Metal Fatigue Failure and Characteristics

Fatigue failure refers to the phenomenon that metal materials fail due to cumulative damage after a certain number of cycles under the action of variable loads. Fatigue failure has a certain degree of contingency and randomness, and the probability of occurrence is relatively high. According to statistics, more than 60% of failures in engineering are fatigue failures.

Fatigue failure is affected by variable loads and cyclic stress

Variable loads refer to loads whose size or direction changes periodically or irregularly with time according to a certain law.

Cyclic stress types can be divided into sine waves, rectangular waves, and triangular waves, with the most common being sine waves.

Fatigue failure classification

According to different stress states, it can be divided into bending fatigue, torsion fatigue, tension and compression fatigue, and composite fatigue.

According to different environments and contact conditions, it can be divided into atmospheric fatigue, corrosion fatigue, high temperature fatigue, thermal fatigue, and contact fatigue.

According to fatigue life and stress level, it can be divided into low-cycle fatigue and high-cycle fatigue. High-cycle fatigue has a longer life, Nf>10*5, and a lower fracture stress, σ<σs, so it is also called low-stress fatigue. Low-cycle fatigue Nf is 10*2~5, the fracture stress is low, σ≥σs, and plastic deformation often occurs, which is also called high-stress fatigue.

Fatigue fracture has the following characteristics:

1. Low-stress fracture under cyclic load. The stress at the time of fracture is lower than the tensile strength of the material, or even lower than the yield strength. Relatively speaking, the lower the stress, the longer the service life, and the higher the stress, the shorter the service life. Below a certain threshold value, fatigue failure will not occur.

2. Brittle fracture. Since fatigue is a low-stress fracture, whether it is a plastic material or a brittle material, there is often no sign before fatigue fracture occurs, which is characterized by suddenness and great harm.

3. It is sensitive to various material defects, such as cracks, brittle inclusions, uneven organization, surface damage, etc. Fatigue originates from local weak links, and stress concentration areas such as cracks and material surface steps are most likely to become fatigue origins. Figure 3 shows a fatigue fracture originating from surface brittle inclusions.