Nuts and bolts are versatile. Failure of the bolts in the home may not be a serious problem, but if it occurs at a nuclear power plant, it may have catastrophic consequences. For this reason, failure analysis of damaged bolt connections is particularly important.
There are many reasons for bolt failure. In fact, this is usually caused by many factors.
According to Laurent Dastas, bolting analysis specialist at Alstom Transport, there are four root causes of bolt failure:
1. Forget to tighten the bolts.
2. The tightening tolerance of the tightening tool is not accurate enough.
3. The steel grade of the fastener is incorrect.
4. The tightening sequence is incorrect.From a technical point of view, bolt failures fall into two main categories: static failure and fatigue (also known as "dynamic failure"). Laurent Dastas said: "Using the electronic key to simultaneously record the couple and angle values ​​in the tightening operation ensures that the bolts are tightened for each assembly according to these four parameters."
Static failures are often easy to discern, such as overloading, over-tightening of assembly components, accidental external loads, or "failed products" can result in static failure.
Fatigue failure is often much more complicated because fatigue is formed during the work cycle. Materials such as fasteners, threads of bolts, or any component in an assembly may first crack and then exacerbate and spread to the positive section (pressure section) of the screw, causing the assembly to fail completely.
Zouhair Chaib, Nord-Lock senior technical expert, said: "In general, crack-related damage is the most dangerous failure for assembly components that are bolted."
Assembly components that fail fatigue may be properly tightened at first, but under external load, the bolted joint will begin to lose its original preload due to slack or looseness.
Chaib said: "Once the preload is lost, it is difficult to stop because the alternating stress and sliding between the parts will increase. Under this repeated sliding, the preload will be lost more and more. ."
There is also an increase in alternating stress, which will eventually lead to fatigue cracking. Under external and cyclic loads, fatigue cracks continue to spread. If it spreads to the screw, it will completely fail once its load reaches the fatigue load. Corrosion or corrosion of the screw material, as well as the cutting quality of the thread or the unexpected load (impact), can induce fatigue cracks.
If multiple screws are involved, one of them will fail and overload the adjacent screws. Overloading can cause chain failures in a very short time, and the screw failure will "infect" each other.
Chaib pointed out: "In terms of fatigue failure, you never know when fatigue, cracks and assembly components will fail completely. Under normal circumstances, fatigue failures are too late to prevent, which is the most unexpected situation we are not willing to see. ."
One way to find out the cause of failure is to use the "Ishikawa map" (ie fishbone diagram) analysis, which is often used by the Nord-Lock Technology Center to help customers solve problems.
Chaib said: "In the event of a failure, the customer must first ensure that the environment is safe, and then replace the existing assembly components and replacement parts. Customers must take pictures of all parts and number them one by one, and also be careful not to touch the failed surface, contact surface Or thread. We also need our customers to provide an accident and failure description to quickly identify the problem."
In addition, before the customer sends the parts to the technical center for analysis, it is necessary to do the anti-corrosion work of the parts, so as to avoid the analysts making the wrong assumptions and mistakenly believe that the crack failure is caused by corrosion.
Chaib said: "Customers do this to help us quickly and accurately analyze the situation and find answers and solutions as quickly as possible. To do this, you don't have to replace the assembly components. For example, customers can choose to replace parts with hidden dangers. To make production sustainable."
Customers often encounter limitations and need to resume production as soon as possible. Therefore, it is important to inspect and verify assembly components during transportation. At the same time, customers and expert analysts can work together to determine the best and most accurate solution to replace existing assembly components.
At the Nord-Lock Technology Center, we inspect all parts and take photos one by one. In some cases, we will use a 3D microscope to compare and contrast factors, and we will request an external collaborative lab for additional analysis. In the end, all the results are analyzed and the whole problem is revealed.
Chaib said: "We perform vibration, torque and pre-tightening tests in the laboratory and verify the use of the screws to fully ensure that our solutions are flawless."
He continued: "At the Technology Center, we analyze a variety of situations, identify the cause of the problem and propose technical countermeasures. We take into account many aspects, including economic, practical and operational factors."
Case of counterfeit pretensioner:
Superbolt pretensioners are widely used because of their good pre-tightening accuracy and high preload force with low torque.
A customer used a multi-push bolt in a structure that was produced by other manufacturers. After a period of time, the structure failed, and the customer asked Nord-Lock to investigate the cause.
Chaib said: "We use a three-dimensional microscope to analyze failures and find that the main cause of failure is fatigue, and the pre-tightening force accelerates fatigue."
From the observation of the intersection of the end of the push bolt, it can be known that the preload force applied at the beginning is normal, but looking at the rotational position of the jack bolt, it will be found that all the multi-push bolts have significant differences. One of the jacking bolts rotates to zero, which is caused by the self-looseness of the jacking bolt. Chaib said: "We also checked the resistance of the counterfeit pretensioner to self-looseness. The conclusion is that the imitation does not take this factor into account. Comparing the replicas with our pretensioners, they have the same diameter and the same material. The outer diameter is also the same, but in the former, some details of the multi-push bolt are ignored."
The static bearing capacity of this pretensioner is good, but there is a problem of self-looseness, which leads to fatigue failure.
Clamping length case:
A customer uses a standard bolting solution that causes component damage. The customer replaced the components with new bolts, nuts, flat washers and gaskets and took the damaged parts for inspection. The customer also sent CAD to provide information on the current situation and external load.
The customer used a gasket to increase the clamping length, thinking that this would solve the problem, but the same failure occurred after restarting the machine.
Chaib said: "After analyzing the customer's components, we think it is fatigue failure. The pre-tightening force of this set of components is very high. In fact, the high preload is not dangerous, but it is a good way. It seems that we have to investigate in depth, another Find the cause of the failure."
Finally, we conclude that in the current situation, there was a high shear load, which is a lateral load. The customer needs to use the same clamping length as before (no gasket required), but the preload can be increased by increasing the torque and increasing the screw rating as a fastening solution. For lateral loads, the best solution is to use a fastening solution to increase and maintain the preload.
Chaib said: "We recommend that customers use Nord-Lock lock washers to prevent the components from loosening themselves. With our solution, the customer needs to make some modifications to the components: with two Nord-Lock lock washers, with a high grade Bolts (10.9), then fixed with appropriate torque."
Thread failure case:
Half of the components of a customer failed, but the threads were not significantly damaged.
Chaib said: "We ask our customers to provide more information on the estimated load and the operating conditions of the components. The customer claims that the assembly is subjected to impact loads, not fatigue loads."
"We observed its outer contour and found a slight plastic deformation at the end of the thread. We also observed the axial trace along the thread and noticed the problem of inconsistent outer diameter."
The analyst envisioned a scenario that could lead to failure: the tolerance of the outer diameter and material properties was insufficient to support the circumferential stress due to the thread angle and axial load. At full load, the first portion is stretched (radial elongation), and due to the action of the threaded fine teeth, the second portion slips away from the first portion.
Chaib said: "In order to verify this scenario, we performed FEM (Finite Element Method) calculations, comparing the measured diameter with the FEM diameter (after applying the axial load), and comparing the thread shape obtained by FEM with the reality. shape."
The FEM results are similar to the actual situation, so the scenario envisioned by the analyst is recognized. We have proposed the following practical solutions:
Increase the outer diameter of the threaded portion.
Use coarse thread.
Use a material that is not easily deformed (high Young's modulus).
Optimize the shape of the first part.The most common causes of bolt failure:
1. Human error.
2. Material defects, such as materials containing impurities that cause fatigue cracks.
3. The tool is not properly calibrated.
4. Design issues.
5. Underestimate external loads or make false assumptions.
6. Calculate problems such as incorrect torque or stress calculations for the bolts.Overview of bolt failure analysis:
After the bolt assembly fails, the customer should:
1. Ensure the environment around the failed component is safe.
2. Take a photo of all the parts.
3. Number all parts.
4. Prevent parts from corroding.
5. Describe the accident and failure conditions in detail.Nord-Lock Technology Center approach:
1. Identify the problem.
2. Check all parts and take pictures of all damaged parts.
3. Compare and contrast the relevant factors with a 3D microscope.
4. Use the Ishikawa map to identify possible factors that cause failure.
5. Perform related tests such as vibration, torque and preload testing.
6. Request a collaborative laboratory to conduct additional analysis.
7. Analyze the results, identify the main simulation scenarios and submit them to the customer.
8. Develop practical solutions (economic, safe, easy to install).
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