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ULTRASONIC TESTINGUltrasonic testing is an inspection method that has experienced revolutionary advances in the last 2 decades. Ultrasonic imaging, phased array and many other computerized applications of ultrasound have changed our day-to-day industrial inspections in the last 10 years.
More rigorously, ultrasonic testing is a nondestructive testing method capable of locating both surface and subsurface discontinuities in components. Ultrasonic testing can also be used to measure the thickness of materials, a common method of evaluating metal losses due to corrosion or erosion.
Ultrasound is sound above the human detection level (20 kHz). Produced by a piezoelectric crystal, it is mechanical in nature. The crystal converts electrical energy to mechanical vibrations and back to electrical energy. A couplant (liquid medium) transfers ultrasound from the crystal containing transducer to the component tested; it eliminates the air barrier between parts. When the sound beam encounters a discontinuity in the component examined, some sound reflects back to the transducer. An electronic instrument displays the reflected sound after converting it into electrical energy. Discontinuities are located by interpretation of the electronic signal in conjunction with sound path calculations.
Ultrasound measures thickness based on sound travel time from the transducer through the component thickness until the sound is reflected back to the transducer. The time is directly proportional to the component thickness.
Automated (encoded) ultrasonic testing is described in detail on the automated ultrasonics page.
Ultrasonic inspections can be performed on metallic and non-metallic materials including wood and concrete. However, the inspection depends on the acoustic properties of the material. Some austenitic stainless steels are very poor ultrasound transmitters.
Ultrasonic inspection is commonly performed on welds, castings and forgings, valves, shafts, machined parts, pressure vessel category A, B, C, and D welds, and on the search for plate delaminations.
The areas to be scanned must be clean, accessible and free of immediate obstructions. Thin wall paints and other coatings are acceptable if they are not disbonded and/or flaking. IRISNDT personnel have performed AUT scans at temperatures as high as 650 F; however, the higher the temperature, the more prejob preparation required.
Test results are interpreted in accordance with specifications and/or customer requirements. The reports have detailed drawings of the defect positions or locations.
