Main Failure of High-Strength Anchor Cage Bolts

Fatigue is a form of failure in which high strength anchor cage bolts are cracked or broken under repeated action of cyclic stress and strain. 
Wind Turbine Foundation Cage Completed

Fatigue is a form of failure in which high strength anchor cage bolts are cracked or broken under repeated action of cyclic stress and strain. Fatigue is a process of cumulative mechanical damage and delayed failure. Including crack initiation and crack propagation, occurs at a yield strength below the material and abruptly fails without significant deformation. Therefore, fatigue is one of the most dangerous forms of failure compared to other forms of failure for high strength anchor bolt.

Usually,  connecting bolts on the vehicle are pulled-pull fatigue failure. For example, wind turbines have complex and harsh service environments. They are subjected to dynamic load shocks and corrosive environments. The fatigue fracture of wind turbine foundation bolts is as high as 85%. It is concluded that main failure mode of high-strength anchor cage bolts is fatigue.

Challenges for High-Strength Anchor Bolt Manufacturing

High-end mechanical equipment brings unprecedented opportunities for high-strength bolt manufacturing. It will be difficult to achieve the expected goals without solving the “three major problems”, and it is even harder to say that it has reached the international advanced level. Solving the “3 major problems” needs to take care of problems: first, independent intellectual property rights; second, economic affordability; third, long-life, high-reliability, and structural weight-reduction technologies.

Wind Turbine Anchor Cage Detection

First of all, to change the concept, we must recognize the time and space, China must develop high-end machinery manufacturing, master key technologies and enhance core competitiveness, and abandon those flickering words that change rhetoric , and become second-hander of technology in developed countries, market-changing technology and other errors. The concept from the past “introduction, imitation – backward, re-introduction, imitation…”, liberation from paste, copy, and cottage products, speeding up development of fastener heat treatment is an urgent task.

Secondly, it is a revolution to fully understand “3 major problems” and realize the long life, high reliability and structural weight reduction of high-strength bolts:

  1.  From the “formed” manufacturing of high-strength bolts to the technological revolution of anti-fatigue manufacturing; Solve the “with or without” transformation into a key high-strength bolt to support the sustainable development of high-end machinery manufacturing;
  2.  From the final inspection of indicators to the management revolution of process control; 
  3. From low-tech, passive to high-tech, active practitioners Quality revolution; 
  4. From cheap labor, abuse of the environment and resources to high value-added, humanized, low-emission, environmentally friendly environmental revolution.

"3 Major Problems" of High-Strength Anchor Bolts

Wind Turbine Foundation Tower

High-strength bolts are generally made of high-strength low-alloy steel, ultra-high-strength steel and high-temperature alloy. With the development of high-end mechanical equipment, the strength of alloys for high-strength bolts will continue to increase. Material for high strength anchor bolt is featured with high fatigue strength, and light in weight, small in size, long in life, and highly reliable. However, outstanding weakness of high-strength materials is that fatigue strength is sensitive to stress concentration.

The fatigue failure analysis results of a large number of high-strength bolts show that more than 80% of fatigue failures are caused by surface damage, decarburization at head bar intersection, obvious fine cracks in thread processing, or discontinuities in the machining of the tool marks and surface corrosion, quenching. The tissue is uneven for high concentration of stress.

High-strength bolts are generally made of high-strength low-alloy steel, ultra-high-strength steel and high-temperature alloy. With the development of high-end mechanical equipment, the strength of alloys for high-strength bolts will continue to increase. Material for high strength anchor bolt is featured with high fatigue strength, and light in weight, small in size, long in life, and highly reliable. However, outstanding weakness of high-strength materials is that fatigue strength is sensitive to stress concentration.

The fatigue failure analysis results of a large number of high-strength bolts show that more than 80% of fatigue failures are caused by surface damage, decarburization at head bar intersection, obvious fine cracks in thread processing, or discontinuities in the machining of the tool marks and surface corrosion, quenching. The tissue is uneven for high concentration of stress.

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