8. New Technology Roots
The electromagnetic, noncontact transducer (EMAT) is particularly well suited to ultrasonic inspection problems where it is difficult or impossible to couple the sound into the object being tested through a liquid or grease layer. For inspecting buried natural gas pipelines, the inspection process must be carried out by a self contained instrument package blown through the pipe by the gas stream. This demands that the transducers move with speeds of 10 to 20 mph a long the pipe and give continuous · operation over distances of 50 to 100 miles. Under the sponsorship of the American Gas Association, it has been demonstrated that EMAT's designed to launch and detect Lamb waves which propagate around the circumference of the pipe can meet these inspection requirements. · The special transducers designed for this purpose used conductors laid down in a meander line configuration with the spacing between the adjacent lines chosen to be equal to one half of the wave length of the first antisymmetric Lamb wave mode of the pipe wall (a flexure wave) at the operating frequency of 130 KHz. These Lamb waves were focused into a beam of energy and directed around the pipe circumference so that when they encountered a longitudinal stress corrosion crack or a region of pit corrosion, the reflection of energy could be used to detect the location and approximate size of the defect. Both transmitters and receivers were fabricated and the detection of simulated defects machined into the outside surface of a 36-inch dia. by 40-foot long section of pipe buried at the Science Center has been demonstrated. In addition, special experiments were performed to demonstrate that the EMAT's performed satisfactorily when the metal moves past them at speeds up to 20 mph (27 ft/sec, 1/3 inches/millisecond).
Alers, George A., "Buried Gas Pipeline Inspection with EMAT's" (1978). Proceedings of the ARPA/AFML Review of Progress in Quantitative NDE, September 1976–June 1977. 56.