Nondestructive testing (NDT)

Nondestructive testing (NDT) is any method used to test a product or substance that does not cause damage to or destruction of the item being tested. This testing can be as simple as a visual inspection—the original form of nondestructive testing—or it can involve a number of chemical or imaging procedures to allow inspectors to see things beyond visual capabilities. NDT has applications in a number of industries and helps to ensure the reliability and safety of many products.

Background

It is unclear exactly when people began using means other than vision to inspect manufactured objects. There is evidence that the ancient Greeks and Romans used oil and flour to find cracks in marble as far back as 750 BCE. Blacksmiths and bell makers struck an object and listened to its sound to detect flaws in the metal; experience taught them which tone indicated the metal was whole and free of defects.

These methods developed somewhat organically, however, and were not specifically considered nondestructive testing. The first recorded instance of someone using a method to test the soundness of an object without destroying it happened in 1868, when an Englishman named S. M. Saxby wrote an article describing a process for magnetizing gun barrels and using a compass to find areas with cracks or other weaknesses. Within a few years, this technique was being used to find problems in metal train rails and turbines.

After German physicist Wilhelm Conrad Röntgen discovered X-rays in 1895, he noted that one of their potential uses might be to find defects in objects, though this use was not fully explored for another three decades. During the 1930s, X-rays were used to identify potential problems in a number of products, most notably those that were welded. These uses grew after the foundation of the American Industrial Radium and X-Ray Society in 1941. This organization became known as the American Society for Nondestructive Testing (ASNT). Other early types of nondestructive testing included the use of liquid dyes that could penetrate metals and other substances to reveal weaknesses and ultrasonic testing, both of which were developed during the latter half of the nineteenth century and put into widespread use in the early twentieth century.

Overview

Nondestructive testing goes by a number of names. Nondestructive examination, nondestructive evaluation, and nondestructive inspection are three of the most common. The testing can be done while items are being manufactured, prior to distribution, or while in use. This enables companies to determine if products meet standards and will meet expectations for their lifespan. The testing can also be used to find flaws during use that can present a safety hazard or create an interruption in productivity. It has a significant advantage over its alternative, destructive testing, in that the item is not destroyed or damaged by testing and can continue to be used after testing, assuming it passes the test.

There are two main categories of NDT: conventional and advanced. Conventional methods are often simpler and may be less costly than more advanced methods. Some of the most popular and widely used conventional methods include the following:

• acoustic emission testing, which detects stress within an object by listening to the sounds it makes
• infrared testing, which uses special light rays to look for weakened areas
• liquid penetrant testing, which uses a fluorescent or glowing dye to reveal flaws
• electromagnetic and magnetic particle testing, which use magnetic fields to identify weaknesses
• ultrasonic testing, which uses ultrasound waves to uncover problem areas
• various forms of X-ray or radiographic imaging to "see" inside objects

Inspectors still also rely on their eyes and experience to identify potential problems during a visual inspection.

Many of the advanced forms of NDT are variations of the more conventional forms. For instance, advanced radiographic forms of testing include computerized radiography, computerized tomography (CT scan), and digital radiography. Ultrasonic testing can be enhanced through the use of long-range testing, phased array testing, and a number of other variations, such as electromagnetic acoustic transducer testing, the automated ultrasonic backscatter technique, immersion testing, and internal rotary inspection. Electromagnetic testing can be enhanced with alternating current field measurements and eddy current testing, as well as magnetic flux leakage testing. In addition, lasers, including holographic lasers, and vibration technology can be used to test substances.

NDT testing is important for a number of reasons. Failure of a part can create a dangerous situation for workers and others. For instance, a weld that weakens can allow a large, heavy piece of equipment to fall, or a coupling that develops an internal crack can allow hazardous materials to spill. This can endanger people who are near the item when it fails and create hazards that can harm others as well. Equipment failures can also cost companies large sums of money in lost inventory, time, and replacement or repair costs. For the company that manufactures the item, failures can result in costly damage to its reputation and/or lawsuits.

A number of industries rely very heavily on NDT. These include the aerospace, defense, automotive, chemical, and oil and gas industries. In these industries, it is critical for those involved to have confidence in the equipment and parts being used. For this reason, companies test their products at several stages before and during installation. Testing is often done during use as well to ensure that wear and tear has not caused damage that could pose a risk.

In some instances, nondestructive testing is also done after a failure to determine why the failure occurred without destroying anything that might obscure the cause. For example, if an airplane crashes from an unknown cause, parts that are suspected of being linked to the cause might be examined with radiography or ultrasonic techniques to look for any hidden weaknesses or failures that might have led to the accident. Investigators may be reluctant to cut parts open to examine them directly, because in doing so, they might inadvertently cut through and obscure a hidden problem.

NDT has become so important that a number of organizations maintain codes and standards for what testing should be done and when it should be done in various stages of a product's life. Some testing might be required during manufacturing, while other equipment may require testing at specific points during its lifespan. These tests provide a cost-effective way to ensure the safety and reliability of equipment and materials and are essential to the profitability and success of many industries.

Bibliography

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"Nondestructive Testing Standards." ASTM International, www.astm.org/Standards/nondestructive-testing-standards.html. Accessed 7 Nov. 2024.

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