The principle and characteristics of ultrasonic testing (UT)
Ultrasonic Testing, the industry referred to as UT, is the most widely used in industrial Nondestructive Testing (Nondestructive Testing), the highest frequency and rapid development of a nondestructive testing technology. Can be used in product manufacturing quality control, raw material inspection, improve the process and other aspects, but also one of the indispensable means of equipment maintenance.
The main applications of ultrasonic inspection are the detection of macroscopic defects inside workpieces and the measurement of material thickness.
According to different characteristics, ultrasonic detection can be divided into a variety of different methods:
(1) Classification according to principle: ultrasonic pulse reflection method, Diffraction Time ofFlight Diffraction (TOFD), etc.
(2) Classified by display mode: A-type display, ultrasonic imaging display (B, C, D, P scanning imaging, dual array imaging, etc.).
Principle of ultrasonic detection
Ultrasonic detection, in essence, is the use of ultrasonic waves and the interaction of matter: reflection, refraction and diffraction.
(1) What is ultrasound?
We call the mechanical waves that can cause hearing sound waves, with a frequency between 20-20000Hz, and the mechanical waves with a frequency higher than 20000Hz are called ultrasonic waves, which are inaudible to humans. For the detection of metal materials such as steel, we commonly use ultrasonic waves with a frequency of 0.5~10MHz. (1MHz=10 to the sixth power Hz)
(2) How to send and receive ultrasonic waves?
The core element of ultrasonic probe is piezoelectric crystal, which has piezoelectric effect: under the action of alternating tension and compression stress, the crystal can produce alternating electric field.
When the high frequency electrical pulse excites the piezoelectric crystal, the inverse piezoelectric effect occurs, and the electrical energy is converted into sound energy (mechanical energy), and the probe intermittently emits ultrasonic waves in the form of pulses, that is, pulse waves. When the probe receives ultrasonic waves, a positive piezoelectric effect occurs, converting sound energy into electrical energy.
The conventional probe used for ultrasonic detection is generally composed of piezoelectric wafer, damping block, joint, cable, protective film and shell, which is generally divided into two categories: straight probe and inclined probe, and the latter usually has a diagonal block that makes the wafer and the incident surface at a certain Angle.
The following diagram shows a typical inclined probe structure
The following is a physical picture of the inclined probe:
The probe model :2.5P8* 12K2.5, its parameters are:
a) 2.5 represents frequency f: 2.5MHz;
b) P represents the chip material is: lead zirconate titanate ceramic, with good temperature stability, excellent electrical properties, easy to manufacture and low price;
c) 8*12 represents rectangular chip size: 8mm*12mm;
d) K2.5 represents: the tangent value of the Angle of refraction of the inclined probe is 2.5, that is, tan(68.2°)=2.5, and its refraction Angle is 68.2°.
Type A display of ultrasonic pulse reflection method working principle:
The pulse wave generated by the sound source enters the workpiece, and the ultrasonic wave propagates forward in a certain direction and speed in the workpiece. When encountering the interface with different acoustic impedance on both sides (the difference in acoustic impedance is often caused by some discontinuity in the material, such as cracks, pores, slag inclusion, etc.), part of the sound wave is reflected, and the detection equipment accepts and displays: analyzes the information such as the amplitude and position of the sound wave, and evaluates whether the defect exists or the size and position of the defect.
Type A shows the characteristics of the ultrasonic pulse reflection method
1. Scope of application
Suitable for metal, non-metal and composite materials and other parts.
a) Raw materials and parts testing: steel plate, steel forgings, aluminum and aluminum alloy plates, titanium and titanium alloy plates, composite plates, seamless steel pipes, etc.
b) Butt welding joint inspection: steel butt joints (including fillet welds, T-welded joints, support frames and structural parts), aluminum and aluminum alloy butt joints
Below is a steel butt joint: T-welded joint.
2. the advantages of the ultrasonic pulse reflection method shown in type A
a) Strong penetration ability, can detect the internal defects of the workpiece within a large thickness range. For metal materials, thin wall pipes and plates with a thickness of 1~2mm can be detected, and steel forgings several meters long can also be detected.
b) Defect location is more accurate.
c) The detection rate of area defects is higher.
d) High sensitivity, can detect the internal size of the workpiece very small defects. The theoretical sensitivity of ultrasonic detection is about half of the ultrasonic wavelength, and when the detection object is steel, the ultrasonic inclined probe with a frequency of 2.5MHz is used, and the sensitivity is about 0.65mm.
e) The detection cost is low, the speed is fast, the equipment is light, harmless to human body and the environment, and the field use is more convenient.
3. the limitations of the ultrasonic pulse reflection method displayed in type A
a) Accurate qualitative and quantitative analysis of defects in the workpiece still needs to be further studied.
b) It is difficult to perform ultrasonic inspection on workpiece with complex shape or irregular shape.
c) The location, orientation and shape of the defect have a certain influence on the detection result.
d) Workpiece material, grain size, etc., have a greater impact on detection.
e) The display of test results is not intuitive, and there is no direct witness record of test results.
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