Radiographic flaw detection is a method of flaw detection using penetrability and linearity of rays. Although these rays are not directly detectable by naked eye like visible light, they can lighten photographic film and can also be received by a special receiver.
Rays commonly used for flaw detection include X-ray and gamma rays emitted by isotope, which are called X-ray flaw detection and γ-ray flaw detection respectively. When these rays penetrate (irradiate) a substance, the greater density of substance, the more intensity of rays decreases, that is, the lower intensity of rays that can penetrate substance. At this time, if you use a photographic film to receive, light sensitivity of film is small; if you use an instrument to receive, signal obtained is weak.
Therefore, when radiation is used to irradiate parts to be inspected, if there are defects such as pores, slag inclusions, etc., material density of radiation passing through defective path is much smaller than that of path without defects, and its intensity is reduced less, that is, transmitted intensity is greater. If film is used to receive, light sensitivity is greater, and plane projection of defect perpendicular to ray direction can be reflected on film; if other receivers are used, meter can also be used to reflect projection of defect perpendicular to ray direction and ray transmission.
It can be seen that, under normal circumstances, it is not easy to find cracks in radiographic inspection, or radiographic inspection is not sensitive to cracks. Therefore, radiographic inspection is most sensitive to volumetric defects such as pores, slag inclusions, and incomplete penetration. Flaw detection is suitable for volume flaw detection, but not suitable for area flaw detection. In addition, radiographic flaw detection is generally more expensive.

Frequency range of sound waves that people's ears can directly receive is usually 20 Hz to 20 kHz, that is, sound (sound) frequency. Frequency lower than 20Hz is called infrasound, and frequency higher than 20kHz is called ultrasonic. Several megahertz ultrasonics are commonly used in industry for flaw detection.
Ultrasonic frequency is high, propagation is strong, it is easy to propagate in solid, and it is easy to reflect when it encounters interface formed by two different media, so it can be used for flaw detection. Usually, with an ultrasonic probe in good contact with surface of workpiece to be inspected, probe can effectively transmit ultrasonic waves to workpiece, can receive ultrasonic waves reflected from (defect) interface, at the same time convert it into an electrical signal, and then transmit it to instrument for processing.
According to speed of ultrasonic propagation in medium (often called speed of sound) and time of propagation, location of defect can be known. The larger defect, the larger reflecting surface, and the greater reflected energy. Therefore, size of each defect (equivalent) can be found according to reflected energy. Commonly used flaw detection waveforms include longitudinal wave, transverse wave, surface wave, etc. The former two are suitable for detecting internal defects, the latter is suitable for detecting surface defects, but requires high surface conditions.

 

Magnetic particle inspection is a magnetic inspection method based on principle of magnetic flux leakage. When lines of magnetic force pass through ferromagnetic material and its products, a leakage field will be generated at (magnetic) discontinuity, forming a magnetic pole. At this time, when dry magnetic powder is sprinkled or magnetic suspension is poured, magnetic poles will adsorb magnetic powder and produce obvious magnetic marks that can be directly observed with naked eye. Therefore, defects of ferromagnetic materials and products can be displayed by means of magnetic marks.
Magnetic particle inspection can detect tiny defects that are exposed on the surface and cannot be directly observed with naked eye or with aid of a magnifying glass. It can also detect near-surface defects that are not exposed but are buried a few millimeters below surface. Although this method can also detect volumetric defects such as pores, inclusions, and incomplete penetration, it is more sensitive to area defects and is more suitable for checking cracks caused by quenching, rolling, forging, casting, welding, electroplating, grinding, fatigue, etc.
There are many ways to display defects in magnetic flaw detection, which can be displayed with magnetic powder or without magnetic powder. Display with magnetic powder is called magnetic particle flaw detection. Because of its intuitive display, simple operation and people's willingness to use it, it is one of the most commonly used methods. It is habitually called magnetic flux leakage detection that does not use magnetic particle display. It often uses induction coils, magnetotubes, Hall elements, etc. to reflect defects. It is more hygienic than magnetic particle detection, but not as intuitive as the former.
As current magnetic inspection mainly uses magnetic powder to show defects, people sometimes refer to magnetic particle inspection directly as magnetic inspection, and its equipment is called magnetic inspection equipment. Magnetic particle inspection is widely used in inspection of automobile engine parts.

Eddy current testing is that alternating magnetic field generated by alternating current acts on conductive material to be tested to induce eddy current. If there is a defect in the material, it will interfere with generated eddy current, that is, form an interference signal. When interference signal is detected by eddy current flaw detector, condition of defect can be known.
There are many factors that affect eddy currents, that is, there are abundant signals in eddy currents. These signals are related to many factors of materials. How to separate useful signals from many signals is a difficult problem for current eddy current researchers. Over years, some progress has been made, and some problems can be solved under certain conditions, but it is still far from meeting requirements of scene and needs to be vigorously developed.
Distinguishing feature of eddy current testing is that it can work on conductive materials, not necessarily ferromagnetic materials, but it has a poor effect on ferromagnetic materials. Secondly, surface finish, flatness and boundary of workpiece to be inspected have a great influence on eddy current flaw detection. Therefore, eddy current flaw detection is often used for flaw detection of non-ferromagnetic workpieces such as copper tubes with relatively regular shapes and smooth surfaces.

Penetration testing is a method of testing using capillary phenomena. For parts with smooth and clean surfaces, use a colored (usually red) or fluorescent, highly penetrating liquid to coat surface of parts to be explored.
If there are micro-cracks on the surface that cannot be directly detected by naked eye, liquid will penetrate to its roots along cracks due to strong permeability of liquid; penetrating liquid on the surface will be washed away, and then a display liquid with greater contrast (often White). After a while, because crack is very narrow, capillary phenomenon is significant. Penetrating liquid that has penetrated into crack will rise to surface and spread, showing a thicker red line on white substrate, which shows shape of crack exposed on the surface, so it is often called colored flaw detection.
If penetrating liquid is a fluorescent liquid, liquid that rises to surface from capillary phenomenon will fluoresce under irradiation of ultraviolet lamp, which can better show shape of crack exposed on the surface. Therefore, penetrant inspection at this time is often directly called fluorescent inspection. This flaw detection method can also be used for metal and non-metal surface flaw detection. Flaw detection liquid used in it has a relatively large odor and is often toxic.
In addition to above five conventional methods, in recent years, there have been some new flaw detection methods such as infrared and acoustic emission.