The safety of the bolt is related to the safety of the flap. The safety of the bolt directly affects the safety of the flight. Therefore, the bolt is a very important inspection site in the usual locomotive work.
Analysis of Causes of Cracks in 2 Flanges of Rails
As mentioned above, the bolted joint is often the part where the structure is subjected to a large force. And the cross section of the member at the bolt hole is abrupt, so that there is local stress concentration. If the stress concentration is further aggravated for some reason, the bolt will generate fatigue cracks after a certain number of cycles of alternating load.
Through a large number of fatigue accidents and experimental observations, it is found that the causes of bolt crack formation and the factors affecting the service life of bolts are as follows: 2.1 Crack bolts formed due to scratches have a certain use time due to wear, scratches, impacts. , and corrosion, etc., resulting in repeated relative movement between the bolt and the bolt hole, although the relative displacement is small, only one thousandth of a millimeter or less, due to the sudden change of the section and the impact load, The scratch forms a source of fatigue, thereby forming cracks and expanding.
2.2 Corrosion due to environmental problems There is data showing that the life of carbon steel and high-strength steel will decrease in humid air. The aircraft used in coastal areas is particularly corrosive, and corrosion points and corrosion pits formed by corrosion will form. The stress concentration source provides the necessary conditions for the formation and expansion of cracks.
2.3 If the bolts and bolt holes have different precisions, the bolts of the bolt holes will be inconsistent, which will cause impact on the bolts and form fatigue cracks.
2.4 When the bolts are assembled, the washers are not flattened or the thickness is inconsistent, resulting in the inclination of the nut after tightening. This tilt not only causes high contact stress concentration on the contact surface, but also the eccentricity of the load may cause the bolt to bend, resulting in no The necessary bending moment creates additional stress in the bolt.
2.5 Bolts too tight also have a certain impact on the bolts. Tightening too tightly increases the total load on the bolt, which reduces the fatigue life of the bolt; if it is too loose, it will increase the impact of the alternating load. In addition, the impact friction between the bolt and the bolt hole will also reduce the service life of the bolt and cause unnecessary influence on the bolt hole.
3 bolt ultrasonic testing
3.1 Detection principle
The ultrasonic longitudinal wave method is a method in which an ultrasonic probe transmits a pulse wave to a test piece to detect a defect of the test piece according to the condition of the reflected wave. According to the cause of cracks in the bolt, considering the need of in-situ detection, ultrasonic longitudinal wave method is selected for detection.
3.2 Selection of instruments and probes 3.2.1 Selection of testing instruments Ultrasonic detectors are the main equipment for ultrasonic testing. The detectors should be selected according to the detection requirements and site conditions before testing. The detection of the bolt is on-site inspection. We choose the MUT-1 handheld intelligent ultrasonic flaw detector. The instrument is small in size, light in weight, easy to carry, and has a high-brightness display and intelligent operating software for continuous operation. 4 hours, suitable for field testing.
3.2.2 Selection of probe and detection frequency The selection of probe and frequency should be considered according to the condition of the test piece, as well as the nature and location of the defect. The choice of the probe is mainly the choice of frequency, wafer size, type and angle. .
The frequency has a large impact on the detection. High frequency, high sensitivity and resolution, good directivity, and good for detection. However, the frequency is high, the length of the near-field region is increased, the attenuation is increased, and it is unfavorable for detection. In the actual test, it is necessary to comprehensively analyze and consider various factors and select the frequency reasonably. Generally, a lower frequency is selected as much as possible while ensuring the sensitivity of the detection.
Considering various conditions and factors, in order to achieve the best detection results, we choose a longitudinal wave straight probe with a frequency of 2.5MHz.
3.2.3 Coupling conditions The selected couplant is a liquid medium that is applied between the probe and the surface of the workpiece to allow ultrasonic energy to pass into the test piece. The main purpose is to fill the unevenness and gap of the detection surface, remove the air, and achieve a good coupling state by achieving full contact. According to the site conditions, select 932 aviation low temperature grease.
3.2.4 Preparation of test block In order to obtain the test block state and bolt consistency, a set of inspected bolts was used as the inspection standard, and the crack depth was 3 mm at the second thread, which was used as the basis for sensitivity calibration.
3.3 Detection
3.3.1 Adjustment of scanning speed
Place the probe on the test surface of the test block and couple it well with 932 low temperature aviation grease. Repeat the “horizontal†and “deep fine-tuning†knobs so that the leading edge of the start wave (T) and the front edge of the bottom wave (B) are aligned to the “0†scale and the “10†scale, respectively. At this point, the instrument detection range is the bolt length.
3.3.2 Adjustment of detection sensitivity Adjust the “Attenuator†knob so that the height of the crack reflection wave (F) reaches 80% of the full scale of the screen.
3.3.3 Scanning 932 low temperature aviation grease will be used to couple the tested bolts to the inspection surface. Place the probe on the surface of the bolt and rotate the probe to observe the waveform change.
4 waveform analysis
In the case of longitudinal wave flaw detection, when the diffused sound beam meets the sidewall of the workpiece, according to the mode conversion principle, it is possible to produce a modified transverse wave: if the deformed transverse wave has a chance to directly return to the probe, since the longitudinal straight probe is not sensitive to the transverse wave, basically Can cause interference. If the deformed transverse wave can return to the side wall from the original path and then convert to the deformed longitudinal wave return probe, an interference wave will be formed. The longitudinal wave sound path reflected by the interference wave on the screen can be calculated by the following formula: w=b+eclct, the longitudinal wave path corresponding to the W t-interference wave; b-longitudinal sound path; the actual e-deformation wave Sound path; cl-longitudinal sound velocity in the workpiece; ct-sonic wave sound velocity in the workpiece; when the longitudinal wave is flawed, the diffusion angle is generally small, that is, the incident angle of the diffused sound beam to the side wall is close to 90°, which can be considered as b≈a -dtg can be derived: e=d 1 cosα then W≈a+d(1 cosα?
Clct -tgα) because sinα= ctcl can be derived to obtain W t≈a+d (clct) 2 -1姨 for steel parts, substituting longitudinal and transverse wave sound velocity calculations, get W t =a+1.53d for bolt inspection, often The waveform conversion appears as shown. It can be seen from the calculation that the deformed transverse wave sound path lags the direct defect echo by 0.76d. Therefore, there is a modified echo at 0.76d intervals on the screen. On the screen, the variant wave appears behind the defect wave and thus does not interfere with the identification of the defect wave.
5 Conclusion
After theoretical analysis and a large number of experiments, according to the assembly characteristics of the flaps, the causes of cracks in the bolts are analyzed. The in-situ detection method of bolt cracks is proposed, and the causes of the variant waves are analyzed. The feasible method of identifying the variant waves is proposed. . The bolts are detected by the above method, which avoids the occurrence of breakage accidents in the flaps of the flaps during flight, and ensures flight safety.
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