Application of Piezo Ceramics Used for ultrasonic non-destructive testing

The development history and characteristics of ultrasound

Acoustics, as a branch of physics,it is a science that studies the occurrence, propagation, reception, and effects of sound waves. Before 1940, there were only single-crystal piezoelectric materials, which made ultrasonic wave not widely used. In the 1970s, PLZT transparent piezoelectric ceramics were developed. The development of piezoelectric materials has greatly promoted the development of the ultrasonic field.

The total frequency of sound waves is 10-4 Hz to 1014 Hz. Generally, sound waves with a frequency of 2 × 104 Hz to 2 × 109 Hz are called ultrasonic waves. Ultrasound, as a part of sound waves,which follows the basic law of sound wave propagation, but piezo ceramic disc crystal also has some outstanding features: ① The directionality of ultrasound waves is good: Ultrasonic waves have the same directivity as light waves, and can be directionally transmitted through a special design. Effective detection in the detection object, the higher the ultrasonic frequency, the better the directivity; ② it is strong ultrasonic penetrating ability for most media, it has a strong penetrating ability; ③ high ultrasonic energy: The working frequency of ultrasonic detection is much higher than the frequency of sound waves and has high energy; ④ Ultrasonic waves have short wavelengths and small diffraction phenomena: because of their high frequency and short wavelength, its reflections caused by object defects are easy to find, so they are often used for flaw detection. It is these characteristics that make ultrasound widely used in non-destructive testing.

Basic principles of ultrasonic testing

Ultrasonic waves propagate in different media. When is encountering the interface, the ultrasonic waves will undergo reflection, refraction and wave mode conversion. Reflections occur at heterogeneous interfaces with different acoustic impedances. When ultrasonic waves are perpendicularly incident from medium 1 to medium 2, the reflectivity at the interface R. The larger the difference in acoustic impedance between the two media, the more the reflection of ultrasonic waves at the interface strong. The larger the acoustic impedance difference between the inclusions in the Pzt material piezoelectric ceramic and the substrate, the greater the possibility of detection.

Detection content

Piezo ceramics detected by ultrasonic method can be divided into porosity detection and surface defect detection.Detection porosity of piezo ceramic The strength, elastic modulus, and density of piezo ceramics are directly related to the porosity. The porosity of some piezo components determines whether it can be used. There is also a certain relationship between the transmission speed of ultrasonic waves and porosity (or density) in piezoceramics.which can be obtained through actual measurement, and then the porosity of piezo ceramics can be obtained by using equations.

The stomata are reduced to randomly oriented ellipsoids, which are uniformly distributed in an isotropic matrix. The composite micromechanical model is used to calculate the speed of sound and compare it with the measuring values. The porosity is calculated. When the theoretical density of piezo ceramic materials is known, the porosity can be calculated from the bulk density. There are many methods to measure the body density. The Archimedes method is commonly used. The internal defects of piezo ceramics can also be detected by the water immersion flaw detection method. When the defects are detected by the water immersion longitudinal wave vertical flaw detection method, the beam path can be read directly. To detect small defects, a lower frequency probe can be used instead.

 Surface defect detection

For piezo ceramics, surface defects of the same shape and size are more likely to cause damage than internal defects, so the detection of surface defects is particularly important. The surface defects of piezo ceramics are usually detected by water immersion surface wave method. The ultrasonic wave (longitudinal wave) is obliquely incident on the surface of the object to be immersed in the water. When the incident angle θc is greater than the second critical angle θC, the refracted sound waves all propagate along the surface of the workpiece, forming a surface wave equation: Cl 1-longitudinal in water Velocity of sound; Cr2——Sonic surface velocity of water immersion in the workpiece.

The surface wave wavelength is shorter than the shear wave wavelength, and the attenuation is also greater than the shear wave. At the same time, it only propagates along the surface. When it encounters sharp corners or edges, there will be strong reflection echoes. The greater the curvature, the stronger the reflection. When the flooded surface wave propagates on the surface of the test block, energy can leak into the water, so after only a few millimeters, the height of the reflected wave of the flooded surface wave is significantly reduced. In view of the small amplitude characteristic of the reflected wave transmission distance, the piezo ceramic ring transducer should be as close to the defect as possible. In addition to the above methods, comb electrode transducer method, ultrasonic microscope method and laser scanning ultrasonic microscope method can be used for the detection of modern piezo ceramic surface defects. So far, there are not many researches on ultrasonic ceramic flaw detection in modern China, the accumulated data is small, and there are no standard test blocks with strong objectivity. The research directions that need to be strengthened in the future are the development and application of new flaw detection methods and the development of high-performance probes. Of course, with the rapid development of information technology, which is embodied by computer technology, modern ultrasonic non-destructive testing technology is also developing towards digital signal processing and inspection imaging.