Several Factors That Must Be Known To Affect Bolt Fatigue!
In the past, it was believed that metal fatigue, to some extent, was due to changes in the metallurgical properties of materials. Somehow, metals fail prematurely under alternating loading.
This is a well-known problem and the cause of many product failures, including some failures involving loss of life.
Now, after the summary of fastener industry in recent years, fatigue is caused by one or more cracks passing through a material.
Fatigue shows progressive cracking
Cracks can start from some existing defects, such as inclusions in metal, or at high stress points, such as notches, and grow slowly with each load.
It may require millions of repeated loads (called stress cycles) to actually detect cracks.
With the increase of crack length, the remaining material bears more and more stress, because the area to maintain the load is smaller and smaller. When the crack actually reaches the critical length, it will always pass through the material, leading to complete failure.
It takes years for a fatigue crack to pass through the bolt
The term "fatigue failure" is often used to describe part damage due to sustained loading. The fatigue damage is equivalent to the crack length.
In some critical applications, the use of dye penetrants or even regular inspection of bolt cracks by x-ray is required to ensure that there are no detectable cracks. (Cracks may exist at microscopic scales, i.e. below the detection threshold of the measurement technique.)
Bolt is a typical multi notch part, and its fatigue performance will be obviously affected by many factors such as bolt structure, size, material and manufacturing process. Compared with notched pieces of the same material, their fatigue strength is usually significantly lower.
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 rod, and the transition fillet between the bolt head and the rod. Due to the sudden change of the section, there is also a high stress concentration in these parts. Here, we list 10 factors that affect the fatigue properties of bolts. Please look at the figure below to find the corresponding bolt locations.
Factors Affecting Bolt Fatigue Strength
01. Thread surface quality
The surface roughness of the thread has a great influence on the fatigue life of the bolt. For example, when the roughness of 40CrNiMo steel bolts with M6-1.0 threads decreases 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 thread will produce deformation strengthening layer and high residual compressive stress, which plays a great role in preventing the initiation and early growth of fatigue cracks; At the same time, it will also reduce the surface roughness of the tooth valley, which is conducive to improving the fatigue strength of the bolt. However, if the thread is rolled and then heat treated, the above favorable factors will disappear. Therefore, in order to improve the fatigue performance of bolts, the threads 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 service life of the wire rolling die. In addition, if the quality of rolling wire is not good enough, micro cracks or similar contact fatigue peeling phenomenon occur 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.
03. Distance between nut end face and thread
The test shows that the closer the nut end face is to the starting position of the thread, the earlier the bolt will fail. This is because the position where the bolt starts to thread is usually the roughest place for rolling, and there is greater stress concentration. The first thread of the bolt pair has the most concentrated stress, and if the first thread is close to the starting position, the fatigue strength will decrease. Therefore, a distance of more than 2 threads between the first thread of the bolt pair and the place where the thread starts will eliminate this hidden danger.
The influence of thread valley shape and radius size.
04. Shape and size of thread thread
When the bolt is stressed, stress concentration will occur at the thread valley, and its value depends on the shape of the valley to a large extent. Change the shape of the tooth valley. For example, the smoother the tooth valley of the thread is, the smaller the stress concentration is, and the higher the fatigue strength is. In general, the thread fatigue strength of the flat bottomed valley is the lowest. If round tooth valley is used instead of flat bottom tooth valley, the fatigue strength of bolts can be improved. The size of the bolt also affects the fatigue characteristics. The larger the diameter is, the lower the fatigue strength is; 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 also at the bottom of the head. The crack that starts to sprout 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 holder. A small angle between the bolt head and the holder (also known as the nut end face), such as 2 degrees, will have an unpredictable negative impact on the fatigue strength. This phenomenon often occurred in the past when the object to be supported is a weldment (the weldment will usually have stress relief after welding, and the structure shape will change).
06. Stress distribution
The stress distribution on the nut is uneven, and a large amount of load is actually carried by the first few buckles. Therefore, a large number of bolt pair fatigue occurs in the first two threads of the nut. Therefore, we can see that the improvement that makes the stress evenly distributed in the bolts will increase the fatigue strength.
07. Metallurgical defects of steel
Some bolts will not be cut after cold heading or cold drawing, so the surface defects of raw materials will always remain on the surface of finished parts.
The serious decarburization layer on the bolt surface is a weak area. 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 decarburized layer are very low, so it is very easy to wear and trip (the thread is sheared) failure, and it is very easy to become a source of fatigue cracks, causing early fatigue failure.
08. Improve the stress distribution of screw thread of bolt pair
For improving the stress distribution between screw threads of bolt pairs, the fatigue life is increased; The investigation shows that it can also be realized by changing the shape of the nut. A groove is made on the end face where the nut contacts the holder, which 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. For example, change the material of the nut to another material to make its elastic modulus different from that of the bolt; Another example: making the threads of bolts and nuts into different pitches; Alternatively, use pointed threads.
09. Tighten the bolt to the designed pre tightening force
In many cases, the most effective way to improve the fatigue life of the bolt pair is to tighten the bolt to the design preload. Normally, a bolt that is tightened in place only carries 5% (or less) of the dynamic load. Therefore, a bolt fastened in place has a strong resistance to fatigue load. This is because the alternating load on the bolt is very small, so the alternating stress generated in 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 that the bolt preload does not reach the design value, which exposes the bolt to the bending moment stress, leading to early failure.
