Needless to say, threaded fasteners have a wide range of applications, so once a fastener fails, it will easily lead to mechanical failure and even major accidents. Because the most intuitive phenomenon of fastener failure is that the bolt breaks or the bolt and nut separate, many friends think that the quality problem of the bolt or nut causes the fastening failure, but ignore the problems in design and installation.

Here, I would like to share with you the 7 failure modes of threaded fasteners, so that you can analyze and solve the problem of fastener failure in a targeted manner.

Assembling, twisting, pulling and breaking

The failure of this kind of fastener is characterized by obvious necking and elongation at the fracture site. The failure modes of assembly twisting and pulling fractures are mainly due to the friction coefficient of the connecting surface being too small; the torque applied when tightening or pre-tightening is too large, the sleeve and thread are not axial when applying torque, and the speed when applying torque is too fast; the performance of the workpiece itself It is caused by factors such as insufficient strength and poor perpendicularity between the fastener surface and the thread centerline.

Threads are broken by shear force

The fracture site of this failure mode usually has a spiral shape and no obvious necking. Common causes are that the threads are stuck during the tightening process, such as thread deformation, inconsistent tooth shapes of interconnections, welding slag on the threads, etc.; the section where the bolt is screwed in is blocked, such as the nut being a blind hole. But the effective thread depth is not enough.

Fracture after use at stress-concentrated areas

This kind of failure is mostly manifested in the head of the bolt and the right-angle part where the head and the threaded rod transition. The reasons are that the fillet at the right angle between the head and the threaded rod is too small; there are defects in the plastic streamline of the head when the bolt is cold-formed; and the verticality between the connected surface and the bolt is poor.

fatigue fracture

This kind of breakage mainly occurs during use after bolting. Insufficient pre-tightening force; excessive clamping force attenuation; unqualified bolt size and performance; mutual cooperation between workpieces, assembly environment, and operating conditions that cannot meet design requirements are common factors that cause fatigue fracture.

delayed rupture

Hydrogen embrittlement is the main factor causing delayed fracture. It refers to the 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. Generally speaking, self-tapping screws, elastic washers and bolts with electroplated surface treatment are prone to hydrogen embrittlement.

Workpiece torque alarm

Failures that occur during the assembly process of bolts that control torque through the angle method are mostly in the form of workpiece torque alarms. The main reason is that the assembly torque control range of the workpiece is unreasonable. For example, the set control range is too small, and the control range shifts upward or downward. If the pre-tightening angle is not reached, the torque reaches the upper limit and the alarm occurs. For example, the friction coefficient of the workpiece itself exceeds the upper limit. , the workpiece fitting friction coefficient exceeds the upper limit, interference between parts, etc. causes the assembly torque to rise sharply; the torque lower limit alarm occurs during normal assembly, such as the friction coefficient of the workpiece itself exceeds the lower limit or the workpiece fitting friction coefficient exceeds the lower limit, and the fitting torque when the workpiece is screwed in is greater than the initial Torque, that is to say, excessive screwing-in torque consumption, which is common in the tightening process of lock nuts.

Thread sliding thread

Thread slippage is a common failure form in threaded connections. Its manifestation is that the torque cannot be applied during assembly. After disassembly, it will be found that all or part of the threads have been ground flat, and the surface hardness of the nut hole in the case of bolt threads is low; The dimensions of the internal and external threads match, but the contact area of the connecting pair that is intended to match is small. For example, the number of interlocking thread threads is small, and the threads do not contact within the pitch diameter. In other words, the precision fit is not good, and the contact between the bolt thread and the nut thread is not enough.