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Ultrasonic Inspection - UT


ULTRASONIC INSPECTION is a nondestructive method in which beams of high-frequency sound waves are introduced into materials for the detection of surface and subsurface flaws in the material. The sound waves travel through the material with some attendant loss of energy (attenuation) and are reflected at interfaces. The reflected beam is displayed and then analyzed to define the presence and location of flaws or discontinuities.


Most ultrasonic inspection is done at frequencies between 0.1 and 25 MHz well above the range of human hearing, which is about 20 Hz to 20 kHz. Ultrasonic waves are mechanical vibrations; the amplitudes of vibrations in metal parts being ultrasonically inspected impose stresses well below the elastic limit, thus preventing permanent effects on the parts.


Advantages and Disadvantages
Superior penetrating power, which allows the detection of flaws deep in the part. Ultrasonic inspection is done routinely to thicknesses of a few meters on many types of parts and to thicknesses of about 6 m
 (20 ft) in the axial inspection of parts such as long steel shafts or rotor forgings
 High sensitivity, permitting the detection of extremely small flaws
 Greater accuracy than other nondestructive methods in determining the position of internal flaws, estimating their size, and characterizing their orientation, shape, and 
nature
 Only one surface needs to be accessible Operation is electronic, which provides almost instantaneous indications of flaws. This makes the method suitable for immediate interpretation, automation, rapid scanning, in-line production monitoring, and process control.
 With most systems, a permanent record of inspection results can be made for future reference
 Volumetric scanning ability, enabling the inspection of a 4 volume of metal extending from front surface to back surface of a part
 Nonhazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity Portability
 Provides an output that can be processed digitally by a computer to characterize defects and to determine material properties


The disadvantages
Manual operation requires careful attention by experienced technicians
Extensive technical knowledge is required for the development of inspection procedures
Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect
Discontinuities that are present in a shallow layer immediately beneath the surface may not be detectable
Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected 
Reference Standards are needed, both for calibrating the equipment and for characterizing flaws
Some of the major types of equipment that are ultrasonically inspected for the presence of flaws are:
Mill components: Rolls, shafts, drives, and press columns Power equipment: Turbine forgings, generator rotors, pressure piping, weldments, pressure vessels, nuclear fuel elements, and other reactor components
Jet engine parts: Turbine and compressor forgings, and gear blanks
Aircraft components: Forging stock, frame sections, and honeycomb sandwich assemblies
Machinery materials: Die blocks, tool steels, and drill pipe 
Railroad parts: Axles, wheels, track, and welded rail
Automotive parts: Forgings, ductile castings, and brazed and/or welded components

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