T.O. 33B-1-15-80SECTION VULTRASONIC THICKNESS MEASUREMENT5.5 ULTRASONICTHICKNESSMEASUREMENT.NOTEState-of-the-art instruments provide highly accurate thickness measurements from0.005 inch up to several inches. These instruments not only measure thicknesses ininches/mm, but can also determine the velocity of the material under test.5.5.1 Applications.Examples of applications for ultrasonic thickness measurement are as follows:a. Checking part thickness when access to the backside is not available.b. Checking large panels in interior area where a conventional micrometer cannot reach.c. Maintenance inspections for checking thickness loss due to wear and/or corrosion.5.5.2 GeneralPrinciples.Two basic methods of measuring thickness ultrasonically are the pulse-echo method and the resonance method.5.5.2.1 Pulse-EchoMethod.The pulse-echo method is now the most commonly used ultrasonic thickness measurement method. This method usesthe basic principle defined by the following equation:d = vtWhere:d = distance (inches)v = velocity (inches per second)t = time (seconds)The ultrasonic instrument measures time between the initial front and back surface signals or between successivemultiple back reflection signals. Since the velocity for a given material is a constant, the time between these signals isdirectly proportional to the thickness. Calibration procedures are used to obtain a direct readout of test part thickness.Depending on the instrument and material under test, ranges from 0.005 inch to several feet can be measured withpulse-echo thickness measurement instruments. The accuracy depends on the surface condition, the search unit and theinstrument. On smooth surfaces (63 microinches or less) maximum accuracy of ±0.0001 inch can be obtained on thelower ranges for some digital-readout instruments. On other ranges ±0.5% of full scale is a typical accuracy.5.5.2.2 ResonanceMethod.This method uses an instrument which applies continuous (as opposed to pulsed) electrical energy to the search unit.The frequency of this energy is continuously changing. Therefore, the wavelength of the sound transmitted by thesearch unit is continuously changing also, but it is changing inversely in proportion to the velocity of the material beingtested (l = v/f). When the search unit is coupled to a test part, and when one of the transmitted wavelengths is amultiple of the thickness of the part, the piezoelectric element in the search unit vibrates with a higher amplitude.When this occurs, the transducer is said to be in resonance with the part. If the instrument is calibrated on a referencestandard so that the peaks in the transducer element vibration amplitude correspond to known reference thicknesses,the instrument will indicate unknown thickness of a test part. Resonance equipment has been largely replaced bypulse-echo equipment. For this reason, the resonance method will not be discussed further in this chapter.
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