Non-destructive testing (NDT) at TWI

Summary

This video script offers an insightful introduction to non-destructive testing (NDT) techniques, essential for maintaining structural integrity without damaging components. It covers six common NDT methods: magnetic particle inspection, liquid penetrant inspection, radiography, ultrasonic testing, eddy current testing, and thermography. Each method's principle, application, and benefits are outlined, highlighting the importance of these techniques in various industries for detecting flaws and ensuring safety.

Takeaways

• 🔨 **Destructive vs. Non-Destructive Testing**: Destructive testing damages the component, making it unusable post-test, while non-destructive testing (NDT) allows for continued use of the component.
• 🧲 **Magnetic Particle Inspection (MPI)**: MPI is used to find surface and near-surface discontinuities in ferromagnetic materials by applying a magnetic field and observing the pattern of ferrous particles.
• 🌈 **MPI Techniques**: MPI can be performed using color contrast or fluorescent methods, with the latter being more sensitive for detecting defects.
• 💧 **Liquid Penetrant Inspection**: This NDT method is used for non-ferromagnetic materials to find surface defects by applying a colored penetrant that is drawn into surface-breaking defects.
• 📼 **Radiography**: X-rays are used to detect internal flaws in materials; digital radiography is becoming more prevalent over traditional wet radiography due to its efficiency and image quality.
• 🔊 **Ultrasonic Testing (UT)**: UT uses high-frequency sound waves to detect discontinuities within a material; it's effective for various applications, including the inspection of welds and pressure vessels.
• 🌊 **Time-of-Flight Diffraction (TOFD)**: A type of UT that is particularly useful for sizing planar flaws, though it can be less reliable with certain material conditions.
• 📊 **Phased Array Systems**: These systems use ultrasonic beams at various angles for inspections, offering a more efficient method for detecting flaws in welds and other structures.
• 🌐 **Surface Ultrasonics**: Used for detecting surface-breaking cracks, especially in petrochemical plants and civil engineering structures like bridges.
• 🌊 **Long-Range Guided Wave Ultrasonics**: This technique uses low-frequency waves for detecting corrosion and erosion in pipes, providing a quick screening method for long pipe lengths.
• 🔗 **Eddy Current Testing**: A method for detecting surface-breaking cracks and material composition variations, especially effective for non-ferritic materials.
• 🔥 **Thermography**: Uses infrared cameras to detect temperature anomalies in components, useful for remote inspection of equipment, including those operating at high temperatures.

Q & A

What is the primary difference between destructive and non-destructive testing?

Destructive testing leaves you without a serviceable component or test specimen once the testing is over, whereas non-destructive testing allows the structure to remain in daily service after testing.

What is Magnetic Particle Inspection (MPI) used for?

MPI is used to find surface and just beneath the surface discontinuities in ferromagnetic materials by introducing a magnetic flux into the material and using ferrous particles that are drawn to the site of flux leakage.

How do color contrast and fluorescent techniques in MPI differ?

In color contrast MPI, a thin layer of white paint is applied and black ink is sprayed on, showing defects as black against white. The fluorescent technique uses a fluorescent medium and a fluorescent lamp to make defects appear as bright green, which is more sensitive than color contrast.

What is Liquid Penetrant Inspection and how does it work?

Liquid Penetrant Inspection is used for locating surface defects on non-ferromagnetic materials. It involves cleaning and degreasing the component, applying a colored penetrant, and then using a powder developer to draw the penetrant out of flaws, making them stand out.

How does Radiographic testing work and what are its requirements?

Radiographic testing uses X-rays to detect flaws within a material. It requires access to both sides of the component, with a sensitive film on one side to capture the X-ray image, which reveals flaws by showing where the X-rays are absorbed or transmitted differently.

What is the difference between wet and digital radiography?

Wet radiography uses photographic films and requires traditional darkroom technology, while digital radiography relies on substances that fluoresce when exposed to X-radiation, converting this fluorescence into a digital signal for easier manipulation and storage.

What is Ultrasonic Testing (UT) and how does it locate discontinuities?

UT is a method that uses ultrasonic signals above audible frequencies to detect discontinuities. The signal is passed from a probe into the component, and when it encounters a discontinuity, it reflects back. By measuring the time it takes for the signal to return, the location of the discontinuity can be determined.

What is the purpose of Time-of-Flight Diffraction (TOFD) in UT?

TOFD is used mainly for sizing planar flaws oriented perpendicular to the surface. It is less reliable with a high density of defects, scattered inclusions, or coarser grained materials.

How do Phased Array systems in UT differ from standard UT?

Phased Array systems offer the possibility of performing inspections with ultrasonic beams of various angles and focal lengths using a single array of transducers, making it quicker and with reduced scanning requirements compared to standard UT.

What is the application of Long-Range Guided Wave Ultrasonics and what advantages does it offer?

Long-Range Guided Wave Ultrasonics uses low-frequency guided waves to detect corrosion and erosion in pipe work. It provides 100% initial screening coverage of a long length of pipe and only requires local access to the pipe surface, making it suitable for various pipe work applications.

How does Eddy Current Testing work and what is its primary use?

Eddy Current Testing involves placing a circular coil carrying an alternating current near an electrically conductive specimen. The alternating current generates a changing magnetic field that induces eddy currents in the test object, which are useful for detecting surface-breaking or near-surface cracking and variations in material composition.

What is Thermography and how is it used in non-destructive testing?

Thermography involves using an infrared camera to detect abnormally hot or cold areas on a component. It is a remote technique suitable for equipment operating at elevated temperatures and can be used for inspection of low conductivity materials like composites.

Outlines

00:00

🔍 Introduction to Non-Destructive Testing (NDT)

The first paragraph introduces the concept of non-destructive testing (NDT), contrasting it with destructive testing. It explains that destructive testing methods, such as tensile tests, leave components unusable after testing, which is not ideal for structures in daily use. NDT, on the other hand, allows for the inspection of materials without causing damage. The paragraph then delves into various NDT methods, starting with magnetic particle inspection (MPI), which uses magnetic flux to detect surface and subsurface discontinuities in ferromagnetic materials. It also discusses color contrast and fluorescent MPI techniques. The paragraph further explains liquid penetrant inspection for non-ferromagnetic materials, radiography using X-rays, and the shift from wet to digital radiography, highlighting the benefits of digital radiography.

05:07

🌐 Advanced NDT Techniques: Ultrasonic and Eddy Current Testing

The second paragraph focuses on advanced NDT techniques such as ultrasonic testing (UT) and eddy current testing. It describes how UT uses high-frequency sound waves to detect discontinuities within a material. The paragraph details different UT methods, including time-of-flight diffraction, Toft, phased array, and surface ultrasonics, each with specific applications like sizing planar flaws and inspecting welds. It also covers long-range guided wave ultrasonics for detecting corrosion in pipes. The paragraph concludes with a discussion on eddy current testing, which uses alternating currents to induce eddy currents in conductive materials, helping to detect surface or near-surface defects and material composition variations.

10:13

🔥 Thermal Inspection Techniques: Thermography

The third paragraph introduces thermography, a thermal inspection technique that uses infrared cameras to detect temperature variations on a component's surface. It explains how modern thermal cameras can identify minute temperature differences and record thermal images for analysis. The paragraph highlights the remote nature of thermography, making it suitable for various plant areas, especially those with equipment operating at high temperatures. It also mentions active thermography, a method where heat is applied directly to the component for inspecting low conductivity materials like composites.

Keywords

💡Destructive Testing

Destructive testing refers to a method of evaluating the properties of materials or components by subjecting them to stresses until failure, which results in the destruction of the test sample. In the context of the video, this is contrasted with non-destructive testing (NDT), highlighting the importance of preserving the integrity of the component for further use. An example from the script mentions that after destructive tests like tensile tests, the tested items such as a chassis box section or a car are destroyed.

💡Non-Destructive Testing (NDT)

Non-destructive testing is a group of testing methods used to evaluate the properties of a material, component, or system without causing damage. The video emphasizes NDT as a critical approach in industries where the tested components need to remain functional after inspection. The script introduces several NDT techniques, such as magnetic particle inspection and ultrasonic testing, which are used to detect flaws without destroying the test specimen.

💡Magnetic Particle Inspection (MPI)

Magnetic particle inspection is an NDT method used to detect surface and near-surface discontinuities in ferromagnetic materials. The process involves magnetizing the material and then applying a magnetic powder which is attracted to any magnetic leakage fields created by defects. The script describes MPI as a technique that uses a magnetic flux and ferrous particles to reveal flaws, with two types mentioned: color contrast and fluorescent.

💡Liquid Penetrant Inspection

Liquid penetrant inspection is another NDT technique used to detect surface-breaking defects in non-ferromagnetic materials. It involves applying a colored or fluorescent penetrant to the surface of the material, which seeps into any cracks or defects. After a period, the excess penetrant is wiped off, and a developer is applied to draw the penetrant out of the flaws, making them visible. The script uses this technique as an example of how NDT can reveal surface defects without damaging the component.

💡Radiography

Radiography is an NDT method that uses X-rays or gamma rays to detect internal flaws in materials. The script explains that radiography does not require surface preparation and can be used to inspect components from one side by placing a sensitive film on the other side. The X-rays create a latent image on the film, which is darker in areas where more radiation passes through, indicating flaws. The video also mentions the transition from wet radiography to digital radiography, which uses fluorescence and digital signals for analysis.

💡Ultrasonic Testing (UT)

Ultrasonic testing is a generic term for various methods that use ultrasonic waves to detect flaws within materials. The script describes how UT works by sending ultrasonic signals from a probe into the component; when these signals encounter a discontinuity, they reflect back, and by measuring the time it takes for the signal to return, the location of the flaw can be determined. The video discusses different types of UT, including time-of-flight diffraction and phased array systems, which are used for various applications such as inspecting welds and pressure vessels.

💡Phased Array

Phased array is a specific application of ultrasonic testing that allows for inspections with ultrasonic beams of various angles and focal lengths using a single array of transducers. The script highlights its use in finding manufacturing flaws and in-service issues like fatigue cracking and corrosion. It is particularly useful for rapid inspections of welds in pipelines and thick-section pressure vessels, showcasing its efficiency over conventional ultrasonic testing methods.

💡Surface Ultrasonics

Surface ultrasonics is a technique used to detect surface-breaking flaws that are oriented perpendicular to the surface of the component. As mentioned in the script, this method is used to find flaws like environmental cracking in petrochemical plants and fatigue cracks in civil engineering structures. Surface waves travel along the surface, penetrating only a short depth, making it ideal for detecting surface-related defects.

💡Long-Range Guided Wave Ultrasonics

Long-range guided wave ultrasonics is a technique that uses low-frequency guided waves to detect corrosion and erosion in pipe work. The script emphasizes its advantage in providing 100% initial screening coverage of long pipe lengths with only local access needed for the transducer array. This method is suitable for various pipe diameters and wall thicknesses, making it a valuable tool in industries like petrochemical processing and oil and gas transmission.

💡Eddy Current Testing

Eddy current testing is an NDT method that uses a coil carrying an alternating current to induce eddy currents in a conductive material. These currents interact with the material and can be affected by flaws or changes in material composition. The script explains that standard eddy current testing is a near-surface technique used for detecting surface-breaking or near-surface cracking. It is also used for special applications in materials with varying conductivities, such as inspecting airframe components for cracking or corrosion thinning.

💡Thermography

Thermography is a thermal inspection technique that uses infrared cameras to detect abnormally hot or cold areas on components. The script describes how modern thermal cameras can detect minute temperature differences and record thermal images for analysis. Thermography is a remote technique suitable for various plant areas, especially for equipment operating at high temperatures. The video also mentions active thermography, which applies heat directly to the component for inspecting low conductivity materials like composites.

Highlights

Destructive testing leaves you without a serviceable component post-testing, unlike non-destructive testing (NDT).

Non-destructive testing is crucial for structures in daily service.

Magnetic Particle Inspection (MPI) detects surface and sub-surface discontinuities in ferromagnetic materials.

MPI uses a magnetic flux and ferrous particles to reveal defects.

Color contrast MPI involves applying white contrast paint and black ink to highlight defects.

Fluorescent MPI is a more sensitive technique, using fluorescent medium and a lamp to reveal defects.

Liquid Penetrant Inspection is used for non-ferromagnetic materials to find surface defects.

Radiography uses X-rays to detect internal flaws in materials.

Digital radiography is replacing traditional wet radiography with benefits like no chemical processing and faster image viewing.

Microfocus radiography offers high-definition radiographs, useful for inspecting tiny defects.

Ultrasonic Testing (UT) uses ultrasonic signals to detect discontinuities within a component.

Time-of-Flight Diffraction (TOFD) is a type of UT used for sizing planar flaws.

Phased array systems in UT allow for inspections with various angles and focal lengths using a single array of transducers.

Surface ultrasonics is used for detecting environmental cracking and fatigue cracks in structures.

Long-range guided wave ultrasonics uses low-frequency waves to detect corrosion and erosion in pipe work.

Eddy current testing is used for detecting surface breaking or near-surface cracking in conductive materials.

Thermography uses infrared cameras to detect temperature differences, indicating potential issues.

Active thermography is used for inspecting low conductivity materials like composites.

Transcripts

testing testing materials testing and structural testing destructive or

non-destructive the difference is clear-cut destructive testing leaves you

without a serviceable component or test specimen once the testing is over the

sharply test the vend test the tensile test this chassis box section this pipe

section and this car they're all destroyed in the testing process

well that's no use of your structure is in daily service so that's where

non-destructive testing comes in

in this short introduction to non-destructive testing we're going to

be looking at half a dozen of the most common types of NDT and also take a

glance at a few of the less traditional newcomers to the field well let's get

started with magnetic particle inspection we use MPI as it's known to

find surface and just beneath the surface discontinuities in ferromagnetic

materials and we do it by introducing a magnetic flux into the material an ink

containing extremely fine ferrous particles is applied to the surface of a

ferromagnetic material whilst a magnetizing field is applied floors

cause leakage of magnetic flux which creates new magnetic poles so the

ferrous ink particles are drawn to the site of the flux leakage field

disturbance and the inspector sees it there are a couple of types of MPI color

contrast and fluorescent with color contrast a very thin layer of white

contrast paint is applied magnetic force is also applied and black ink spray on

the surface below the excess away and any defects or discontinuities show up

black against white with the fluorescent technique we coat the component under

inspection with a fluorescent medium while the part is magnetized and

a fluorescent lamp discontinuities or defects show up vividly as bright green

it's a more sensitive technique than the color contrast method now liquid

penetrant inspection works on non ferromagnetic materials for locating

surface defects only clean and degrease the component then apply a brightly

colored penetrant which in due course is drawn into the surface breaking defects

after a specified period the excess penetrant is wiped off and a powder

developer applied it works rather like blotting paper drawing the penetrant out

of the flaws so that they stand out in sharp contrast to the surrounding

material after a specified bleed time the defects are recorded now radiography

is an entirely different type of non-destructive testing

x-rays are produced by directing a stream of high velocity electrons across

an evacuated chamber against an angled target material the x-ray is passed

through almost every material and can be detected by a sensitive film on the

other side however a high density material will absorb x-rays while low

density materials allow a greater proportion of them to pass through for

radiographic inspection there's no need for surface preparation before testing

but you do need to have access to both sides of the component the film which is

highly sensitive to x-rays is placed on one side and the x-ray source is

carefully located opposite the x-ray is passed through the component and create

a latent image on the film on the other side any volumetric flaws or

discontinuities in the component allow more of the radiation onto the film and

this creates a darker image on the radiograph in recent years wet

radiography which uses photographic films and traditional darkroom

technology has been overtaken by digital radiography now digital radiography

relies on the ability of certain substances to fluoresce when they are

exposed to x-radiation the fluorescence that's emitted from the substance is

made to create an analog electronic signal which can be converted to a

digital signal by means of an analog to digital converter

during testing a fluorescent coated surface is placed behind the test

specimen as the radiation passes through the specimen it causes different levels

of fluorescence to occur on the coated surface depending on the dosage received

and this is controlled by the specimen geometry this is then converted to a

digital signal and displayed on a screen

microfocus test equipment differs from standard

radiography kit in that the focal spot size is extremely small typically in the

micron range the benefit is that it produces radiographs with extremely high

definition the equipment lends itself well to producing large magnification

images while retaining good definition of tiny defects some of the benefits of

digital radiography over wet radiography are obvious there's no need for chemical

processing the exposure to image viewing time is shorter the images are easily

stored on a hard drive and they can be easily manipulated and enhanced on

computer well now let's look at ultrasonic testing UT as it's known is

actually a generic title for several different methods of generating and

observing ultrasonic signals they're all different but use the same basic

principle they all use a signal above the audible frequencies hence the word

ultra sonic this is passed from a probe into the component under investigation

the signal bounces or is reflected when it comes across a discontinuity if we

can measure how long the signal takes to return

knowing the speed at which it travels we can tell where the discontinuity is

time-of-flight diffraction another type of UT is used mainly for sizing plane

our floors orientated perpendicular to the surface

however it's less reliable when there's a high density of defects when the

material contains scattered inclusions and also with coarser grained materials

Toft is widely used in the petrochemical and nuclear industries for the

inspection of butt wells in pressure vessels and process pipe work it's often

used to provide critical floor sizing data for input into engineering critical

assessments phased array systems work quite differently they offer the

possibility of performing inspections with ultrasonic beams of various angles

and focal lengths using a single array of transducers so what's it used for

well finding the same manufacturing flaws like lack of sidewall fusion lack

of root penetration lack of root fusion and porosity in service floors like

fatigue cracking stress corrosion cracking

corrosion and erosion and parent material flaws like

inclusions and lamellar tearing only it does it quicker and with a reduced

amount of scanning a specific application of phased arrays lies in

pipe girth weld inspection particularly in installation of long pipe lines where

it replaces conventional systems with many probes this technique is also used

for the rapid inspection of thick section pressure vessel welds turbine

disks shafts and blade routes Filat worlds and

welds in coarse-grained materials so now to surface ultrasonics it tends to be

used for the detection of environmental cracking in petrochemical plant and also

fatigue cracks in civil engineering structures like bridges as the name

suggests surface waves travel along the surface of components penetrating to a

depth of only one ultrasonic wavelength it's used to find Plano floors that

break the surface and lie at 90 degrees to the scanned surface such as fatigue

cracks in Weld toes and also stress corrosion cracking finally long-range

guided wave ultrasonics make you search low-frequency guided waves to detect

corrosion and erosion in pipe work the main advantage of the technique is that

it provides 100% initial screening coverage of a long length of pipe and

only requires local access to the pipe surface at those points where the

transducer array is attached it's suitable for use on pipe diameters above

50 millimetres and on wall thicknesses up to 40 millimetres

it's used specifically to find both internal and external corrosion and also

erosion in thermally insulated coated and buried pipe work corrosion under

pipe supports and hidden welded joints so it finds its niche in petrochemical

process pipe work oil and gas transmission lines gas manifolds

offshore risers jetty lines and power station boiler tubes now to eddy current

testing in standard eddy current testing a circular coil carrying an alternating

current is placed close to an electrically conductive specimen the

alternating current generates a changing magnetic field which interacts with the

test object and induces eddy currents standard eddy current testing is

essentially a near-surface technique it's useful for detecting surface

breaking or near surface cracking and variations in material composition eddy

current methods are used for surface inspection of both non ferritic and

ferritic materials eddy current penetration is deeper on non ferritic

materials so it's also used for special applications with thin components such

as in the inspection of multiple layer airframe components for cracking or

corrosion thinning lastly to thermography

thermography is the simplest of all thermal inspection techniques it

involves using an infrared camera to look for abnormally hot or cold areas on

a component operating under normal conditions most modern thermal cameras

are able to detect temperature differences of less than a quarter of a

degree Celsius and they enable thermal images to be recorded on video or

computer disk for future analysis thermography is a remote technique for

use in most areas of plant but is particularly suited to equipment

operating at elevated temperatures active thermography where heat is

applied directly to the component is used for inspection of low conductivity

materials like composites well thermography just about winds up

this brief introduction to the popular non-destructive testing techniques to

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