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What is Non-Destructive Testing (NDT)?


Non-destructive testing (NDT) is used by industry to evaluate the properties of a material, component, structure or system without causing damage.

NDT is also known as non-destructive examination (NDE), non-destructive inspection (NDI) and non-destructive evaluation (NDE).

This article is one of a series of TWI frequently asked questions (FAQs).

Methods of Non-Destructive Testing 

Current NDT test methods include:

Acoustic Emission Testing (AE)

This method uses a localised external force such as a pressure or temperature change or mechanical load to create short lived high frequency stress waves. These stress waves create small material displacements or plastic deformation on the surface which can be detected by sensors.

Electromagnetic Testing (ET)

This testing method uses an electric current or magnetic field which is passed through a conductive part. There are three types of electromagnetic testing, including eddy current testing, alternating current field measurement (ACFM) and remote field testing (RFT).

Eddy current testing uses an alternating current coil to induce an electromagnetic field into the test piece, alternating current field measurement and remote field testing both use a probe to introduce a magnetic field, with RFT generally used to test pipes.

Guided Wave Testing (GW)

Ideal for testing pipes over long distances, guided wave testing uses ultrasonic wave forms to reflect changes in the pipe wall, which are then sent to a computer for control and analysis.

Ground Penetrating Radar (GPR)

This geophysical NDT method sends radar pulses through the surface of a material or subsurface structure, such as rock, ice, water or soil. The waves are reflected or refracted when they encounter a buried object or material boundary with different electromagnetic properties.

Laser Testing Methods (LM)

Laser testing falls into three categories including holographic testing, laser profilometry and laser shearography.

Holographic testing uses a laser to detect changes in the surface of the material which has been subjected to stress such as heat, pressure or vibration. The results are then compared to an undamaged reference sample to show defects.

Laser profilometry uses a high speed rotating laser light source and miniature optics to detect corrosion, pitting, erosion and cracks by detecting changes in the surface via a 3D image generated from the surface topography.

Laser shearography uses laser light to create an image before the surface is stressed and a new image is created. These images are compared to one another to determine if any defects are present.

Leak Testing (LT)

Leak testing can be broken down into four different methods - bubble leak testing, pressure change testing, halogen diode testing and mass spectrometer testing.

Bubble leak testing uses a tank of liquid, or a soap solution for larger parts, to detect gas (usually air) leaking from the test piece in the form of bubbles.

Only used on closed systems, pressure change testing uses either pressure or a vacuum to monitor the test piece. A loss of pressure or vacuum over a set time span will show that there is a leak in the system. 

Halogen diode testing also uses pressure to find leaks, except in this case air and a halogen-based tracer gas are mixed together and a halogen diode detection unit (or 'sniffer') is used to locate any leaks.

Mass spectrometer testing uses helium or a helium and air mix inside a test chamber with a 'sniffer' to detect any changes in the air sample, which would indicate a leak. Alternatively, a vacuum can be used, in which case the mass spectrometer will sample the vacuum chamber to detect ionized helium, which will show that there has been a leak.

Magnetic Flux Leakage (MFL)

This method uses a powerful magnet to create magnetic fields which saturate steel structures such as pipelines and storage tanks. A sensor is then used to detect changes in magnetic flux density which show any reduction in material due to pitting, erosion or corrosion.

Microwave Testing

This method is restricted to use on dielectric materials and uses microwave frequencies transmitted and received by a test probe. The test probe detects changes in dielectric properties, such as shrinkage cavities, pores, foreign materials or cracks and displays the results as B or C scans.

Liquid Penetrant Testing (PT)

Liquid penetrant testing involves the application of a fluid with low viscosity to the material to be tested. This fluid seeps into any defects such as cracks or porosity before a developer is applied which allows the penetrant liquid to seep upwards and create a visible indication of the flaw. Liquid penetrant tests can be conducted using solvent removable penetrants, water washable penetrants or post-emulsifiable penetrants.

Magnetic Particle Testing (MT)

This NDT process uses magnetic fields to find discontinuities at or near the surface of ferromagnetic materials. The magnetic field can be created with a permanent magnet or an electromagnet, which requires a current to be applied.

The magnetic field will highlight any discontinuities as the magnetic flux lines produce leakage, which can be seen by using magnetic particles that are drawn into the discontinuity. 

Neutron Radiographic Testing (NR)

Neutron radiography uses a beam of low energy neutrons to penetrate into the workpiece. While the beam is transparent in metallic materials most organic materials allow the beam to be seen, allowing the structural and internal components to be viewed and examined to detect flaws.

Radiographic Testing (RT)

Radiographic testing uses radiation passed through a test piece to detect defects. X-rays are commonly used for thin or less dense materials while gamma rays are used for thicker or denser items. The results can be processed using film radiography, computed radiography, computed tomography or digital radiography. Whichever method is used, the radiation will show discontinuities in the material due to the strength of the radiation.

Thermal/Infrared Testing (IR)

Infrared, or thermal, testing measures surface temperatures as heat flows through, to, or from an object. A thermal imaging device, or infrared camera, is used view this thermal or infrared radiation and show any corrosion damage, delamination, disbonding, voids, inclusions or similar defects. 

Ultrasonic Testing (UT)

Ultrasonic testing sends high frequency sound throught the part being examined in a simliar manner to naval SONAR. Should the sound strike a material with a different acoustic impedance some of the sound will reflect back to the sending unit and appear on a visual display. The time it takes to send the sound back can be used to determine if there are any faults.

There are several variants of ultrasonic testing, including straight beam, angle beam, immersion testing, through transmission, phased array and time of flight diffraction.

Vibration Analysis (VA)

This process uses sensors to measure the vibration signatures from rotating machinery in order to assess the condition of the equipment. The types of sensors used include displacement sensors, velocity sensors, and accelerometers. 

Visual Testing (VT)

This most common NDT technique involves the operator looking at the test piece. This can be aided by the use of optical instruments such as magnifying glasses or computer-assisted systems (known as 'Remote Viewing').

This method allows for the detection of corrosion, misalignment, damage, cracks, and more. Visual testing is inherent in most other types of NDT as they will generally require an operator to look for defects.

The Advantages of using NDT

 Non-destructive testing has a number of distinct advantages, the most obvious of which is that the pieces being tested are left undamaged by the process, allowing for an item to be repaired rather than replaced should any problems be found. 

NDT is also a very safe testing method for operators, with most techniques being harmless to humans, although some types of test - such as radiographic testing - still need to be conducted under strict conditions. NDT can also help prevent injury or fatalities by ensuring structures, components and machinery is safe.

Non-destructive testing is also a very accurate way of inspection since the tests are repeatable and a number of tests can be used together to correlate results. 

These testing methods are also economical. Unlike destructive testing, NDT can prevent the need to replace an item before malfunction occurs without destroying the piece itself.

NDT also offers operators peace of mind, knowing that equipment is functioning as it should, preventing future accidents and determining any measures that can be taken for life extension. 

Non-Destructive Testing (NDT) Services

TWI has one of the most definitive ranges of NDT services.


Where is Non-Destructive Testing used?

Non-destructive testing is used in most industries, including aerospace, automotive, power, marine, and oil and gas.

Learn NDT with TWI

Whether you are already working in NDT and wish to further your skillset or want to advance your career into this area, TWI offers NDT training and certification as both off-the-peg and bespoke courses, including e-learning options.

Find out more about NDT training

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