Fatigue Failure Of An M16 Flanged Bolt

When it comes to bolts there are many different things one needs to take into account in order to ensure the longevity of their construction. While bolts are usually a fairly easy thing to assemble, there are also a number of things that can potentially go wrong causing you to have to add new bolts to whatever you were making. If a bolt is being used in machinery these failures can often mean that the machines are going to stop working causing even greater problems.

 

 

Fatigue Failure of an M16 Flanged Bolt is one of the many things that you need to consider when it comes to the assembly of your projects. Fatigue failure is a lot more common than what many people expect, but more importantly, fatigue failure is actually the reason why over 90% of bolt breakages occur. Naturally, with that statistic in mind, it becomes quite obvious why fatigue failure is one of the things that you will need to consider. This is why we have created this guide that will help you fully understand what causes fatigue failure as well as how to potentially create the right conditions to avoid fatigue failure.

 

 

What is Fatigue failure?

Fatigue failure shows that during the installation of a bolt, there was a lack of preload. The lack of preload can also occur after the installation has been completed. To visually understand what fatigue failure looks like in most metals you can imagine a thin metal rod that you start bending back and forth several times. Over time the place where the bending occurs is going to suffer from fatigue failure which will lead to it breaking. That same visual image can be applied to a bolt if you want to get a better view of how a bolt can actually break because of fatigue failure.
Another way of understanding fatigue failure is by realizing that bolts are going to fail when the stress they have to deal with is more than what their strength level is. Over time this could lead the bolt to fail and could potentially result in fatigue failure.

 

 

When do bolts fail from fatigue failure?

 

Bolts will usually fail from fatigue failure when they have been exposed to cyclic loading which exceeds a prespecified amplitude. Bolted joins will in general have to go through Axial load fatigue tests in order to figure out what degree of cyclic loading is necessary for them to fail. There is also a global testing standard set forth specified as ISO 3800:1993. Generally, the specific stress amplitude is commonly repeated for anywhere between 104 and 107 cycles until fatigue failure occurs.

 

 

What are the stages of fatigue failure?

 

There are four stages to fatigue failure. What is important to remember about these is that whether you are dealing with high or low cycles, the same basic steps take place in the lead-up to complete failure caused by fatigue failure. The four stages are:
1. Crack initiation
2. Crack growth stage I
3. Crack growth stage II
4. Failure

There are a few different important things to remember when it comes to these stages. One of them is that in general, the crack growth phase is able to be stopped if the loads are small enough that they would fall below a critical threshold. Still, there is always the possibility of fatigue cracks growing from defects and cracks as small as 10 μm. What is certain is that if the rate of growth increases, then generally the stress is also going to be increasing which would lead to fatigue striations being visible on the top of the fracture surface. 

 

 

How strong is an M16 Flanged Bolt and what is its endurance?

 

Often times the endurance is going to depend on the manufacturer and supplier that you are using for your bolts. If you want high-quality bolts you can always purchase them through AYA Fasteners, a website that provides you with hundreds of options without you needing to stress about your bolts being insufficient to complete the job you need them for. 
Approximately a hexagon bolt M16x2.0 Class 5.8 usually has an endurance limit of around 10 Mpa. 

 

 

 

 

 

Why does fatigue failure usually occur? 

 

There are four reasons why bolts fatigue failure is common these include: 
1. Cyclical loading is leading to the bolt self loosening
2. The bolt was improperly installed
3. Not torquing the bolt
4. Insufficient joint design
Remember that fatigue failure is the result of a lack of preload. This information alone is often enough to protect you from having to experience fatigue failure since for the most part, you can ensure that the correct preload has been used in every single case and project that you are using bolts for.

 

 

What is cyclic loading?

 

Cyclic loading refers to when loads are applied, removed, and reapplied in a repetitive pattern or form. This continuous and repeated application of load will cause variations in stresses and tensions which could ultimately lead to the bolt self-loosening.

 

 

What is bolt preload?

 

Bolt preload as a term refers to the tension that is created when two materials are being brought together and held in place by a bolt. When the tension is at its optimal preload, then the working load will be evenly distributed from the bolt to the rest of the assembly. As a result, the bolt will not have to deal with bearing its full complete load ever again. This automatically reduces the chances of failure fatigue.

 

 

Is preload the same as torque?

 

Put simply these two values are not the same. However, there is a linear relationship between the two. On a completely theoretical level, every torque value has a corresponding preload value. The exact value of these depends as there is no set friction coefficient. Instead, there is a minimum and maximum and the friction coefficient will usually have a value set between those two.

 

 

Why is preloading important?

Preloading helps improve accuracy in the radial and axial directions. This improved accuracy is the reason why preload is crucial. It is also important to remember that in most cases higher the preload the fewer chances of shag there are.

How can you prevent fatigue failure of bolted joints?
To avoid the fatigue failure of bolted joints the most important thing you can do is ensure that the correct preload has been applied. You should also make sure that the preload is not getting lost during operation. It is important to remember that applying the correct preload can be difficult at times, however, that does not mean that you should not attempt to apply the correct preload.

 

 

What conditions might make it hard to avoid fatigue failure in bolts?

 

There are many challenges when it comes to fatigue failure, especially considering that self-loosening can often have dire effects on the process and longevity of a bolt. When it comes to a bolted joint the conditions that you must be most aware of relate to changes in temperature and the environment. What is important to remember is that sustained high or low temperatures might lead to significant variations in friction conditions.
Oxidation and corrosion, which is particularly common in areas with high humidity can also be conditions that will greatly affect the effectiveness of a bolt. In fact, a nut and bolt that have corroded a lot will be almost impossible to disassemble without the bolt being destroyed. As such, in those situations, fatigue failure is almost a guarantee.
Another thing to consider is whether there will be a lot of vibration in the machinery that you have used the bolt for. Excessive vibration might not only lead to self-loosening, but at times it is also possible for it to lead to fatigue failure. As such, that is something to consider ahead of time in order to ensure that the construction will hold for as long as possible

 

 

Conclusion

 

One of the big problems with fatigue failure is that it can often be the result of the conditions that surround your nuts and bolts. From variations in temperature to corrosion and oxidation there are a few different ways for your bolt to end up being affected and ultimately failing. Apart from the conditions that your bolt is being placed and left in, there is also the matter of installation. If the correct preload is not applied during the installation process, then it is close to guaranteed that the bolt in question would not suffer from fatigue failure. As such there are many early variations to consider when it comes to your bolts being safely and securely installed.

 

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