Where Lies The Strength of High-Strength Bolts?

1. Background

On November 28, 2021, a research team led by Professor Han Dong from the School of Materials Science and Engineering at Shanghai University, in collaboration with Hebei Longfengshan Foundry Co., Ltd., Qifeng Precision Technology Co., Ltd., Zhoushan 7412 Factory, Jiangsu Metallurgical Technology Research Institute, Shanghai University (Zhejiang) Institute for Advanced Equipment Materials, and Shanda New Materials (Taizhou) Institute, successfully developed ultra-high strength steel fasteners B17.8 and B19.8. This achievement was the result of more than a year of joint efforts, using a full-process collaborative model of "material production—fastener manufacturing—service evaluation." Based on advanced steel material theories and utilizing high-purity iron provided by Longfengshan Foundry, they developed manufacturing technology for 16.8 and 19.8 grade bolts.

2. What Are High-Strength Bolts?

High-Strength Friction Grip Bolts (HSFG), commonly known as high-strength bolts, are often misunderstood due to the omission of the terms "friction" and "preload" in everyday use.

Misconception 1: High-strength bolts are simply bolts with a material grade higher than 8.8.

In reality, the fundamental difference between high-strength bolts and ordinary bolts lies not in the material strength, but in how they handle loads. The key feature of high-strength bolts is the application of preload to utilize static friction to resist shear forces. According to British and American standards (like BS EN 14399, ASTM A325, and ASTM A490), high-strength bolts only include grades 8.8 and 10.9, whereas ordinary bolts cover multiple grades such as 4.6, 5.6, 8.8, 10.9, and 12.9. Thus, material strength alone is not the defining factor.

3. Advantages of High-Strength Bolts

Comparing the tensile and shear strength of ordinary 8.8-grade bolts and high-strength 8.8-grade bolts based on the GB50017 standard reveals that ordinary bolts of the same grade typically have higher design values. So, where does the "strength" of high-strength bolts come from?

To answer this, one must analyze their design working conditions and elastic-plastic deformation characteristics:

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Ordinary Bolts: When overloaded, the shank undergoes plastic deformation, eventually leading to shear failure. Before absorbing shear forces, there is slip between the connected plates, after which the shank comes into contact with the plates to absorb the load through deformation.

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High-Strength Bolts: When static friction is insufficient to resist shear, the connected plates may slip (at the limit state). However, even after slipping, the bolt shank can still handle shear forces through its own elastic-plastic deformation.

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Misconception 2: High-strength bolts have higher load-bearing capacity.

In fact, the tensile and shear strength of individual high-strength bolts are not necessarily higher than ordinary bolts. The advantage lies in their design: the connection point does not permit any slippage, which means minimal deformation, high stiffness, and greater safety margins. These features make them especially suitable for high-stiffness connections like main beam joints, aligning with the seismic design principle of "strong joints, weak components."

4. Comparison Between High-Strength Bolts and Ordinary Bolts

Due to differences in load-bearing principles, the construction and inspection processes for ordinary bolts and high-strength bolts differ significantly. Ordinary bolts generally have slightly higher mechanical performance requirements than high-strength bolts of the same grade. However, high-strength bolts must also meet additional impact toughness standards.

In practical applications, bolts can be identified by markings. Note that the torque calculations for high-strength bolts differ between British and American standards. Generally, ordinary bolts cost about 70% of high-strength bolts due to the latter's higher toughness (impact energy) requirements.

5. How to Improve the Fatigue Strength of Bolts

Under complex loading conditions, fatigue failure is the most common failure mode for high-strength bolts. According to a 1980 study of 200 bolt failure cases, over 50% were due to fatigue damage. Therefore, enhancing fatigue resistance is crucial.

Characteristics of bolt fatigue fractures include:

1. The maximum stress is significantly lower than the material's static strength limit, even below the yield strength.

2. The fracture exhibits brittle characteristics with little plastic deformation.

3. The fracture is a result of accumulated micro-damage.

Common failure locations include:

· The first thread of the nut (65%)

· The transition point between threads and the shank (20%)

· The rounded transition between the bolt head and the shank (15%)

Strategies to improve fatigue strength:

1. Optimize design to reduce stress concentration: Use larger transition radii, relief grooves, and reinforced thread designs to minimize stress concentration.

2. Enhance manufacturing processes:

a.Heat Treatment: Performing heat treatment before thread rolling introduces residual compressive stress, delaying crack propagation. It's also essential to prevent surface decarburization, which can reduce fatigue strength by approximately 20%.

b.Phosphating Treatment: Reduces friction during thread rolling, improves surface roughness, and enhances fatigue resistance.

3. Apply appropriate preload: Increasing preload induces residual compressive stress at the thread roots, improving fatigue resistance and reducing the risk of fatigue failure by minimizing joint loosening.

Conclusion

In summary, the advantage of high-strength bolts does not lie in their inherent load-bearing capacity but rather in their high stiffness and fatigue resistance at connection points. This makes them uniquely suited for structures requiring high stiffness and safety margins.