Effect of Lubricant on Helicopter Bolt Phased Array Ultrasonic Detection Capability

The on-site inspection of the hub shock absorber bolts for in-service helicopters can reduce maintenance costs. However, in the assembly structure of bolts-lubricating oil-fasteners, the difference in acoustic impedance caused by lubricating oil may lead to missed inspection and misjudgment of bolt surface defects. To detect the bolt cracks in the hub damper of the in-service helicopter, the cracks of the bolt “R” chamfer and the screw of the blot are detected by the phased array ultrasonic test (PAUT). Meanwhile, to verify the influence of lubricant on crack detection, the PAUT results of the bolts which screw surface coated with lubricant was compared with that found in the air. The results show that, regardless of the presence or absence of lubricant, the phased array fan scan image can find defects in the bolt “R” chamfer and screw with a depth of more than 0.1 mm. And by comparing the percentage difference between the echo amplitude of the surface coated with lubricant and the defect in the air, the difference ranges from 1% to 6%. After conversion, the difference between the two echoes is about 1 dB. Therefore, a 1 dB increase in the parameter settings can be set for eliminating the negative influence of the lubricant on the screw detected.