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Fastener technology: inspection of bolt holes of helicopter blades
Sensitivity determination Since the bolt hole is covered by the flat washer, the in-situ detection can only be performed if the crack exceeds the washer. Macroscopically detectable cracks are not allowed in the use of helicopter moving parts. If the initial crack length of the tail rotor blade root joint is set to a0u003d5.18mm (equal to a0u003d1.0mm from the edge of the washer), it is equivalent to allowing a certain tail rotor blade with a certain length. In order to ensure safety, the length of the crack flying, the damage rate is set to Ru003d10-7.
According to the material formula and the damage tolerance spectrum of the tail rotor blade, the crack growth life Lt under different failure rates can be determined, which is the relationship curve between the crack growth life and the failure rate. The two curves in the figure are calculated from the edge of the hole, and the initial crack length a0u003d1.25mm and a0u003d5.18mm (a0u003d5.18mm from the edge of the hole, which is equivalent to a0u003d1.0mm from the edge of the washer).
Crack growth life Lt/t The relationship curve of crack growth life and failure rate. If the crack formation life is considered, the crack propagation life from the initial crack propagation to the complete failure of the structure is also considered. The damage rate of the initial crack formation life of the component is Rs, and the damage rate of the component from the initial crack to complete fracture is R. As two independent events, if the two occur at the same time, the component is considered to be broken or failed. The damage rate R is: Ru003dRsR For the moving parts of the helicopter, the damage rate is required to be controlled within Ru003d10-6. It is safe for us to set the damage rate of the tail blade with an initial crack of 5.18mm as Ru003d10-7.
According to the analysis, cracks exceeding 1.0mm of the washer are allowed. Therefore, the web area around the gasket can be inspected to determine whether the bolt hole has a dangerous defect. If the crack exceeds the washer, when the eddy current detection probe approaches, the eddy current generated in the cracked area is different from the eddy current generated in the intact area, and the changes in the probe coil voltage and impedance are also different. During the inspection process, as long as we take this If the difference is detected, it can be judged whether there is a defect.
The eddy current flaw detector not only guarantees the reliability of detection but also considers the practicability, the WT-5 eddy current flaw detector is used. Probe selection When using an unshielded probe to detect the web area around the gasket, the gasket must interfere with the probe. To avoid interference, a shielded pen probe is used.
The production of the test block is to make the state of the test block consistent with the bolt hole of the web. Remove a bolt from the scrapped tail blade, and process a radial crack in the bolt hole by wire cutting. The crack exceeds the washer by 1.0mm , The external surface paint layer and internal surface treatment conditions remain unchanged.
The detection probe is first adjusted on the test block. Put the probe on the intact area around the washer of the test block, and adjust the zero position knob so that the pointer of the meter is at the 0 position. Then put the probe on the natural crack on the test block, and adjust the sensitivity knob appropriately so that the indicated value is about 50% of the full scale. At this time, the calibration of the instrument is completed.
Conclusion It is feasible to use the eddy current flaw detection method to detect the tail blades of this model. This method can be extended to other helicopter tail blade joint webs and rotor joint webs.