An introduction to Non-Destructive Testing?
In industries today, quality assurance of products is carried out with the help of rigorous testing processes on parts and components of any product. The physical properties of materials such as impact resistance, ductility, yield, ultimate tensile strength, fracture toughness, and fatigue strength are tested, and the functionality of a component is decided.
The only concern with such a mode of quality assurance is that most of these tests are destructive in nature. A destructive test is done on a limited number of samples (referred to as lot sampling), rather than on the materials, components, or assemblies being put into service. 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. Such a form of test poses huge impacts on the economical front, as these parts cannot be reused. Plants and factory operations can be disrupted due to the non-functionality of this equipment after an invasive test.
This brings us to the solution, which is a non-destructive testing (NDT).
Non-destructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components, or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed, the part can still be used. NDT is also known as non-destructive examination (NDE), nondestructive inspection (NDI), and non-destructive evaluation (NDE).
So, what are the major advantages of using NDT?
There are several 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. Non-destructive testing has been in existence for longer than many people assume. The first proper non-destructive testing technique was developed in the very late stages of the nineteenth century when X-Rays were invented by a German physicist named Wilhelm Conrad Rontgen.
Of course, the oldest form of non-destructive testing is visual testing, a means of simply inspecting a component for surface flaws by sight alone. This is still a common, and very effective procedure today, carried out by skilled technicians who know exactly what to look for in a component. Many people forget that this is a form of non-destructive testing, as it is, of course, a means of examining something without causing any detriment to it. In this case, a patient, may or may not have an illness.
To summarise the main advantages, include:
- Safety: The tests are carried out to ensure product safety, and to make sure the person carrying out the work on any machinery or components is safe.
- Reliability: Any given piece of equipment or machinery can undergo a range of non-destructive tests which will remove the risk of any inaccuracy of result, or oversight.
- Cost-Effective: These types of tests can also give insights that can result in the effective replacement or repair of components or equipment before a real malfunction or breakdown occurs, which will save more money in the long term.
Now let us go ahead and discuss the various forms of NDT Test methods that are employed in the market today. Test method names often refer to the type of penetrating medium or the equipment used to perform that test.
Let us discuss some of these methods in detail below:
1. 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 of the structure detect the AE. It is even possible to detect AE from plasticization 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.
2. 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 uses a probe to introduce a magnetic field, with RFT generally used to test pipes.
3. 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.
4. 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.
5. 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.
6. 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 the material due to pitting, erosion, or corrosion.
7. 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.
8. 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.
9. Neutron Radiographic Testing (NR)
Neutron radiography uses a beam of low-energy neutrons to penetrate 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.
10. 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-color 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 analyzed to determine the condition of a component. Often used to detect near-surface delamination or bonding defects in composites.
11. 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).
12. Vibration Analysis (VA)
This process uses sensors to measure the vibration signatures from rotating machinery to assess the condition of the equipment. The types of sensors used include displacement sensors, velocity sensors, and accelerometers.
13. Visual Testing (VT)
Visual testing is 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 using 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.
14. 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.
NDT offers a novel solution for companies to develop sustainable solutions in terms of quality control that is both economical and ecologically responsible. It poses a solution that is progressing to be the definite need of the hour!