Is There A Defect in The Thread Of The Fastener Bolt? Which Testing Technique Is Better?
Aug 06, 2024
Fasteners bolts, as connecting components, have a wide range of applications. For example, bolts are an important connection method in the rail transit industry, mainly used for connecting important components such as brake disc clamps and gearboxes. Of course, the heat treatment and thread processing of bolts during the manufacturing process can cause serious quality problems such as heat treatment cracks, irregular knife marks, shape defects, etc. In order to enable everyone to quickly and accurately find whether the fastener bolts have defects, Xiaorui will tell you in the following text which testing technique is better.
The following is a comparison of the process and detection sensitivity through penetration testing, magnetic particle testing, and eddy current testing of the bolt thread after fatigue testing, in order to obtain a more suitable detection method for the bolt thread.
1. Penetration testing
Penetration testing is a non-destructive testing technique based on the principle of capillary action to inspect surface opening defects in non porous materials. The working principle is to apply a dye containing penetrant solution to the surface of the specimen to be inspected, and under capillary action, it penetrates into the surface opening defects. Then, excess penetrant solution on the surface is removed and dried, and a developer is applied. The penetrant solution that penetrates into the defects will re infiltrate into the surface of the workpiece under capillary action, forming an enlarged display. Based on the defect display, the quality assessment of the surface opening defects of the workpiece is carried out. The following is a brief description of the testing process.
(1) Testing materials: Select four defective 18CrNi4WA bolts that have undergone fatigue testing and are numbered 1 #, 2 #, 3 #, and 4 #, respectively.
(2) Penetration detection system: solvent removal type dye penetration method - solvent suspension imaging agent.
(3) The penetration testing process involves pre cleaning, application of penetrant, removal of penetrant, and imaging.
Pre cleaning: Use cleaning agent to thoroughly remove oil stains from the threaded parts of the 4 test bolts. After cleaning, dry them thoroughly to prepare for the next process. Due to the very small spacing between the bolt threads used in the experiment, the cleaning effect of the cleaning agent may not be very good. Therefore, the cleaning time can be appropriately extended to ensure that the oil stains and other pollutants at the thread or opening defects are thoroughly cleaned to ensure the effectiveness of penetration testing.
Apply penetrant: Spray the penetrant evenly on the threaded area, and the threaded area should be completely wetted by the penetrant. The infiltration time should be at least 20 minutes to ensure good infiltration effect for small fatigue cracks. The entire infiltration process should ensure that the penetrant remains moist on the tested surface.
Removing penetrant: Removing penetrant is a key step in penetration testing, and insufficient cleaning can cause excessive background masking of related displays; Excessive cleaning may also remove all the penetrant that has infiltrated into the defect, leading to the failure of penetration testing. Regarding the process of removing penetrant from bolt threads, first use a clean and lint free cloth to remove excess penetrant, and then fold a corner with a certain thickness using shaftless paper and insert it into the threaded area to wipe. The threaded area should have a light pink base color.
Imaging: The test bolt uses a spray can wet solvent based imaging agent. Before applying the imaging agent, the spray can must be shaken for 3-5 minutes to evenly distribute the powder that has settled to the bottom of the can in the solvent. The applied imaging agent should form a uniform thin film on the threaded area, and the imaging time is generally 5-10 minutes.
(4) Test results: Only 1 # and 4 # of the 4 test bolts showed defects (see Figure 1 and Figure 2). The surface defects shown in Figure 1 are point like and linear defects at the second thread position. Based on experience, the actual defect may be a linear defect where the points and lines are not connected together. It may be due to the penetration of penetrant into the defect between the points and lines being washed away during intermediate cleaning. The defect shown in Figure 2 is a linear defect at the second thread position; The surface display on the right side of the linear defect should be a false display caused by insufficient removal of the penetrant. The absence of defects in the threaded parts of bolts 2 # and 3 # may be due to insufficient removal of penetrant, resulting in excessive background defects being masked.
2. Magnetic particle testing
Magnetic particle testing technology is to magnetize ferromagnetic materials or workpieces directly by passing current or placing them in a magnetic field. Under certain conditions, a leakage magnetic field is generated at the defect site, and magnetic particles or magnetic suspensions are applied to the surface of the workpiece. The leakage magnetic field at the defect site attracts the magnetic particles to form a magnetic particle pile up. Based on the location, shape, and size of the magnetic particle pile up, the nature and size of the defect can be determined
The residual magnetism method was used for this bolt magnetic particle testing test. For example, on the one hand, when using the continuous method to detect electromagnetic induction and pouring magnetic suspension, if the electrification time is long, there will be more magnetic particles adsorbed on the threaded parts with small spacing, which can easily form excessive background; After the residual magnetization method is used to detect the magnetization of the workpiece, pour 2-3 times of magnetic suspension to fully wet the workpiece. At this time, the threaded part will not produce excessive background magnetic marks, making it easier to observe. On the other hand, the residual magnetic induction intensity of the bolt in this test is greater than 0.8T, and the coercive force is greater than 1 kA/m, so the residual magnetic method can be used for detection.
2.1 Testing process:
(1) Testing method: Residual magnetism wet fluorescent magnetic particle testing.
(2) Testing equipment: CJW-1000 bolt magnetic particle flaw detector.
(3) Test samples: 4 bolt samples that have undergone fatigue testing.
(4) Ultraviolet irradiance: 2600 μ W/cm2.
(5) Fluorescence magnetic suspension concentration: 0.1 mL/100 mL.
(6) Perform sensitivity verification.
2.2 Magnetic Particle Testing Process
(1) Clean the oil stains and impurities from the threaded part of the bolt.
(2) Turn on the flaw detector and stir the magnetic suspension thoroughly for 10 minutes. Inject 100 mL of magnetic suspension into the concentration precipitation tube and let it stand for 40 minutes. Then read the volume of magnetic powder in the precipitation tube.
(3) Place the ultraviolet radiation illuminance meter on the threaded part to verify the intensity of ultraviolet light.
(4) Clamp the bolt, turn off the axial magnetization and turn on the longitudinal magnetization, with a power on time of 0.25~1 s.
(5) Stop magnetizing and remove the bolt. Apply magnetic suspension to the threaded part of the bolt by pouring it 2-3 times to ensure sufficient wetting of the threaded part.
(6) Let the bolt stand horizontally for 10 seconds (allowing residual magnetic suspension in the threaded area to flow away) and observe the magnetic trace display under ultraviolet light.
(7) Measure the demagnetization of magnetic trace size.
2.3 Test results
Only 1 # and 4 # of the 4 test bolts show defects, as shown in Figures 3 and 4. Figure 3 shows linear displays of approximately 8mm and 12mm at the second thread position. Figure 4 shows a linear display of approximately 8mm at the second thread position. No defect magnetic marks were found on bolts 2 # and 3 #, which may be due to the small size of the defect not forming enough leakage magnetic field to adsorb the magnetic powder accumulation.
3. Eddy current testing
The principle of eddy current testing is that a coil with alternating current passing through it approaches a conductor, and the alternating magnetic field generated by the alternating current induces eddy current in the workpiece. The properties of the workpiece and the presence or absence of defects can affect the phase and magnitude of eddy currents, which in turn affect the magnetic field and cause changes in the voltage and impedance of the coil. By measuring the changes in coil voltage or impedance, the presence or absence of defects in the workpiece can be analyzed. The detection feature is that the detection coil does not need to contact the workpiece or couple with the medium, and the detection speed is fast.
3.1 Testing method
Use a multi frequency eddy current flaw detector to perform eddy current testing on the bolt thread area.
3.2 Test results
(1) Eddy current testing parameters
Magnetizing equipment: TEDDY+A eddy current flaw detector (see Figure 5).
Probe: Placement type specialized bolt thread detection probe (see Figure 6).
Excitation frequency: 100 kHz~500 kHz.
Sensitivity adjustment: The same material bolt test block has an artificial crack with a depth of 0.3 mm in the threaded part.
(2) Eddy current testing results
The eddy current testing of the threaded parts of bolts numbered 1 #, 3 #, and 4 shows the results as shown in Figures 7 to 9. The left side of the figure shows an artificial crack with a depth of 0.3 Imm, while the right side shows a defect in the test bolt.
4. Test conclusion
Penetrating magnetic particle and eddy current tests were conducted on the threaded parts of four bolts that underwent fatigue testing. The results showed that defects were detected in bolts 1 #, 3 #, and 4 #. Among them, all three detection methods for bolts 1 # and 4 # showed that bolt 3 # only exhibited defect signals under eddy current testing.
(1) Penetration testing: detecting point and line defects (see Figure 1), which should actually be line defects (as verified in Figure 3), but failing to display the complete defect morphology results in low detection sensitivity; In addition, there are many penetration testing processes, and the testing time for one bolt is nearly 30 minutes. It is also very difficult to remove excess penetration fluid at the root of the thread. Incomplete removal can easily cause excessive background and reduce sensitivity.
(2) Magnetic particle testing: Defects can be clearly seen in the threaded parts of bolts 1 # and 4 #, but no magnetic traces are displayed in bolts 2 # and 3 #. This may be due to the small size of the defects, which did not form sufficient leakage magnetic field to adsorb the accumulation of magnetic particles. In addition, the residual magnetism method should be used for the threaded part of the bolt. The residual magnetism method requires the bolt coercive force to be 1 kA/m and the residual magnetic field strength to be above 0.8 T, so some bolts cannot be tested using this method.
(3) Eddy current testing: It can detect defects that cannot be detected by the above two methods with high detection sensitivity and no coupling medium required. It can complete the detection in 30 seconds with high efficiency and fast speed. Eddy current testing uses electrical signals to characterize defects, so the displayed results can be digitized, stored, reproduced, and the data can be easily automated for testing.
In summary, eddy current testing at bolt thread locations has relatively high sensitivity and fast detection speed, and can be prioritized as a method for detecting surface defects at bolt thread locations.
