Ultrasonic Testing Principle

Ultrasonic Testing Principle

Date:Jun 06, 2018

Shorter than ordinary sound waves, wave lengths of ultrasound make a good direction, but also through the opaque material, this feature has been widely used in ultrasonic testing, thickness, distance measurement, remote control and the technology of ultrasonic imaging. Ultrasonic imaging is a technology that USES ultrasound to present the inner image of opaque objects. From the transducer of ultrasonic acoustic lens focused on opaque sample, the ultrasonic carried from the sample passes was as part of the information (such as the ability of reflection, absorption and scattering of sound waves), the acoustic lens converge on piezoelectric receiver, the electrical signal input amplifier, using scanning system can turn opaque sample image displayed on the screen. The device is called an ultrasonic microscope. Ultrasonic imaging technology has been widely applied in medical examination, in the manufacturing of microelectronic device used for inspection on large scale integrated circuit, is used for displaying alloys of different compositions in the materials science area and grain boundary, etc. Acoustical holography is an ultrasonic interference principle of record and reproduce the three-dimensional image of opaque acoustic imaging technology, its principle and optical holography are basically the same, just record means different (see holography). With the same ultrasonic signal source motivation two transducer be placed in a liquid, they launched two coherent beam of ultrasound: a beam through the object studied after become a wave, a bunch of reference wave. Object wave and reference wave coherent superposition acoustical hologram formed on the surface of the liquid, with laser beam acoustical hologram, using a laser reflection on acoustical hologram diffraction effect and get things back like, usually with a camera and television sets for real-time observation.

The meaning of ultrasonic cleaning

Ultrasonic cleaning effect, is more than human hearing audio transmission wave in the liquid. When the ultrasonic propagation in the detergent, due to the sonic is a longitudinal wave, longitudinal wave to promote the role of the media can make the liquid pressure changes, resulting in many tiny vacuum bubble, referred to as "cavitation effect". When the bubble compression blasting, can produce powerful impact, can be in fixation of objects within the corner dirt scattered, and enhance the wash wash effect, owing to the length of the ultrasonic frequency takanami, strong penetrating power, so to have a crack or hidden complex structure of the cleaning, can achieve amazing wash effect

Ultrasonic cleaning is based on cavitation, that is, in the cleaning fluid in the rapid formation of numerous bubbles and rapid implosion. The resulting shock will strip the dirt off the inside and outside surfaces of the workpiece immersed in the cleaning solution. With the increase of ultrasonic frequency, the number of bubbles increases and the blasting impact is weakened. Therefore, high-frequency ultrasound is especially suitable for cleaning small particle dirt without breaking the surface of the workpiece. The expansion of cavitation bubbles and the burst (implosion) bubbles are created by applying high-frequency (ultrasonic), high-intensity sound waves to liquids. Any ultrasonic cleaning system, therefore, must have three basic elements: cheng fang cleaning fluid in the tank, converts electrical energy into mechanical energy of high frequency electrical signal transducer and ultrasonic generator.

Transducers and generators

The most important part of ultrasonic cleaning system is the transducer. There are two kinds of transducer, one is magnetic transducer, which is made of nickel or nickel alloy. A piezoelectric transducer made of lead zirconate titanate or other ceramics.

When a piezoelectric material is placed into an electric field of varying voltage, it deforms. This is called the "piezoelectric effect". Magnetic transducers, by contrast, are made of materials that deform in a changing magnetic field. No matter what kind of transducer is used, the most basic factor is usually the intensity of cavitation effect.

Ultrasonic waves, like other sound waves, are a series of pressure points, a wave that compresses and expands alternately (as shown below). If the sound energy is strong enough, the liquid is pushed away at the expansion stage of the wave and bubbles are formed. In the compression stage of wave, these bubbles burst or implode in liquid instantly, producing a very effective impact force, especially suitable for cleaning. This process is called cavitation. Sound waves of compression and expansion are analyzed theoretically, the burst of the cavitation bubble will produce more than 10000 psi pressure and the high temperature of 20000 ° F (11000 ° C), and in the instant blast will burst quickly outward radiation. Energy released by a single cavitation bubble is very small, but every second for millions of cavitation bubble burst at the same time, the cumulative effect will be very strong, produce the powerful impact of contamination of the workpiece surface spalling, this is all the characteristics of ultrasonic cleaning. If the ultrasonic energy is large enough, cavitation will occur everywhere in the cleaning solution, so ultrasound can effectively clean small cracks and holes. Cavitation also promotes chemical reactions and speeds up the dissolution of surface membranes. But only in a certain area of the liquid pressure is lower than the gas pressure inside a bubble matches will produce cavitation phenomenon in the area, so the generated by the transducer of ultrasonic wave amplitude is large enough to satisfy this condition. The minimum power required to produce cavitation is called the cavitation critical point. Different liquids have different cavitation critical points, so ultrasonic energy must exceed the critical point to achieve the cleaning effect. That is, cavitation bubbles can only be produced if the energy exceeds the critical point for ultrasonic cleaning.

Importance of frequency

Noise is produced when the working frequency is low (within the range of human hearing). When the frequency is less than 20kHz, the working noise not only becomes very high, but may exceed the safety noise limit stipulated by the occupational safety and health law or other regulations. In applications where high power is required to remove dirt without considering the surface damage of the workpiece, a lower cleaning frequency ranging from 20kHz to 30kHz is usually chosen. The frequency of cleaning within this frequency range is often used to clean large, heavy or high density materials. The 20KHz magnetic transducer and 25KHz piezoelectric transducer Cavitation Relative Strength to 40 kHz frequencies are commonly used to clean smaller, more sophisticated parts or to remove tiny particles. High frequencies are also used in applications where damage is not allowed on the surface of the workpiece. Using high frequencies improves cleaning performance in several ways. As the frequency increases, the number of cavitation bubbles increases linearly, producing more intense shock waves that allow them to enter smaller gaps. If the power remains constant and cavitation bubbles decrease, the energy released by cavitation bubbles will decrease correspondingly, which effectively reduces the damage to the workpiece surface. Another advantage of high frequencies is that they reduce the viscous boundary layer (the Bernoulli effect), allowing ultrasound to 'detect' extremely small particles. This situation is similar to that of small rocks at the bottom of a clear stream when the water level in the stream drops. The company offers a range of intermediate frequencies of 40kHz, 80kHz, 120kHz and 170kHz. Products with a frequency of 350kHz can be selected when cleaning extremely small particles. The company recently launched a MicroCoustics system for such occasions at a frequency of 400kHz.

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