TIMEOUT FOR TECH
Figure 4A: Ultrasonic testing equipment customized for rail inspection. Courtesy of Herzog.
ULTRASONIC
TESTING OF STEEL Seeing invisible defects using inaudible sounds.
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BY GARY T. FRY, PH.D., P.E., VICE PRESIDENT, FRY TECHNICAL SERVICES, INC.
elcome to “Timeout for Tech with Gary T. Fry, Ph.D., P.E.” Each month, we examine a technology topic that professionals in the railway industry have asked to learn more about. This month, our topic is fatigue defect detection in steel. Fatigue defects are a common cause of failure of steel components that resist dynamic loads. For this reason, fitness for service assessments of such components nearly always include specific inspections that are designed to detect the presence of fatigue defects. Unfortunately, detecting the defects can be quite difficult. This is primarily because fatigue cracks are usually exceptionally thin, having measurable length and width but negligible thickness. As a result, even when a large fatigue defect extends from the surface of a component, it might be apparent only as a very thin, faint line that is not easily seen with the unaided eye—very easily missed. Sometimes fatigue defects do not extend to the surface of the component, remaining 44 Railway Age // March 2022
completely encased within. Such defects are termed internal defects or subsurface defects and are essentially invisible. This is common with fatigue defects in railway wheel rims and rail heads. For example, Figure 1 (right) illustrates a typical transverse fatigue defect in a rail head. The defect is tinted in green and is located near, but a bit below, the top surface of the rail. The important question is this: How can such defects be detected? One way to do this is with acoustic wave energy. Figure 2 (p. 45) is a photograph of a gondola floating through a Venetian canal on a quiet summer afternoon. As the boat glides along, the bow of the boat and the gondolier’s oar generate small waves. If we look closely at the magenta square inset in the photograph, we can see that some of those waves are reflected back into the canal after encountering the wall of a building. Wave motion is the underlying principle of technology that allows us to create images of internal body structures in medical examinations and to “see” invisible fatigue defects inside solid steel. It is also how some animals, in complete darkness,
can locate and capture food while avoiding obstacles, competitors and predators. The technology is called ultrasound, ultrasonic non-destructive testing (NDT), ultrasonic testing (UT) or echolocation. As the name implies, ultrasonic testing involves high-frequency sound energy. Specifically, ultrasonic means beyond human hearing. The frequencies used in ultrasonic inspection are usually in a range
Figure 1: Schematic illustration of a rail head fatigue crack. Commonly, cracks in rail heads and wheel rims are completely contained and invisible. railwayage.com