What is Non-Destructive Testing (NDT)?
Non-destructive testing (NDT) is a testing and analysis technique used by industry to evaluate the properties of a material, component, structure or system for characteristic differences or welding defects and discontinuities without causing damage to the original part. NDT also known as non-destructive examination (NDE), non-destructive inspection (NDI) and non-destructive evaluation (NDE).
This frequently asked question has been created to provide a breakdown of what NDT is, an introduction to each of the methods, the difference between NDT and destructive testing and the advantages of using this analysis technique.
Non-Destructive Testing Methods
The most common NDT test methods include:
Visual Testing (VT)
Visual testing also known as visual inspection is one of the most common 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.
Magnetic Particle Testing (MPI)
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.
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.
Ultrasonic Testing (UT)
Ultrasonic Testing entails the transmission of high frequency sound into a material to interact with features within the material that reflect or attenuate it. Ultrasonic testing is broadly divided into Pulse Echo (PE), Through Transmission (TT) and Time of Flight Diffraction (ToFD).
Pulse Echo Inspection
This technique introduces a sound beam into the test material surface. The sound will travel through the part, either reaching the rear wall of the material and then returning to the transducer or returning early when reflected from a discontinuity within the part. If the acoustic velocity is known, the time interval recorded is then used to derive the distance travelled in the material.
Through Transmission Testing
TT uses separate transducers to emit and receive the sound. The transmit probe is positioned one side of the test sample and the receive transducer is positioned on the other side. As the sound passes through the component, it is attenuated by features within it, such as porosity. Thickness measurement is not normally possible with this technique.
Time of Flight Diffraction (ToFD)
Diffraction is the process of a change in wavelength in sound as it interacts with a discontinuity in a material. This mechanism is used in situations where a true reflection cannot be obtained but sufficient diffraction occurs to alter the time of flight of the sound in a pitch-catch arrangement. This method is used to detect the tip of a defect that resides perpendicular the probe contact surface. ToFD is also used for rear wall inspection for detection of corrosion.
Immersion Testing
The requirement to wet couple the ultrasound probe to the part can be a challenge for large or complex geometric samples. For convenience these parts are immersed in water – typically in an immersion tank. This method is usually enhanced by actuators that move the part and/or the probe within the tank during ultrasonic inspection.
Air Coupled Testing
Certain inspections and materials cannot tolerate the application of wet coupled and so in certain circumstances air coupled ultrasound testing may be performed. This entails the application of sound through an air gap. This typically entails the use of lower frequency inspection.
Electromagnetic Acoustic Transducer (EMAT) Testing
EMAT Testing is a type of non-contact inspection method that uses electromagnetic sound generation and reception without immediate contact or wet coupling with the part. EMATs are of particular use for excessively hot, cold, clean, or dry environments. As with conventional ultrasound, EMATs can produce normal and angled beams as well as other modes, such as guided waves.
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.
Automated Inspection
The benefit of automation is achieved by the integration of NDT sensors with standard commercially available industrial robots as well as collaborative robots, also known as “cobots”. Custom written software for acquiring and visualising data creates a seamless and intuitive user experience that can be adapted to specific needs.
TWI has developed several highly capable automated inspection systems suited to both research and development work as well as production inspection.
Phased Array Ultrasonic Testing (PAUT)
PAUT probes are different from conventional UT probes in that they consist of an array of individual elements that can be pulsed independently. By controlling the times at which each element is fired, sound beams can be focussed or steered. By sweeping the beam through a range of angles or depths, cross-sectional views can be generated using one probe where several probe and wedge combinations may have been required with conventional UT. A virtual probe can be created from a number of elements and this can be electronically indexed along the length of the array to create a wide paintbrush scan.
What is the Difference Between Destructive and Non-Destructive Testing?
Destructive testing destroys or changes the part in some way such that even if it passes the test it is no longer fit for service. Examples might be tensile testing, 3 point bend test or macro sectioning. NDT does not destroy or change the part such that it is still fit for service if it passes the test.
There are 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.
It 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. This testing technique 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 is cost effective as it can prevent the need to replace an item before malfunction occurs without destroying the piece itself.
This testing technique 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.
It is also useful for testing of welds and verification of welding procedures to ensure that a welding process has been completed to the correct specification within the bounds of quality control, for example to make sure that the base metal has reached the correct temperature, cooled at the specific rate and that compatible materials have been used to prevent welding defects.
It is used in most industries, including oil and gas. aerospace, automotive, power, and marine.
Conclusion
Non-destructive testing is a valuable technique used by many industries to evaluate the properties of a material, component, structure or system without causing any damage.
There are various types of NDT such as visual inspection, radiography, ultrasonic testing, magnetic particle testing and penetrant testing. Each type has its own advantages depending on the application.
Overall, NDT is a very versatile and useful technique that can help identify problems or defects in materials or structures without causing any damage.