Current NDT test methods include:
Acoustic Emission Testing (AE)
This is a passive NDT technique, which relies on detecting the short bursts of ultrasound emitted by active cracks under a load. Sensors dispersed over the surface the structure detect the AE. It is even possible to detect AE from plasticisation in highly stressed areas before a crack forms. Frequently a method for use during proof tests of a pressure vessel, AE testing is also a continuous Structural Health Monitoring (SHM) method, for example on bridges. Leaks and active corrosion are detectable AE sources too.
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.
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.
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 (IRT)
Infrared testing or thermography uses sensors to determine the wavelength of infrared light emitted from the surface of an object, which can be used to assess its condition.
Passive thermography uses sensors to measure the wavelength of the emitted radiation and if the emissivity is known or can be estimated, the temperature can be calculated and displayed as a digital reading or as a false colour image. This is useful for detecting overheating bearings, motors or electrical components and is widely used to monitor heat loss from buildings.
Active thermography induces a temperature gradient through a structure. Features within it that affect the heat flow result in surface temperature variations that can be analysed to determine the condition of a component. Often used to detect near surface delaminations or bonding defects in composites.
Ultrasonic Testing (UT)
Ultrasonic testing sends high frequency sound through the part being examined in a similar 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.
As well as conventional ultrasonic testing methods (as above), there is also a variant known as dry coupled ultrasonic testing, which doesn’t require the use of coupling liquids.
There are several variants of ultrasonic testing, including straight beam, angle beam, guided wave testing, phased array ultrasonic testing (PAUT), immersion testing, through transmission, phased array, advanced ultrasonic backscatter technique (AUBT), and time of flight diffraction:
This ultrasonic testing technique mounts the transducer on an angled wedge (also known as a probe). The transducer transmits a sound beam into the part to be tested at a designated angle. The most common angles used for inspection are 45°, 60° and 70°. If a governing code doesn’t specify the angle and frequency to be used the operator should select a combination that will correctly inspect the parts to be tested.
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. Guided wave testing can be carried out using medium or long range tests – guided wave medium range ultrasonic testing (GW MRUT) and guided wave long range ultrasonic testing (GW LRUT). GW MRUT techniques cover an area of 25mm to 3000mm, while GW LRUT covers distances greater than this and can be used to inspect areas over hundreds of metres from one location.
Electromagnetic Acoustic Transducer (EMAT) Testing
This non-destructive testing technique uses electromagnetic sound generation and reception without the use of contact or couplant. As the sound is directly generated within the material adjacent to the transducer, EMAT is of particular use for automated inspection, and hot, cold, clean, or dry environments. This emerging test method can be used for flaw detection, thickness measurement, and material property characterisation. As a result, EMAT has found applications in industries including automotive, rail, pipelines, and boiler and pressure vessel industries.
This testing method uses a tank of water to provide a coupling medium for the sound beam between the transducer and the part to be tested. The UT machine is mounted on a moveable platform (called a bridge) on the side of the tank allowing it to travel along the length of the tank. A swivel mount is used so that the transducer can be moved along the x,y and z axes so that it can be orientated so the sound beam penetrates the part at the correct angle. Round parts are often mounted on rollers so that can be rotated as the transducer moves, allowing the test to reach the full circumference.
Phased array testing uses multiple elements in the probe so that they can be activated individually. Being able to vary the time of activation for each element means that the sound beam can be ‘steered’ and the data combined to create a visual image representing a cross-section of the part to be inspected.
Straight Beam Testing
This simple form of ultrasonic testing sends sound waves through the test piece. Should these waves meet an internal reflector the sound will return to the transducer faster than any waves that are being reflected from the back wall of the test piece. This reflection shows a discontinuity, which could be caused by cracking, porosity or slag in the material.
This inspection uses two transducers, one on either side of the test part. One transducer sends the signal while the other acts as a receiver. Should there be any reflectors in the part, the sound reaching the receiver will be reduced.
Time of Flight Diffraction (ToFD)
Similar to through transmission, time of flight diffraction also uses two transducers – one to send and the other to receive the signals. This technique sends a lateral wave signal and a backwall reflection to determine if there are any discontinuities. If a discontinuity is located, the ultrasound waves diffract and show up on a monitor. This method is ideal for detecting discontinuities and defects with sharp geometries. ToFD can be performed with several sets of transducers.
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)
Visual testing also known as visual inspection is one of the most common NDT techniques which 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.