are used for monitoring and observing changes in the electrical and mechanical systems.

3. Non-destructive Testing Conductivity and Resistivity Meters

These are the devices that are used for evaluating the electrical conductivity and resistivity of a substance to specify the traits of its materials.

4. Ultrasonic Instruments

These instruments aim to use beams of high-frequency wave signals to regulate and measure the components of the material.

Click on the link to watch an Ultrasonic Testing device called Sonaphone.

5. X-Ray Instruments

X-ray instruments penetrate gamma radiation or X-rays that capture the internal picture of the material for the screening process.

6. Corrosion and Electrochemical Instruments

To determine corrosion and electrochemical circumstances in machines, batteries, field appliances, colloids, etc.

Top 5 NDT Companies in India

There are many Non-destructive Testing Companies in India that provide excellent services, out of which we have handpicked the top 5 non-destructive testing companies in India.

• OnestopNDT
• Arora Technologies Limited
• NDTS India
• Sai Heatreaters & Non-Destructive Testing PVT. LTD.
• Mayuresh Engineers and Fabricators

Difference Between Non-destructive Testing And Destructive Testing

The most significant difference between the Non-destructive Testing and Destructive Testing two lies in their names.

In Non-destructive Testing, the part of the substance doesn't get destroyed in the process and remains capable of providing a good service.

But in destructive testing, the element gets damaged and can no longer be fit for use.

Some NDT Methods are macro sectioning, tensile testing & 3-point blend testing.

Conclusion

These NDT Methods are chosen based on factors such as the type of material being tested, the specific defect being sought, and the accessibility of the test area.

Combining multiple NDT Techniques is often employed for comprehensive inspections.

By using a combination of NDT Techniques, inspectors can obtain a more accurate and detailed assessment of the material's integrity.

This approach allows for a more thorough examination, ensuring that any potential defects or flaws are detected. 

Key Takeaways

• NDT is a crucial tool in manufacturing, construction, and aerospace for ensuring structural integrity without causing damage. 
• It uses various techniques, such as Ultrasonic Testing, radiographic testing, magnetic particle testing, and more. 
• Each NDT Method has its advantages and limitations, and its effectiveness depends on careful considerations and adherence to best practices. 
• The industry is constantly evolving, with future trends in Acoustic Emission Testing being discussed and shaping the future. 
• Understanding the pros and cons of each method is essential for accurate defect detection.

FAQs

Q: Why is NDT necessary?

A: NDT is crucial for inspecting materials without causing damage, ensuring the safety and integrity of structures and components.

Q: How does Ultrasonic Testing work?

A: Ultrasonic Testing uses high-frequency sound waves to detect flaws within a material by analyzing the reflected waves.

Q: Are there safety concerns with Radiographic Testing?

A: Yes, safety is a top priority in Radiographic Testing due to the involvement of X-rays or gamma rays. Strict safety measures must be followed.

Q: What are the common applications of Magnetic Particle Testing?

A: Magnetic Particle Testing is commonly used in the automotive and construction industries for detecting surface and near-surface defects.

Q: Why is Visual Testing considered essential despite its simplicity?

A: Visual Testing is quick, cost-effective, and can detect visible surface defects, making it an essential and widely used NDT Method.

Q: In which industries are eddy Current Testing prevalent?

A: Eddy Current Testing is prevalent in industries like aerospace and automotive for inspecting conductive materials.

Q: What are the niche applications of Acoustic Emission Testing?

A: Acoustic Emission Testing is often used in monitoring pressure vessels, pipelines, and other critical structures for early detection of defects.

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