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Nondestructive Testing
| Article
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10253 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
8 / 1986 |
4,168 Words |
| Author
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David I. Lewin
David I. Lewin writes on science, technology, and medicine
from Washington, D.C. |
If a nineteenth-century factory worker wanted to check a newly made train wheel, he would strike it and listen to the sound. Today the worker would subject the wheel to X rays or ultrasound, pass it by a magnetic coil, or perhaps even probe it with a laser beam.
The art of testing an object for defects without destroying it or changing its usefulness is called either "nondestructive testing (NDT)" or "nondestructive evaluation (NDE)." It can be traced back centuries to egg candling and metal ringing. Within the past hundred years this art has become a science based on discoveries about the structure of matter and matter's interactions with energy. Nondestructive tests ensure the safety and reliability of manmade objects as different from each other as airliners and automobiles or industrial boilers and microchips. As new materials with unique properties come into use, academic and industrial researchers seek to develop appropriate tests.
"The biggest challenge in nondestructive evaluation is understanding how the probing energy interacts with the material to give the information you want," said Thomas Yolken of the National Bureau of Standards (NBS) in Gaithersburg, Maryland, a Washington, D.C., suburb. According to Yolken, who heads up NDE activities at this federal laboratory, almost every form of energy is used by nondestructive techniques: electromagnetic energy (light, X rays, gamma rays, magnetic fields), mechanical vibrational energy (ultrasound), and thermal energy. Subatomic particles such as neutrons and electrons are also used to study materials nondestructively.
Ten years ago, Yolken would have described NDE as a technology based largely on empirical knowledge. The practitioner searched for a needle in a haystack - a potential cause of failure. Finding the "needle" required in-service inspection: stripping down an aircraft engine to look for cracks or signs of wear; inspecting a pipeline for leaks, or for flaws that could produce leaks; and certifying that large turbines at electric power plants are fit for continued use.
In manufacturing, NDE has been used either at the end of a process or between major steps to separate "good" products from "bad." If a weld was tested and found unacceptable, it would be ground out and redone. If a bad part could not be reworked to meet the manufacturer's standards, it would be discarded. "The positive feature of this is that you don't sell bad products," Yolken said. "The negative feature is that you don't find your problems until you have a lot of value added, and that is
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