Fatigue failure is one of the most common failure modes of mechanical components. Statistical data shows that 50% to 80% of all mechanical part failures are attributed to fatigue damage. Prior to fatigue fracture, components usually exhibit no obvious plastic deformation or pre-failure signs. Nevertheless, fatigue fracture occurs abruptly and destructively, often leading to severe equipment faults and safety accidents. Fatigue damage follows specific initiation and propagation laws and eventually develops into complete fatigue fracture.
Definition of Fatigue: Metal fatigue refers to the continuous deterioration of mechanical properties of metallic materials under repeated cyclic action of alternating stress or strain.
Definition of Fatigue Fracture: When metallic materials are subjected to long-term cyclic alternating stress and strain, local microstructural changes and continuous propagation of internal defects occur, resulting in the degradation of mechanical properties and eventual complete fracture of components. This failure process is defined as fatigue fracture. The stress that induces fatigue fracture is generally far lower than the static strength limit of the material. Fatigue fracture is characterized by sudden occurrence, high locality, and high sensitivity to surface and internal defects.
1. Macroscopic Morphology Analysis of Fracture Surface
The research object of this case is a fractured stud bolt used in wind turbine equipment. The macroscopic morphological characteristics of the fracture surface are analyzed as follows:
1. The lower left and right sides of the fracture surface present irregular concave and convex morphologies. The left side shows an arc-shaped convex notch, while the right side is distributed with multiple tiny serrated notches. The bottom arc of the fracture forms a sharp cutting edge.
2. A large central area of the fracture serves as the crack propagation region, covered with distinct beach marks, which are typical macroscopic features of fatigue fracture.
3. Rust traces are observed on the fracture surface of the crack initiation zone and the beach mark propagation zone, caused by long-term exposure to the external environment.
4. The instantaneous fracture zone presents a wide and fine shear lip with a smooth and bright fracture surface free of rust. This indicates that the final fracture occurred rapidly and belongs to ductile instantaneous fracture.
5. The lower shank of the fractured stud features a smooth and bright surface with obvious friction traces, forming a smooth arc surface generated by long-term repeated friction.
6. No obvious necking or deformation is found at the fracture position, which excludes the possibility of overload tensile fracture.
2. Macroscopic Failure Mechanism Analysis
1. Cracks initiate at the lower side of the stud bolt. The smooth cutting edge, serrated notches and arc-shaped gaps at the bottom shank form stress concentration regions, which are the original crack initiation sources.
2. The beach mark features in the crack propagation zone are prominent. In the initial propagation stage, the spacing of beach marks is small, indicating stable and slow crack growth under cyclic tangential stress. In the later propagation stage, the spacing increases significantly, demonstrating accelerated crack propagation and rising failure risk.
3. The wide, fine and bright shear lip in the instantaneous fracture zone shows typical ductile fracture characteristics, proving that the stud possesses well-matched strength and toughness with excellent comprehensive mechanical properties.
4. The smooth arc surface on the stud side is a typical friction morphology. It indicates that the stud endured long-term lateral alternating stress, reciprocated continuously during operation, and generated repeated friction and extrusion with the gear hole wall, finally forming a polished smooth surface.
Macroscopic Conclusion: The fracture of the stud is a typical bending fatigue fracture. The instantaneous fracture zone presents favorable ductile fracture characteristics without brittle fracture features.
3. Microscopic Morphology Analysis
To further clarify the fracture mechanism, scanning electron microscope (SEM) microscopic detection was carried out on the fracture specimen to observe the microstructural features of different fracture regions.
1. SEM observation reveals distinct and regular fatigue striations in the crack propagation zone, which are the core microscopic features of metal fatigue fracture and correspond well with the macroscopic beach marks, confirming fatigue failure.
2. A large number of uniformly distributed dimples are observed in SEM images, which are typical microscopic manifestations of excellent material toughness. This matches the wide shear lip in the macroscopic fracture zone and verifies the reasonable coordination of strength and plasticity of the stud.
3. No large-area cleavage fracture morphology is found on the whole fracture surface. Only a small amount of quasi-cleavage features appear in local areas, alternating with dimples. This indicates that the material has low brittleness and an optimal match of strength and toughness.
Microscopic Conclusion: The original surface of the stud retains turning tool marks with high surface roughness, forming weak stress concentration areas. During equipment operation, the stud bears continuous alternating tangential bending stress, and the surface tool marks are polished into a smooth arc surface by repeated friction. Under long-term high-frequency reciprocating bending, collision and friction, microcracks preferentially initiate at rough surface defects and form fatigue sources. With the continuous action of alternating loads, the cracks propagate gradually and finally lead to high-cycle bending fatigue fracture. The macroscopic and microscopic morphological features are highly consistent, proving that the sudden fracture is caused by accumulated bending fatigue damage.
4. On-site Investigation and Working Condition Analysis
To restore the actual failure working condition, two on-site investigations were conducted at the wind turbine site. Relevant information including equipment operation duration, assembly process, installation position, fracture morphology and maintenance records was verified to clarify the failure conditions.
More than thirty 1.5 MW wind turbines have been in operation for over three years, and only one stud bolt fractured, which is an individual failure case. On-site inspection shows that the fractured stud was installed with positional deviation. One side of the shank closely fitted the gear hole end face without any gap, while the other side had a large assembly clearance. The installation deviation caused the stud to bend and reciprocate continuously with turbine operation, generating long-term alternating lateral loads and inducing potential fatigue failure risks.
The closely fitted side of the stud was gapless and inaccessible to tools, proving that this position endured extremely high cyclic tangential stress and caused great difficulty in later disassembly. Under continuous axial vertical alternating stress, the side of the stud fitting the hole wall experienced repeated bending, impact and friction with the gear hole. The original turning marks were completely worn away, forming a mirror-like smooth arc surface with obvious friction characteristics.
The wind turbine operates at a rotational speed of 17 r/min for 15 hours per day, generating approximately 15,300 cycles daily. The total operating cycles reach 2.8×10⁶ within half a year and 2.0×10⁷ within three and a half years. The stud swings within a small amplitude of about 4 mm with low stress amplitude and low overall stress level, belonging to typical low-stress high-cycle fatigue working conditions. The fracture is induced by long-term accumulated bending fatigue under alternating lateral loads, which is a typical case of bolt bending fatigue fracture.
5. Comprehensive Summary
1. The failure mode of the stud is confirmed as bending fatigue fracture, rather than overload fracture, brittle fracture or fracture caused by material defects.
2. The root cause of failure is improper assembly process. The positional deviation leads to unilateral close fitting of the stud with the gear hole, resulting in sustained alternating tangential bending stress. Fatigue damage accumulates continuously during high-cycle equipment operation and eventually causes bending fatigue fracture.
