What Is High-Strength Bolt?

Ultra High-Strength Friction Grip Bolt can reduce weight and increase installation space by reducing their own size under the same clamping force.

Therefore, the function and volume of the connected parts can be optimized, so that the equipment can achieve the purpose of overall weight reduction and performance optimization.

So what is high strength bolt?

What is the High-Strength Friction Grip Bolt?

I will show you today.

What Is High-Strength Bolt? 1

Physical fasteners of grade 16.8 and 19.8

1. What is High-Strength Bolt?

High strength bolt is also known as high strength friction grip bolt, which is abbreviated as HSFG in English.

It can be seen that the high-strength bolt mentioned in construction is the abbreviation of high-strength friction grip bolt.

In daily communication, only the words “friction” and “grip” are omitted, but many engineering and technical personnel have misunderstood the basic definition of High-Strength Friction Grip Bolt.

Myth 1:

Bolts with material grade more than 8.8 are “high strength bolts”?

The core difference between high-strength bolts and ordinary bolts is not the strength of the materials used, but the form of stress.

The essence is whether to apply preload and use static friction to resist shear.

In fact, in the British standard specifications, the high-strength bolts (HSFG BOLT) mentioned in the American standard specifications are only grade 8.8 and grade 10.9 (BS EN 14399 / ASTM-A325&ASTM-490), while ordinary bolts include 4.6, 5.6, 8.8, 10.9, 12.9, etc. (Table 2 of Clause 11 of BS 3692);

It can be seen that the strength of materials is not the key to distinguish high-strength bolts from ordinary bolts.

Table2 Strength grade designations of steel bolts and screws

Strength grade designation4.64.85.65.86.88.810.912.9
Tensile strength Rm min.N/mm240040050050060080010001200
Yield stress Re min.N/mm2240320300400480
Stress at permanent set limit R0.2 N/mm26409001080

2. What is the strength of high-strength bolts?

According to GB50017, calculate the tensile and shear strength of single ordinary bolt (class B) grade 8.8 and high-strength bolt grade 8.8.

What is the strength of high-strength bolts

Through calculation, we can see that under the same grade, the design values of tensile strength and shear strength of ordinary bolts are higher than those of high-strength bolts.

So where is the “strength” of high-strength bolts?

In order to answer this question, we must start with the design working state of the two bolts, study the law of their elastic-plastic deformation, and understand the limit state of design failure.

What Is High-Strength Bolt? 2

Stress-strain curves of ordinary bolts and high-strength bolts under working conditions

Limit state at design failure

Ordinary bolt: the screw itself has plastic deformation that exceeds the design permission, and the screw is sheared.

For ordinary bolt connection, relative slip will occur between the connecting plates before it begins to bear shear, and then the bolt rod contacts the connecting plate, resulting in elastic-plastic deformation and bearing shear.

High strength bolt: the static friction between the effective friction surfaces is overcome, and the relative displacement of the two steel plates occurs, which is considered as failure in design considerations.

For high-strength bolt connection, the friction force first bears the shear force.

When the load increases to the point where the friction force is insufficient to resist the shear force, the static friction force is overcome, and the connecting plate slides relatively (limit state).

But at this time, although it is damaged, the bolt rod can still use its own elastic-plastic deformation to bear the shear when it contacts the connecting plate.

Myth 2:

High bearing capacity is high strength bolt?

According to the calculation of a single bolt, the tensile and shear design strength of high-strength bolts are lower than that of ordinary bolts.

The essence of its high strength is that when the joint works normally, no relative slip is allowed, that is, the elastic-plastic deformation is small and the joint stiffness is large.

It can be seen that under the given design node load, the node designed with high-strength bolts may not save the number of bolts, but its deformation is small, its stiffness is large, and its safety reserve is high.

High strength bolts are suitable for main beams and other positions requiring high node stiffness, which conforms to the basic seismic design principle of “strong nodes, weak members”.

The strength of high-strength bolts does not lie in the design value of their own bearing capacity, but in the large stiffness, high safety performance and strong damage resistance of their design joints.

3. Comparison between high-strength bolts and ordinary bolts

There are great differences in construction inspection methods between ordinary bolts and high-strength bolts because of their different design stress principles.

Inspection requirements for ordinary bolts and high strength bolts

Bolt

Standard

Stretching

AKV

Chemical analysis

Hardness test

Loading force

yield strength
MPa

Tensile strength
MPa

Extensibility
%

percentage reduction of area

J

HRC

HV10

kN

Common 8.8

BS3692
ISO898

640

830

12

52

R

23-34

256-335

212

Experimental value of a batch

885

959

29

53

R

29.8

294

High strength 8.8

BS4395-1
ISO891

635

827

12

52

30J

R

25-34

260-330

207

Experimental value of a batch

817

944

24

58

33J

R

28.9

The mechanical performance requirements of ordinary bolts of the same grade are slightly higher than those of high-strength bolts, but high-strength bolts have more impact energy acceptance requirements than ordinary bolts.

What Is High-Strength Bolt? 3
  • a. British standard grade 8.8 high strength bolt connection pair identification, bolts, nuts and washers (BS4395);
  • b. British standard grade 8.8 high strength bolt connection pair identification, bolts, nuts and washers (BSEN14399);
  • c. American Standard Grade 8.8 high strength screw connection pair identification, bolts, nuts and washers (ASTM A325);
  • d. British standard grade 8.8 ordinary bolt connection pair identification, bolts, nuts and washers (BS3692).

The marking of ordinary bolts and high-strength bolts is the basic method for on-site identification of bolts of the same grade.

Since the values of torque value calculation for high-strength bolts in British and American standards are different, it is also necessary to identify the bolts of the two standards.

The price of ordinary bolts are about 70% of the price of high-strength bolts.

Combined with the comparison of their acceptance requirements, it can be concluded that the premium part should be to ensure the impact energy (toughness) performance of the material.

4. How to improve the fatigue strength of bolts?

No matter what complex load it bears, the common failure form of high-strength bolts is fatigue failure.

As early as 1980, experts studied 200 cases of bolt connection failure, of which more than 50% were fatigue failure.

It is very important to improve the fatigue resistance of high-strength bolts.

The fatigue fracture of bolts has the following characteristics:

1. The maximum stress of fatigue fracture is far lower than the strength limit of the material under static stress, even lower than the yield limit.

2. The fatigue fracture is brittle sudden fracture without obvious plastic deformation.

3. Fatigue fracture is the result of micro damage accumulation to a certain extent.

For bolts, the failure forms are mainly the plastic deformation of the thread part and the fatigue fracture of the screw, among which:

  • 65% of the damage occurred in the first thread connected with the nut;
  • 20% of the damage occurred at the transition between thread and polished rod;
  • 15% of the damage occurred at the fillet between the bolt head and the screw.
What Is High-Strength Bolt? 4

1. Optimize design to reduce stress concentration

Strictly control the ending size of bolts to eliminate stress concentration:

a. Use large transition fillets

b. Cutting unloading groove

What Is High-Strength Bolt? 5

c. Cutting undercut at the end of thread

d. Optimizing the inclination angle of bolt head can also effectively reduce stress concentration

What Is High-Strength Bolt? 6

e. Use reinforced thread

The main differences between the reinforced thread and the ordinary thread are the minor diameter d1 of the external thread and the root transition fillet R.

The main characteristics of the reinforced thread are that the small diameter d1 is larger than the ordinary thread, the root transition fillet radius increases R, and the stress concentration of the bolt is reduced.

There are specific requirements for R: R+=0.18042P, rmin=0.15011P, where P is the pitch, while the ordinary thread has no such requirements, and it can even be a straight section.

reinforced thread vs common thread

2. Improve manufacturing process

Strengthening the control of the heat treatment and surface treatment process in the manufacturing process of bolts can effectively improve the fatigue of bolts.

a. Heat treatment

Bolts are first heat treated and then rolled into shape, which will produce large residual compressive stress in the bolt, so as to slow down the formation and development of cracks and improve the fatigue strength of bolts.

During heat treatment, decarburization should also be prevented, and the fatigue strength of bolts without surface decarburization and with surface decarburization should be compared.

Because the carbon in the decarburized layer is oxidized, the number of cementite in the metallographic structure is less than that in the normal structure, so its strength or hardness is lower than that in the normal structure in terms of mechanical properties.

The fatigue strength of bolts usually decreases by 19.8% under the condition of surface decarburization.

b. Phosphating

Phosphating treatment of bolt surface is to prevent rust and stabilize the friction during assembly.

However, phosphating treatment can also reduce wear.

In the process of thread rolling, reducing the friction between the thread of the thread rolling wheel and the screw thread will have a positive effect on the stress distribution on the bolt thread after thread rolling and reducing the surface roughness of the thread.

3. Set appropriate preload

The screw tension of ordinary bolt connection is mainly borne by the front three tooth stressed thread.

When the initial preload is large enough, plastic deformation will occur locally at the root of some threads, and residual stress will occur at the root of these threads.

The residual compressive stress at the root of the thread can improve the fatigue strength of the thread.

At the same time, the thread after plastic deformation can also improve the stress distribution of the thread and reduce the contact pressure on the thread.

This also improves the fatigue strength of the thread.

The greater the preload, the greater the ability of the bolt connection to resist the separation of the connection, and the stronger the ability to resist the relaxation of the preload.

At the same time, the actual effective fatigue strength of bolted connections is also greater.

Therefore, increasing the pre tightening force of bolt connection is conducive to improving the ability of bolt connection to resist fatigue failure under cyclic external load, and reducing the risk of fatigue failure of bolt connection under the action of vibration impact force and limited overload.

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