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 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 (below) 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.
Read the full article at RailwayAge.com.
Photo Courtesy of Herzog.