Analysis of ultrasonic thickness gauge display value is too large or too small reasons
In the actual test work, often encountered thickness gauge value and design value (or expected value) compared to the obvious large or small, the reasons are analysed as follows:
(1), laminated materials, composite (non-homogeneous) materials. To measure the uncoupled laminated material is impossible, because ultrasonic waves can not penetrate the uncoupled space, and can not be uniformly propagated in the composite (non-homogeneous) material. For equipment made of multiple layers of material cladding (like urea high-pressure equipment), the thickness measurement should pay special attention to the thickness gauge indicates only the thickness of the layer of material in contact with the probe.
(2), the wrong choice of sound speed. Before measuring the workpiece, according to the type of material preset its speed of sound or according to the standard block back-measure the speed of sound. When a material calibration instrument with a material (commonly used test block for steel) and then go to measure another material, will produce erroneous results.
(3), the effect of temperature. General solid materials in the speed of sound with its temperature increases and decreases, there are test data show that the hot material every increase of 100 ° C, the speed of sound decreased by 1%. For high-temperature in-service equipment often encountered this situation.
(4), the influence of the coupling agent. Coupling agent is used to exclude the air between the probe and the object under test, so that ultrasonic waves can effectively penetrate the workpiece to achieve the purpose of detection. If the choice of type or use of improper methods, will cause errors or coupling sign flicker, can not be measured. In practice, due to the use of coupling agent used too much, resulting in the probe away from the workpiece, the instrument shows the value of the coupling agent layer thickness value.
(5), the measured object (such as pipelines) within the deposit, when the deposit and the workpiece acoustic impedance difference is not large, the thickness gauge shows the value of the wall thickness plus the thickness of the deposit.
(6), the influence of metal surface oxides or paint coverings. Metal surface produced dense oxide or paint anti-corrosion layer, although the combination of close to the base material, no name obvious interface, but the speed of sound in the two substances in the propagation speed is different, thus causing errors, and with the thickness of the cover different, the size of the error is different.
(7), when there are defects within the material (such as inclusions, interlayers, etc.), the display value of about 70% of the nominal thickness (at this time to use ultrasonic flaw detector for further defect detection).
(8), the impact of stress. In-service equipment, pipelines, most of the presence of stress, the stress condition of the solid material has a certain impact on the speed of sound, when the direction of stress and propagation direction, if the stress is compressive stress, the stress effect of the workpiece elasticity increases, the speed of sound; conversely, if the stress is tensile stress, the speed of sound slows down. When the stress and wave propagation direction is not the same, fluctuations in the process of mass vibration trajectory by stress interference, wave propagation direction deviation. According to the data, the general stress increases, the speed of sound increases slowly.
