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Ultrasonic thickness measurement error analysis

Ultrasonic thickness measurement error analysis
Ultrasonic thickness gauge thickness measurement is based on the principle of reflection of an ultrasonic pulse, when an ultrasonic pulse emitted by the probe through the measured object reaches the material interface, the pulse is reflected back to the probe through the accurate measurement of ultrasonic propagation time in the material is determined measuring the thickness of the material. Where enables the ultrasonic wave at a constant speed in its internal propagation of various materials can be used on this principle measurement. Actual detection work, often encountered than ultrasonic thickness gauge showing the value and design value (or expected value), significantly larger or smaller, articles for these for some reason analysis.

Ultrasonic thickness gauge showing the value of the design value (or expected value) compared to actual detection work, often encounter significantly larger or smaller, and reasons are analyzed as follows:

1, laminated materials, composite (non-homogeneous) material. To be measured without the coupling of the laminated material is not possible, due to the ultrasonic waves can not penetrate without a coupling space, and can not spread the material of the composite (heterogeneity) uniform. Thickness measurement by the multilayer materials bandage made equipment (such as high-pressure urea equipment), special attention to the value of ultrasonic thickness gauge showing only that the layer thickness of the material in contact with the probe.

2, the speed of sound wrong choice. The preset measurement of the workpiece according to the type of material the speed of sound, or according to the speed of the standard block anti measured aloud. (Commonly used test block for steel) went to the measurement of another material with a material correction instrument will produce incorrect results.

3, the influence of temperature. The speed of sound in general solid material decreases as its temperature rises, the experimental data show that the hot material for each additional 100C, the speed of sound, a decrease of 1%. For high temperature service equipment often encounter this situation.

4, the influence of the coupling agent. The coupling agent is used to the exclusion of air between the probe and the measured object, so that the ultrasonic energy can penetrate effectively into the workpiece to achieve the detection purpose. If you choose to type or use of improper, will result in errors or coupling logo flashes, can not be measured. Actual use, due to the excessive use of coupling agent, resulting in the probe away from the workpiece, the instrument showing the value of the coupling agent layer thickness values.

5, the measured object (such as pipelines) within sediments when the sediment and the acoustic impedance of the workpiece or less ultrasonic thickness value of wall thickness and sediment thickness.

6, the metal surface oxide layer or paint coverage. Metal surface a dense oxide or corrosion paint layer, although in combination with the matrix material closely, innominate significant interface, but the speed of sound in the propagation velocity of the two substances is different, resulting in error, and with the cover thickness is different, the size of the error also different.

7, when the material internal defects (such as inclusions, laminated, etc.), the displayed value is approximately the nominal thickness of 70% (in this case to use ultrasonic flaw the further defect detection).

8, the impact of stress. Equipment in service pipes most of the presence of stress, the stress state of the solid materials have a certain impact on the speed of sound, when the stress direction consistent with the direction of propagation, if the stress is compressive stress, the stress of the workpiece elastic increase to accelerate the speed of sound; contrary , if the stress is tensile stress, the sound velocity slower. When the stress wave propagation in different directions to the fluctuations in the process of particle vibration trajectory by


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