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Ultrasonic thickness measurement before use
Sep 28, 2018

Ultrasonic thickness measurement before use

Let's talk about the following small series, before using the ultrasonic thickness measurement

(1) The surface roughness of the workpiece is too large, resulting in poor coupling between the probe and the contact surface, low reflection echo, and even failure to receive echo signals. For surface rust, in-service equipment, pipelines, etc., which have extremely poor coupling effects, can be treated by sand, grinding, frustration, etc. to reduce the roughness, and at the same time, the oxide and paint layers can be removed to expose the metallic luster, so that the probe A good coupling effect can be achieved by the coupling agent with the test object.

(2) The radius of curvature of the workpiece is too small, especially when the small diameter tube is thick, because the surface of the common probe is a plane, and the contact with the curved surface is point contact or line contact, and the sound intensity transmission is low (coupling is not good). A small diameter probe (<6mm) can be used to accurately measure curved materials such as pipes.

(3) The detection surface is not parallel to the bottom surface, and the sound wave is scattered by the bottom surface, and the probe cannot receive the bottom wave signal.

(4) Castings and austenitic steels are caused by uneven tissue or coarse grains. When ultrasonic waves pass through them, they cause severe scattering attenuation. The scattered ultrasonic waves propagate along complex paths, which may cause the echoes to annihilate, resulting in no display. . A low-frequency coarse crystal dedicated probe (2.5MHz) is available.

(5) There is some wear on the probe contact surface. The surface of the commonly used thickness measuring probe is made of acrylic resin. The long-term use will increase the surface roughness, resulting in a decrease in sensitivity, resulting in incorrect display. It can be sanded with 500# sandpaper to make it smooth and ensure parallelism. If it is still unstable, consider replacing the probe.

Ultrasonic thickness measurement before use

(1) There are a large number of corrosion pits on the back of the object to be tested. Due to rust spots and corrosion pits on the other side of the object, the sound waves are attenuated, resulting in irregular readings and, in extreme cases, no reading.

(2) There is sediment in the measured object (such as pipeline). When the sediment and the acoustic impedance of the workpiece are not much different, the thickness gauge shows the wall thickness plus the thickness of the deposit.

(3) When there are defects inside the material (such as inclusions, interlayers, etc.), the displayed value is about 70% of the nominal thickness. At this time, an ultrasonic flaw detector or a thickness gauge with a waveform display (such as MVX, PVX of Dakota, USA) can be used. Or CMX, etc.) further defect detection.

(4) The effect of temperature. Generally, the speed of sound in solid materials decreases with increasing temperature. Test data shows that for every 100 °C increase in hot material, the speed of sound drops by 1%. This is often the case with high temperature in-service equipment. High temperature special probes and high temperature coupling agents (300-600 ° C) should be used. Do not use ordinary probes.

(5) Laminated materials, composite (non-homogeneous) materials. It is impossible to measure uncoupled laminates because ultrasonic waves cannot penetrate uncoupled spaces and cannot propagate at a constant rate in composite (non-homogeneous) materials. For equipment made of multi-layer material (like urea high-pressure equipment), special care should be taken when measuring thickness. The thickness gauge indicates only the thickness of the material that is in contact with the probe.

Ultrasonic thickness measurement before use

(1) The influence of the coupling agent. The coupling agent is used to exclude the air between the probe and the object to be measured, so that the ultrasonic wave can effectively penetrate the workpiece for inspection purposes. If the type is selected or the method of use is improper, it will cause an error or the coupling mark will flash and cannot be measured. Since a suitable type is selected depending on the use, a low viscosity coupling agent can be used when used on a smooth material surface; a highly viscous coupling agent should be used when used on a rough surface, a vertical surface, and a top surface. High temperature couplings should be used for high temperature workpieces. Secondly, the coupling agent should be used in an appropriate amount and evenly applied. Generally, the coupling agent should be applied to the surface of the material to be tested, but when the measurement temperature is high, the coupling agent should be applied to the probe.

(2) The sound speed selection is incorrect. Before measuring the workpiece, preset the speed of sound according to the type of material or reverse the sound speed according to the standard block. When the instrument is calibrated with one material (commonly used for steel) and another material is measured, erroneous results will result. It is required to correctly identify the material before measuring and select the appropriate speed of sound.

(3) The effect of stress. Most of the in-service equipment and pipelines have stresses. The stress state of solid materials has a certain influence on the speed of sound. When the stress direction is consistent with the direction of propagation, if the stress is compressive stress, the stress will increase the elasticity of the workpiece and accelerate the speed of sound; If the stress is tensile stress, the speed of sound is slowed down. When the stress and the wave propagation direction are different, the vibration trajectory of the particle 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.

(4) The effect of metal surface oxide or paint overlay. The dense oxide or paint anti-corrosion layer produced on the metal surface, although tightly combined with the matrix material, has no obvious interface, but the speed of sound propagation in the two materials is different, resulting in errors, and the thickness of the cover varies with the thickness of the cover. It is also different.

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