T.O. 33B-1-15.5.3.3.4If there are two or more areas on the test part of different thickness (within paragraph 5.5.3.3 limits) whichcan be measured both ultrasonically and mechanically or optically, these areas may be used as the standards.5.5.4TestPartPreparation.5.5.4.1SurfaceContamination.All foreign matter that might interfere with the thickness measurements shall be removed. Examples of suchmatter are loose scale, paint, dirt, and rust. For maximum accuracy, paint should be removed in the area tobe measured. Paint can introduce errors in the measurements up to three times the maximum thickness ofthe paint. Metallic plating on the surface of the test part (Cr, Cd, Ni, etc.) will not significantly affect theaccuracy of the readings; this plating is usually relatively thin (0.0005 inch).5.5.4.2SurfaceRoughness.The surface finish of the test part affects the accuracy of the reading as shown in Table 5-7. If the surface ofthe test part is pitted or irregular, consistent readings will not be obtained. Variations as great as the depthof the pits or irregularities will be introduced. If permitted by the applicable manual or the prime depot, localareas may be sanded to provide smooth surface for increased accuracy in the thickness measurements.5.5.5SpecialConsiderations.5.5.5.1CorrosionPitting.The effect on thickness measurements of corrosion pits on the back surface of the test part depends on thesize of the pits and the size of the search unit. The depth of large pits (search unit diameter or greater) can bemeasured by subtracting minimum readings from maximum readings obtained on adjacent areas of the testpart. Smaller pits will generally cause a broadening of the back surface ref lection signals and sometimes areduction in amplitude, due to scattering of the sound beam. These effects can be observed on instrumentsequipped with waveform displays. Smaller pits also lower the average thickness readings of the test part.5.5.5.2CurvedSurfaces.Measurements of curved surfaces require reference standards in accordance with paragraph 5.5.3.3.1. Inaddition, for convex radii less than one inch or concave radii less than three inches, shoes are required toadapt the search unit to the curved surface. In addition, detailed procedures for taking the measurementsmust be obtained from the applicable NDI manual or the depot level engineering activity. On all curvedsurfaces, it is recommended that an instrument with a waveform display be used. Small-diameter searchunits (1/4 inch or less) are also recommended. When making a measurement on a curved surface, the backsurface signals should be maximized by rocking the search unit on the surface until the back surface signalspeak and the thickness reading is at a minimum. The minimum thickness reading should be recorded as thetest part thickness.5.5.6CalibrationandMeasurement.NOTEAccurate thickness measurements require that the reference standards andthe test part have temperatures equal within 10oF. Calibration shall beperformed in the same physical location as the measurements on the test part.Adequate time should be allowed for the reference standards to reach the testpart temperature. When using puls-eco or resonance method to measurethickness of materials, exact horizontal linearity of equipment is crucial andmust be checked prior to calibration and measuring thicknesses. See processcontrol chapter for procedures.If only a basic ultrasonic instrument is available and no specific instructions are given, the followingprocedure may be used.a.Obtain two reference standards in accordance with paragraph 5.5.3.3.b.Select a search unit that will resolve the back surfaces of the reference standards.c.Decide which divisions on the horizontal scale of the display graticule would be convenient locationsfor the echo signals from the back surfaces of the references. For example, if the reference standardsChange 25-83
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
