Views: 1 Author: Site Editor Publish Time: 2019-09-23 Origin: Site
The high-frequency pulse circuit of the ultrasonic flaw detector generates a high-frequency pulse oscillating current to be applied to the piezoelectric ceramic crystal in the ultrasonic transducer (probe), which excites the ultrasonic wave and transmits it to the workpiece to be inspected, and when the ultrasonic wave propagates in the workpiece to be inspected, When a defect (heterogeneous) is encountered on the acoustic path (the propagation path of the ultrasonic wave), a reflection will be generated at the interface, and the reflected echo is received by the probe into a high-frequency pulse electrical signal input to the receiving amplifier of the flaw detector. After that, on the display screen of the flaw detector, an echo waveform (graphic) is proportional to the echo sound pressure is displayed. The size of the linear piezo tubes can be estimated according to the amplitude of the displayed echo, and the horizontal line on the display screen can be adjusted to be proportional to the propagation time (distance) of the ultrasonic wave in the medium (commonly known as "calibration"), then the position of the defect in the workpiece can be determined based on the position of the echo on the horizontal scan line of the display screen. The position of the bottom echo of the workpiece on the horizontal scanning line can also be used to determine the thickness of the workpiece. The space occupied by the ultrasonic waves is called the ultrasonic field, It includes the near field (N is the near field length) and the far field. The sound pressure distribution in the near-field region is not uniform, and the sound pressure in the far-field region changes monotonously as the distance increases. The length of the near-field region is related to the diameter of the transducer wafer and the wavelength of the ultrasonic wave, and the ultrasonic beam in the near-field region is converged, at the end of the near-field region, that is, at the transition point from the near-field region into the far-field region. The beam diameter is the smallest (so this point is also called the natural focus). After entering the far field, the beam will diverge at a certain angle.
The slope of the beam edge is expressed by the semi-diffusion angle ,the semi-diffusion angle of the beam is the same. It is related to the wafer diameter of the piezo ceramic disc crystal transducer and the wavelength of the ultrasonic wave. Therefore, in the ultrasonic detection, in order to evaluate the defect size according to the amplitude of the echo, when the size of the workpiece to be inspected is small and falls within the range of the near-field region, it is usually necessary to use the reference comparison test block for comparative evaluation, the material of the reference test block, The acoustic characteristics should be the same or similar to the object to be tested, and contain specific artificial reflectors of known size (such as flat bottom holes, transverse holes, column holes, grooves, etc.), and the detecting echo echo amplitude and the same sound.The amplitude of the reflector echoes of the process (ultrasonic propagation path) is compared, and the size of the defect equivalent expressed by the size of the artificial reflector is obtained.
In the far field detection, due to the large size of the workpiece, it is difficult to prepare the test piece of the corresponding size in advance, and it is inconvenient to carry and use. In view of the fact that the sound pressure in the far field changes monotonously with the increase of the distance, the echo sound pressure changes of various artificial reflectors are regularly ruled, so the distance-amplitude curve can be calculated by calculation or pre-measurement. (referred to as the AVG method or the DGS method) to determine the detection sensitivity and to assess the equivalent size of the defect. It must be pointed out that the size of the defect is equivalent evaluated in the ultrasonic test means that the echo amplitude of the defect is the same as the echo amplitude of the artificial reflector of a certain size, but the actual size of the defect is not the same as the size of the standard artificial reflector. Because the amplitude of the echo of the defect is affected by various factors such as the material of the workpiece to be inspected and the nature, size, shape, orientation, surface state of the defect itself, and also related to the self-characteristics of the ultrasonic wave, the "equivalent" is introduced. The concept of a considerable amount is used as a measurement of the size of defects. For example, we say that the ultrasonic inspection found that there is a defect of Φ2mm diameter flat bottom hole at a certain position, which means that the echo amplitude of the defect is Φ2mm diameter flat bottom hole at the same position in the workpiece (the bottom surface of the flat bottom hole is echo beam axis is perpendicular, and the coaxial echo amplitude is the same, however the actual area size of the defect is often greater than the bottom surface area of the Φ2 mm diameter flat bottom hole. In addition, according to the results of ultrasonic testing to determine the nature of the defect (qualitative) problem has not been well resolved, currently rely mainly on the tester's practical experience, technical level and material properties of the workpiece to be inspected, processing characteristics,conditions, etc. Understand to make a comprehensive subjective judgment. The general steps of the ultrasonic pulse reflection method to detect the attack:
(1) Selection of ultrasonic detecting surface - When the ultrasonic beam is perpendicular to the direction in which the defect extends in the workpiece, or perpendicular to the defect surface, the best reflection can be obtained, and the defect detection rate is the highest. Therefore, on the workpiece to be inspected, the surface of the workpiece that can make the ultrasonic beam as perpendicular as possible to the direction in which the defect may exist is selected as the detection surface. The right figure shows the ultrasonic inspection surface of the common workpiece.
Method for detecting surface requirements
Contact method of longitudinal wave detection ≤3.2μm
Longitudinal wave detection by water immersion ≤6.3μm
Contact method of transverse wave detection ≤3.2μm
Contact farleigh wave (surface wave) detection ≤0.8μm
Contact flange wave (plate wave) detection ≤1.6μm
If the surface of the test piece does not meet the test requirements, special surface preparation should be carried out, or special remedial measures (such as special coupling method or sensitivity compensation) should be taken.
Determination of the coupling method - When there is air between the ultrasonic probe and the workpiece to be inspected, the ultrasonic waves will be reflected and cannot enter the workpiece to be inspected. Therefore, a coupling medium is required between them, and depending on the coupling method, it can be divided into contact method, the ultrasonic probe is in direct contact with the workpiece detection surface, in which oil, transformer oil, grease, glycerin, water glass (sodium silicate Na2SiO3) or industrial glue, chemical paste,which are used as coupling agents, or commercialized. special coupling agent for ultrasonic testing. Water immersion method - There is a certain thickness of piezoelectric ceramics ring between the ultrasonic probe and the workpiece detection surface. The thickness of the water layer varies depend on the thickness of the workpiece, the speed of sound of the material and the inspection requirements, but the water quality must be clean, from bubbles and impurities,they have a wetting ability on the workpiece.
The temperature should be the same as the workpiece to be inspected, otherwise it will cause greater interference to the ultrasonic inspection. Contact method and water immersion method are the two main coupling methods used in ultrasonic testing. In addition, there are various special coupling methods such as water gap method, water jet column method, overflow method, carpet method and roller method. (4) Preparation of test conditions ,selecting an appropriate ultrasonic flaw detector, ultrasonic probe, reference standard test block (or calculation program using calculation method or distance amplitude curve, AVG or DGS curve, etc, and the instrument before the test calibration (time baseline correction, initial sensitivity setting, etc.) (5) Inspection Scan - Scan the ultrasonic probe on the inspection surface of the workpiece to be inspected, and ensure that the ultrasonic beam covers all the areas to be inspected. (6) Defect assessment - locate and mark the defects found (the depth and horizontal position of the defect in the workpiece), the quantitative (defect size, area, length), and if necessary, determine the nature or type of the defect. , that is, qualitative assessment. (7) Recording and Judgment - Record the test results, judge whether the test is qualified or not according to the technical conditions and acceptance criteria, draw the test conclusion, and issue the test report. (8) Processing - Mark the workpieces that have found the problem, isolate them for processing, and pass the qualified marks to the next production process or turnover program. The above is the most basic procedure for ultrasonic pulse reflection detection. In the actual product inspection, the inspection should be carried out according to the requirements of specific inspection specifications or test procedures. Ultrasonic pulse reflection detection is the most widely used method in ultrasonic testing, not only in industrial ultrasonic thickness gauge, but also in other fields such as thickness measurement, fish detection, underwater sonar, ocean sounding, seabed topography and geology. Structural detection, medical ultrasound diagnosis widely utilize the reflection characteristics of ultrasonic waves.
