Brief Analysis of Ultrasonic Thickness Measurement Method in Nondestructive Testing

There are many methods for measuring thickness in non-destructive testing, and there are also many corresponding instruments. In addition to the general mechanical method, the commonly used thickness gauges are: X-ray thickness gauge, ultrasonic thickness gauge, magnetic thickness gauge, current thickness gauge, etc. Compared with thickness gauges made using other principles, ultrasonic thickness gauges have the advantages of being small, light, fast, high-precision, battery-powered, and dirt accumulated in the container does not affect the measurement accuracy. Therefore, in recent years, most of the thickness gauges used in industry for thickness measurement are ultrasonic thickness gauges.

Ultrasonic thickness gauges were first used in my country in the 1960s. As early as the 1960s, my country designed and manufactured the CCH-J-1 pulse reflection thickness gauge, which was a head type, and mass-produced for users. Now, digital technology has been applied to manufacture the UTM-101H digital thickness gauge.

In recent years, pulse reflection thickness gauges have developed very rapidly. Due to the use of integrated circuits, digital thickness gauges are small enough to be held in the palm of your hand and weigh no more than one kilogram. The lower limit of measurement is reduced to 0.25 mm, the upper limit is generally several hundred mm, and the accuracy can reach 0.01 mm.

Based on the principle, ultrasonic thickness gauges can be divided into two types: resonance type and pulse type. The following focuses on pulse reflection thickness gauges.

(I) Basic principles

In principle, the pulse reflection thickness gauge measures the round-trip propagation time t of the ultrasonic pulse in the material, that is: d=c*t/2

If the sound speed c is known, then the round trip propagation time t of the ultrasonic pulse in the material can be measured to obtain the material thickness d. This thickness gauge can directly indicate the thickness using a meter or digital tube, and is extremely convenient to use.

The block diagram of the thickness gauge circuit workpiece principle is shown in the figure on the right. The transmitting circuit outputs a periodic electrical pulse with a very short rise time and a very narrow pulse, which is added to the probe through a cable, exciting the piezoelectric sheet to generate a pulse ultrasonic wave. The ultrasonic wave emitted by the probe enters the workpiece and forms multiple reflections on the upper and lower surfaces of the workpiece. The reflected wave passes through the piezoelectric sheet and then becomes an electrical signal. It is amplified by the amplifier, and the propagation time t of the sound wave between the two surfaces is measured by the calculation circuit, and finally converted into thickness for display.

(II) Overview of the pulse reflection thickness gauge circuit

The pulse reflection thickness gauge circuit is mainly composed of the main controller, transmitting circuit, receiving amplifier circuit, calculation circuit, thickness display and other parts.

Several major parts are discussed below.

1. Transmitter circuit

This circuit generates a narrow transmission pulse signal after being triggered by a pulse from the main controller, so that the transducer (probe) transmits an ultrasonic pulse. In order to improve the resolution and reduce the lower limit of the thickness gauge range, the transmission pulse pulse should have a short rise time and a narrow pulse width. It should be pointed out here. The factors that affect the transmission pulse width include both circuit problems and probe manufacturing problems. In order to improve the sensitivity and expand the upper limit of the measurement range, a large transmission power is required.

At present, pulsed ultrasonic thickness gauges all use transistor circuits and integrated circuits. In order to increase the emission intensity, the power supply voltage of several volts needs to be increased to tens to hundreds of volts through a DC converter and supplied to the transmitting tube to increase the emission intensity.

2. Receiving amplifier circuit

It mainly receives and amplifies the reflected signal from the bottom of the workpiece. The amplitude of the reflected signal from the bottom is affected by the intensity of the transmitted pulse, as well as the surface finish, coupling, material and bottom surface of the workpiece being measured. Therefore, the amplitude of the reflected signal varies greatly. In order to make the instrument sensitive enough, the amplifier circuit is required to have a higher gain. However, excessively increasing the gain will widen the pulse width and affect the measurement accuracy.

3. Calculation circuit and thickness display

Digital thickness gauges are the most convenient to read and have high measurement accuracy, up to ±0.01 mm. Currently, pulse ultrasonic thickness gauges produced at home and abroad are mainly digital. The following discusses the calculation circuit and thickness display of digital thickness gauges.

The transmitting pulse and the amplified bottom surface reflection signal (or two adjacent bottom surface reflection signals) trigger the thickness gate control circuit, which outputs a square wave whose width is proportional to the ultrasonic wave propagation time in the workpiece being measured. This square wave is used to control the opening and closing of the gate circuit.

The high-frequency oscillator outputs a series of high-frequency oscillation signals. During the period when the gate circuit is open, these high-frequency signals enter the calculator through the gate and are counted. Finally, the digital tube displays the number of high-frequency oscillations within the gate opening time. The count is proportional to the width of the gate opening square wave, and the square wave width is proportional to the thickness of the workpiece. Therefore, the count is proportional to the thickness. The frequency of the high-frequency oscillator is adjustable. The oscillation frequency can be adjusted according to different materials so that the number of oscillations within the gate opening time is equal to the thickness of the workpiece, so that the digital tube directly displays the thickness.

The domestic UTM-101H digital ultrasonic thickness gauge has an accuracy of ±0.1 mm and a measuring range of 1.2-220 mm.

When the measurement accuracy is 0.1 mm, for steel, the corresponding sound wave propagation time is about 0.03 microseconds, which means that one cycle of high-frequency oscillation is required to be 0.03 microseconds, that is, the oscillation frequency is adjusted to about 30 MHz. At this time, the counter displays the thickness value in units of 0.1 mm. If the measurement accuracy is to reach 0.01 mm, the high-frequency oscillation frequency is about 300 MHz. However, it is too difficult to make a digital circuit that works at such a high frequency. The actual solution is to add a square wave signal expansion circuit before the counting circuit to accurately expand the door opening time by 10 times. In this way, the measurement accuracy can reach 0.01 mm with an oscillation frequency of about 30 MHz.

(III) Use and precautions of thickness gauge

The first step is to select the probe. The probe for thickness measurement should generally be selected based on the thickness measurement range, measurement accuracy and workpiece conditions. Dual-chip probes are better because they have a wider measurement range and a lower limit.

The surface treatment requirements for the workpiece when measuring thickness are the same as those for flaw detection. When measuring the thickness of a rough surface, the polishing area is not large, and the key is to make it flat.

The selection of coupling agent is the same as that of flaw detection. For measuring the thickness of small diameter pipe walls and solid walls, glycerin or water glass is more suitable.

When measuring, in order to avoid false readings caused by multiple reflections of the coupling agent film or other clutter signals, the indication must be stable and can be repeated before reading the data. False readings are unstable in most cases. When using a dual crystal probe, for workpieces that generally have no directional problems. The probe placement direction is irrelevant, and each measuring point can be measured once. For pipeline thickness measurement, the probe should be placed in a direction so that its sound insulation layer is perpendicular to the pipeline axis. For other thickness measurements related to the probe placement direction, each measuring point should be measured twice (measure once, then rotate the probe 90° and measure again), and mark and record them.

When measuring pipes, especially small diameter pipes, carefully swing the probe left and right to make it perpendicular to the pipe wall, so as to obtain a stable and accurate thickness indication. If the instrument works normally but cannot measure the thickness, first check whether the finish of the workpiece is qualified. Check whether the coupling agent is good or not. If the finish is not good enough, polish it again. If the coupling agent is too thin, change it to one with higher viscosity. If it still cannot be measured, it may be caused by severe internal corrosion. You can move the probe a little bit and measure again.

During thickness measurement, attention should be paid to two situations: multiple readings or defect reflections. When the readings differ greatly from the expected values, the cause should be analyzed to see whether it is a multiple reading (readings too large) or a defect reflection (readings too small). At this time, if there are other types of thickness gauges or flaw detectors, they should be used for auxiliary measurements to identify the cause.

To measure high-temperature workpieces during operation, high-temperature probes and special coupling agents are required.

Usually, the acoustic impedance of the sediment in the pipeline is very different from that of the pipe material, so it has no effect on the thickness measurement. However, in some cases, such as when refining natural oil with a high sulfur content in a refinery, a strong iron salt sediment is formed in the pipeline and container, and its acoustic impedance is close to that of steel. At this time, the measured wall thickness may be the sum of the pipe wall thickness and the sediment thickness. Therefore, be particularly careful when measuring. When you are in doubt about the measured thickness, use a small hammer to knock on the pipe wall a few times, and then measure again.