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Analytical analysis and characterization techniques are used throughout the technology, science, materials and engineering industries. Many of these analytical techniques are separated into classes and one way of defining techniques is whether they are destructive or not, i.e. does the techniques cause damage to the sample during analysis. The class of non-destructive techniques is large and encompasses many different types of instruments and techniques, and we’re going to look at what non-destructive testing (NDT) encompasses in this article.
Non-destructive testing—commonly referred to as NDT—is the common name for the tests that are performed on a material which does not cause damage to the sample. It is an area that can go by many names, with it also being referred to as non-destructive examination, non-destructive inspection, and non-destructive evaluation. Unlike destructive testing methods, non-destructive testing offers a way to analyze the softest and most delicate of samples, as well as very large structures.
Where destructive testing methods will induce some strain, mechanical abrasion, chemical change, radiation damage or some other way of breaking the sample to analyze it, non-destructive testing methods tend to use electromagnetic waves as they do not damage the sample while, at the same time, providing information on it.
Non-destructive testing methods are often split into various categories and these are often governed by the type of wave which is used in the analysis. Some of the most common examples of this are magnetic fields, ultrasonic waves and X-rays that make up the common magnetic particle testing (MPT), ultrasonic testing (UT), and radiographic testing (RT) classes, respectively. These are not the only examples of non-destructive testing areas, but merely showcase some of the different electromagnetic waves which can be used.
Because non-destructive testing involves methods that use different electromagnetic waves, the sample being measured is not limited to a certain size—which is often the case with many destructive methods where there are sample size limits. This is why large construction samples can be tested and the construction/civil engineering industry is one of the industries that have been benefitted by the different non-destructive testing methods available. Moreover, unlike destructive testing, non-destructive testing can also be performed on manufacturing and assembly lines to analyze the samples in real-time.
Another area available to those who use non-destructive testing over destructive testing is when an item needs to be repaired, as it enables the damaged areas to be identified (and subsequently repaired) without causing further damage to the item. One area of common use nowadays is during the fabrication process of a material/construction itself, as the methods available can show the operators if the joining and fusing of materials are being performed to a high standard and are not likely to break and cause potential harm to anyone installing/working on the materials (as well as anyone in the vicinity of the build). So, the scope available, and the industries that utilize non-destructive testing methods are widespread.
Aside from not destroying the sample, there are several reasons why non-destructive testing is used, and it can also be used to gain a lot of information about the sample being analyzed. One other advantage is that the operators are never at any risk because the waves used are generally harmless to humans. There is one exception, and that is when X-rays are used—because, just like any area in the medical field, the X-rays need to be used in a controlled environment so that the operator is not exposed to excessive amounts of radiation. But, on the whole, they are a safe class of methods for operators and the local environment where they are used. The other key advantage is that they have high repeatability and the results from multiple tests can be correlated together easily, which has made them an ideal set of methods for quality control applications.
In terms of the information available with non-destructive testing methods, it does vary between the different techniques used. Overall, the operator can obtain a wide range of information about the physical properties of a material, and this includes the impact resistance, ductility, yield, tensile strength, fracture toughness and fatigue strength of the material. However, non-destructive techniques are best utilized when the aim is to see if there are any issues with the material in question (rather than its beneficial properties), including whether there are any discontinuities, differences in material characteristics, corrosion damage, delamination, disbonds, voids, or inclusions in the sample. Overall, they have become a useful class of techniques for the many reasons outlined above.
Sources and Further Reading
American Society for Nondestructive Testing: https://www.asnt.org/MinorSiteSections/AboutASNT/Intro-to-NDT
TWI: https://www.twi-global.com/technical-knowledge/faqs/what-is-non-destructive-testing
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