Fracture modes of metals

Metallic materials play a dominant role in engineering applications, and their fracture behavior directly affects the safety and reliability of structures. A deep understanding of metal fracture modes is crucial for material selection, design, processing, maintenance during use, and failure analysis. Metal fracture modes are diverse and can be classified in various ways. Each type has its unique formation mechanism, macroscopic and microscopic characteristics, and impact on material properties.

Classification by the Degree of Plastic Deformation Before Fracture

Ductile Fracture

• Formation Mechanism: Ductile fracture is a fracture mode in which significant plastic deformation occurs in metals before fracture. When a metal is subjected to external forces, dislocations move and multiply within the crystal. As stress increases, dislocations become entangled and accumulate, forming dislocation pile-ups. Simultaneously, slip systems in the crystal are activated, resulting in slip deformation. When the deformation reaches a certain level, micropores form in localized areas. These micropores grow and aggregate under stress, eventually leading to material fracture.

• Macroscopic Characteristics: The fracture surface of ductile fracture typically exhibits a cup-cone or fibrous shape. In tensile tests, the specimen exhibits significant necking after fracture, with the fracture surface forming an angle of approximately 45° with the tensile axis. The fracture surface is rough, with obvious traces of plastic deformation, including necking and shear lips.

• Microscopic Features: Under a microscope, numerous dimples are visible on the ductile fracture surface. Dimple formation is a typical microscopic feature of micropore aggregate fracture, and its size, shape, and depth are related to the material’s composition, microstructure, and stress state. Generally, the larger and deeper the dimples, the better the material’s toughness.

• Impact on Material Properties: Ductile fracture is typically a slow process. Before fracture, the material undergoes significant plastic deformation, absorbing more energy and thus exhibiting better impact resistance and fracture toughness. In engineering applications, for structures subjected to dynamic or impact loads, high toughness is generally desirable to prevent brittle fracture.

Brittle Fracture

• Formation Mechanism: Brittle fracture is a fracture form in which metals undergo almost no plastic deformation before fracture. Its formation is mainly related to factors such as the material’s crystal structure, defects, and stress state. For example, in body-centered cubic metals, brittle fracture easily occurs at low temperatures or high strain rates. Defects such as cracks and inclusions in the material become stress concentration sources. When the stress reaches the material’s fracture strength, the crack propagates rapidly, leading to fracture.

• Macroscopic characteristics: The fracture surface of a brittle fracture is generally smooth, perpendicular to the direction of normal stress, and shows no obvious traces of plastic deformation. The fracture surface exhibits a crystalline or herringbone pattern, with the tips of the herringbone pointing towards the crack origin.

• Microscopic characteristics: The microscopic characteristics of brittle fracture depend on the crystal structure and fracture mechanism of the material. For cleavage fracture, cleavage steps, river patterns, etc., can be seen on the fracture surface; for intergranular fracture, the fracture surface has a candy-like appearance.

• Impact on material properties: Brittle fracture is sudden, with no obvious warning signs before fracture, thus posing a significant danger. In engineering applications, brittle fracture should be avoided whenever possible. Toughness can be improved by selecting appropriate materials, controlling heat-treatment processes, and optimizing stress states.

Classification by Fracture Path

Transgranular Fracture

• Formation Mechanism: Transgranular fracture refers to a fracture form in which the crack penetrates the interior of a grain. It can be ductile or brittle. When a crack propagates within a grain, it is influenced by crystallographic planes, such as slip and twin planes, which can change its propagation direction.

• Macroscopic and Microscopic Characteristics: The macroscopic characteristics of transgranular fracture depend on the fracture type (ductile or brittle). Microscopically, in ductile transgranular fracture, dimples are distributed within the grain; in brittle transgranular fracture, such as cleavage fracture, cleavage steps and river patterns appear.

• Impact on Material Properties: The occurrence of transgranular fracture is closely related to the crystal structure and microstructure of the material. Generally, fine-grained materials have higher transgranular fracture toughness because grain boundaries can hinder crack propagation. The resistance to transgranular fracture can be improved by refining the grain size and enhancing the material’s microstructure.

Intergranular Fracture

• Formation Mechanism: Intergranular fracture refers to a fracture form in which the crack propagates along the grain boundaries. Grain boundaries are weak points in materials. When impurities, second-phase particles, or stress concentrations are present at grain boundaries, microcracks are more likely to form. These microcracks propagate along the grain boundaries under stress, eventually leading to intergranular fracture.

• Macroscopic and Microscopic Characteristics: The fracture surface of intergranular fracture macroscopically exhibits a candy-like appearance and is dark in color. Under a microscope, cracks can be seen clearly propagating along the grain boundaries, where second-phase particles or impurities may be present.

• Impact on Material Properties: Intergranular fracture typically significantly reduces the strength and toughness of materials. In engineering applications, intergranular fracture should be avoided whenever possible. Measures such as controlling the chemical composition of the material, optimizing heat-treatment processes, and reducing grain-boundary defects can improve the material’s resistance to intergranular fracture.

Classification by Fracture Mechanism

Cleavage Fracture

• Formation Mechanism: Cleavage fracture is a brittle fracture form in which crystals in metals fracture along specific crystallographic planes (cleavage planes) under normal stress. When stress reaches a certain value, microcracks will form on the cleavage surface. These microcracks rapidly propagate and connect, leading to cleavage fracture of the material.

• Macroscopic and Microscopic Characteristics: Macroscopically, the cleavage fracture surface is smooth and perpendicular to the direction of normal stress. Microscopically, cleavage steps and river patterns are visible on the fracture surface. Cleavage steps are step-like structures formed by the intersection of cleavage surfaces of different heights; river patterns are composed of many small cleavage facets, with their flow direction pointing towards the crack origin.

• Impact on Material Properties: Cleavage fracture is a typical form of brittle fracture, which greatly damages the toughness of the material. Increasing the material temperature, reducing the strain rate, and refining the grain size can suppress cleavage fracture to some extent.

Fatigue Fracture

• Formation Mechanism: Fatigue fracture is a fracture form that occurs in metals under alternating stress after a certain number of cyclic loading cycles. Under alternating stress, microcracks will form inside the material. These microcracks gradually propagate with increasing cycle count, eventually leading to fatigue fracture of the material.

• Macroscopic and Microscopic Characteristics: Fatigue fracture surfaces typically exhibit three regions: the fatigue initiation region, the fatigue crack propagation region, and the instantaneous fracture region. The fatigue initiation region is the origin of the fatigue crack and is generally smooth. The fatigue crack propagation region displays a shell-like or beach-like pattern, formed due to cyclic stress during crack propagation. The instantaneous fracture region forms when the material fractures and exhibits characteristics similar to those of a static-load fracture.

• Impact on Material Properties: Fatigue fracture is one of the most common failure modes in engineering structures and is highly hazardous. Fatigue fractures can be reduced by improving material fatigue strength, optimizing structural design, and controlling stress levels.

The fracture modes of metals are complex and diverse. Different fracture modes have different formation mechanisms, macroscopic and microscopic characteristics, and impacts on material properties. In practical engineering applications, various factors should be comprehensively considered based on the material’s usage conditions and requirements, and effective measures should be taken to enhance the material’s fracture resistance, ensuring the safety and reliability of the structure.