The characteristic of torsion fracture is obvious necking elongation at the fracture location. The common causes of torsion fracture are mainly due to the small coefficient of friction of the mating surface; excessive torquing or pretightening torque, the socket and the thread being misaligned during torquing, the torque being applied too quickly; the strength of the part itself is not enough and the perpendicularity between the fastening surface and the thread centerline is out of tolerance.
The fracture of the threaded shear is generally spiral-shaped without obvious necking. The common causes of threaded shear failure are that the thread is stuck during tightening, such as thread deformation, inconsistent thread profiles, presence of welding slag, etc.; the cross-section of the bolt is resisted, such as insufficient effective thread depth for nuts with blind hole.
The fracture after the concentrated stress on the screwed rod is commonly seen in the head of the bolt and the right angle transition between the head and the screwed rod. The common causes of fracture at stress concentration areas are small radius at the right angle transition between the head and the screwed rod; defects in the plastic streamline at the head during bolt cold heading. Poor perpendicularity between the mating surface and the bolt.
During the usage of bolted connections, the primary mode of failure is often fatigue fracture. Common reasons leading to fatigue fractures include insufficient preload, excessive clamp force decay, substandard bolt dimensions or properties, and inadequate fulfillment of design requirements by the interplay of parts, assembly environments, and usage conditions.
Delayed fracture is often attributed to hydrogen embrittlement. Hydrogen embrittlement involves trace amounts of hydrogen entering the steel during production processes such as electroplating and welding. Under the influence of internal residual or externally applied stresses, hydrogen embrittlement causes material embrittlement and even cracking. Fasteners prone to hydrogen embrittlement include self-tapping screws, spring washers, and bolts with surface treatments of electroplating of Grade 8 or higher.
Torque alarms in components commonly occur during the assembly process when torque is controlled through angular methods. Failure modes and reasons for fastener torque alarms include:
After assembly completion, the final torque of the component is higher than the upper control limit or lower than the lower control limit. This occurs when the torque control range of the component is unreasonable, indicated by excessively narrow control ranges or offsets either upwards or downwards.
Failure to achieve the preset angle, reaching the upper limit alarm. The reasons for this include exceeding the friction coefficient limit of the component itself, exceeding the friction coefficient limit of the component fit, or interference between components, leading to a sharp rise in assembly torque.
Normal assembly with a lower limit torque alarm. This occurs when the friction coefficient of the component itself exceeds the lower limit or the friction coefficient of the component fit exceeds the lower limit. Excessive initial torque is common, indicating excessive torque consumption during bolt insertion, especially when tightening lock nuts.
Thread stripping is a common occurrence in threaded connections. The main causes of thread stripping include decarburization of threads, resulting in a feeling of inability to apply torque during assembly and discovering flattened threads, or reduced surface hardness on the bolt thread or nut hole. Other causes include:
Simultaneously, incorrect hole assembly during assembly can also cause thread slippage. If the assembly is not aligned with the hole, excessive tightening can result in thread slippage. Additionally, a thread friction coefficient that is too low can contribute to thread slippage. Factors such as surface coatings, surface roughness, inappropriate surface lubricants, and foreign objects in the bolt threads or threaded holes can damage the threads. Variations in pitch, angle, and damaged threads between the bolt and nut can also lead to thread slippage.
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