Analysis Of The Causes Of High-Strength Bolt Fracture And Preventive Strategies

In engineering practice, people often mistakenly believe that "the higher the strength of a bolt, the less likely it is to break." However, in fact, cases of high-strength bolt fractures are more common. This phenomenon stems from the comprehensive effects of multiple dimensions such as material characteristics, installation processes, usage environments, and quality control. The following is a systematic analysis from a technical perspective.

I. Material Characteristics: The Contradiction between High Strength and Low Toughness

High-strength bolts (such as grade 8.8 and 10.9) meet the requirements of high-load working conditions by increasing the material strength. However, their mechanical properties follow the law of "negative correlation between strength and toughness":

 

The higher the strength, the higher the hardness (for example, the hardness of a grade 10.9 bolt can reach HRC 32 - 39), but the elongation significantly decreases (usually ≤12%, while that of ordinary bolts is ≥20%), resulting in a decrease in material toughness.

When the load exceeds the design limit, high-strength bolts will directly undergo brittle fracture due to the lack of a plastic deformation buffer process, while ordinary bolts can give a warning of failure through obvious deformation.
Core logic: High-strength bolts are mostly used in high-stress scenarios (such as bridges and wind power equipment). Their working loads often approach the yield strength of the material (about 80% of the tensile strength), and a slight overload can trigger a fracture. In contrast, ordinary bolts are used in low-stress environments, and the actual loads are far lower than the limit values.

II. Installation Process: The Influence of Torque Control and Friction Coefficient

1. Fracture Caused by Torque Overload

High-strength bolts achieve connection fastening through torque pre - tightening. The standard pre - tightening force is usually set at 70% - 75% of the yield strength of the material. When the applied torque exceeds this threshold, the stress - concentrated areas of the bolt shank (such as the root of the thread) are prone to generating cracks due to overload, ultimately leading to fracture.
Key influencing factors:

 

Tool accuracy: If the accuracy error of the torque wrench (the national standard requires ±5% - ±15%) is not calibrated, the actual torque may far exceed the design value.

Operator qualification: Untrained operators may misread the torque parameters or use incorrect tightening methods (such as not tightening in stages).

2. Failure Caused by Fluctuations in the Friction Coefficient

The actual pre - tightening force in a bolt connection is closely related to the friction coefficient between the nut and the connected part. When the friction coefficient decreases due to excessive lubrication (such as applying excessive talcum powder) or surface contamination (oil stains, rust), the pre - tightening force generated under the same torque decreases, causing the bolt to bear additional shear or tensile loads. Conversely, an excessively high friction coefficient will reduce the torque conversion efficiency, resulting in insufficient pre - tightening force or overload fracture.

III. Usage Environment: Fatigue Damage and Abnormal Working Conditions

1. The Cumulative Effect of Fatigue Fracture

Under alternating loads (such as vibration and impact), high - strength bolts are prone to generating fatigue cracks at the fillet where the head meets the shank. These cracks are difficult to detect with the naked eye in the initial stage and gradually expand as the number of cycles increases, ultimately leading to sudden fracture.
Risk factors:

 

Over - service life: Some enterprises reuse bolts to reduce costs, exceeding their fatigue life (usually the designed number of cycles is ≤10⁶ times).

Design defects: Insufficient consideration of the dynamic loads in the actual working conditions leads to the bolts being in a high - stress state for a long time.

2. Secondary Fracture Caused by Loose Tightening

When the high - strength bolt does not reach the specified pre - tightening force, there is a large gap in the connection pair, which will produce a slip - impact effect under dynamic loads. For example, in a drilling device, a loosely tightened bolt will bear additional shear force due to uneven torque transmission and break instantly due to overload. Studies have shown that in the case of insufficient pre - tightening force, the shear stress borne by the bolt can increase by 3 - 5 times.

IV. Quality Defects: Hidden Dangers in Materials and Processes

1. Sub - standard Material Performance

Deviation in chemical composition: The content of elements such as carbon, manganese, and sulfur in the steel does not meet the standards (for example, an excessive carbon content leads to an increase in brittleness).

Excessive inclusions: Non - metallic inclusions (such as alumina and sulfides) in the raw materials become crack sources.

2. Defects in the Heat Treatment Process

Improper quenching temperature: An excessively high temperature causes the grains to become coarse, reducing toughness; an excessively low temperature results in insufficient hardness.

Inadequate tempering: Residual stress is not eliminated, leaving hidden micro - cracks inside the bolt.

V. Systematic Preventive Measures

Installation stage: Use a high - precision torque wrench (calibrated regularly), perform the step - by - step operation of "initial tightening - final tightening", and operators must be certified.

Friction coefficient management: Control the surface treatment process (such as Dacromet coating), and prohibit the use of lubricants that affect the friction performance.

Life - cycle management: Establish a bolt replacement cycle system and conduct regular ultrasonic flaw detection on high - stress parts.

Quality traceability: Strictly control the procurement of raw materials (require third - party test reports), and add a metallographic analysis link to the heat treatment process.

Conclusion

The reliability of high - strength bolts is the result of the coupling of multiple factors. It is necessary to implement refined management throughout the whole process of materials, processes, and usage. Through scientific design, standardized operation, and continuous monitoring, the fracture risk can be significantly reduced, ensuring the safety of engineering structures.