The method is based on detection of magnetic stray fields over the defects with ferromagnetic particles. The method is suitable for control of parts of ferromagnetic materials.
In the part placed in a magnetic or electromagnetic field excited by the magnetic flux. In the defective areas with low magnetic permeability, the magnetic lines of force beyond parts, bypassing the defect and forming a nonuniform magnetic field scattering. For detection of cracks on the surface cause the ferromagnetic particles suspended in a fluid - oil or kerosene. In an inhomogeneous magnetic field the particles are pulled together in the place of greatest density of magnetic field lines, that is, to defects.
Informative parameter of a method serves as a display figure in the form of strips deposited powder, repeating the configuration of the defect. The method provides the detection of surface microcracks with a width of 1 to 3 µm and a depth of 10...50 microns and subsurface defects under non-magnetic coating, such as a chromium layer, at a depth of 0.1 to 0.3 mm, a large subsurface defects section 2..3 mm2 at a depth of 1..3 mm .
The method is implemented in two main varieties - the control of the applied magnetic field (applied magnetic alloys and for large parts) and residual magnetization. Stationary detectors allow you to control components up to a length of 1600 mm and a diameter of 800 mm.
Method advantages: relatively high sensitivity, ease of implementation, possible the same type of test with different form and dimensions of the parts, low labor intensity.
The main disadvantages - the necessity of removing the protective coatings of thickness greater than 0.03 mm and the complexity of the demagnetization on some parts.
Capillary methods
Methods of this group based on the ability of the wetting liquid to fill the narrow cavity.
The surface of the part applied with a painted or luminescense liquid penetrating into the cavity defects. After removal of the indicator liquid with the surface of the part and applying a developer (white paint or powder) remaining in the cavity of the defect of liquid projects outward, forming a visible to the eye or luminescent in the ultraviolet rays of the display image.
The method enables detection of cracks with a width over 1 µm, a depth of more than 10 microns and a length of over 100 µm.
The advantages of the method include high visibility of results and the sensitivity, the accuracy of determining the shape and size of the defect, simplicity, and the ability to control parts of any shape and of any material.
Weaknesses - high labor intensity and duration of monitoring (full cycle can be up to 3 hours), unreliable in detection, closed oxide film, and the toxicity of certain chemicals.
Ultrasound methods of control
The method is based on the analysis of the process of propagation of elastic waves in the material of the part. The waves are reflected from defects and acoustic properties are the density and speed of sound are different from the properties of the material.
Control consists of applying a short probe ultrasonic pulse and the detection of the reflected echo signals on a cathode ray tube flaw detector. Informative parameters are the amplitude of the echo signal and its position on the screen. For the generation of ultrasonic pulses is used piezoelectric transducer.
The method is suitable for inspection of almost all aircraft alloys. The only exceptions are the coarse-grained high-temperature alloys ZHS-6 and WEL-12. The grain size of these alloys up to 8 mm, is significantly greater than the length of the acoustic wave, which dramatically increases the dissipation of energy. Therefore, the alloys of this family are not prosvechivayut at frequencies of commercial flaw detectors.
The sensitivity of the method for crack width of up to 30MKM, the depth of about 100 µm, length 1...2 mm.
The main advantages of the method, the possibility of detecting internal defects in one-way access and the ability to detect gas-saturated inclusions, e.g., titanium nitride TiN.
Disadvantages - the inability to accurately assess the nature and size of the defect, it is difficult to control parts with design reflectors (grooves, holes, etc.) and the need to develop special transducers for each class of parts.
The latter feature is associated with the need to ensure a reliable contact with the surface of the transducer with the workpiece and causes a number of difficulties for the control of parts of complex shape. So, to identify the blank disk of discontinuity flaws of metal, oriented in different ways relative to the external surfaces, requires the use of several different converters structure: straight and oblique, with a working surface, flat or curved shapes.
Radiation control method
Methods of this group based on the different absorption of radiation (alpha, beta, gamma, x-ray, etc.) materials with different density. X-ray or gamma radiation penetrating through the material and recorded on a special film that has a different intensity after passage through the sections of the test object of different density. Informative parameter of these methods is the degree of darkening of various parts of the film.
Radiographic metol provides detection of defects, the length of which is 1...2% of the thickness of the illuminated object, the graphical gamma - 2...4%. The smallest width of opening of the detected cracks of about 100 microns.
The advantage of the method is to obtain objective documents x-ray or gamma images.
Disadvantages - the complexity of monitoring equipment, the need to protect staff from the action of radiation, the duration and high cost of control. With this in mind, radiation techniques are used mainly to control all-in-one modular units, for example, is made using honeycomb structures, etc.