Methods of non-destructive testing

Non-destructive testing is the use of acoustic, optical, magnetic and electrical properties, in the absence of damage or does not affect the use of the object under the premise of the performance of the object being inspected, the detection of the existence of defects or inhomogeneities in the object being inspected, giving the size, location, nature and number of defects and other information, and thus determining the state of technology in which the object is being inspected (eg, qualified or unqualified, the remaining life, etc.) The general name of all the technical means.
Commonly used non-destructive testing methods: Ultrasonic Testing (UT), Magnetic Particle Testing (MT), Liquid Penetration Testing (PT) and X-ray Testing (RT).
Ultrasonic testing
UT (Ultrasonic Testing) is one of the non-destructive testing methods in industry. Ultrasonic waves into the object encountered defects, part of the sound wave will be reflected, the transmitter and receiver can analyze the reflected wave, it can be exceptionally accurate measurement of defects, and can show the location and size of the internal defects, determine the thickness of the material and so on.
Ultrasonic inspection advantages:
1, the penetration ability is large, for example, in the steel in the effective detection depth of up to 1 meter or more;.
2, for plane-type defects such as cracks, interlayers, etc., the detection sensitivity is higher, and can determine the depth and relative size of defects;
3, lightweight equipment, safe operation, easy to realize automated inspection.
Disadvantages:
Not easy to check the complex shape of the workpiece, requires a certain degree of smoothness of the inspected surface, and the need for coupling agent to fill the gap between the probe and the inspected surface to ensure adequate acoustic coupling.
Magnetic particle inspection
First of all, let's understand the principle of magnetic particle inspection. After the magnetization of ferromagnetic materials and workpieces, due to the existence of discontinuities, the magnetic lines of force on the surface of the workpiece and near the surface of the local distortion, and produce a leakage field, adsorption of magnetic powder applied to the surface of the workpiece, forming a visually visible magnetic trace in the appropriate light, thus showing the location, shape and size of the discontinuity.

The applicability and limitations of magnetic particle inspection are:
1, magnetic particle flaw detection is suitable for detecting discontinuities on the surface and near surface of ferromagnetic materials that are very small in size and have extremely narrow gaps that are difficult to see visually.
2, magnetic particle inspection can be a variety of cases of parts detection, but also a variety of types of parts to be detected.
3,Cracks, inclusions, hairline, white spots, folding, cold segregation and loose and other defects can be found.
4, magnetic particle inspection can not detect austenitic stainless steel materials and welds welded with austenitic stainless steel welding electrodes, and can not detect copper, aluminum, magnesium, titanium and other non-magnetic materials. For the surface of shallow scratches, buried deeper holes and with the workpiece surface angle less than 20 ° delamination and folding is difficult to find.
Liquid penetration detection
The basic principle of liquid penetration detection, the surface of the part is coated with fluorescent dyes or coloring dyes, in a period of time under the action of the capillary, the penetrating liquid can penetrate into the surface opening defects; after removing the excess penetrating liquid on the surface of the part, and then coated with a developer on the surface of the part.
Again, under the action of the capillary, the developer will attract the permeate retained in the defect, the permeate seeps back into the developer, and under a certain light source (ultraviolet light or white light), the trace of the permeate at the defect is displayed, (yellow-green fluorescence or bright red), so as to detect the defect's morphology and distribution status.
The advantages of penetrant detection are:
1, can detect a variety of materials;
2, with high sensitivity;
3, the display is intuitive, easy to operate, low testing costs.
And the disadvantages of penetration testing are:
1, not suitable for checking the workpiece made of porous loose material and rough surface workpiece;
2, penetration testing can only detect the surface distribution of defects, it is difficult to determine the actual depth of defects, and therefore it is difficult to make a quantitative evaluation of defects. The detection results are also affected by the operator.
X-ray Inspection
The last, ray detection, because X-rays through the irradiated object will have loss, different thicknesses of different substances on their absorption rate is different, and the negative is placed on the other side of the irradiated object, because of the intensity of the ray is different and produce the corresponding graphic, film evaluators can be based on the image to determine whether the object is defective and the nature of defects within the object.

Applicability and limitations of radiographic inspection:
1. It is more sensitive to detecting volumetric defects, and it is easier to characterize the defects.
2, Radiographic negatives are easy to retain and have traceability.
3, Visualize the shape and type of defects.
4, the disadvantages can not locate the buried depth of defects, while the detection of limited thickness, the negative needs to be sent specifically to wash, and a certain amount of harm to the human body, the cost is higher.
In short, ultrasonic, X-ray flaw detection is suitable for detecting internal defects; where ultrasonic for more than 5mm, and the shape of the regular parts, X-ray can not locate the buried depth of defects, radiation. Magnetic particle and penetration flaw detection is suitable for detecting defects on the surface of parts; among them, magnetic particle flaw detection is limited to detecting magnetic materials, and penetration flaw detection is limited to detecting open defects on the surface.