1. Classification by principle
Ultrasonic flaw detection methods are classified according to the principle, which can be divided into pulse reflection method, penetration method and resonance method.
1. Pulse-echo method
The ultrasonic probe emits pulse waves into the test piece, and the method of detecting the defects of the test piece according to the reflected wave is called the pulse reflection method. Pulse reflection methods include defect echo method, bottom wave height method and multiple bottom wave method.
2. Penetration method
The penetration method is a method to judge the defect condition based on the energy change after the pulse wave or continuous wave penetrates the test piece. The penetration method often uses two probes, one for receiving and the other for sending, which are respectively placed on both sides of the specimen for detection.
3. Resonance method
If the sound wave (continuous wave with adjustable frequency) propagates in the tested workpiece, when the thickness of the test piece is an integer multiple of the half-wavelength of the ultrasonic wave, it will cause resonance, and the instrument will display the resonance frequency. When there are defects in the test piece or the thickness of the workpiece changes, the resonant frequency of the test piece will be changed, and the method of judging the defect and the change of the workpiece thickness according to the resonant frequency characteristics of the test piece is called the resonance method. The resonance method is often used to measure the thickness of test pieces.
2. Classified by waveform
According to the waveform used in flaw detection, it can be divided into longitudinal wave method, shear wave method, surface wave method, plate wave method, climbing wave method and so on.
1. Longitudinal wave method
The method of using a straight probe to emit longitudinal waves for flaw detection is called the longitudinal wave method. At this time, the beam is vertically incident on the detection surface of the test piece, and penetrates into the test piece with a constant wave pattern and direction, so it is also called the vertical incidence method, or vertical method for short.
The vertical method is divided into single crystal probe reflection method, dual crystal probe reflection method and penetration method. Commonly used single crystal probe reflection method.
The vertical method is mainly used for flaw detection of casting, forging, rolling materials and their products. This method has a good effect on the detection of defects parallel to the detection surface. Due to the limitations of the blind area and resolution, the reflection method can only find the defects inside the test piece beyond a certain distance from the detection surface.
When propagating in the same medium, the velocity of the longitudinal wave is greater than that of other wave modes, the penetration ability is strong, and the sensitivity of grain boundary reflection or scattering is poor, so the thickness of the detectable workpiece is the largest among all wave modes, and it can be used for Flaw detection of coarse-grained materials.
2. Shear wave method
The method in which the longitudinal wave is incident obliquely on the detection surface of the test piece through wedges, water and other media, and the method of using the wave mode conversion to obtain the shear wave for flaw detection is called the shear wave method. Since the transverse beam penetrating into the test piece forms an acute angle with the detection surface, it is also called the oblique beam method.
This method is mainly used for flaw detection of pipes and welds; for flaw detection of other test pieces, it is used as an effective auxiliary means to find defects that are not easy to find by vertical method.
3. Surface wave method
The method of flaw detection using surface waves is called surface wave method. This method is mainly used for specimens with smooth surfaces. Surface waves have very short wavelengths and are greatly attenuated. At the same time, it only propagates along the surface, and is sensitive to the cladding, oil, dirt, etc. on the surface, and is greatly attenuated. Using this feature, the defect can be located by touching with oil on the sound beam propagation direction and observing the change of the echo height of the defect.
4. Plate wave method
The method of using plate wave for flaw detection is called plate wave method. It is mainly used for flaw detection of simple-shaped test pieces such as thin plates and thin-walled pipes. During flaw detection, plate waves fill the entire test piece, and internal and surface defects can be found.
5. Climbing wave method
3. Classification by the number of probes
1. Single probe method
The flaw detection method that uses one probe to both transmit and receive ultrasonic waves is called the single-probe method, and the single-probe method is commonly used.
2. Dual probe method
The method of flaw detection using two probes (one transmitting and one receiving) is called the double probe method, which is mainly used to find defects that are difficult to detect with a single probe
3. Multi-probe method
The method of using more than two probes combined in pairs for flaw detection is called the multi-probe method.
4. Classification according to the contact mode of the probe
1. Direct contact method
A thin layer of couplant is coated between the probe and the detection surface of the test piece, so it can be regarded as a direct contact between the two. This method is called the direct contact method.
This method is easy to operate, the flaw detection pattern is relatively simple, the judgment is easy, and the defect detection sensitivity is high, so it is the most widely used method in actual flaw detection. However, the roughness requirements of the detection surface of the tested part are relatively high.
2. Liquid immersion method
The method of immersing the probe and the workpiece in the liquid and using the liquid as the coupling agent for flaw detection is called the liquid immersion method. The couplant can be oil or water.
The liquid immersion method is suitable for test pieces with rough surfaces, the probe is not easy to wear, the coupling is stable, the detection results are repeatable, and it is convenient for automatic flaw detection.
The liquid immersion method is divided into full immersion and partial immersion
