9 factors that screw people must know that affect bolt fatigue
The bolt is a typical multi-notched part, and its fatigue performance will be obviously affected by many factors such as the structure, size, material and manufacturing process of the bolt. The fatigue strength is usually significantly lower than that of notched parts of the same material.
The bolt is a typical multi-notched part, and its fatigue performance will be obviously affected by many factors such as the structure, size, material and manufacturing process of the bolt. The fatigue strength is usually significantly lower than that of notched parts of the same material. In addition to the thread, the other weak parts that affect the fatigue performance of the bolt are the transition between the thread and the shank and the transition fillet between the bolt head and the shank. There are also high stress concentrations at these locations due to sudden changes in cross-section. Here, we list 10 factors that affect the fatigue characteristics of bolts: 1. Thread surface quality
The surface roughness of the thread has a great influence on the fatigue life of the bolt. For example, when the thread is M6-1.0 40CrNiMo steel bolt, when the roughness is reduced from 0.08 to 0.16 to 0.63 to 1.35, the fatigue strength is reduced by 33%; When it is 0.16 to 0.32, the fatigue strength decreases by 21%.
2. The influence of thread rolling process
The rolling thread will produce a deformation strengthening layer and a high residual compressive stress, which plays a great role in preventing the initiation and early expansion of fatigue cracks; at the same time, it will also reduce the surface roughness of the valley, which is beneficial to the fatigue strength of the bolt. improvement. However, if heat treatment is carried out after rolling the thread, the above advantages will disappear. Therefore, from the perspective of improving the fatigue performance of the bolt, the thread should be rolled after heat treatment. However, there is another problem at this time, that is, the hardness of bolts, especially high-strength bolts, is usually high after heat treatment, which reduces the life of the rolling die. In addition, if the quality of the thread rolling is not good enough, micro-cracks or spalling phenomenon similar to contact fatigue occurs on the surface or root of the thread, the effect of improving the fatigue performance of the bolt is not obvious, and even the fatigue performance will be reduced.
3. The distance between the end face of the nut and the thread
Testing has shown that the closer the nut face is to the threading location, the earlier the failure of the bolt will result. This is because the position where the bolt is threaded is usually the roughest part of the rolling process, and there is a greater stress concentration. The first thread of the bolt pair has the most concentrated stress, and making the first thread close to the threading position will lead to a decrease in fatigue strength. Therefore, the distance between the first buckle of the bolt pair and the threading place is more than 2 buckles, which will eliminate this hidden danger. Fourth, the thread shape and size
When the bolt is stressed, a stress concentration occurs at the thread valley, and its value depends to a large extent on the shape of the valley. Change the shape of the valley, such as the smoother the valley groove of the thread, the smaller the stress concentration and the higher the fatigue strength. In general, flat-bottomed valleys have the lowest thread fatigue strength. If the flat-bottomed valley is replaced by a circular valley, the fatigue strength of the bolt can be improved. The size of the bolt also has an effect on the fatigue properties, the larger the diameter, the lower the fatigue strength; the same applies to bolt threads.
5. Cracks at the bottom of the screw head
Fatigue cracks usually start at the bottom of the thread, but they also often start at the bottom of the screw head. Crack initiation at the bottom of the screw head is usually caused by improper design of the diameter of the transition arc of the screw head (stress concentration caused by improper transition arc diameter), or the bolt is installed on an inclined support superior. A small angle between the bolt head and the support (which can also be understood as the end face of the nut), such as 2 degrees, will have an inestimable negative impact on the fatigue strength.
6. Distribution of stress
The stress distribution on the nut is uneven, and a large amount of load is actually carried by the first few buttons. Therefore, a large amount of bolt pair fatigue occurs in the first and second buckles of the nut head. Therefore, we can see that the improvement of the buttons that make the stress evenly distributed in the combination of the bolt pair will increase the fatigue strength.
Seven, steel metallurgical defects
Some bolts are not machined after cold heading or cold drawing, so the surface defects of the raw material remain on the surface of the finished part.
The severe decarburized layer on the surface of the bolt is a weak area on it. During the thread rolling process after cold heading, due to the large deformation of the steel surface, most of the decarburized layer will be squeezed into the top area of the thread. The strength and hardness of this decarburized layer are very low, so it is very prone to wear and tripping (the thread is sheared) failure, and it is very easy to become the source of fatigue cracks, resulting in early fatigue failure.
8. Improve the stress distribution of the bolt pair thread
For improving the stress distribution between the screw threads of the bolt pair, the fatigue life can be improved; the investigation shows that it can also be achieved by changing the topography of the nut. Making a groove on the end face of the nut in contact with the holding object can increase the fatigue life by 25%. This improvement is especially suitable for large size bolts. Of course, there are other ways to make the stress distribution of the bolt-nut combination more even, such as: changing the material of the nut to another material, so that its elastic modulus is different from that of the bolt; another example: making the thread of the bolt and the nut for different pitches; alternatively, use pointed threads.
Nine, the bolt is fastened to the design preload
In many cases, one of the most effective means to improve the fatigue life of the bolt pair is to tighten the bolt to the design preload. Typically, a bolt that is fastened in place carries only 5% (or even less) of the dynamic load. Therefore, a bolt pair that is fastened in place is very resistant to fatigue loads. This is because the alternating load acting on the bolt is very small, so the alternating stress generated inside the bolt is also very small, usually far below the limit that the bolt can withstand. When fatigue failure occurs, nine out of ten reasons are because the bolt preload does not reach the design value, thereby exposing the bolt to bending moment stress, which in turn leads to early failure.