Corrosion failure analysis of bolt on Tesla
Although the small bolt is not impressive, but it is used as a series of automotive key parts of the fastener, once the corrosion fracture failure, light car failure, heavy casualties.
This year, there have been many accidents at Tesla, and one after another global recalls. According to the website of China’s State Administration of market supervision and administration, Tesla motor (Beijing) Co., Ltd. filed a recall plan with the State Administration of market supervision and administration. It is decided to recall 3183 imported model X Series vehicles produced from April 15, 2016 to October 16, 2016 from June 7, 2020.
When some vehicles within the scope of this recall are exposed to high corrosive environment such as strong deicing salt for a long time, the bolts that fix the steering gear motor on the steering gear housing may be corroded and broken, resulting in the displacement of the steering gear motor and the slip of the drive belt, resulting in the weakening or loss of steering power, which may increase the risk of vehicle collision, and there are potential safety hazards. This article will take Tesla recall as an example to analyze the causes of bolt fracture failure, bolt selection and assembly should pay attention to.
Tesla will replace new steering gear steel bolts for vehicles within the scope of recall, and add anti-corrosion coating on the steering gear motor housing and motor fixing bolts; if the fixing bolts are found damaged or damaged during disassembly, the steering gear will be replaced free of charge.
This is not the first time Tesla has recalled its products due to steering power problems.
In early 2018, Tesla announced that it would recall 123000 models cars worldwide due to power steering system failure. The recall covers all models cars that were manufactured before April 2016. The reason for the recall is that corrosion of certain bolts in cold weather may cause power steering wheel failure. In the cold environment, the road salt splashed on the road for snow melting may accelerate the corrosion rate of the problem parts.
It is not difficult to find that in 2018, when Tesla recalled the vehicles for the first time due to specific bolts, it delineated the scope of all models cars that were manufactured before April 2016, but in fact, some model X Series cars produced from April 15 to October 16, 2016 also had the same problem, and these vehicles continued to drive for two years with potential safety hazards, It is only today that the problem has been discovered and made public.
We still don’t know whether the second recall of the same problem is caused by Tesla’s intention to conceal other vehicle problems at the time of its first recall, or whether the enterprise is not conducive to the investigation of the same potential problems of other products.
Fortunately, there is no report of serious consequences due to the above problems. However, as an electric vehicle manufacturer with modern advanced technology, Tesla “fell” twice on a small bolt, which inevitably makes the outside world question Tesla’s ability in quality control and product potential risk prevention and control.
Fracture analysis of Tesla bolt
Table of Contents
- 1 Fracture analysis of Tesla bolt
Failure analysis of common fasteners
- 2.1 The failure of general fasteners includes the following:
- 2.2 The cause of the failure comes from one or a combination of the following three aspects:
- 2.3 Here are a few examples of fastener failures that everyone sees daily:
- 2.4 Look at the cause of failure from the manufacturing and assembly process of fasteners
- 3 Recommendations for the selection of fasteners
- 4 Precautions for assembly of fasteners
Description of recalled bolt connection parts
This recall is due to the problem of the connecting bolts of the electronic power steering gear housing. The application parts of the recalled bolts are as follows: electronic power steering gear, steering gear motor, steering gear motor connecting bolts and steering gear motor and steering gear housing after assembly hole.
Analysis of possible causes of recall
It can be seen from the recall instructions that the connecting bolts are made of aluminum alloy, and the housings of the motors and gearboxes are also made of aluminum alloy. Under normal circumstances, bolts of this material will not easily cause corrosion. However, it is mentioned in the recall statement that the bolts were broken due to corrosion caused by deicing agents.
According to the Tesla engineer of Model S, the frequent use of calcium or magnesium salt instead of sodium chloride (table salt) in the snow melting agent will cause greater corrosion.
Snow melting agents are mainly divided into two categories:
- One type is: organic snow melting agent with potassium acetate as the main component. Although this type of snow melting agent has a good snow melting effect and no corrosion damage, its price is too high and generally only suitable for airports and other places;
- The other is: chlorine salt snow melting agents, including sodium chloride, calcium chloride, magnesium chloride, potassium chloride, etc., commonly known as “deicing salt”. Its advantage is that it is cheap, and the price is only 1/10 of the organic snowmelting agent, but its corrosion to large-scale public infrastructure is very serious.
According to the paper “New Method for Determination of Corrosion of Chloride-type Snow Melting Agent Carbon Steel” written by Wang Shuxuan and others, when the solution concentration is less than 4%, the weight loss rate of corrosives is CaCl2>MgCl2>NaCl, so when the concentration is less than 4 %, calcium salt and magnesium salt snow melting agent will cause greater corrosion.
After the bolt is connected and assembled, there is basically no exposed part, and in principle, it will not cause corrosion of the bolt itself. The possible reason is that the deicing agent will penetrate into the threaded hole, causing corrosion of the thread and threaded hole, especially after the threaded hole is corroded, if the design engagement length of the threaded hole is relatively short, it may cause the sliding tooth of the threaded hole. Or even if the tooth is not slippery, the pre-tightening force will decrease, which will cause the fatigue fracture of the bolt under the action of long-term fatigue load.
From the description of the recall measures, sealants are used to protect the bolt head, motor and gearbox housing and their contact surfaces, which will reduce corrosion, especially the bolt head and gearbox housing, gearbox housing and motor housing Corrosion between bodies.
It can also be seen from these measures that there is no problem in the design or quality of the bolt itself, mainly due to the corrosion of the connected parts, especially the corrosion between the contact surfaces, plus the wall thickness of the gearbox housing flange It is relatively thin, resulting in greater rigidity of the connected part. If a certain amount of corrosion occurs on the contact surface, a slight change in the clamping length of the connected part will also cause a decrease in the pre-tightening force, which will result in a fatigue load. There will be problems with openings on the joint surface, which will result in greater fatigue stress, and fatigue fracture will occur if the bolts can be broken.
In addition, steel grade 8.8 bolts are usually used here, and aluminum alloy bolts are used in Tesla’s instructions. If they are consistent with the usual 8.8 grade bolt specifications, it may cause insufficient clamping force;
If a larger first-class specification is used with the 8.8 bolt specification, such as the M8 specification, and other parts use shared parts, if other vehicle parts are borrowed, the wall thickness of the aluminum alloy connected parts will become thinner, which will Cause a series of problems such as insufficient strength.
The main reason for the recall of the steering gear motor mounting bolts here is: due to the effect of the deicing agent, the connected parts, especially the corrosion effect between the joint surfaces, cause a slight reduction in the clamping length of the connected parts, thereby reducing the bolts The pre-tightening force will eventually cause the fatigue fracture of the bolt under the action of the fatigue load.
Usually, the bolts here are made of 8.8 grade steel bolts. Tesla uses aluminum alloy bolts, which may cause low bolt strength and insufficient pre-tightening force. Under the action of external load, it will cause openings in the connected parts, resulting in Larger fatigue stress may cause fatigue fracture of bolts.
There is another possible reason: the existing mature parts are borrowed from the connecting parts, the larger aluminum alloy bolts cause insufficient engagement length, or the threaded holes are enlarged and the wall thickness becomes thinner, resulting in insufficient strength.
Failure analysis of common fasteners
The failure of general fasteners includes the following:
- ① The assembly is broken when twisted and pulled;
- ② The thread is broken by shearing force;
- ③ The stress concentration part breaks after use;
- ④ Fatigue fracture;
- ⑤ Delayed fracture;
- ⑥ Parts torque alarm;
- ⑦ Thread sliding teeth.
The cause of the failure comes from one or a combination of the following three aspects:
- ① Quality problems in the fastener manufacturing process lead to fastener failure;
- ② The selection of fasteners or the use of components (including man-made) causes fasteners to fail;
- ③ Fastener assembly problems cause fastener failure.
Here are a few examples of fastener failures that everyone sees daily:
- 1) In order to determine whether a product design is reliable, there are many ways to perform destructive testing to verify confirmation. Usually, what many manufacturers like most is deliberately destroying a component in a controlled environment (given operating conditions and load spectrum) (for example, testing until the fastener breaks) to determine one or more components (including tightening The performance of parts), and finally get some reasons leading to product failure. The failure of fasteners caused by this aspect is caused intentionally. Whether this method and the result are completely reasonable is not easy to evaluate.
- 2) There is also the problem that consumers frequently abuse products intentionally or unintentionally. For example, a person spends more time fiddling with his heavy briefcase and suitcase on a low-priced table than working on the table. Soon, the fasteners supporting the table will loose or break and fail. This type of fastener failure may cause personal injury, but the manufacturer is unlikely to recall the product because the customer’s improper use caused the problem.
- 3) Volvo Cars recalled three models in March 2017 because the bolts used to fix the side curtain airbags failed. The quality of the bolt manufacturing process for fixing the airbag is not effectively controlled, and may quickly break due to internal hydrogen embrittlement. Volvo engineers believe that the entire airbag design is structurally reasonable, but the entire airbag assembly needs to be replaced due to potential defects in the fastener manufacturing process.
- 4) Loose fasteners may eventually fail. Two years ago, the off-road vehicle manufacturer Polaris had to recall all Commander models because the input shaft fasteners were loose and allowed the shaft to move along the length of the power steering spline. In extreme cases, this movement may cause the shaft to be completely separated from the spline.
- 5) Engineers from General Motors Corporation discovered loose fasteners on several models in 2014, which caused the company to recall more than 500,000 vehicles this year. The Chevrolet Impala has a “strongly shaped” fastener that is not tightened to the required torque. GMC ATV, Buick Regal, Lacrosse and Cadillac SRX front seats and passenger seats moved up and down freely, causing a crash and three people were injured. According to statistics, in 95% of failures, fasteners have problems during installation or maintenance, and another 5% are due to the use of wrong fasteners.
Look at the cause of failure from the manufacturing and assembly process of fasteners
Fasteners have one thing in common with all other manufactured products: the better the manufacturing control, the better the performance. In the manufacturing process, several factors affect the quality of fasteners:
- 1) If the temperature reaches about 700 degrees Celsius, metal decarburization may occur during the heat treatment, and there is not enough protective atmosphere in the furnace. This may cause the threads to become soft and fall off.
- 2) Quenching and tempering (or stretching) is one of the most commonly used heat treatment processes for steel fasteners. Fasteners should be tempered within a few minutes after being removed from quenching and before being completely cooled. Otherwise, it may cause quenching cracking, premature failure or shorter service life than normal service life (see Figure 1).
3) During the forming process of the fastener head, it is very important that the metal grain flow lines in the correct direction. The crystal grain streamline that moves sharply to the fillet at the junction of the bolt head and the rod cannot produce good flow (see Figure 2). This may make the fastener head easily broken during installation.
Fig.2 During the head forming process, it is very important that the metal grain flow lines shape in the correct direction. The image on the left represents an acceptable particle flow that meets SAE USCAR8 specifications.
Everyone needs to inspect the fasteners during the manufacturing process to determine the appropriate grain flow pattern. The inspector uses hydrochloric acid and water to boil the head of the sample fastener for 1 minute, so that the grain size can be easily evaluated. flow. For example, all head bolts and screws used by our automotive customers must meet the grain flow pattern of the SAE USAC8 specification.
4) When the torque is too large, if the threads close to the head are too close, the pressure on the head will increase. This situation may also lead to failure from head to shaft. Because this defect will increase the radial stress, it will accelerate hydrogen embrittlement when it appears in the fastener (see Figure 3).
Fig.3 Hydrogen embrittlement is a common cause of fastener failure. The arrow in the photo indicates the load stress area.
When hydrogen is absorbed in the steel during the electroplating process of fasteners and moves along the grain boundaries to stress concentration, internal hydrogen embrittlement will occur. This may cause sudden catastrophic failure under load.
5) Most threaded fasteners are treated with electroplating or protective coating to prevent rust or corrosion, and then test their corrosion resistance according to ASTM B117 (salt spray test). This process may cause damage to the protective coating.
6) In the final test and assembly process, each fastener will suffer some more or less damage, resulting in the protective coating may be damaged. Premature red (iron oxide) rust may appear during use.
7) Another common problem is dip spin coating. It provides excellent corrosion protection, but when too much electroplating fills the head of the small screw, it may cause unsmooth drilling.
The geometry, material, heat treatment, surface treatment and other factors of the fastener will affect its performance during the service life of the product from assembly to installation. Therefore, fasteners should always be regarded as engineering parts rather than simple standard parts.
Recommendations for the selection of fasteners
High-quality fastening requires a perfect match between the fastener and the connected material and the fastener (if it is a bolt) and the nut. For example, using grade 8 bolts with grade 5 nuts will produce a clamping force lower than expected.
Due to the miniaturization of products, the correct selection of fasteners is more important than ever. Fastener handling not only requires the same load, but also uses lighter and thinner materials in a smaller space.
- (1) Repeated use is not recommended. The thread on the reused nut is slightly softer than the thread on the mating bolt. In addition, due to the increased thread friction, these threads will be compressed and constantly lose the clamping load each time they are used. You can see the effect of this situation on the wheel studs, which usually fail after driving about 120,000 miles.
- (2) It is recommended not to use different types of bolts or too long bolts in the same part. In the former case, the fastener with higher hardness will eventually carry most of the load. The thread on the bolt that is too long will have relatively little shock absorption at the connection, which will actually lead to premature metal fatigue in the area.
- (3) Aesthetics is a reasonable consideration. Manufacturers usually hope that fastener heads can enhance the appearance of assembled products. For example, Harley-Davidson uses chrome-plated and stainless steel nuts and bolts to assemble motorcycle sprockets, consoles and air filters. These fasteners must still be able to resist vibration and environmental humidity.
- (4) Another way for manufacturers to prevent fasteners from failing is to only purchase fasteners that fully comply with all design specifications. A few years ago, Boeing began to implement its policy of refusing to supply all substandard fasteners to the enterprise more strictly. This move is in response to the Federal Aviation Administration (FAA) plan to fine Boeing US$2.75 million because the company installed substandard fasteners on the 777 aircraft in 2008.
Precautions for assembly of fasteners
The correct manufacturing and selection is an important first step to prevent fastener failure. During the tightening process, ensure that the specified requirements are met (in many cases, especially in field maintenance, it may happen that the specifications are not met or over tightened), and the integrity of the fastener thread is maintained.
- (1) Over-tightening may cause fasteners to break during assembly, which is obvious, and may also cause thread peeling, which may not be so obvious. The latter is particularly dangerous because the integrity of the bolted connection may be compromised and cannot be detected by the assembler.
- (2) A correct understanding of the lubricity of fasteners (friction and torque coefficient, etc.) helps the end user to obtain the best clamping load when using torque. However, unwanted lubricants on fasteners, such as a bit of oil in the hands of the fitter, can cause overtightening. This will change the torque tension value at the joint by increasing the clamping force. When the fastener is not sufficiently tightened to a lower clamping load, the periodic or fluctuating load exceeding the clamping load may quickly lead to fatigue.
- (3) The external load of the unfastened fastener is greater than expected. There may be some small abnormal noises such as squeaks and creaks, until catastrophic fatigue failure.
- (4) Insufficient tightening may also be caused by loose inserts. For example, when the bolt is embedded in a soft material, the joint cannot be fully compressed. Improper nut assembly is another cause of insufficient tightening. When the nut is screwed down too fast, a rebound effect will occur, and the tightening part will rebound slightly. In addition, due to torsional relaxation within the bolt length range, tightening the bolt head instead of the nut will reduce the clamping load by 10% to 15%.
- (5) The threads need to be kept as clean as possible, despite a small amount of lubricant, to facilitate installation and disassembly. Even tiny particles on the threads will reduce the friction coefficient of the fastener during installation. This will increase the joint tension and stress, making the fasteners prone to failure. The average friction coefficient of fasteners is 0.15, but it varies according to the lubricant and fastener coating.
- (6) Sometimes it is possible to repair the thread cross-thread by reassembling the parts at different angles. However, if the thread geometry is not accurate, you may need to repair or replace parts. If the thread cross-threading problem is not repaired, the fastener may not be installed in the correct position, causing the thread to be unable to withstand the specified clamping force.
- (7) When pressure and friction cause the bolt thread to jam on the thread of the nut or threaded hole, wear will occur. If the fastener is stuck due to wear, it is usually necessary to cut off the bolt or remove the nut to complete the disassembly.
Source: Network Arrangement – China Fasteners Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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