Basic knowledge of ultrasonic flaw detection
The general concept of 1.1 ultrasonic
Define, ultrasonic and basic physical quantities
Definition: ultrasonic ultrasonic (Ultrasound) refers to the frequency of more than 20000 Hz (Hertz, Hz) which surpass hearing sound high limit, which belongs to the mechanical wave.
(a) the ultrasound has three basic physical quantities, namely frequency (f), wavelength (lambda), speed of sound (C). Frequency (Frequency) is in every second, the number of medium vibration, represented by F, units of Hertz (Hz); speed of sound (Speedofsound) refers to the speed of sound wave propagation in the propagation medium, denoted by C; wavelength (Wavelength for the completion of a completely vibration time propagation distance, to lambda said. The relationship between the three is: c=f or λ =c/f λ, media material called medium of ultrasonic propagatio
(two) the same frequency of ultrasonic propagation in different media, velocity is not the same.
(three) acoustic impedance (Acousticimpedance) is used to represent an important physical medium ultrasonic propagation ability, the product of its value by the medium density and acoustic velocity of C in the medium of the decision, namely: Z= ρ C, unit Kg / m2 · s.
The physical properties of two, ultrasonic
(a) the ultrasonic propagation in the medium, to meet different acoustic impedance of the interface and the interface thickness is much larger than the wavelength, can produce reflections (Reflection). Acoustic impedance differences of the soft tissue of the body is very small, as long as the acoustic impedance of 1 ‰ difference, can produce reflection. Because the acoustic impedance of the soft tissues of the human body than air acoustic impedance is much greater, ultrasonic junction at which almost all be reflected, so the ultrasonic inspection with coupling agent, the same reason, ultrasound are not suitable for examination of lung, bone and soft tissue around the great acoustic impedance of different organs. Ultrasonic diagnostic apparatus is the use of reflection of human tissue for ultrasonic extraction, medical diagnostic information from the acoustic reflection in.
(two) when the velocity interface on both sides at the same time not phonophoresis, second medium, the propagation direction will change the refraction (Refraction). Sound waves from a small to medium speed medium speed when the incident sound loudly, through the interface of two media after refraction refraction angle is larger than the incident angle. When the incident angle is greater than critical angle (90 degrees), refraction wave corresponding disappear, appear total reflection. We are in the ultrasonic inspection, should be as far as possible the beam perpendicular to the interface, avoid the incident angle is too large, otherwise it will cause a reflection of the actual location and display the location of dislocation happened, and even the emergence of total reflection, resulting in ultrasound cannot check the organs and tissues of the interface the following.
(three) when the obstacle is equal to or less than lambda / 2, ultrasonic will bypass the obstacles and continue to advance, this phenomenon is called diffraction (Diffraction), the ultrasonic wavelength shorter (i.e. higher frequencies), can find the obstacle is small, that is to say the resolution is better, ultra sound images more clear, but through the organization ability.
(four) the obstacles encountered rough surface or minimal in ultrasonic propagation in (or a group of small obstacles form), there will be a part of the energy to be scattering (Scattering)
(five) the process of ultrasonic propagation in medium, with the increase of the propagation distance, the acoustic energy decreased gradually, this phenomenon is called acoustic attenuation (Acousticattenuation). Attenuation of sound absorption and medium on the acoustic wave (Acousticabsorption), and the scattering and beam spread and other reasons, the absorption is the main attenuation factor.
(six) the Doppler effect (Dopplereffect) for the relative motion between the sound source and the receiver and the induced acoustic frequency change phenomenon. When the source and receiver are oppositely, acoustic frequency receiver received higher than the sound source frequency, as they go in the opposite direction, the receiving frequency emitted below the sound source frequency, the frequency difference of frequency shift (Frequencyshift). Ultrasonic source and receiver Doppler ultrasound instruments are installed in the probe (Huan Nengqi), probe, Huan Nengqi sends out the ultrasonic backscattering, issued by the movement of the red blood cells, then the receiver Huan Nengqi receives the echo. The ultrasonic echo the receiving transducer received frequency after two times the Doppler effect, so the sending and receiving ultrasonic frequency difference and flow compared with two times speed proportional to transducer.