Fibrous Fracture and Crystalline Fracture

来阐述纤维状断口和结晶状断口

Metal fatigue, also known as material fatigue or structural fatigue, is a failure pattern caused by cyclic loading of a material or structure. Metal fatigue can occur in any material, but it is very common in structural metals. Metal fatigue can be divided into two types: fibrous fractures and crystalline fractures.

Fibrous Fractures

Fibrous fractures occur in metals that have been repeatedly loaded at less than the yield point. This type of fracture occurs in metals that have been repeatedly subjected to low-level cyclic loading. The fatigue process begins on an atomic level, with small amounts of plastic deformation caused by the loading. As deformation continues, cracks form and propagate through the material, resulting in visible fracture surfaces.

The fracture surface of a fibrous fracture is characterized by a rough, interlocking grain structure that resembles the structure of woven cloth. Under a microscope, one can see alternating bands of smooth and rough areas, which results from the progressive, interlocking process of crack formation.

Crystalline Fractures

Crystalline fractures occur in metals that have been repeatedly loaded at above the yield point. In cases where the load exceeds the yield point, permanent deformation takes place, resulting in localized yielding in the material. As the deformation continues and each stress cycle is passed, cracking starts and the failure process accelerates.

Unlike fibrous fractures, crystalline fractures are characterized by a smooth feeling. When viewed under a microscope, the fracture surface reveals numerous, small, randomly-oriented cracks. These cracks form due to the localized brittle failure that occurs as a result of plastic deformation and the subsequent stress cycles.

Although fibrous and crystalline fractures differ in their physical characteristics, the fatigue process in both types of fracture is the same. Both types of fracture begin with low-level cyclic loading and occur as a result of progressive crack formation, resulting in eventual failure. To prevent fatigue, metal structures must be designed to withstand the cyclic loading, preferably under the strength of the metals yield point.

描述 Fibrous fracture and crystalline fracture are two common types of fracture in material sciences. Fibrous fracture is a fracture which occurs when the deformation forces are either tensile or shearing. It is characterized by numerous fibrillations and is common in cold worked or brittle materials such as cemented carbides and paper. The fibrillations in fibrous fractures reflect the unidirectional nature of the deformation. The cross section of the fractured surface typically looks like ragged, irregular waves or undulations. The surface shows no visible crystalline structure.

Crystalline fracture is the fracture of a material which has been subjected to significant compressive or tensile stresses. It is usually associated with brittle materials such as ceramics and glass. The fracture surface typically reveals a distinct crystalline structure with sharp corners. The crystalline structure is a result of the stressed atoms interlocking with each other in a regular pattern. The characteristic “grainy” appearance of crystalline fracture is remarkably different from the “ragged” surface of fibrous fracture.

Overall, fibrous fracture and crystalline fracture are two distinct and recognizable types of fracture. The fibrous fracture is best described as ragged, irregular and without any crystalline structure whereas the crystalline fracture is characterized by sharp corners and a grainy surface. The two types of fracture are easily distinguishable and can yield information about the mode of fracture and the way the material was deformed.