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Failure Analysis damaged bolted

Nuts and boltsHas many uses. Home of the bolt failure may not have all right, but if it occurred in the nuclear power plant, it could have disastrous consequences. For this reason, failure analysis of damaged bolted it is particularly important.
Cause bolt failure are many, in fact, this is usually caused by multiple factors.
Alstom Transport bolting Laurent Dastas analysts believe that the root cause of the failure of four bolts:
1. forget to tighten the bolts.
2. Tighten the fastening tool tolerances enough.
3. grade steel fasteners error.
4. Tighten the operation sequence error.

      From a technical perspective, bolt failure can be divided into two categories: static and fatigue failure (also known as "dynamic failure"). Laurent Dastas said: "The use of electronic keys simultaneously recorded tightening operation couple and angle values ​​to ensure press these four parameters at each fastening bolt assembly."

      Static failure are usually easy to identify, such as overload, assembly fitting tighten unwanted external load or "substandard products" will cause static failure.

      Fatigue failure is often much more complex because of fatigue in the work cycle is formed. Material (such as fasteners, threaded bolt or fitting assembly any part) may appear first crack, then intensified and spread to the normal section of the screw (pressure section), which led to the complete failure of the component assembly.

       Nord-Lock Advanced Technical Expert Zouhair Chaib said: "Generally speaking, for the use of bolt assembly component, crack-related damage is the most dangerous fault."

      Fatigue failure of the assembly components, may initially be tightened correctly, but in the external load due to relaxation or self-loosening and other reasons, the bolt connections will start to lose their preload.

      Chaib said: "Once you start losing preload, it is difficult to stop, because the alternating stress and slip between the parts will increase again in this slide, the preload will lose more and more. . "

      Continued to increase along with alternating stress, which will eventually lead to fatigue cracks. In the external cyclic loading and fatigue cracks will continue to spread. If the spread to the screw, once it reaches the fatigue load weight, will be completely ineffective. The screw corrosion or impurities in the material, and thread cutting quality or accidental load (impact), will induce fatigue cracks.

      If it involves more starsScrews, Then one of a failure, it will cause the screw next to the overload. In a very short time overload will trigger a chain failure Needless long time, it will fail screw each other "contagious."

      Chaib said: "Regarding the fatigue failure is concerned, you never know fatigue, cracks and assembled components completely ineffective when they will occur under normal circumstances, the fatigue failure are far more unprepared, which is why we want to see the most unexpected situations. . "

      One way to identify the cause of failure, is to use the "Ishikawa diagram" (ie fish bone diagram) analysis, Nord-Lock technology centers often use this method to help customers solve problems.

      Chaib said: "Once a failure occurs, the customer must first ensure environmental safety, and then replace the existing assembly components and replacement parts for all customers to be photographed individually numbered and components, but also careful not to touch the surface of the failure, the contact surface. or thread. we also need to provide customers with the case described the accident and failure to quickly identify the problem. "

      In addition, before the customer will be sent to technical spare parts center for analysis, the need to do the work of anti-corrosion parts, so as not to make the wrong assumptions analysts, mistaken crack failures are caused by corrosion.

      Chaib said: "This will help our customers quickly and accurately analyze the situation, to find answers and solutions as quickly as possible to do the work, you do not have to replace the assembly fitting example, customers can choose to replace the existence of hidden parts. , so that production can be sustained. "

      Customers often encounter various limitations, and the need to resume production as soon as possible. Thus, during transport inspection and verification of assembly components is essential. At the same time, customers and analysts can be an expert to cooperate, to determine the best and most accurate solution to replace the existing assembly components.

      In the Nord-Lock Technology Center, we will check all of the components, and the damaged parts of each photograph. Under most circumstances, we will use three-dimensional microscope to compare and contrast the factors, but also external collaboration requests additional analysis laboratories. Ultimately, all of the results of their analysis, the whole problem will be the truth.

      Chaib said: "We were in a laboratory vibration, torque and preload test and inspection screws use to completely ensure that our proposed solution foolproof."

      He continued means: "In the technology center, we will analyze a variety of conditions, identify the cause of the problem and propose countermeasures we put a lot of technical aspects into consideration, including economic, practical and operational factors."

Counterfeit pretensioner case:

      Superbolt preload preload has a good accuracy, only a small torque can achieve high preload, and therefore widely used.

      A customer has used more push bolt in a structure, then this is a bolt other manufacturers. Over time, a structure fails, the Nord-Lock customers should investigate the cause.

      Chaib said: "We use three-dimensional microscope to analyze the failure and found that the main cause of fatigue failure, and then let preload fatigue faster."

      Viewed from the top end of the bolt pushing the junction, can be learned preload initially applied is normal, but the view of the rotational position of pushing the bolt, you will find all the more push bolt has a significant difference. One of the top push the bolt is rotated to zero, which is pushing its own loose bolts caused. Chaib said: "We also checked for self-imitation pretensioner loose resilience concluded that this factor did not imitate imitations and compare our pretensioner can find them.Screw threadThe same diameter, the same material, diameter is the same, but in the former, some details more push bolts are ignored. "

      The pretensioner static endurance is good, but there are loose on their own problems, which can lead to fatigue failure.

Clamping length Case:

      A client uses a standard bolting solutions has led to damage to the components. Customer Component Replacement newbolt,Screw nut,Flat washers and spacersAnd photographed the damaged parts inspection. Customers also sent CAD, in order to provide the circumstances and the external load information.

      Customers have increased with the length of the clamping pads, so that the problem can be solved, and then restart the machine but the same failure.

      Chaib said: "After analysis of customer components, we believe that fatigue failure is high preload set of components is actually high preload are not dangerous, but it seems a good idea we have in-depth investigation, the other... failure to find a reason. "

      Finally, we conclude that: In the circumstances, there is a very high shear load, which is a lateral load. Customers need to use the same as before clamping length (without pads), but you can increase torque and improve the level screw to increase preload, as fastening solutions. For lateral load, the best way is to use fastening solutions to increase and maintain the preload.

      Chaib said: "We recommend that customers use the Nord-Lock lock washers to prevent loosening of the components on their own With our solution, customers need to make some changes to components: two Nord-Lock lock washer, with a high level bolt (10.9), then the proper torque is fixed. "

Failure Cases thread:

      Half a client component fails, but the thread but no significant damage.

      Chaib said: "We ask the customer to provide more information, as well as estimates of operating conditions described component load customers said this component is subjected to impact load, rather than fatigue load.."

      "We observe its outer profile, threaded end found a slight plastic deformation. We also observed along the axial trace of the thread, and noted the inconsistency of the outer diameter of the problem."

      Analysts envisioned a scenario could lead to failure: diameter tolerance and material properties, insufficient to support the hoop stress due to angular and axial thread load generated. Under full load, the first part of the stretch (radial elongation), but due to fine thread teeth, while the second part will slip out of the first part.

      Chaib said: "To test this scenario, we conducted a FEM (finite element method) calculations, (after application of axial load) diameter measured diameter and FEM were compared, also compared the results of FEM thread form and reality shape."

      FEM results and the actual situation is similar, so analysts recognized its scenarios. We propose the following practical solutions:

Increasing the outside diameter of the threaded portion.
Use coarse thread.
Easy to use deformable material (high Young's modulus).
The first part of the shape optimization.

The most common cause of failure of bolts:
1.? Human error.
2.? Material defects, such as material containing impurities lead to fatigue cracks.
3.? Improper calibration tool.
4.? Design.
5.? Underestimate external loads or make wrong assumptions.
6.? Computational problems, such as stress or bolt torque calculated incorrectly.

Bolt Failure Analysis Overview:
After the failure of the bolt assembly, the customer should:
1.? To ensure environmental safety failures surrounding components.
2.? Pictures to all parts.
3.? To all the part numbers.
4.? Member corrosion prevention.
5.? Detailed description of the accident and failure status.

Nord-Lock technology-centered approach:
1.? Pinpoint the problem.
2.? Check all parts and to take pictures of all damaged parts.
3.? Compare and contrast microscope with 3D-related factors.
4.? Identify factors that may lead to failure by Ishikawa diagram.
5.? Related tests, such as vibration, torque and preload test.
6.? Seek cooperation laboratories other analyzes.
7.? Analysis to identify key simulation scenarios and submitted to customers.
8.? Develop practical solutions (economic, security, ease of installation).
　　　　　　　　　　　　　　　　　　　　   This article is reproduced: Ekintong   
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