Material-related Factors

Quality ultrasonic thickness gages can be highly accurate when used properly, however there are a number of factors related to the test material, part geometry, and user skill and care that can affect the degree of accuracy achieved in a given application. This section discuss the most common of those factors.

Material-related Factors

The physical properties of the material being measured will be one of the factors that affects measurement range and accuracy in ultrasonic gages. This includes both acoustic and geometrical factors.

1. Acoustic Properties of the Test Material

There are several conditions found in certain engineering materials that can potentially limit the accuracy and range of ultrasonic thickness measurements:

• Sound Scattering: In materials such as cast stainless steel, cast iron, fiberglass, and composites, sound energy will scatter from individual grain boundaries in castings or boundaries between fibers and maxtrix within the fiberglass or composite. Porosity in any material can have the same effect. Gage sensitivity must be adjusted to prevent detection of these spurious scatter echoes. This compensation can in turn limit the ability to detect a valid return echo from the back wall of the material, thereby restricting measurement range.

• Sound Attenuation or Absorption: In many polymers such as low density plastics and in most types of rubber, sound energy is attenuated very rapidly at the frequencies used for ultrasonic gaging. This attenuation typically increases with temperature. The maximum thickness that can be measured in these materials will often be limited by attenuation.

• Velocity Variations: An ultrasonic thickness measurement can be accurate only to the degree that material sound velocity is consistent with the gage's velocity calibration. Some materials exhibit significant variations in sound velocity from point to point. This happens in certain cast metals due to the changes in grain structure that result from varied cooling rates, and the anisotropy of sound velocity with respect to grain structure. Fiberglass can show localized velocity variations due to changes in the resin/fiber ratio. Many plastics and rubbers show a rapid change in sound velocity with temperature, requiring that velocity calibration be performed at the same temperature where measurements are being made.

• Phase Reversal or Phase Distortion: The phase or polarity of a returning echo is determined by the relative acoustic impedances (density × velocity) of the boundary materials. Ultrasonic gages assume the customary situation where the test piece is backed by air or a liquid, both of which have a lower acoustic impedance than metals, ceramics, or plastics. However, in some specialized cases, such as measurement of glass or plastic liners over metal, or copper cladding over steel, this impedance relationship is reversed and the echo appears phase reversed. In these cases, it is necessary to change the appropriate echo detection polarity in order to maintain accuracy. A more complex situation can occur in anisotropic or in homogeneous materials such as coarse-grain metal castings or certain composites, where material conditions result in the existence of multiple sound paths within the beam area. In these cases, phase distortion can create an echo that is neither cleanly positive nor negative. Careful experimentation with reference standards is necessary in these cases to determine the effects on measurement accuracy.

2. Physical Properties of the Test Material

The size, shape, and surface finish of the test piece must also be considered in establishing the limits of measurement range and accuracy.

• Surface Roughness of the Test Piece: The best measurement accuracy is obtained when both the front and back surfaces of the test piece are smooth. If the contact surface is rough, then the minimum thickness that can be measured will be increased because of sound reverberating in the increased thickness of the couplant layer. Echo amplitude may be reduced because of inefficient coupling. Additionally, if either the top or bottom surface of the test piece is rough, it may cause distortion in the returning echo due to the slightly different multiple sound paths seen by the transducer, resulting in measurement inaccuracies.

In the case of corrosion measurements, loose or flaking scale, rust, corrosion, dirt on the outside surface of a test piece will interfere with the coupling of sound energy from the transducer into the test material. Thus, any loose debris of this sort should be cleaned from the specimen with a wire brush or file before measurements are attempted. Generally it is possible to make corrosion measurements through thin layers of rust, as long as the rust is smooth and well bonded to the metal below. Some very rough cast or corroded surfaces may have to be filed or sanded smooth in order to insure proper sound coupling. It may also be necessary to remove paint if it is flaking off the metal.

• Curvature of the Test Piece: A related issue involves the alignment of the transducer with respect to the test piece. When measuring on curved surfaces, it is important that the transducer be placed approximately on the centerline of the part and held as steadily as possible on the surface. In some cases, a spring-loaded V-block holder may be helpful for maintaining this alignment. In general, as the radius of curvature decreases, the size of the transducer should be reduced, and transducer alignment will become progressively more critical. For very small radii, an immersion approach with a focused transducer will be necessary. In some cases it will be useful to observe a waveform display as an aid in maintaining optimum alignment. Also, on curved surfaces it is important to use only enough couplant to obtain a reading. Excess couplant will form a fillet between the transducer and the test surface where sound will reverberate and possibly create spurious signals that may trigger false readings.

• Taper or eccentricity: If the contact surface and back surfaces of the test piece are tapered, eccentric, or otherwise angled or misaligned with respect to each other, the return echo will be reduced in amplitude and potentially distorted due to the variation in sound path across the width of the beam. Accuracy of measurement will be reduced. Typically the measured thickness will represent an approximate integrated average of the changing thicknesses within the beam diameter. In cases of significant misalignment, no measurement will be possible, because the reflected beam will form a v-path away from the transducer and thus not be received. This effect becomes progressively greater as material thickness increases.

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