Failure Analysis of Connecting Bolts of Locking Mechanism in Aircraft

The connecting bolt of locking mechanism in aircraft fractured during the repair process. To investigate the cause of the fracture, failure analysis was conducted using characterization instruments such as stereomicroscope, scanning electron microscope, metallographic microscope, and energy dispersive spectrometer. The macroscopic and microscopic morphology, micro zone composition, metallographic structure, and microhardness of the fracture surface and screw teeth of the fractured bolt were analyzed, and compared with the unbroken bolt. The results indicate that the cause of bolt fracture is overload. As there is no clear torque requirements in the process documents, in order to study the stress distribution of bolts during the tightening process and confirm scientific tightening torque values, finite element simulation analysis was conducted. It was found that the calculated maximum stress position matched the actual fracture location, verifying that overload is the main cause for bolt failure. It is determined that the reasonable assembly torque should not exceed 12 N·m, and no fracture occurs after improving the process.