Some people may ask, how can a bolt made of iron produce fatigue? In fact, after carbon steel materials are produced into the bolt products we want, during the continuous use, if some technical parameters and mechanical properties do not meet the requirements at the beginning, over time, it will form a force on its local area. When this force reaches the critical value, the bolt will have a slight crack, and the generation of this crack is only the first step of fatigue. When the number of cycles reaches a certain level, the crack will directly break. This is the fatigue phenomenon and result of the bolt.
So why do carbon steel bolts produce fatigue? Is it true that the higher the strength of the bolt, the easier it is to produce fatigue? First of all, the fatigue of bolts has nothing to do with strength. It is just that the strength requirements of ordinary bolts are not so high, so its application environment will not cause excessive fatigue effects on bolts. The application environment of high-strength bolts has certain tensile performance requirements, which invisibly increases the fatigue effect on bolts. Therefore, the fatigue of bolts we encounter in daily life is generally high-strength bolts, but it does not mean that ordinary bolts will not produce fatigue, but when we use ordinary bolts, we do not have high requirements for them.
Let's look at the reason why bolts produce fatigue. It is the change of local stress during the cyclic use process, which imposes a certain amount of damage on the weak points of the bolts, and finally forms cracks. So its process should be like this. First, the stress erodes the bolt points, and then cracks are formed in the bolt parts. After a period of time, the cracks become larger and larger, and at a certain critical point, the bolt suddenly breaks. After a long period of analysis, we found that this fatigue stress does not require too much external force to achieve. Sometimes the stress generated by the bolt is much lower than the yield strength of the bolt. Therefore, after the fatigue fracture of the bolt, it will be found that the broken port can not see any external force to deform or bend it.
After the above analysis, we can appropriately change some process foundations during the manufacturing process to make the bolts resist the occurrence of this fatigue. Let's take a look at a schematic diagram:
The figure above shows the shape of the thread. We can make the thread spacing into this shape with an R angle. Because fatigue fracture occurs between threads and under the head, we can change some basic manufacturing processes of the thread to effectively avoid fatigue. We can compare it with ordinary threads:
The thread above is an ordinary thread. The thread teeth form a right angle, and this right angle reacts directly to the change of stress, so this right-angle thread is prone to fatigue fracture. As analyzed above, in addition to the thread, the bottom of the bolt head is also a disaster area for fatigue fracture. Let's take a look at the schematic diagram:
The principle is the same as the thread R angle. We can also make it into an R angle within the allowable range at the junction of the bolt head and the thread.
