Three Typical Bolt Failure Modes: Tensile, Shear and Fatigue Fracture
Nearly all bolt breakage accidents do not happen accidentally. Hidden metal deformation and micro cracks will accumulate for a long time before final fracture. Industrial engineering divides bolt failure into three standard types: tensile fracture, transverse shear fracture and fatigue fracture. Engineers can judge assembly errors, material defects and design flaws directly through fracture morphology, which is essential for long-term structural safety maintenance.
1. Tensile Fracture
This failure happens when axial bearing load exceeds bolt ultimate tensile strength. It mainly results from insufficient bolt strength grade, over-tightening operation or sudden structural overload. The bolt shaft will elongate visibly with reduced cross-section before breaking, showing fibrous fracture texture without sharp fragments. Most tensile failures are caused by improper fastener type selection in early engineering design.
2. Transverse Shear Fracture
Shear damage is triggered by lateral offset force acting on bolt joints. Common inducements include missing supporting washers, unreasonable joint layout and inaccurate stress calculation for flange connections. Different from ductile tensile breakage, shear fracture occurs instantly with smooth and flat fracture surface, without any plastic deformation. It is the most destructive hidden danger for bridge and highway engineering fasteners.
3. Metal Fatigue Fracture
As the most concealed bolt defect, fatigue fracture occurs under long-term cyclic vibration load. Even if the working load is lower than the rated bearing capacity, repeated stress changes will expand tiny internal thread cracks gradually. Fatigue failure accounts for more than half of wind power, power tower and automotive fastener breakdown accidents, which is hard to detect through daily appearance inspection.
4. Fault Comparison and Preventive Measures
| Failure Type | Root Cause | Engineering Prevention Solution |
| Tensile Fracture | Low strength grade, excessive tightening torque | Match grade 10.9/12.9 bolts, standard torque control |
| Shear Fracture | Lateral offset load, unstable assembly | Install reinforced flat washers, optimize joint structure |
| Fatigue Fracture | Continuous mechanical vibration | Equip anti-loosening washers and damping gaskets |
Our factory implements full mechanical batch inspection, including tensile, shear and fatigue resistance tests. All export fasteners are attached with official test reports. We provide professional fastener matching schemes for vibration-intensive and heavy-load global engineering projects to eliminate connection fracture risks fundamentally.