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Non-destructive testing (NDT) methods are a valuable set of techniques for many industries, especially in the materials, construction, civil engineering, and medical industries. NDT methods are a set of techniques that can be used to analyze a sample—from a small sample to a large material face in a construction project. This is all carried out without damaging the material under analysis. Therefore, NDT techniques can be used on several types of materials, including those which cannot be used with other destructive testing methods.
There are many different method classifications that fall within non-destructive testing - too many to document in detail in a single article, especially when there are many different techniques associated with each method classification. In this article, we are going to focus on the most common areas, including:
- Magnetic particle testing (MT)
- Liquid penetrant testing (PT)
- Radiographic testing (RT)
- Ultrasonic testing (UT)
- Electromagnetic testing (ET)
- Visual testing (VT)
However, non-destructive testing also encompasses automatic emission testing (AE), guided wave testing (GW), laser testing methods (LM), leak testing (LT), magnetic flux leakage (MFL), neutron radiographic testing (NR), infrared testing (IR), and vibration analysis (VA), but these methods are less widely used, hence the focus on the big six testing areas.
Magnetic Particle Testing (MT)
MT methods are a class of NDT methods which use one or more magnetic fields to detect any discontinuities, cracks or breaks in and around the surface of a ferromagnetic material. The magnetic field is induced in these methods by either a permanent magnet or an electromagnet, with specific examples being yokes, prods, coils, heads and central conductors. When the magnetic field encounters some form of discontinuity in the surface of the material (that is in the transverse direction to the magnetic field), it generates a magnetic flux field that does not carry outside of the material. So, when ferromagnetic particles (which are often colored, but can be a powder or suspension) are put onto the material, they gravitate to the area where there is a localized flux field and gather there—and, because they are different colors, the area with a crack can be visually identified.
Liquid Penetrant Testing (PT)
PT methods work by using a low-viscosity fluid that can penetrate any cracks, fissures and voids in a material. The fluids can then be seen by the tester as the liquids are often colored or are fluorescent (so a black light is needed for these liquid penetrants). There are different fluids that can be used. The different fluids often govern the different technique classifications within this category, alongside the ways in which they can be removed afterwards so that they don’t cause damage to the sample. Examples (which take the name from both considerations) include water-washable, solvent-removable and post-emulsifiable methods. The liquid is not kept in the material too long to avoid damage and is completely removed post-analysis. It is a method which can be used on both magnetic and non-magnetic materials, but it is not a good option for porous materials (for obvious reasons).
Radiographic Testing (RT)
Radiographic testing involves exposing the material being analyzed to radiation, most commonly X-rays or gamma radiation. The radiation penetrates and passes through the material, where it is collected by a detector. The detectors determine the intensity of the waves which reach it against the incident radiation to determine if there are any anomalous areas/discontinuities. Other factors which are important for determining any issues in the material include the thickness of the material and the distance between the source and receiver. There are many different types of techniques used, which vary between the type of radiation and detector used, including film radiography, computed radiography (CR), computed tomography (CT), and digital radiography (DR).
Ultrasonic Testing (UT)
UT methods use sound waves to analyze material and are very similar techniques to SONAR. The different techniques vary in many different areas, with differences in the operating principle, types of wave, sound wave path and equipment varying between the techniques in this method class.
Some of the techniques used include straight beam, angle beam, immersion testing, through transmission, phase array, pulsed echo, immersion, air-coupled, full matrix capture, virtual source aperture and time of flight diffraction methods.
Ultra-high frequency waves are used in all the methods and can be either a compression or shear wave. While there are differences between the techniques, the general mechanism is that the sound wave is sent towards the material with a known speed. The time required to reach the receiver is known for each part of the material, so if it differs from the norm, then the operator knows that there is something abnormal in the material—such as a discontinuity, crack or void.
Electromagnetic Testing (ET)
There are a few methods that fall within the realm of ET, but eddy current testing is by far the most used, with others including alternating current field measurements and remote field testing. The methods are similar to magnetic particle testing, in that an induced magnetic field is used. Much like the MT methods, the discontinuities within the material will cause localized changes in the magnetic field, where the magnetic field flows around them and the feedback in the system can be used to identify these changes. In the case of eddy current testing, it is slightly different in that an eddy current is used and any discontinuities will cause an identifiable change in the eddy current density. Software can be used from the feedback data to isolate and identify the location of any discontinuities in the material.
Visual Testing (VT)
Visual testing is simply a way of looking at the surface of a material to see if there are any surface cracks. Because of its simplicity, it is one of the most widely used methods in industry. Some of the ways that this can be performed is through direct viewing, line of sight viewing, or through the use of various optical components such as magnifying glasses, mirrors, charge-coupled devices (CCDs) or computer-assisted viewing systems. Some of the information that can be obtained using this approach includes misalignment of parts, physical damage, cracks and whether there is any corrosion.
Sources and Further Reading
TWI: https://www.twi-global.com/technical-knowledge/faqs/what-is-non-destructive-testing
ASNT: https://www.asnt.org/MinorSiteSections/AboutASNT/Intro-to-NDT
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