Fatigue results in progressive cracking.
A crack can begin from an existing flaw, such as an inclusion in the metal, or at a high-stress point, such as a notch, and slowly grow with each loading.
It may take millions of cycles of repeated loading (known as stress cycles) to actually detect the crack.
As the crack length increases, the remaining material bears increasingly greater stress as the area to support the load decreases. When the crack reaches a certain critical length, it continues through the material, resulting in complete failure.
It can take years for a fatigued crack to penetrate a bolt.
The term “fatigue failure” is often used to describe damage to parts caused by sustained loading. This type of damage is equivalent to the length of the crack.
In critical applications, it is necessary to use dye penetrant or even X-ray inspection to periodically check for bolt cracks to ensure that no detectable cracks exist. (Cracks may exist at a microscopic level, i.e. below the detection threshold of measurement techniques.)
Factors Affecting Bolt Fatigue Strength
Bolts are a typical multi-notched component, and their fatigue performance will be significantly affected by various factors such as structure, size, material, and manufacturing processes. Compared with notched parts of the same material, their fatigue strength is generally significantly lower.
In addition to the screw thread, another weak area that affects the fatigue performance of bolts is the transition between the screw thread and the shaft, as well as at the transition fillet between the bolt head and the shaft. Due to the sudden change in the cross-section, there is also a high stress concentration in these areas.
In this regard, we have listed the top 10 factors that affect the fatigue characteristics of bolts. Please refer to the following figure to locate the corresponding position of the bolt.
1. Thread Surface Quality
The surface roughness of the thread has a significant impact on the fatigue life of the bolt. For example, in a 40CrNiMo steel bolt with an M6-1.0 thread, when the roughness is reduced from 0.08-0.16 to 0.63-1.35, the fatigue strength decreases by 33%.
For a bolt with an M12-1.5 thread, when the surface roughness is reduced from 0.08-0.16 to 0.16-0.32, the fatigue strength decreases by 21%.
2. Impact of Thread Rolling
Thread rolling produces a deformation hardening layer and residual compressive stress, which play a vital role in preventing the initiation and early propagation of fatigue cracks.
At the same time, it also reduces the surface roughness of the thread, which is conducive to improving the fatigue strength of the bolt.
However, if heat treatment is performed after thread rolling, the above favorable factors will disappear.
Therefore, from the perspective of improving the fatigue performance of bolts, thread rolling should be performed after heat treatment.
However, another problem arises at this time: the hardness of bolts, especially high-strength bolts, is usually higher after heat treatment, which reduces the service life of the thread rolling die.
In addition, if the quality of thread rolling is not good, microcracks or contact fatigue peeling phenomena similar to those occurring on the surface or at the root of the thread may occur, and the effect of improving the fatigue performance of the bolt is not obvious, and may even reduce the fatigue performance.
3. Distance between the nut end face and thread
Tests show that the closer the distance between the nut end face and thread position, the earlier the bolt will fail. This is because the position where the bolt starts to thread is usually the roughest area of the rolled thread, which leads to a greater concentration of stress.
The first threaded section of the bolt assembly is where stress is most concentrated. Bringing this first threaded section close to the starting thread position will result in a decrease in fatigue strength.
Therefore, keeping a distance of at least two pitches between the first threaded section of the bolt assembly and the starting thread position will eliminate this hidden danger.
4. Thread tooth shape and size
When a bolt is under load, stress concentration occurs at the thread valley, and its value largely depends on the shape of the valley.
Changing the shape of the valley, such as making the thread valley smoother, reduces stress concentration and increases fatigue strength.
Generally speaking, the fatigue strength of a flat-bottomed thread valley is the lowest.
By replacing the flat-bottomed thread valley with a rounded one, the fatigue strength of the bolt can be improved. The size of the bolt also affects its fatigue characteristics. The larger the diameter, the lower the fatigue strength. This applies to the bolt threads as well.
5. Cracks at the bottom of bolt heads
Fatigue cracks usually originate at the bottom of the thread, but they can also start at the bottom of the bolt head.
Cracks that start at the bottom of the bolt head are usually caused by improper design of the transitional arc diameter of the bolt head (which causes stress concentration due to an improper transitional arc diameter), or because the bolt is installed on a tilted support.
A small angle between the bolt head and the supporting member (which can also be understood 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 on welded components (welded components typically undergo stress release and changes in structural shape after welding).
6. Stress distribution
The stress distribution on the nut is uneven, and a large amount of load is actually borne by the first few threads.
Therefore, a lot of fatigue in bolt assemblies occurs in the first and second threads of the nut. So, improving the average stress distribution on the threads where the bolt assembly is joined will improve its fatigue strength.
7. Steel metallurgical defects
Some bolts are not machined after cold heading or cold drawing, so surface defects in the raw material remain on the surface of the finished part.
A severe decarburization layer on the surface of the bolt is a weak area, and during the rolling process after cold heading, due to the large amount of deformation on the steel surface, most of the decarburization layer is squeezed into the top area of the thread.
This decarburized layer has low strength and hardness, making it prone to wear and stripping (thread being cut off), ultimately leading to early fatigue failure and becoming a source of fatigue cracks.
8. Improving stress distribution between bolt threads
To improve the stress distribution between bolt threads and increase fatigue life, investigations have shown that this can also be achieved by changing the shape of the nut.
Making a groove on the end face of the nut that contacts the supporting member can increase fatigue life by 25%. This improvement is especially suitable for large-sized bolts.
Of course, there are other ways to achieve more even stress distribution between the bolt and nut, such as changing the material of the nut to one with a different elastic modulus from the bolt, making the bolt and nut threads have different pitches, or using pointed threads.
9. Tightening bolts to the designed preload force
In many cases, the most effective way to increase the fatigue life of a bolt assembly is to tighten the bolt to the designed preload force.
Typically, a properly tightened bolt bears only 5% (or even less) of the dynamic load.
Therefore, a bolt that is tightened to the proper torque has strong resistance to fatigue loads. This is because the alternating loads applied to the bolt are small, resulting in small alternating stresses generated inside the bolt, which are usually well below the limit that the bolt can bear.
When fatigue failure occurs, the reason is often that the bolt preload force did not reach the design value, exposing the bolt to bending stresses and leading to early failure.