DMTA-10022-01EN, Rev. C, January 2015Chapter 562couplant than larger-diameter transducers. In all modes, tilting the transducerdistorts echoes, and causes inaccurate readings, as noted below.Curvature of the test pieceA related issue involves the alignment of the transducer with respect to the testpiece. When measuring on curved surfaces, it is important that the transducer beplaced approximately on the centerline of the part, and held steadily as close tothe surface as possible. In some cases, a spring-loaded V-block holder may behelpful in maintaining this alignment. In general, as the radius of curvaturedecreases, the size of the transducer should be reduced, and transducer alignmentbecomes more critical. For very small radii, an immersion approach is necessary.In some cases, it may be useful to observe the waveform display as an aid tomaintaining optimum alignment. Observe the best method for holding atransducer with the aid of a waveform display. On curved surfaces, it is importantto use only enough couplant to obtain a reading. Excess couplant forms a filletbetween the transducer and the test surface where sound reverberates, andpossibly creates spurious signals that may trigger false readings.Taper or eccentricityIf the contact surface or back surface of the test piece is tapered or eccentric withrespect to the other, the return echo is distorted due to the variation of the soundpath across the width of the beam. The accuracy of the measurement is thereforereduced. In severe cases, no measurement is possible.Acoustic properties of the test materialThere are several conditions in certain engineering materials that can potentiallylimit the accuracy and range of ultrasonic thickness measurements:• Sound scattering:In materials such as cast stainless steel, cast iron, fiberglass, and composites,sound energy scatters from individual crystallites in the casting, or fromboundaries of dissimilar materials within the fiberglass or composite.Porosity in any material can have the same effect. Adjust the instrumentsensitivity to prevent detection of these spurious scatter echoes. Thiscompensation can in turn limit the ability to discriminate a valid return echofrom the back wall of the material, thereby restricting the measurement range.• Sound attenuation or absorption:In many organic materials, such as low-density plastics and rubbers, soundenergy is attenuated very rapidly at the frequencies used for ultrasonicgaging. This attenuation typically increases with temperature. The maximumthickness that can be measured in these materials is often limited byattenuation.
