Bolt Fracture: Influencing Factors and Mechanism

The bolt is a common fastener.

If the bolt breaks slightly, it must be shut down for maintenance.

If it breaks badly, it will cause machine damage and human death.

Simply replacing the broken bolt cannot completely eliminate the risk of re-breaking, so it is necessary to analyze the breaking factors and improve them.

From the individual point of view, bolt fracture factors vary widely.

When the number of bolt fracture samples is large, some common features can be found.

1. Phase characteristics and correlation analysis of influencing factors of bolt fracture

The life cycle of bolts can be divided into four stages: design, manufacturing, use and maintenance, failure, etc., and the use of interruption cracks is the most harmful.

In order to avoid bolt fracture failure, it is necessary to understand the complete set and subset of the influencing factors of bolt fracture in the life cycle and different stages of bolts.

Table 1 and Table 2 are obtained after statistical analysis on the influencing factors of 227 bolts fracture.

Table 1 Statistical Analysis of Factors Affecting Fracture in Bolt Fracture Samples by Stages

Total

227

Single stage factor breaking

Manufacture

81

Design

34

Material Science

14

Use

14

Two stage factor combination causes breakage

Design+Manufacturing

27

Manufacturing+Materials

17

Manufacturing+use

14

Design+use

7

Design+Materials

1

Use+Material

1

Broken by combination of 3 stages+material factors

Design+Manufacture+Use

7

Design+Material+Manufacturing

2

Material+Manufacture+Use

7

3 stages+materials

1

Table 2 Correlation Analysis of Bolt Fracture with Three Stages and Materials in Bolt Fracture Samples

Correlation classification

Proportion/%

Manufacturing related

independent

81

combination

75

Total

156

68.7

Design related

independent

34

combination

45

Total

79

34.8

Related to materials

independent

14

combination

29

meter

43

18.9

Related to use

independent

14

combination

37

Total

51

22.5

Bolt fracture may be caused by a single factor or a combination of multiple factors.

Table 2 shows that 77.5% of bolt fracture samples are caused by pre use factors, while 68.7% of bolt fractures are related to manufacturing.

2. Factors and mechanisms affecting fracture at different stages of bolt life

There are many factors affecting the fracture of bolts during their life cycle.

Due to the limited space, only the influence mechanism of factors with high frequency of influence is preliminarily analyzed.

2.1 Factors and mechanisms affecting bolt fracture in design stage

The data in Table 3 shows that the factors that have the greatest impact on bolt fracture in the design stage are improper material selection, external causes of bolts, bolt structure design, small diameter and temperature difference load caused by different materials, etc.

The first five factors account for 85 items, accounting for 82.5% of the 103 items in the design stage.

2.1.1 Effect of improper material selection on bolt fracture

Material selection is an important part of bolt design.

The items affected by improper material selection in bolt fracture samples are shown in Table 4.

Environmental and stress corrosion, insufficient or excessive material strength, and unsuitable heat treatment process occurred 47 times, accounting for 81.0% of 58 times.

Table 3 Classification and Frequency of Factors Affecting Bolt Fracture at Design Stage in Bolt Fracture Samples

Affected Items

103

Improper material selection

42

External influence: vibration and insufficient rigidity of connectors

17

Bolt structure design

14

Small diameter

7

Temperature difference load caused by different materials

5

Number and arrangement of bolts

Anti loosing

3

Length and connection method

3

Improper design requirements

3

other

5

(1) Impact of environment and stress corrosion on bolt fracture

Stress corrosion is the failure phenomenon of materials under the joint action of static stress (mainly tensile stress) and corrosion.

Under the action of stress and corrosion medium, the surface oxide film of the bolt is damaged due to corrosion.

The damaged surface and the undamaged surface form anodes and cathodes respectively.

The anodic current density is very high.

The damaged surface is further corroded. With the effect of tensile stress, cracks gradually form at the damaged area, and the cracks gradually expand until they break.

In order to prevent stress corrosion, materials with strong resistance to stress corrosion shall be selected first.

For example, chromium nickel austenitic stainless steel with low manganese content shall be selected in the environment containing sulfide high temperature water.

Secondly, the bolt structure should be reasonably designed to reduce stress concentration.

Improve the corrosion environment, such as adding corrosion inhibitor in the corrosion medium.

Use metal or non-metal protective layer to isolate the effect of corrosive medium.

(2) The influence of poor bolt material performance on bolt fracture.

The material performance includes multiple indicators. The poor performance refers to that the selected bolt material is incompatible with the service environment.

The use of materials beyond their engineering capacity will lead to bolt fracture.

Taking the fracture of the coupling bolt of a 200 MW unit as an example, the coupling bolt was originally made of 35 steel, which was not quenched and tempered and was of coarse widmanstatten or banded structure.

After the bolt fracture, it was analyzed that it was not appropriate to use it to manufacture the coupling bolt, and 40CrNiMo steel was used instead, which improved the comprehensive mechanical properties of the bolt material.

However, when using 40CrNiMo steel, attention should be paid to the matching of the hardness of bolts and coupling materials.

When analyzing the broken bolt, it was found that the fretting wear of the bolt with hardness of (260~280) HB would cause damage to the bolt hole.

If 40CrNiMo steel is used to improve the hardness of bolts to obtain high fatigue strength, adverse consequences may occur.

Therefore, it is necessary to carry out comprehensive mechanical property test to make the bolt have low notch sensitivity, matching hardness and bending fatigue strength before changing to use.

(3) Impact of excessive or insufficient strength on bolt fracture

It is not difficult to understand that insufficient strength is easy to cause bolt fracture, but the relationship between excessive strength and bolt fracture is easy to be ignored.

High strength bolts not only increase the sensitivity of notch stress concentration, but also have hydrogen embrittlement sensitivity.

Generally, hydrogen induced cracks will occur when the hydrogen content in steel exceeds 5 ppm. However, for high-strength steel, even if the hydrogen content in steel is less than 1 ppm, under the effect of stress, the hydrogen atoms in lattice gap will be concentrated at the stress concentration place generated by the notch through diffusion.

The hydrogen atoms interact with the dislocation, so that the dislocation line is pinned and can no longer move freely, thus making the body brittle.

2.1.2 External factors affect bolt fracture

(1) Effect of vibration on bolt fracture

The vibration response of the connecting bolt mainly depends on the modal characteristics of the connecting bolt and the vibration excitation transmitted to the bolt by the connecting piece.

After the connecting bolt between a transmission and a power take-off was broken, the modal test of the long bolt showed that the natural frequency of the first bending mode was 1155 Hz and the modal damping ratio was 0.67 under the condition of 45 N · m tightening torque.

The vibration response test of the transmission power take-off under the working condition of the engine shows that: when the transmission system is working, the long bolt is excited by the obvious vibration with the main vibration frequency of 1 000 ~ 1 500 Hz, the first bending frequency of the long bolt is within this frequency band, and the damping ratio is very small, the resonance amplification effect is significant, resulting in a large bending resonance response of the bolt, and a large bending dynamic stress on the threaded connection, the connecting bolts broke early.

(2) Impact of insufficient rigidity of connected parts

Insufficient rigidity of the connected parts not only generates vibration, but also causes uneven stress on bolts.

The anchor bolts of a marine diesel engine broke quite frequently.

The analysis results show that the main engine has large vibration, especially vertical vibration, which is caused by the poor rigidity of the base – bilge.

After the wedge positioning block of the host positioning support is welded firmly, the anchor bolt is not broken anymore, because its rigidity has been strengthened.

Both ends of the steel plate of the drum shell of a hoist are connected to the flange plate with M22 bolts.

There is no reinforcing support ring or circumferential lining ring inside the drum, forming a simply supported beam along the axis.

The rigidity is poor. Under the working condition, the middle of the drum shell has the largest deformation, which makes the connecting bolt M18 bear the maximum force and causes the bolt at this point to break, while the connecting bolts near the flange plate at both ends of the drum have not broken.

2.1.3 Effect of bolt hole structure on bolt fracture

The main factor affecting the bolt fracture in the bolt and screw hole structure is that the transition fillet is too small, including the transition fillet at the thread root, screw and bolt head, undercut, etc., which not only generates stress concentration, but also because the transition fillet is small, it is easy to generate large internal stress in the heat treatment, so microcracks or crack tendency appear, reducing the bolt bearing capacity.

The combined effect of external load and internal stress makes the load borne by the bolt exceed its limit.

As a result, the bolt was broken.

The main bearing bolt of a DF 7B locomotive diesel engine was broken.

After the modification, the middle bolt hole was eliminated, which increased the bearing area of the main bearing bolt head by 45% and the strength of the thread part by a large margin.

At the same time, due to the cancellation of the inner screw hole, the stress concentration caused by the inner hole thread structure is eliminated, and the fatigue strength of the bolt is increased.

2.2 Factors and mechanisms affecting bolt fracture in manufacturing stage

Heat treatment quality, machining quality, processing transition fillet too small, fitting and assembly quality, and bolt forming process are the main factors affecting bolt fracture in the manufacturing stage.

There are 141 factors, accounting for 89.2% of 158. See Table 5 and Table 6.

Table 5 Classification and Items of Factors Affecting Bolt Fracture in Manufacturing Stage

Item

158

Heat treatment quality

71

Machining quality

40

The fillet is too small

11

Fit and assembly quality

10

No integral forging or forming process defect

9

Root surface defect of screw tooth

2

Coating and corrosion fracture

1

other

14

Table 6 Classification and Items of Factors Affecting Bolt Fracture by Heat Treatment

Item

87

Heat Treatment Process Design and Process Quality

33

hydrogen embrittlement

14

Decarburization and partial overburn

11

Dysplasia

8

High hardness and low plasticity

6

Surface or center carburization

5

Quenching quality and pretreatment intelligence

5

Heat treatment and material conflict fastener leather rope

1

others

4

2.2.1 Effect of heat treatment on bolt fracture

Heat treatment process design and process quality, hydrogen embrittlement, decarburization and local overburning, poor structure, high hardness and low plasticity are the main factors affecting bolt fracture, accounting for 82.8% of 87 items.

(1) Effect of heat treatment process design and process quality on bolt fracture

An example is given to illustrate the influence of improper heat treatment process on bolt fracture. When hypoeutectoid quenched and tempered steel 42CrMo is used as bolt material, and the section size is large (such as ≥ 500 mm), it is difficult to reach the yield ratio of 0.9 with conventional quenching and tempering treatment.

In order to achieve this goal, it is necessary to reduce the tempering temperature, that is, use medium temperature tempering or lower temperature tempering.

In this way, although the strength can be improved, the toughness will also be reduced, and defects will be left in the metallographic structure (the second type of temper brittleness).

The test results of a batch of bolts are: high strength (σb>1200 MPa), high hardness (HBS>400), and the metallographic structure is tempered troostite, which fully proves this point.

Research at home and abroad shows that the higher the strength of steel is, the greater the sensitivity to cracks is.

When σb>1200 MPa, if the toughness is insufficient, it is easy to cause low stress brittle fracture.

(2) Effect of hydrogen embrittlement on bolt fracture

Some connections require bolts to have high strength and be used in corrosive environments, which requires anti-corrosion treatment for bolts.

Some anti-corrosion processes, such as chromium plating, are prone to hydrogen embrittlement.

The research shows that the higher the material strength level is, the greater the hydrogen embrittlement sensitivity is, and the greater the crack growth speed is.

In various steel microstructures, the general order of susceptibility to hydrogen embrittlement from high to low is martensite, upper bainite, lower bainite, sorbite, pearlite and austenite, and the high strength is based on the corresponding metallographic structure.

Hydrogen embrittlement can be divided into hydrogen embrittlement caused by internal hydrogen and external hydrogen. Internal hydrogen is generated in the manufacturing process, while external hydrogen is penetrated in the use process.

In general, the internal hydrogen will crack or fracture the bolt before or after use. When the external hydrogen enters the material, it requires an accumulation process to make the hydrogen gradually reach the content of damage.

Therefore, it takes a long time to fracture the bolt.

In order to avoid hydrogen embrittlement fracture caused by electroplating, it is recommended to use hydrogen embrittlement free coatings, such as zinc chromium coatings widely used in automotive, aerospace and other industries.

2.2.2 Effect of machining quality on bolt fracture

In the process of bolt manufacturing, the defects such as wrinkles, folds and microcracks formed due to improper processing often cause further cracking or expansion of bolt threads during rolling or rolling forming or heat treatment.

Especially, these defects are often distributed at the root of bolt threads.

The microcracks at the root of bolt threads are prone to stress concentration under the action of cyclic stress or load.

Fatigue sources preferentially initiate at these defects and form multi-source fatigue fracture.

A machining streak was observed on the fracture surface of the heater bolt of a 350 MW gas turbine unit.

The streak was located at the junction of the screw and the bolt head.

There was a large corrosion pit on the streak, indicating that there was obvious crevice corrosion before the bolt cracked.

The appearance inspection also found that the surface of the bolt polished rod was rough, which not only became the source of stress concentration, but also provided conditions for crevice corrosion and stress corrosion.

2.3 Analysis of material factors and frequency affecting bolt fracture

In the sample of broken bolts, the influencing factors and items of materials on bolt fracture are shown in Table 7, including 39 items of inclusions, material quality, metallurgical defects, and excessive chemical element content, accounting for 86.7% of the 45 items.

Table 7 Factors and Items Affecting Bolt Fracture by Materials

Item

45

Inclusion

16

Material quality

10

Metallurgical defect

7

Chemical elements of materials

6

Segregation

4

Microcrack

2

(1) Influence of inclusions on bolt fracture

When magnesium, calcium in foreign inclusions and sulfur, manganese, chromium and other elements inside the material segregate towards the grain boundary, grain boundary embrittlement will occur in local areas, which is equivalent to potential cracks.

However, if the size of inclusions in bolts is too large, especially near the surface layer, the initiation and propagation of fatigue cracks in bolts will be promoted.

The banded distribution of MnS inclusions in steel also increases the susceptibility to hydrogen induced cracking.

(2) Effect of supplied material properties on bolt fracture

In some service environments, it is obviously not enough to only pay attention to the strength and hardness of bolt materials.

Plasticity, impact toughness, corrosion resistance, notch sensitivity, and performance difference between room temperature and working temperature need to be comprehensively considered.

The material performance nonconformity here means that the supplied material does not meet the design requirements.

After the gas heater bolts of a gas turbine unit were broken, according to the chemical composition and metallographic analysis of the bolt materials, the broken bolts were not made of 304 type stainless steel specified in the design, but were cast after several different stainless steel materials were remelted, and their corrosion resistance could not meet the requirements, resulting in galvanic corrosion between the bolts and the blind plate due to different electrode potentials at the beginning.

Although the broken bolts on the flange of the intermediate pressure governing valve of a steam turbine generator were qualified in the spot check of the mechanical test strength and impact toughness at room temperature, both of them were unqualified in the test at the operating temperature of 540 ℃.

(3) Influence of metallurgical defects of materials on bolt fracture

The existence of looseness, bubble, slag inclusion and internal crack in bolt materials significantly reduces the actual allowable stress of materials.

The macro and micro analysis of the fracture surface of a high strength bolt shows that after the fracture starts from the crack source, the fracture process expands in a rapid and unstable manner until it breaks.

This is due to the fact that there are many micro defects such as microcracks and micropores in the material, which reduces the actual allowable stress, which is also a prerequisite for rapid unstable crack growth.

The formation of these microcracks is related to the incomplete degassing and slagging during smelting and the incomplete elimination of subsequent forging.

2.4 Analysis of service factors and frequency affecting bolt fracture

The pre tightening force, uneven tightening force, improper tightening method, installation and other problems are the main factors affecting the bolt fracture in the use stage.

These three items occurred 69 times, accounting for 75.0% of the 92 times, as shown in Table 8.

(1) Effect of pre tightening force on bolt fracture

A clamp connection seal structure is not accessible because it is under high temperature and high pressure conditions.

The operator uses a special wrench about 1m long to tighten the clamp bolt until it can not be tightened, so that the bolt preload has far exceeded the allowable stress of the bolt.

The bolt stress further increases after the pipeline pressure rises, which eventually leads to the bolt breaking in a short time.

In one group of connecting rod bolts, once one bolt has insufficient preload, there will be a large gap between the connecting rod journal and the bearing bush.

At this time, the load must be transferred to another bolt.

When the crankshaft runs at high speed, the bolt will bear a large alternating impact load and bending moment, causing fatigue and fracture, and then the second bolt will be overloaded and fractured.

(2) Uneven fastening force and improper fastening method

When maintaining a compressor, the maintenance personnel did not use a torque wrench according to the designed pretightening force. Instead, they used a sledgehammer to hit a solid head wrench.

The pretightening force was controlled by experience.

The pretightening force of the bolt was greater at a convenient location for hammering, while the pretightening force of the bolt was smaller at a inconvenient location for hammering.

According to the fracture distribution of the compressor cylinder head bolts, most of the bolts at the convenient location for hammering are broken, which is consistent with the analysis results.

3. Conclusion

(1) The life cycle quality objectives of mechanical products are based on the realization of the life cycle quality of all parts including bolts.

It is an innovative practice to establish the concept of part level life cycle quality to establish the set of influencing factors of bolt fracture.

(2) The life cycle quality of bolts is affected by multiple stages and multiple factors.

The establishment of a set of factors affecting bolt fracture is conducive to the overall planning and selection of these factors, so as to achieve the life cycle quality objectives of bolts.

(3) Gradually improve the set of influencing factors of bolt fracture, and supplement and modify the teaching materials and relevant documents to support the practice of the concept of life cycle quality of bolts and mechanical products.

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