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Failure analysis of bolt corrosion

Although the small bolt is not impressive, it is used as a fastener to connect the key parts of the automobile in series. Once the corrosion fracture failure occurs, it will lead to vehicle failure in light and casualties in heavy cases.
According to the website of the State Administration of market supervision and administration, Tesla Motors (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 manufactured from April 15, 2016 to October 16, 2016 from June 7, 2020.
When some vehicles within the scope of this recall have been exposed to high corrosion environment such as strong deicing salt for a long time, the bolts fixing the steering gear motor to the steering gear housing may be corroded and broken, resulting in displacement of the steering motor and slipping of the drive belt, resulting in the weakening or loss of steering power, which may increase the risk of vehicle collision and have potential safety hazards. This paper 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 the steering gear steel bolts for the recalled vehicles, and add anti-corrosion coating on the steering gear motor housing and motor fixing bolts; if the fixing bolts are found to be damaged or damaged during disassembly, the steering gear will be replaced free of charge.
According to foreign media reports, on February 12, 2020, Tesla decided to recall 15000 model x vehicles in North America because of the potential loss of steering power. According to Tesla officials, the recall is applicable to most modelx models manufactured before mid October 2016, while those produced later will not be affected, involving 14193 U.S. cars and 843 Canadian cars, not involving the Chinese market.
This is not the first time Tesla has recalled a product due to potential steering problems
In early 2018, Tesla announced that it would recall 123000 model s cars worldwide due to power steering system failure. The recall covers all model s cars 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 sprayed on the road for snow melting may accelerate the corrosion rate of the problem parts.
From the delivery time of the recalled vehicles mentioned above, it is not difficult to find that in 2018, Tesla first recalled all model s sedans due to specific bolts, but in fact, some models were produced between April 15, 2016 and October 16, 2016 X Series cars also have the same problem, and these vehicles with safety risks continue to drive for two years, until today the problem was discovered and announced.
It is still unknown whether Tesla intends to conceal other vehicle problems at the time of the first recall, or whether the enterprise is not conducive to the investigation of the same potential problems of other products.

Fortunately, there are no reports of serious consequences due to the above problems. However, as an electric vehicle manufacturer integrating modern advanced technology, Tesla has “fallen” on small bolts twice, 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

Recall of bolt connection

This recall is due to the problem of connecting bolts of the electric power steering housing. The application parts of the recalled bolts are as follows: electronic power steering gear, steering gear motor, steering motor connecting bolts and connecting bolt holes after the assembly of steering gear motor and steering gear housing.

Analysis of possible causes of recall

It can be seen from the recall instructions that the connecting bolt is made of aluminum alloy, and the shell of motor and gearbox is also made of aluminum alloy. Under normal circumstances, the bolt made of this material will not easily cause corrosion. However, it was mentioned in the recall note that the bolt was broken due to corrosion caused by deicing agent.
According to Tesla engineers of model s, the frequent use of calcium or magnesium salts instead of sodium chloride (table salt) in deicing agents can cause greater corrosion.

Deicing agents can be divided into two categories

  1. Organic snow melting agent with potassium acetate as the main component. Although this kind of snow melting agent has good snow melting effect and no corrosion damage, its price is too high, and it is generally only applicable to airports and other places;
  2. Chlorine salt snow melting agent, including sodium chloride, calcium chloride, magnesium chloride, potassium chloride, etc., commonly known as “deicing salt”. Its advantage is that it is cheap and its price is only 1/10 of that of organic snow melting agent. However, its corrosion to large public infrastructure is very serious.

According to the paper “new method for determining the corrosivity of carbon steel with chloride type snow melting agent” written by Wang Shuxuan, when the solution concentration is lower than 4%, the weight loss rate of corrosion products is CaCl2 > MgCl2 > NaCl. Therefore, when the concentration is lower than 4%, the deicing agent of calcium salt and magnesium salt will cause greater corrosion.
After the connection and assembly of the bolt, there is basically no exposed part, which will not cause corrosion of the bolt itself in principle. The possible reason is that the deicing agent will penetrate into the threaded hole, resulting in the corrosion of the thread and the threaded hole. Especially after the corrosion of the threaded hole, if the design engagement length of the threaded hole is relatively short, it may cause the sliding teeth of the threaded hole, or even if the thread does not slide, it will also cause the reduction of the preload, which will lead to the fatigue fracture of the bolt under the long-term fatigue load.
The use of sealants from the recall action statement to the bolt head, motor and gearbox housing and their contact surfaces will reduce corrosion, especially between the bolt head and the gearbox housing, and between the gearbox housing and the motor housing.
It can also be seen from these measures that there is no problem with the design or quality of the bolts themselves. The main reason is that the connected parts may suffer from corrosion, especially the corrosion between the contact surfaces. In addition, the thickness of the flange of the gearbox shell is relatively thin, resulting in a relatively large rigidity of the connected parts. If there is a certain corrosion on the contact surface, the clamping length of the connected parts will be very small The change will also cause the decrease of preload, which will lead to the opening of the joint surface under the fatigue load, resulting in larger fatigue stress, and some bolts may appear fatigue fracture.

  • In addition, steel grade 8.8 bolts are usually used here, while aluminum alloy bolts are used in Tesla instructions. If the specifications are consistent with the normal grade 8.8 bolts, the clamping force may be insufficient;
  • If a larger specification is adopted with grade 8.8 bolts, for example, the specification of M8 is adopted, and other parts share the same parts. If other vehicle parts are borrowed, the wall thickness of aluminum alloy connected parts will become thinner, which will cause a series of problems such as insufficient strength.

Summary

The main reason for the recall of the steering gear motor mounting bolts here is: due to the effect of snow melting agent, there is a certain corrosion effect between the connected parts, especially between the joint surfaces, resulting in a small reduction of the clamping length of the connected parts, thus reducing the bolt pre tightening force, and finally causing the fatigue fracture of the bolt under the fatigue load.
Generally, grade 8.8 steel bolts are used here, and aluminum alloy bolts are used by Tesla, which may result in low bolt strength and insufficient preload. Under the action of external load, the opening of connected parts will be caused, resulting in large fatigue stress and fatigue fracture of bolts.
Another possible reason is that the connecting parts borrow the existing mature parts, the larger aluminum alloy bolts lead to insufficient meshing length, or the wall thickness becomes thinner after the threaded hole is enlarged, resulting in insufficient strength.

Failure analysis of common fasteners

Generally, the failure of fasteners includes the following:

  • ① Assembly screw pull fracture;
  • ② The thread is broken by shearing force;
  • ③ The stress concentration part fractured after use;
  • ④ Fatigue fracture;
  • ⑤ Delayed fracture;
  • ⑥ Part torque alarm;
  • ⑦ Thread slippage.

The causes of failure come from one or a combination of the following three aspects:

  • ① The quality problems of fastener manufacturing process lead to fastener failure;
  • ② Fastener selection or component use problems (including artificial) lead to fastener failure;
  • ③ Fastener assembly problems lead to fastener failure.

Here are a few examples of fastener failure that you can see everyday

  • 1) In order to determine whether a product design is reliable, there are many ways to conduct destructive tests to verify the validation. In general, many manufacturers prefer to intentionally destroy an assembly in a controlled environment (given working conditions and load spectrum), such as testing until the fastener breaks, so as to determine the performance of one or more components (including fasteners), and finally obtain some causes of product failure. The fastener failure caused by this aspect is caused by man-made intention. It is not easy to evaluate whether the method and result are completely reasonable.
  • 2) There is also the problem of frequent abuse of products, intentionally or unintentionally. For example, a person spends more time fiddling with his heavy briefcase and suitcase at a low-cost desk rather than working on it. Soon, the fasteners that support the table will become loose or broken and fail, which may cause personal injury, but the manufacturer is unlikely to recall the product because it is the customer’s improper use that caused the problem.
  • 3) Volvo car recalled three models in March 2017 due to a faulty bolt used to secure the side curtain airbag. There is no effective control over the manufacturing process quality of the bolt fixing the airbag, which may break quickly due to internal hydrogen embrittlement. Volvo engineers believe that the whole airbag design is reasonable in structure, but the whole airbag assembly needs to be replaced due to potential defects in the fastener manufacturing process.
  • 4) Loose fasteners may also eventually fail. Two years ago, 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 can cause the shaft to completely separate from the spline.
  • 5) GM engineers found loose fasteners on several models in 2014, leading to a recall of more than 500000 vehicles this year. The Chevy impala has a “strong set” fastener that is not tightened to the required torque. GMC all terrain vehicle, Buick Regal, lacrosse and Cadillac SRX front seats and passenger seats moved up and down freely, resulting in a collision and three people were injured.
  • According to statistics, in 95% of the failures, fasteners have problems in the process of installation or maintenance, and the other 5% is due to the use of wrong fasteners.

Failure causes of fasteners in manufacturing and assembling process

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 ℃, decarburization may occur in the process of heat treatment, and there is not enough protective atmosphere in the furnace. This may cause the thread to soften and fall off.
2) Quenching and tempering (or stretching) is one of the most commonly used heat treatment processes for steel fasteners. Fasteners shall be tempered within a few minutes after removal from quenching and before complete cooling. Failure to do so may result in quenching cracking, premature failure, or shorter service life than normal service life (see Figure 1).

20201201084643 73145 - Failure analysis of bolt corrosion

Figure.1
3) In the forming process of fastener head, the grain streamline of metal is very important in the correct direction. The grain streamline moving sharply towards the fillet at the junction of bolt head and rod can not produce good flow (see Fig. 2). This may cause the fastener head to break easily during installation.

20201201084653 73275 - Failure analysis of bolt corrosion
Fig.2 it is very important for the metal grain streamline to form in the right direction during the head forming process. The image on the left shows acceptable particle flow in accordance with the SAE uscar8 specification.
During the manufacturing process, you need to inspect the fasteners to determine the appropriate grain flow pattern. The inspector uses hydrochloric acid and water to boil the head of the sample fastener in sections for 1 minute, so that the grain flow can be easily evaluated. For example, all head bolts and screws used by your automotive customers must meet the grain flow pattern of the SAE usac8 specification.
4) If the torque is too close to the head, the pressure will increase when the head is too close. This can also lead to head to rod failure. Because this defect increases radial stress, it accelerates hydrogen embrittlement when it appears in fasteners (see Figure 3).

20201201084710 16621 - Failure analysis of bolt corrosion
Fig.3 hydrogen embrittlement is a common cause of fastener failure. The arrow in the picture indicates the load stress area.
Internal hydrogen embrittlement occurs when hydrogen is absorbed in the steel and moves to stress concentration along the grain boundary during the electroplating process of fasteners. This can lead to sudden catastrophic failures under load.
5) Most threaded fasteners are electroplated or coated to prevent rust or corrosion, and then tested for corrosion resistance in accordance with ASTM B117 (salt spray test), which may result in damage to the protective coating.
6) During the final test and assembly, each fastener will suffer some more or less damage, resulting in possible damage to the protective coating. Early red (iron oxide) corrosion may occur during use.
7) Another common problem is dip spin coating. It provides excellent corrosion protection, but when too much plating fills the head of a small screw, it may not drill smoothly.
The geometry, material, heat treatment, surface treatment and other factors of fasteners will affect their performance during the service life from assembly to installation. Therefore, fasteners should always be regarded as engineering parts rather than simple standard parts.

Suggestions on selection of fasteners

High quality fastening requires a perfect match between the fastener and the material to which it is attached and, if it is a bolt, to the nut. For example, using a grade 8 bolt with a grade 5 nut will produce a lower than expected clamping force.
Due to the miniaturization of products, the correct selection of fasteners is more important than ever before. The fastener treatment not only requires the same load, but also uses lighter and thinner materials in a smaller space.

  • (1) Reuse is not recommended. The threads on the reused nuts are slightly softer than those on the mating bolts. In addition, due to increased thread friction, these threads will be compressed and continuously lose the clamping load each time they are used. You can see the effect of this on the wheel studs, which usually fail after driving about 120000 miles.
  • (2) It is recommended not to use different types of bolts or long bolts in the same part. In the former case, fasteners with higher hardness will eventually carry most of the load. Too long threads on the bolt will have less damping at the joint, which will actually lead to premature metal fatigue in this area.
  • (3) Aesthetics is a reasonable consideration, and manufacturers usually expect fastener heads to enhance the appearance of assembled products. For example, Harley Davidson uses chrome plated and stainless steel nuts and bolts to assemble motorcycle sprocket, console and air cleaner. These fasteners must still be resistant to vibration and ambient humidity.
  • (4) Another way for manufacturers to prevent fastener failure is to purchase only fasteners that fully meet all design specifications. A few years ago, Boeing began to enforce more strictly its policy of refusing to supply all unqualified fasteners to the company. The move is in response to the US Federal Aviation Administration’s plan to fine Boeing $2.75 million for installing substandard fasteners on its 777 aircraft in 2008.

Assembly precautions of fasteners

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 the field repair and maintenance, it is possible to meet or over tighten the specification requirements), and maintain the integrity of the fastener thread.

  • (1) Overtightening can lead to fastener fracture during assembly, which is obvious or may cause thread peeling, which may not be so obvious. The latter is particularly dangerous because the integrity of bolted connections may be impaired and cannot be detected by the assembler.
  • (2) A correct understanding of the lubricity of fasteners (friction and torque coefficients, etc.) helps the end user to obtain the best clamping load when using torque. However, unnecessary lubricants on fasteners, such as a little oil in the hands of the fitter, can cause over tightening. This changes the torque tension value at the connection by increasing the clamping force. When fasteners are not fully tightened to a lower clamping load, cyclic or fluctuating loads exceeding that clamping load may quickly lead to fatigue.
  • (3) The external load on the UN tightened fasteners is greater than expected. Small abnormal noises, such as squeaks and rattles, may occur first until catastrophic fatigue failure.
  • (4) Under tightening may also be due to loose inserts. For example, when a bolt is embedded in a soft material, the joint cannot be fully compressed. Improper assembly of nuts is another cause of insufficient tightening. If the nut is unscrewed, it will rebound slightly when the nut is tightened too fast. In addition, tightening the bolt head instead of the nut reduces the clamping load by 10% to 15% due to torsional relaxation over the length of the bolt.
  • (5) Threads need to be kept as clean as possible, despite the presence of a small amount of lubricant to facilitate installation and removal. Even small particles on the thread will reduce the friction coefficient of fasteners during installation. This will increase the joint tension and stress, making the fastener prone to failure. The average friction coefficient of fastener is 0.15, but it varies with lubricant and fastener coating.
  • (6) Sometimes, the parts can be reassembled at different angles to repair thread misalignment. However, if the thread geometry is not accurate, repair or replacement parts may be required. If the problem of thread misalignment is not repaired, the fastener may not be installed in the correct position, resulting in the thread unable to withstand the specified clamping force.
  • (7) Wear occurs when pressure and friction cause bolt threads to become stuck on the threads of nuts or threaded holes. If the fastener is stuck due to wear, it is usually necessary to cut off the bolt or remove the nut to complete the removal.

Source: Network Arrangement – China Bolts 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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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