In industrial production and daily equipment application, bolts serve as core connecting fasteners responsible for fixing, connecting and bearing structural loads. During long-term service, bolts are affected by multiple factors such as load conditions, working environment, manufacturing processes and assembly quality, which may easily lead to fracture failure. This can further cause equipment shutdown, structural failure and even serious safety accidents. This paper systematically introduces the common fracture forms, formation mechanisms, macroscopic fracture characteristics of overload fracture, high-risk fracture positions and fracture states under different loading conditions of bolts, providing technical support for bolt failure prevention, equipment maintenance and process optimization.

I. Common Fracture Forms of Bolts

1. Overload Fracture

Overload fracture occurs when the instantaneous load applied on a bolt exceeds the ultimate tensile strength of the material, belonging to typical static ductile or brittle fracture. This type of fracture happens suddenly without obvious pre-failure signs. For ductile materials, the fracture surface generally presents a cup-cone shape or a 45-degree inclined section with rough surfaces and significant plastic deformation. For high-strength brittle bolts, the fracture surface is relatively flat with negligible plastic deformation.

Fracture Causes: Unreasonable structural design leads to undersized bolt selection and insufficient load margin, resulting in long-term overload operation. Sudden abnormal working conditions such as impact loads and transient overloads during equipment operation can also cause instantaneous over-limit loading. For example, the connecting bolts of crane lifting mechanisms may suffer overload fracture when lifting overweight loads.

2. Fatigue Fracture

Fatigue fracture is the most common failure form of bolts. Under long-term cyclic loads such as alternating tension, compression, bending and vibration, micro fatigue cracks initiate at stress concentration areas. The cracks expand gradually with load cycles, continuously reducing the effective bearing area, and eventually lead to sudden fracture even if the working load does not exceed the rated value. Fatigue fracture occurs without obvious plastic deformation. The fracture surface is generally smooth with typical shell-like or annual ring-like textures.

Fracture Causes: Long-term reciprocating motion and high-frequency vibration of equipment subject bolts to periodic alternating stress. For instance, the connecting rod bolts of automobile engines endure cyclic tension and compression loads from the high-frequency reciprocating movement of pistons and connecting rods, resulting in accumulated fatigue damage and eventual fatigue fracture.

3. Corrosion Fracture

When bolts work in corrosive environments, chemical or electrochemical corrosion occurs on the base material, forming surface defects such as rust and pitting corrosion. These defects reduce the effective bearing area and mechanical strength of bolts, leading to fracture under normal working loads. Corrosion products such as rust layers and corrosion pits can be observed on the fracture surfaces.

Fracture Causes: Bolts serving in humid, salt-spray or acid-base corrosive environments are prone to corrosion failure, such as structural steel facilities near the sea and chemical industrial equipment. For example, the connecting bolts on ship decks are continuously eroded by seawater and salt spray, resulting in material degradation and corrosion fracture.

4. Stress Corrosion Fracture

Stress corrosion fracture refers to brittle fracture occurring under the combined action of constant tensile stress and specific corrosive media. The crack grows slowly in the early stage without obvious failure symptoms, and sudden fracture occurs once the crack reaches the critical size. The fracture surface presents dual characteristics of stress failure and corrosion damage.

Fracture Causes: Certain bolt materials may generate and expand stress corrosion cracks under low constant tensile stress in specific corrosive environments. A typical case is the stress corrosion fracture of austenitic stainless steel bolts in chloride-rich environments.

5. Hydrogen Embrittlement Fracture

During manufacturing or service, hydrogen atoms penetrate and accumulate inside the bolt material, forming hydrogen molecules that produce huge internal pressure. This causes lattice cracking and microcrack propagation, eventually leading to brittle fracture. Hydrogen embrittlement is a typical brittle failure characterized by flat fracture surfaces and no obvious plastic deformation.

Fracture Causes: Hydrogen atoms penetrate into the steel matrix during electroplating, acid pickling, phosphating and other surface treatment processes without adequate dehydrogenation treatment. High-strength steel bolts are highly sensitive to hydrogen embrittlement. Excessive hydrogen ion concentration in plating solution and unqualified dehydrogenation processes are the main inducements of hydrogen embrittlement fracture.

6. Fracture Caused by Manufacturing Defects

Internal and surface defects generated during raw material production and bolt manufacturing form stress concentration sources. Under load, the stress increases sharply at defect positions, inducing crack initiation and rapid propagation, which eventually leads to fracture. Original manufacturing defect features can be clearly observed on the fracture surface.

Fracture Causes: Raw material defects such as inclusions, porosity, shrinkage cavities and segregation; improper control of forging, heat treatment and turning processes causes quenching cracks, grinding cracks, tool marks and scratches.

II. Three Characteristic Zones of Bolt Overload Fracture

1. Fiber Zone

Position: Located at the center of the fracture surface, serving as the crack initiation and initial propagation zone.

Morphological Characteristics: The surface is rough and fibrous with visible plastic deformation and microvoid aggregation, which is a typical feature of ductile fracture.

Formation Mechanism: In the initial fracture stage, the bolt material undergoes plastic rheology under tension. Microvoids generate, grow, aggregate and connect to form the fibrous fracture morphology.

2. Radiate Zone

Position: Located outside the fiber zone, corresponding to the rapid crack propagation stage.

Morphological Characteristics: The fracture surface is relatively flat with clear radial or herringbone textures extending outward from the center.

Formation Mechanism: When the initial crack expands to the critical size, it enters the rapid propagation stage. Severe stress concentration at the crack tip causes rapid transgranular or intergranular tearing, forming radial fracture morphology.

3. Shear Lip Zone

Position: Distributed at the outermost edge of the fracture surface, formed in the final fracture stage.

Morphological Characteristics: The surface is smooth and inclined with typical shear slip features, forming an annular shear lip, which is the final characteristic of ductile fracture.

Formation Mechanism: In the final fracture stage, the residual material slips and tears along the maximum shear plane under high stress, producing plastic shear deformation and forming the shear lip structure.

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III. Common Positions of Bolt Overload Fracture

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1. First Thread Tooth near Nut Bearing Surface

Loads of bolt connections are mainly transmitted through meshing thread teeth. The first thread tooth near the nut bearing surface bears the largest load and presents the most severe stress concentration. It is the most susceptible position for overload fracture during long-term service.

2. Thread Root at the Transition between Bolt Head and Shank

The thread root features abrupt geometric changes and a high stress concentration coefficient. With complex stress states, it becomes the structural weak point of bolts and is prone to fracture under overload and impact loads.

3. Transition Zone between Smooth Shank and Thread Section

Abrupt changes in cross-sectional size and structure at the junction of smooth shank and thread section cause obvious stress concentration and uneven stress distribution. This position easily generates crack initiation under overload conditions and leads to fracture failure.

IV. Bolt Fracture States under Different Loading Forms

1. Fracture under Tensile Stress

Typical cup-cone fracture and obvious overall elongation and necking deformation can be observed. For high-toughness bolts under tensile overload, the final fracture section forms an approximately 45-degree shear angle with the bolt axis, belonging to typical ductile overload fracture, which mostly occurs at the weak thread root position.

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2. Fracture under Impact Stress and Bending Moment

Radial textures and shear lips can be observed on the fracture surface, while the shear lips are incomplete and unevenly distributed without a full annular shape. The fracture presents minor plastic deformation and obvious brittle characteristics, mainly caused by instantaneous impact and bending loads.

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3. Fracture under Combined Torsion and Tension Stress

Under combined torsional and tensile loads, the fracture surface shows obvious swirling torsional textures, as well as crescent or fan-shaped morphologies. The distinct distortion and offset of the fracture are formed by relative slip and tearing of materials along the shear plane under torsional shear force.

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