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How does an ultrasonic flaw detector work

Views:45     Author:Site Editor     Publish Time: 2021-04-14      Origin:Site

The industry commonly uses ultrasound of a few megahertz to detect injuries. This is what a ultrasound flaw detector do. The ultrasonic frequency is high, then the linearity of propagation is high and easy to propagate in solids. Encountering two different media formed by the interface is easy to reflect, which can detect defects. Generally, in a homogeneous material, defects will cause a discontinuity in the material; this discontinuity will often cause acoustic impedance inconsistencies. Through the reflection theorem, we know that the ultrasonic waves at the intersection of two different acoustic impedances will reflect the magnitude of the energy and the media on both sides of the acoustic impedance hub and the difference in the orientation of the interface.


Usually, the ultrasonic probe is used to make good contact with the inspected workpiece's surface, and the search effectively transmits ultrasonic waves to the workpiece. It can receive the ultrasonic waves reflected from the (defective) interface while converting them into electrical signals and then transferring them to the processing instrument. For example, a piece of defect on a workpiece made of steel due to this defect's presence, resulting in the formation of two media, the same interface with different acoustic impedance, when the emitted ultrasound encounters this interface will reflect and send a corresponding signal. The signal transmitted to the instrument, the display corresponds to the horizontal coordinates of a specific location, will show the horizontal coordinates of the reflected wave this location, this horizontal coordinate is the depth of the defect in the material under inspection This horizontal coordinate is the depth of the defect in the material under inspection. Different defects, the height, and shape of the reflected wave can directly reflect the defect's nature.


According to the propagation speed (often called the speed of sound) and propagation time of ultrasonic waves in the medium, we can know the defect's location. When the more significant the fault, the larger the reflective surface, the greater the reflected energy, so according to the reflected energy's size to determine each defect's extent. Commonly used detection waveforms are longitudinal, transverse, and surface waves. The first two are suitable for detecting internal weaknesses, while the latter is ideal for detecting surface defects but requires high surface conditions.



Preparation 1: Ultrasonic flaw detector

  1. Oblique probe calibration    

    Oblique probe calibration usually requires the following steps:

    ① calibration of the incidence point (probe front)

    ② calibration of the probe angle (K value);

    ③ calibration of the material sound speed;

    ④ calibration of the probe zero points.   

  2. The fundamental settings of the flaw detector     

    After the flaw detector is turned on, the first according to the reality of the inspection of the necessary things to set up the detection channel of the flaw detector, scanning method, the apparent tree model surrounding, material sound speed, probe method, gate width, gate origin, gate height, and other fundamental parameters.     

  3. Straight probe zero point calibration

    You can directly adjust the probe zero point for steel forgings because the material sound speed is known. Select the fundamental effect group, then click on the corresponding probe zero on the effect menu. Place the probe on the test block, adjust the sound velocity range of the detector so that the state line measurement (S) of the sound spectrum coincides with the known thickness of the test block. This zero point of the v-probe is the correct probe zero point.

  4. The connection of the flaw detector

    Ultrasonic flaw detector before use to prepare the workpiece to be measured, and then insert the probe cable plug into the socket above the host, screw the plug, and in the probe connector BNC connected to meet the probe. Good use of single probe method, the two connector sockets are equally practical (internal parallel connection), the use of securing double crystal (TR) probe (a wafer launch, a wafer absorption) or two probes (a launch, an absorption), to meticulously connect the launched probe to the left socket, the absorption probe connected to the correct socket. After connecting the flaw detector and pressing the key, the flaw detector emits a short "beep beep" sound, releases the critical finger, the flaw detector active power on. 


Preparation 2: Probe connection calibration

  1. Sweeping agility at least 6dB higher than the benchmark agility.    

  2. Probe movement rate should not exceed 150mm/s. When accepting active alarm device sweeping, not subject to this limit.    

  3. Coupling agent    

    Oil, pastel, glycerin, etc., and does not damage the test workpiece's external surface.   

  4. Detection coverage rate   

    Detection, each sweep of the probe, should ensure that the sound beam's sweep between each other covers more than 15% of the probe diameter.     

  5. Detection surface   

    (1) Acoustic beam sweep to the whole workpiece to be inspected.

    The detection surface and the detection range are determined, should ensure that the acoustic beam sweep to the entire workpiece to be inspected and other products.

    (2) Circumferential and longitudinal detection of oblique probes

    For defects with radial diffusion on the shaft, transverse and longitudinal detection with a straight probe is challenging. Therefore, an oblique probe with a suitable refraction angle needs to be selected for circumferential detection on the shaft's outer surface. At this time, the acoustic beam is shot obliquely on the column, and the acoustic pressure emission rate of radial defects is higher. In reality detection, the oblique probe is consistent with the shaft's circumferential curvature. It can be detected in two ways: single probe or double probe with one sending and one receiving. 

    (3) Straight probe axial and radial inspection

    Straight probes for axial detection, longitudinal and axial incidence, can be found perpendicular to the axis or near the vertical transverse defects axis. Straight probe for radial detection, the probe is placed on the shaft's outer surface. Users can find the central longitudinal beam along the radius of the axis of incidence in the axis of the most common forging deformation formed by the sheet's longitudinal defects. For other differences in forgings, the straight probe detection method also differs; shaft pieces should usually only accept the pancake's axial and circumferential detection. The ring-type should be essential to get the end face radial, circumferential detection of the outer circle.



Ultrasonic flaw detector manual

  1. The probe surface trimming: should be removed from the welding work surface spatter, oxide skin, pits, and rust, etc., the finish is generally lower than ▽ 4. Both sides of the weld flaw detection surface trimming width is usually greater than or equal to 2KT + 50mm, (K: probe K value, T: workpiece thickness). The available choice of K value of 2.5 probes according to the base material of the weld. For example, the thickness of the base material of the workpiece to be tested is 10mm; then, each side should trim the weld 100mm.

  2. The coupling agent's choice should consider viscosity, fluidity, adhesion, no corrosion of the workpiece surface, easy to clean, and economical, combining the above factors to choose paste as coupling agent.

  3. Since the base material's thickness is thin; the detection direction is carried out on one side and two sides.

  4. Because the plate thickness is less than 20mm, so the horizontal positioning method is used to adjust the instrument's scanning speed.

  5. In the process of flaw detection operation, using coarse flaw detection and fine flaw detection. To roughly understand the presence or absence of defects and distribution, quantitative positioning is fine flaw detection. The use of sawtooth sweep left and right sweep, front and back sweep, corner sweep, surround sweep, and several other sweeping methods to find various defects and determine defects' nature.

  6. The detection results are recorded, such as the discovery of internal defects to assess the analysis. If found to have exceeded the typical defects, issued a notice of rectification to the workshop to rectify and re-inspection until qualified.


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