01. Surface quality of threads
The surface roughness of threads has a significant impact on the fatigue life of bolts. When the roughness of 40CrNiMo steel bolts with M6-1.0 threads is reduced from 0.08-0.16 to 0.63-1.35, the fatigue strength decreases by 33%; When the surface roughness of bolts with M12-1.5 threads decreases from 0.08-0.16 to 0.16-0.32, the fatigue strength decreases by 21%.
02. Impact of thread rolling process
Rolling threads will produce a deformation strengthening layer and higher residual compressive stress, which plays a significant role in preventing the initiation and early propagation of fatigue cracks; At the same time, it will also reduce the surface roughness of the tooth valley, which is beneficial for improving the fatigue strength of the bolt. However, if the thread is rolled and then subjected to heat treatment, the above favorable factors will disappear. Therefore, from the perspective of improving the fatigue performance of bolts, the threads should be rolled after heat treatment. But there is another problem at this time, that is, bolts, especially high-strength bolts, usually have higher hardness after heat treatment, which reduces the service life of the rolling mold. In addition, if the quality of the rolling wire is not good enough and micro cracks or similar contact fatigue peeling phenomena occur on the surface or root of the thread, the effect of improving the fatigue performance of the bolt is not significant, and even the fatigue performance may be reduced.
03. Distance between nut end face and thread
Tests have shown that the closer the nut end face is to the threaded position, the earlier the bolt will fail. This is because the position where the bolt starts threading is usually the roughest rolling area, resulting in greater stress concentration. The first thread of the bolt pair is the most stress concentrated, and placing this first thread close to the starting position can lead to a decrease in fatigue strength. So, having a distance of at least 2 threads between the first thread of the bolt pair and the threaded area can eliminate this hidden danger.
The influence of thread valley shape and radius size.
04. Thread pattern shape and size
When a bolt is subjected to force, stress concentration occurs at the thread valley, and its value largely depends on the shape of the valley. Changing the shape of the tooth groove, such as the smoother the groove of the thread, reduces stress concentration and increases fatigue strength. Generally speaking, the fatigue strength of threads in flat bottomed grooves is the lowest. If circular tooth valleys are used instead of flat tooth valleys, the fatigue strength of bolts can be improved. The size of bolts also has an impact on fatigue characteristics, with larger diameters resulting in lower fatigue strength; This also applies to bolt threads.
05. 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. The cracks that begin to appear at the bottom of the screw head are 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 by the bolt being installed on an inclined support. A small angle between the bolt head and the supporting object (also known as the nut end face), such as 2 degrees, can have an immeasurable negative impact on fatigue strength. This phenomenon often occurred in the past when the object being supported was a welded component (which usually undergoes stress release and changes in structural shape after welding).
06. Distribution of stress
The stress distribution on the nut is uneven, and a large amount of load is actually carried by the first few buckles. So, a large amount of bolt fatigue occurs at the first and second buckles of the nut. So we can see that improving the distribution of stress evenly among the few bolts in the joint will increase the fatigue strength.
07. Metallurgical defects in steel
Some bolts are no longer cut after cold heading or cold drawing, so the surface defects of the raw materials remain on the surface of the finished parts.
The severe decarburization layer on the surface of the bolt is a weak area on it. During the rolling process after cold heading, due to the large deformation of the steel surface, most of the decarburization layer will be squeezed into the top area of the thread. The strength and hardness of this decarburization layer are very low, making it prone to wear and tear (thread breakage) failure, and becoming a source of fatigue cracks, leading to early fatigue failure.
08. Improve the stress distribution of bolt pair threads
To improve the stress distribution between bolt threads and increase fatigue life; The investigation shows that it can also be achieved by changing the shape of the nut. Creating a groove on the end face where the nut comes into contact with the reinforcement can increase the fatigue life by 25%. This improvement is particularly suitable for large-sized bolts. Of course, there are other ways to make the stress distribution of the bolt nut joint 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; For example, making the threads of bolts and nuts with different pitches; Alternatively, use pointed threads.
09. Tighten the bolts to the design preload force
Among numerous cases, the most effective way to improve the fatigue life of bolt pairs is to tighten the bolts to the design preload force. Normally, a securely fastened bolt only carries 5% (or even less) of the dynamic load. Therefore, a securely fastened bolt has strong resistance 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 due to the bolt pre tightening force not reaching the design value, which exposes the bolt to bending moment stress and leads to early failure.
