Common problems in ultrasonic Thickness Gauge measurement
In the actual testing work of the ultrasonic Thickness Gauge, it is often encountered that the indicated value of the Thickness Gauge is obviously larger or smaller than the design value (or expected value). The reasons are as follows:
1. Laminated materials, composite (heterogeneous) materials. It is not possible to measure uncoupled laminated materials because ultrasonic waves cannot penetrate uncoupled spaces and cannot propagate at a uniform velocity in composite (heterogeneous) materials. For equipment made of multi-layer materials (such as urea high-pressure equipment), special attention should be paid when measuring the thickness. The indicated value of the Thickness Gauge only indicates the thickness of the layer of material that is in contact with the probe.
2. The speed of sound is wrongly selected. Before measuring the workpiece, preset its sound velocity according to the type of material or reversely measure the sound velocity according to the standard block. When the instrument is calibrated with one material (the commonly used test block is steel) and another material is measured, wrong results will be produced.
3. The influence of temperature. Generally, the sound velocity in a solid material decreases with the increase of its temperature. According to experimental data, the sound velocity decreases by 1% for every increase of 100°C in a hot material. This is often the case for high-temperature in-service equipment.
4. The influence of couplant. The couplant is used to exclude the air between the probe and the measured object, so that the ultrasonic wave can effectively penetrate the workpiece to achieve the purpose of detection. If the type is selected or used improperly, it will cause errors or the coupling mark will flicker, making it impossible to measure. In actual use, due to the excessive use of couplant, when the probe leaves the workpiece, the instrument indicates the thickness of the couplant layer.
5. There is sediment in the measured object (such as a pipeline). When the acoustic impedance of the sediment and the workpiece is not much different, the value displayed by the Thickness Gauge is the wall thickness plus the thickness of the sediment.
6. The effect of metal surface oxide or paint coating. Although the dense oxide or paint anti-corrosion layer produced on the metal surface is closely combined with the base material and has no obvious interface, the propagation speed of the sound velocity in the two substances is different, resulting in errors, and the error varies with the thickness of the covering. Also different.
7. When there are defects (such as inclusions, interlayers, etc.) inside the material, the displayed value is about 70% of the nominal thickness (at this time, an ultrasonic flaw Detector should be used for further defect detection).
8. The influence of stress. Most of the in-service equipment and pipelines have stress, and the stress state of solid materials has a certain influence on the sound velocity. When the stress direction is consistent with the propagation direction, if the stress is compressive stress, the stress will increase the elasticity of the workpiece and accelerate the sound velocity; on the contrary , if the stress is tensile stress, the speed of sound slows down. When the stress and the propagation direction of the wave are different, the particle vibration trajectory is disturbed by the stress during the wave process, and the wave propagation direction deviates. According to the data, the general stress increases and the speed of sound increases slowly.
