Classification, characterization, and features of fatigue fracture

2024,04,07

The change in the properties of a metallic material that occurs under the repeated action of stress or strain is called fatigue. Materials subjected to alternating cyclic stress or strain, caused by local structural changes and the continuous development of internal defects, so that the mechanical properties of the material degradation, ultimately leading to the complete fracture of the product or material, the process is called fatigue fracture, can also be referred to as the fatigue of the metal. The stress that causes fatigue fracture is generally very low, and the occurrence of fatigue fracture is often characterized by suddenness, highly localized and sensitivity to various defects.

No.1 Classification of Fatigue Fracture

1.High-period fatigue and low-period fatigue

Fatigue in which the stress level acting on the part or member is low and the number of cycles of destruction is higher than 100,000 is called high-cycle fatigue. For example, springs, drive shafts, fasteners and other products are generally high week fatigue.

Acting in the component parts of the stress level is high, the number of cycles of destruction is low, generally less than 10,000 times of fatigue, known as low week fatigue. For example, the fatigue damage of pressure vessel and turbine parts belongs to low fatigue.

2. Stress and strain analysis

Strain fatigue - high stress, the number of cycles is low, known as low fatigue;

Stress fatigue - low stress, a higher number of cycles, known as high-cycle fatigue.

Composite fatigue, but in practice, it is often difficult to distinguish between the type of stress and strain, generally both types, so called composite fatigue.

3. According to the type of load classification

Bending fatigue, torsion fatigue, tension fatigue, contact fatigue, vibration fatigue, micromotion fatigue.

No.2 Characteristics of Fatigue Fracture

Macroscopic: crack source → extension zone → instantaneous fracture zone.

Crack source: Surface grooves, defects, or areas of stress concentration are prerequisites for generating a crack source.

Fatigue expansion zone: the section is relatively flat, the fatigue expansion is perpendicular to the stress direction, resulting in obvious fatigue arcs, also known as beach lines or shell lines.

Instantaneous fracture zone: the fatigue crack rapidly expanding to the region of instantaneous fracture, the fracture has metal slip marks, some products instantaneous fracture zone has radioactive streaks and has a shear lip area.

Microscopic: Fatigue fracture is typically characterized by the appearance of fatigue glow.

In some microscopic specimens, there may be disintegration and quasi-disintegration phenomena (crystallographic name for small planes in the microscopic image), as well as microscopic regional features such as tough nests.

No.3 Characteristics of Fatigue Fracture

(1) There is no obvious macroscopic plastic deformation at the time of rupture, and there is no obvious warning before rupture, so it is often a sudden phenomenon that causes damage to or rupture of mechanical parts, which is a very serious hazard.

(2) cause fatigue rupture stress is very low, often lower than the yield strength of static load stress load.

(3) After the fatigue damage, generally able to clearly show at the fracture crack occurrence, expansion and finally fracture of the three components of the region.

Author:

Mr. Richard

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