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34 CHAPTER 6: POTENTIAL FOR RAIL BASE FLAW DETECTION 6.1 Available Technologies for Rail Base Flaw Detection Currently, there are very few nondestructive test methods that have the ability to detect flaws in the base of the rail. TTCI is currently developing an advanced ultrasonic testing technology that will be capable of monitoring the entire rail, including the base. This technology seems to be the potential solution to monitor and detect rail base corrosion. The test system uses a high powered laser to produce ultrasonic waves and air-coupled transducers to receive the signal. Conventional ultrasonic principles are applied in this system, but the noncontact nature of the system provides the ability to go far beyond the inspection capabilities of current systems. Because no contact is required between the rail and the transmitter/receiver, many inspection limitations are removed, which allows this technology to monitor the head, web, and base of the rail, unlike most of the rail flaw detection units in industry today that focus only on the inspection of the railhead (36). Schematics, principles, and the methodology currently under development by TTCI are presented in Figures 21 and 22. This technology is of particular interest for the transit industry because detailed monitoring of the base of the rail can be easily and efficiently conducted. Figure 21. Principles of emerging technologies for rail flaw detection (37). Laser Capacitive Air-Coupled Transducer VSH Rail 130 140 150 160 170 180 190 200 210 220 230 -30 -20 -10 0 10 20 30 Si gn al Am pli tud e Time (μs) SC1.114 130 140 150 160 170 180 190 200 210 220 230 -40 -30 -20 -10 0 10 20 30 40 Sig na l A mp litu de Time (μs) SC1.111No Crack Crack LAHUT Proof of Concept Vertical Split Head Inspection Laser â Air Hybrid Ultrasonic Testing (LAHUT) ÂNon-contact system âavoids rail surface problems ÂCan inspect entire rail section ÂBase  Longitudinal ÂBelow surface head defectsS ign al Am pli tud e Sig na l A mp litu de Si gn al Am pli tud e Sig na l A mp litu de
35 Figure 22. (a) Rail flaw detection vehicle showing (b) location of the lasers and transducer and (c) ultrasonic device monitoring a rail in service (37). 6.2 Current Technologies and Examples of Nondestructive Techniques with Potential for Implementation A variety of NDE methods has become available that are viable candidates for the detection of corrosion. Those that will be considered here are visual, ultrasonic, radiographic, eddy current, electromagnetic acoustic transducer scanning, thermographic inspection, and ground penetrating radar. 6.2.1 Visual Inspection Visual inspection has been and continues to be the primary method for corrosion detection. Visual inspections continue to be one of the more sensitive and reliable inspection methods because careful evaluation of the specimen by touch and sight is a very important part of the process. Visual inspection requires good vision, adequate lighting, and knowledge of what to look for, so only qualified personnel can complete a thorough inspection of the rail. Enhancement of visual inspection can be done by using low powered magnifying glasses, borescopes, or video cameras (38). Unfortunately, visual inspection is only applicable to corrosion that is exposed to the surface; internal and hidden corrosion can easily go unseen.
36 6.2.2 Ultrasonic Inspection Ultrasonic testing (UT) methods can be used to measure the thickness or length of an item. Another common use of ultrasound is for flaw detection. Ultrasound also has the ability to detect liquids inside a component, measure the presence and amount of corrosion in buried anchor rods, and interrogate concrete. Automated ultrasonic inspection systems have been successfully used to detect subsurface corrosion and cracking in piping, storage tanks, and aircraft components. Ultrasonic thickness meters are also commonly used to check pipelines for corrosion. Although these meters can determine wall thickness, it is still very difficult to detect localized internal corrosion. Ultrasonic thickness mapping of specifically selected inspection points provides far more information about the condition of a piping component, tank wall, or floor (38). Automated ultrasonic inspection systems, in particular phased array UT systems, can be successfully used to detect subsurface corrosion on piping, tank walls, aircraft lap-joints and micro-cracking around rivets of high speed aircraft. The use of a phased array UT system can replace several conventional UT probes, and the angle of incidence can be controlled electronically. For more information, refer to reference 39. Phased array systems generate a beam with various probe elements pulsing at slightly different times. By precisely controlling the delays between the probe elements, beams of various angles, focal distance, and focal spot size, an optimization of the beam can be produced. It is possible to change the angle, focal distance, or focal spot size, simply by changing the timing to the various elements. The sectorial scan is a real-time side view generated from a single inspection point, without any physical motion from the probe. Phase array has the advantage that the probe has various transducers that can monitor in various directions or angles at once. 6.2.3 Radiographic Inspection Radiographic inspection provides more information about the condition of a component than any other method; however, it also presents the highest safety risks because it requires the use of dangerous ionizing radiation. It also requires access to both sides of the component being inspected, and it is expensive. Radiographic films have been the primary method to capture images of the component under investigation, but there are several new image-recording methods now becoming available (38). One of the newer image-recording devices, referred to as computed radiography, employs the use of photo-sensitive plates which are used similarly to conventional films. One of the most important differences is that the photostimuable plates are reusable and more efficient at collecting data than film. The dynamic range is far greater with the plates than the film because the latent image is digitized. Other important methods for capturing radiographic images are lens and charge coupled devices, direct imaging flat panel devices, linear arrays, and image intensifiers. All of these devices can be configured into programmable automated scanning systems. The collected image data provided by each of these image collection devices can be enhanced, stored, and retrieved (38). Unfortunately, this method is complicated for implementation into various transit systems.
37 6.2.4 Eddy Current Inspection Eddy current inspection aids in the detection of surface (or near surface) anomalies and is used quite extensively in the aerospace industry. Eddy current technology is also an excellent, inexpensive tool used for material sorting and measuring coating or material thickness. Specialty tubing manufacturers rely on automated eddy current inspection devices to test their products. Eddy current thickness mapping is performed to detect corrosion in aluminum aircraft skins. 6.2.5 Electromagnetic Acoustic Transducer Electromagnetic Acoustic Transducer (EMAT) induces ultrasonic waves in metals without the need for a coupling medium. The method is designed for rapid assessment of corrosion in piping, even if the piping is coated or at temperatures up to 500°F. A volumetric interrogation of the full circumference of a pipe run is accomplished with the EMAT devices astride the top of the pipe. Anomalies that have been detected in long runs of piping by this method are erosion, pitting, and cracking (38). 6.2.6 Thermographic Inspection Thermographic inspection, commonly referred to as infrared inspection, locates defects using thermal characteristics of the specimen. This inspection method is being successfully used throughout the railway industry. Locating hot spot defects in electrical services has been particularly effective. Other uses include the determination of process liquid or catalyst levels in chemical towers and/or columns and boiler tube corrosion characterization with a scanning thermal line device. Thermographic images are acquired in real-time and advanced processing provides information about coating defects and/or thickness variations in the coating and subsurface corrosion spots (38). Lawrence Livermore National Laboratory has developed a dual-band infrared computed tomography system that can search for defects on bridge decks and airplane fuselages. The dual- band infrared images are acquired as maps. The data is then processed, revealing corrosion damage and other surface and subsurface anomalies. 6.2.7 Ground Penetrating Radar Ground penetrating radar systems provide three-dimensional information about reinforced concrete. Ground Penetrating Radar (GPR) requires access to only one surface and is faster, safer, and less expensive than radiography. GPR can provide information concerning rebar that is below the surface and has been found to be useful in evaluating existing structures for continued use, modification, or repair (38).
