There are actually many scenarios of fatigue failure of bolt connections, ranging from aerospace to bicycles. Today we will learn about this knowledge through a video, which is great professional information.
Video material, WiFi is recommended to watch
The main failure modes of threaded fasteners encountered in our work are divided into:
①Assembly, twist, pull and break;
②The thread is twisted off by shear force;
③Broken after use in areas where stress is concentrated;
④Fatigue fracture;
⑤ Delayed fracture;
⑥ Parts torque alarm;
⑦Thread sliding teeth.
Cause analysis of common failure modes
①Assembly, twisting, pulling and breaking:
The characteristic of twist-and-tension fracture is obvious necking and elongation of the fracture area. The common reasons for twist-and-tension fracture are mainly due to the small friction coefficient of the connecting surface; the torque applied when tightening or pre-tightening is too large, and the sleeve and thread are different when applying torque. The shaft and the speed when applying torque are too fast; the performance strength of the part itself is insufficient and the perpendicularity between the fastening surface and the thread centerline is out of tolerance.
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②The thread is twisted off by shear force:
The fracture section that is twisted by shear force generally has a spiral shape without obvious necking. The common reason for the thread to be twisted by shear force is that the thread is stuck during the tightening process, such as: thread deformation, interconnected teeth. The shape is inconsistent, and there are welding slag lights on the threads; the section where the bolt is screwed in is blocked, and if the nut is a blind hole, the effective thread depth is not enough.
③Breaking after use at stress-concentrated areas:
The fracture after use at the stress concentration part is usually manifested in the bolt head and the right angle part where the head and the threaded rod are excessive. The common reason for the fracture of the stress concentration part is that the fillet of the right angle part between the head and the threaded rod is too small; the bolt is cold heading. There are defects in the plastic streamlines of the head. The verticality between the connected surface and the bolt is out of tolerance.
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④Fatigue fracture:
The main fracture during the use of bolted connections is fatigue fracture. Common reasons for fatigue fracture include: insufficient pre-tightening force; excessive clamping force attenuation; unqualified bolt size and performance; mutual cooperation and assembly between parts The environment and operating conditions cannot meet the design requirements.
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⑤Delayed rupture:
A common cause of delayed fracture is hydrogen embrittlement. Hydrogen embrittlement is a trace amount of hydrogen that enters the interior of the steel during the production process (such as electroplating and welding), causing the material to embrittle or even crack under the action of internal residual or external stress. Common fasteners prone to hydrogen embrittlement include: self-tapping nails/elastic washers/bolts with electroplated surface treatment above grade 8.
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⑥Part torque alarm:
Part torque alarms often occur during the bolt assembly process where torque is controlled by the angle method. The failure modes and causes of fastener torque alarm include: 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 excessive set control range. Small, the control range shifts upward or downward.
There is no pre-tightening to the preset angle, and the torque reaches the upper limit and the alarm occurs: the reason is that the friction coefficient of the part itself exceeds the upper limit, the friction coefficient of the parts fits exceeds the upper limit, and the interference between the 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 parts fits exceeds the lower limit. When the parts are screwed in, the fitting torque is greater than the initial torque (that is, the screwing torque consumption is too large), which is common in locking nuts. Tighten.
⑦Thread sliding thread:
Thread slippage often occurs in threaded connections. The main cause of thread slippage is thread decarburization: a common phenomenon is that the torque cannot be added during assembly. After the bolt is removed, it is found that all or part of the thread is smoothed, and the bolt thread or nut The surface hardness of the hole is low; the dimensions of the internal and external threads are matched: the contact area of the matching coupling pairs is small. There are two situations: one is that the number of thread threads for tooth engagement is small, and the other is that the threads do not contact within the pitch diameter (that is, the precision fit is not Well, there is not enough contact between the bolt threads and the nut threads).
At the same time, if the assembly method is not in the right hole, strong tightening will also cause the thread to slip; the friction coefficient of the thread is too small: the surface coating, surface roughness, surface lubricant are unreasonable, and there are foreign matters in the bolt thread or threaded hole, damaging the thread. Variations in the pitch and angle of bolts and nuts can also cause thread slippage.
