Classification of fatigue fracture in metal casting

The fatigue fracture process of metal casting includes fatigue crack nucleation and initiation, fatigue crack propagation and final instantaneous crack. There are different classification methods of fatigue fracture from different angles.

According to the strain classification of the material before fracture, it can be divided into brittle fracture and plastic fracture.

Brittle fracture: the material has no macroscopic plastic deformation or the plastic deformation is very small before fracture, that is, the fracture strain is very small.

Ductile fracture: the material has obvious plastic deformation before fracture, that is, the fracture strain is large.

Generally, the reduction of area of smooth specimen is regarded as the judgment standard of brittle and ductile materials. Higher than 5% is ductile material and lower than 5% is brittle material.

According to the classification of fatigue fracture path, it is generally divided into transgranular fracture, intergranular fracture and mixed fracture.

Transgranular fracture: a fracture in which a fatigue crack extends through the interior of a grain.

Intergranular fracture: fracture in which fatigue cracks propagate along grain boundaries.

Mixed fracture: the fatigue crack in the same crack body may occur transgranular fracture or intergranular fracture, resulting in mixed fracture.

Generally speaking, transgranular fracture has both ductile fracture and brittle fracture. Both ductile fracture and brittle fracture mainly depend on the plastic deformation ability of crystal materials and external environmental factors. When impurity element accumulation occurs at the grain boundary or other factors cause the strength at the grain boundary to be lower than the internal strength of the grain, intergranular fracture is easy to occur under the action of stress. At this time, intergranular fracture may be brittle or ductile.

According to the classification of fatigue fracture morphology, it can be divided into cleavage fracture, quasi cleavage fracture, intergranular fracture, pure shear fracture and micropore aggregation fracture. They correspond to cleavage fracture, quasi cleavage fracture, intergranular fracture and dimple fracture respectively. In most cases, the fracture surface shows mixed fracture, and different areas of macro fracture show different micro fracture morphology.

According to the classification of fatigue fracture causes, it can be divided into overload fracture, creep fracture, environmental fracture, etc.

Overload fracture: fracture of sample or component due to continuous increase of load or sudden increase of working load.

Creep fracture: apply constant stress at high temperature and cause material fracture after deformation for a certain time.

Environmental fracture: such as stress corrosion, hydrogen induced cracking, liquid metal brittleness, etc. Due to the existence of corrosive medium, the crack nucleation and propagation can be caused under low external stress after a certain time until fracture.

When discussing the classification of fatigue cracks, the types of hyperintergranular fracture, transgranular fracture and mixed fracture are proposed. In the actual fracture process, the observation of fracture morphology shows that the crack propagation does not always extend in one direction, but simultaneously in multiple directions. Various obstacles will be encountered in the propagation process, such as the change of crack direction, crack bending, and even secondary cracks. The reason is that the fatigue crack propagation is affected by many factors, such as the external temperature, the change of stress, the material’s own organizational structure, the internal defects of the material and so on.

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