BOLT BREAKAGE OR THREAD STRIPPING AT FACTORY. UNSCREWING AND FATIGUE BREAKAGE DURING LIFETIME OPERATION.
When it comes to bolted joints, there are four main groups of failure modes. This training focuses on explaining their differences and how to avoid them.
Introduction – “how hard can it be?”
The headline of this part of the training is “How hard can it be?”, a common view among engineers regarding the design of bolted joints. Almost all applications include bolted joints, but usually this is still a field of technology with very little focus. This is probably why the bolt joint is responsible for such a large share of the production disturbances and failures during operation.
This training focuses on the four most common failure modes:
1) Bolt breakage or thread stripping during assembly.
2) Bolt breakage within 48 hours after assembly.
3) Unscrewing (problem within a few months).
4) Fatigue breakage.
As shown in the timeline below, the first two failure modes occur before the car/truck leaves the factory. The last two failure modes occur during operation.
Failure during assembly, or shortly thereafter
1A. BOLT BREAKAGE DURING ASSEMBLY
A bolt breakage during assembly is usually easy to detect. It is also simple to replace the broken bolt with a new one.However, it will cause a disturbance in the production.
Possible root causes to bolt fracture are:
– Higher clamping force than expected. A higher clamping force can arise due to higher assembly torque or lower friction.
The friction is dependent on the surface under the bolt head as well as the nut threads – not only the bolt. The assembly speed can also impact the friction, especially on painted surfaces.
– Wrong bolt (wrong property class).
Both at the assembly station and in the bolt production there is a risk of mixing ins. If the standard torque of a 10.9 bolt is used on an 8.8 bolt, there is a high risk that the bolt will break during assembly.
– Wrong mechanical properties of the bolt.
Wrong wire, not sufficienttemperature, too short time in the austenitization furnace, or too slow cooling can create a bolt which doesn’t fulfil the demands (for further information, see the “Hardening” chapter).
Should problems with bolt breakage during assembly arise, use the checklist below to determine the root cause:
1. Check the bolts.
- Do the bolts have the right mechanical properties?
- Is it the right type of bolt?
2. Ask for a torque and angle curve.
- If the bolt builds torque slower than normally -> it’s usually a friction problem.
- If the torque angle curve looks normal but breaks at low torque -> it’s likely a problem with the mechanical properties of the bolt.
3. Perform a friction test.
- Secure that the bolt friction is according to the specifications by performing a standard friction test.
- Secure that the assembly friction is according to the specifications by performing a friction test in the application.
1B. THREAD STRIPPING DURING ASSEMBLY
Female tread stripping is usually a much worse failure mode due to the fact that it can be harder to detect and is usually more expensive and time-consuming to fix.
The triggers for the thread stripping are usually too high clamping force due to low friction, or too high torque. But, as mentioned, in the design guidelines, the bolt should always be the weakest part (exception to this is lowstrength joint, such as a plastic application or steel sheet metal). Therefore, the most common root cause for female thread stripping is engineers not following the design guidelines. Thread stripping can, in exceptional cases, also be caused by the material for the threads not fulfilling the demands.
2. DELAYED FRACTURE/HYDROGEN EMBRITTLEMENT
Hydrogen embrittlement is one of the worst types of failure since it is difficult to detect. During assembly everything is normal, but after a few minutes, or up to 24 hours, the bolts will start to fracture without any added extra load.
When fracture caused by hydrogen embrittlement occurs, three different parameters are present:
– High stress. For example, if the bolt is assembled close to yield, the stress in the material will be high. Rough surface and large inclusions will also lead to high stress due to local stress concentration.
– Presence of hydrogen in the material. Hydrogen can be introduced both during the bolt manufacturing, especially during the surface treatment, but also when the bolt is in use.
– Brittle material. If the bolt has a property class of 9.8 or lower, there is no risk of hydrogen embrittlement. If the property class is 10.9 or higher, hydrogen embrittlement might occur. Casehardened bolt also run an increased risk of hydrogen embrittlement.
The most common method to verify whether the fracture is caused by hydrogen embrittlement or not is to investigate the fracture surface in high magnification using a scanning electron microscope (SEM).
If the fracture is caused by hydrogen embrittlement, the fracture will be brittle and the grain boundaries will be separated. In some cases, micro pores and bird footstep are also visible (see figure below).
Failure during operation (bolted joint exposed to a load)
When the vehicle rolls off the production line, the bolted joint will be exposed to an external load. Depending on the position of the bolted joint, the load can be in axial or/and shear direction, and it can be dynamic (like the wheel) or static (like the safety belt). The bolted joint with a high, dynamic load in shear is the most difficult to design. It is also the bolted joint which fails most often.
3. SELF-LOOSENING /UNSCREWING
Self-loosening failure modes is usually easy to detect. The joined parts separate and/or the bolt is loose/missing. Problems usually occur in joints with high shear loads and/or heavy vibrations.
Probable root causes are:
- Insufficient clamping force (due to high assembly friction, low torque or large settlings/relaxation). If the clamping force is not enough, sliding in the bolted joint will occur and the bolt will start to loosen.
- Low friction between clamped parts. Lower friction increases the risk of sliding between the clamped parts.
- Low thread friction. The friction in the treads stops the bolt from unscrewing. If the thread friction is lower than 0.06, the bolt can get loose due to just a low level of vibration.
To identify the self-loosening root cause, use checklist below:
1. Was the clamping force correct after assembly?
- Check data from the production line (if possible).
- Assembly test in lab. measuring the clamping force.
- Assembly test production line.
2. Is the friction between the clamped parts accordingto specifications?
3. Is the load according to specifications?
4. FATIGUE BREAKAGE
When a bolt breaks after a while during operation, it is usually due to fatigue breakage. By investigating the fracture surface using a light-optical microscope, it is possible to identify whether the fracture is caused by fatigue or not.
Usually, “scratch marks” is the starting point for the fracture. The scratch marks are followed by “river marks”. The distances between the river marks are small close the scratch marks and then increase with each river mark. The last part of the fracture is called the final fracture. The size of the final fracture depends on the load on the bolted joint. If the load is high, the final fracture is big.
The three, most common root causes to fatigue breakage is:
- Wrong design. If possible, it is recommended to avoid large dynamic loads in shear direction.
- External loads.
- Low clamping force.
FAILURE MODES – A SUMMARY
With more focus on the bolted joint field of technology, a large share of the production disturbance and failure during operation could be avoided.
Learn more – take the full training
This is a shortened version of the Bulten Academy training course Failure Modes. To take part of the full version, please contact Bulten Academy – firstname.lastname@example.org.