Mechanical equipment and steel structures are assembled from various components, most of which are fixedly connected by threaded fasteners. Failure of threaded fasteners directly leads to equipment malfunction. In severe cases, it may cause equipment shutdown, structural collapse and even personal injury accidents.

Given the high frequency and serious hazards of fastener failure, technicians need to systematically analyze the inducing factors and formulate targeted rectification and prevention measures to eliminate fastening failures fundamentally.

Fastener failures are mainly divided into two categories. The first category is bolt fracture failure, which causes instantaneous separation of connected structures and usually results in severe equipment faults and safety accidents. The second category includes thread pair loosening and thread slipping of bolts or nuts, which produces tiny relative displacement between connected parts and leads to partial equipment malfunction and reduced operation accuracy.

Fastener failure is a progressive process. Minor loosening without timely treatment will continue to deteriorate and eventually cause complete separation of bolts and nuts, triggering major safety accidents. In practical applications, most personnel misunderstand that bolt fracture is solely caused by material defects and nut loosening by poor nut quality, while ignoring core problems such as unreasonable structural design and non-standard assembly processes. This paper systematically analyzes the causes of various bolt fracture failures from the perspectives of design and assembly.

1 Shear Fracture

Bolt shear fracture mostly occurs in threaded connections under pure preload. The shear fracture surface is located at the joint interface of two connected parts, presenting a small smooth and bright shear area. The specific failure causes are summarized as follows.

1.1 Design Causes

(1) Insufficient friction coefficient at the joint interface or improperly selected bolt specifications leads to inadequate preload. When the friction force at the joint surface is less than the transverse working load, expressed by the formula fF′＜F (where f is the interface friction coefficient, F′ is the bolt preload, and F is the transverse working load), relative slip occurs between connected components. The bolt shank is subjected to extrusion and shear force from the hole wall. Once the shear stress exceeds the shear strength of the bolt material, shear fracture takes place. Moving components under impact loads are more susceptible to such failure. In structural design, load-reducing parts and positioning shoulders can be adopted to bear transverse loads, so that bolts only undertake tensile connection without bearing shear force.

(2) Fasteners without anti-loosening structures are applied in vibrating working conditions. Long-term equipment vibration loosens the thread pair and attenuates bolt preload, resulting in insufficient interface friction and further component slip and bolt shear fracture. For structures working in vibration environments, special anti-loosening fasteners such as Spiralock nuts and prevailing torque lock nuts should be adopted to prevent loosening failure.

1.2 Assembly Causes

Insufficient assembly tightening torque leads to substandard bolt preload and inadequate interface friction. External loads cause relative slip of components and eventually result in bolt shear fracture. Bolt tightening torque is a critical process indicator in steel structure engineering and engine assembly and requires strict control. However, it is often neglected in other industries due to the lack of standardized torque management. In practical failure cases, most thread loosening and fracture faults are caused by inappropriate assembly torque.

The clamping force of a thread pair is generated by rotating nuts or bolts and is positively correlated with assembly torque. To ensure the preload meets design requirements, assembly torque must be clearly specified in process documents and strictly implemented during operation. The assembly torque calculation formula is as follows:

M = KPD

Where: M - Assembly torque (Nm); K - Torque coefficient; P - Designed preload (kN); D - Nominal bolt diameter (mm).

In conventional design, bolt preload is set within 60% to 80% of the material yield strength with a safety factor above 1.2. The yield load of bolts with different specifications and strength grades can be referred to GB/T 3098.1.

The torque coefficient is determined by the friction coefficient of thread pairs and the contact surface between fasteners and connected parts. It is affected by surface treatment, strength precision, geometric tolerance, thread accuracy, support surface roughness and structural stiffness. Surface treatment serves as the dominant factor. Different surface treatments lead to great differences in torque coefficient, up to nearly double. For thread pairs with the same specification and strength, the torque coefficient of phosphating treatment is about 0.13 to 0.15, while that of blackening treatment ranges from 0.26 to 0.30.

Therefore, fasteners with phosphating and blackening treatments produce nearly double preload difference under the same assembly torque. The torque coefficient must be calibrated through experiments. Fastener manufacturers shall strictly control surface treatment processes to ensure consistent torque coefficients of each batch. Users shall not arbitrarily change surface treatment requirements, so as to avoid insufficient preload, bolt stretching or fracture caused by torque coefficient fluctuation.

2 Fatigue Fracture

Fatigue fracture is one of the most common bolt failure modes. Most fatigue failures originate from manufacturing defects, including unsmooth transition fillets under bolt heads, insufficient thread root fillet radius, surface scratches and material inclusions. Meanwhile, non-standard assembly is also an important inducement for fatigue fracture and potential safety accidents.

Taking automobile wheel bolts as an example, the structural design allows wheel bolts to bear only axial preload without direct radial load. Insufficient assembly torque leads to inadequate preload and insufficient friction between the wheel hub and half shaft, causing relative slip during vehicle operation.

During high-speed wheel rotation, the bolt rotates periodically with the hub. The bolt shank is subjected to alternating extrusion force from both sides of the hole wall, resulting in high-frequency cyclic radial loads. Long-term cyclic load action initiates and expands fatigue cracks, eventually leading to bolt fatigue fracture.

When wheel bolt fracture occurs after vehicle operation, inspectors shall first observe the fracture morphology to preliminarily judge fatigue failure before conducting further in-depth analysis.

3 Overload Fracture

Overload fracture refers to tensile fracture when the total axial load exceeds the allowable limit of the bolt material. Since the effective bearing area of the threaded section is smaller than that of the smooth shank with more severe stress concentration, overload fracture mostly occurs at the threaded section, which can be preliminarily identified by fracture morphology.

Grade 8.8 bolts show obvious necking after overload fracture; Grade 10.9 bolts present slight necking; Grade 12.9 high-strength bolts basically have no necking features under normal overload conditions.

Obvious necking may also appear in Grade 8.8 to 12.9 bolts with unqualified materials, inadequate heat treatment, incomplete quenching or insufficient core hardness. Therefore, the judgment criteria for overload fracture are specified as follows:

(1) For fractures with necking, sample testing shall be carried out in accordance with GB/T 3098.1. Overload fracture can be confirmed if the core hardness is qualified.

(2) Metallographic analysis is required for Grade 12.9 high-strength bolts to determine overload fracture, because thread surface defects and material defects may also cause threaded section fracture and cannot be judged merely by appearance.

Overload fracture mainly occurs in bolt connections under axial loads, with three primary causes:

a. Excessively high assembly torque causes the preload to exceed the yield strength of the bolt material, resulting in bolt stretching or fracture during assembly.

b. Excessive assembly preload close to the bolt yield strength. During equipment operation, the bolt bears both residual preload and working load. The superposition of the two loads exceeding the material strength limit causes overload fracture. Under constant working load, higher assembly preload leads to higher residual preload and greater overload risk.

c. Improper bolt selection with insufficient nominal thread diameter failing to meet bearing requirements in structural design.

4 Bolt Fracture Caused by Assembly Eccentric Load

Bolt fracture due to eccentric load is a typical assembly process failure. Obvious crescent-shaped wear marks can be observed on the bolt support surface, with the actual bearing area only accounting for 1/4 to 1/3 of the total support area and severely uneven stress distribution. The bolt shank near the fracture bends opposite to the stress direction, showing typical eccentric bending and fracture characteristics.

Eccentric load failure of bolts is mainly caused by two factors:

First, the contact support surface of connected parts is inclined or uneven, resulting in local stress on the bolt head and forming eccentric load, which is the main cause of this failure case.

Second, excessive straightness error or bending deformation of the bolt shank causes inherent eccentric load during assembly.

The two types of eccentric loads differ significantly in stress position and shank bending direction, which can be distinguished through support surface stress traces and bolt deformation direction.

5 Conclusion

Fastener failures such as fracture, thread slipping and loosening are not always caused by inherent product quality defects. Standardized troubleshooting procedures shall be followed once fastening failure occurs:

First, preserve failed samples completely and protect fracture surfaces from rust, collision and secondary damage to ensure accurate failure analysis.

Second, preliminarily determine the failure type and inducement based on fracture morphology, sample appearance, surface stress traces and actual working conditions.

Third, conduct professional physical and chemical tests including metallographic analysis and hardness detection. If material and heat treatment indicators meet national standards, the failure originates from structural design or assembly process; otherwise, it is confirmed as fastener quality defect.

Fourth, formulate targeted rectification and prevention measures according to the confirmed failure causes, standardize design specifications and optimize assembly processes to avoid recurrence of similar failures.