The main failure modes of threaded fasteners encountered in our work can be divided into: ① Assembly twisting and pulling fracture; ② Thread breaking due to shear force; ③ Fracture after use at the stress concentration site; ④ Fatigue fracture; ⑤ Delayed fracture; ⑥ Part torque alarm; ⑦ Thread slipping.
Cause analysis of common failure modes ① Assembly twisting and pulling fracture: The characteristic of twisting and pulling fracture is obvious necking and elongation at the fracture site. The common causes of twisting and pulling fracture are mainly due to the small friction coefficient of the connection surface; the torque applied during tightening or pre-tightening is too large, the sleeve and the thread are not coaxial when the torque is applied, and the speed is too fast when the torque is applied; the performance strength of the part itself is insufficient and the verticality between the fastening surface and the center line of the thread is out of tolerance. ② Thread breaking due to shear force: The fracture site of the thread breaking due to shear force is generally spiral, without obvious necking. The common cause of the thread breaking due to shear force is that the thread is stuck during the tightening process, such as: thread deformation, inconsistent tooth profiles of the interconnected threads, welding slag lamps on the thread; the section where the bolt is screwed in is blocked, such as the effective thread depth of the nut is not enough for a blind hole. ③ Fracture after use at stress concentration sites: Fracture after use at stress concentration sites is often manifested in the bolt head and the right angle between the head and the threaded rod. The common causes of fracture at stress concentration sites are that the fillet of the right angle between the head and the threaded rod is too small; there are defects in the plastic streamline of the head during cold heading of the bolt. The verticality between the connected surface and the bolt is out of tolerance.
④ Fatigue fracture: The main fracture during the use of bolts after connection is fatigue fracture. The common causes of fatigue fracture are: insufficient preload; excessive attenuation of clamping force; unqualified bolt size and performance; mutual coordination between parts, assembly environment, and use conditions cannot meet the design requirements.
⑤ Delayed fracture: The common cause of delayed fracture is hydrogen embrittlement. Hydrogen embrittlement is the trace amount of hydrogen that enters the steel during the production process (such as electroplating and welding), which causes the material to become brittle or even crack under the action of residual or external stress inside. Common fasteners prone to hydrogen embrittlement are: self-tapping screws/elastic washers/bolts with electroplated surface treatment above grade 8. ⑥ Part torque alarm: Part torque alarm often occurs during the bolt assembly process where torque is controlled by the angle method. The failure modes and reasons for the fastener torque alarm are: After the assembly is completed, the final torque of the part is higher than the upper control limit or lower than the lower control limit: the reason is that the assembly torque control range of the part is unreasonable, which is manifested as the setting of the control range is too small, and the control range is shifted upward or downward.
Not pre-tightened to the preset angle, the torque reaches the upper limit alarm: the reason is that the friction coefficient of the part itself exceeds the upper limit, the friction coefficient of the part exceeds the upper limit, and the interference between parts causes the assembly torque to rise sharply.
Normal assembly, torque lower limit alarm: the reason is that the friction coefficient of the part itself exceeds the lower limit or the friction coefficient of the part exceeds the lower limit, and the fitting torque of the part is greater than the initial torque when the part is screwed in (that is, the screw-in torque consumption is too large), which is common in the tightening of locking nuts. ⑦ Thread slippage: Threaded connections often have thread slippage. The main reasons for thread slippage are thread decarburization: a common phenomenon is that the torque cannot be added during assembly. After the bolt is removed, it is found that the thread is completely or partially ground flat, and the surface hardness of the bolt thread or nut hole is low; internal and external thread size matching: the contact area of the matching connection is small. There are two situations: one is that the number of threads in the connection is small, and the other is that the threads are not in contact within the middle diameter (that is, the precision matching is not good, and the contact between the bolt thread and the nut thread is insufficient).
At the same time, if the assembly method is not correct, forced tightening will also cause thread slippage; the thread friction coefficient is too small: the surface coating, surface roughness, surface lubricant are unreasonable, and there are foreign objects in the bolt thread or threaded hole, damaged threads, and pitch and angle variations between the bolt and nut will cause thread slippage.
