Metal fatigue refers to the process in which materials and components gradually produce local permanent cumulative damage in one or several places under cyclic stress or cyclic strain, and cracks or sudden complete fractures occur after a certain number of cycles. When materials and structures are subjected to repeated changing loads, although the stress value never exceeds the strength limit of the material, it may be damaged even if it is lower than the elastic limit. This phenomenon of material and structure damage under repeated alternating loads is called metal fatigue failure.
In general terms, metals will produce some fine cracks on the metal surface under continuous reciprocating loads. After the cracks on the metal surface accumulate and extend to a certain extent, rapid and strong brittle fracture will occur. When brittle fracture occurs, the metal often does not bear a load that exceeds the tensile/compressive strength of the metal. The reason is that the tensile/compressive strength of the metal is the value obtained under static conditions, and the reason is that under alternating loads, the metal is more likely to reach the strength limit and then produce fatigue failure.
There are two main reasons for fatigue failure of metals.
(1) On the one hand, after a series of processes such as melting and casting, the metal structure inside the finished product is not uniform, which will cause defects and internal stress inside the metal. Good heat treatment can refine the metal structure and eliminate most of the stress. Adding various rare earth elements to the metal can improve the fatigue strength of the metal, thereby increasing the working life of the metal.
(2) On the other hand, there are external factors, which can be summarized into three aspects.
- One is to distinguish by load type, such as impact fatigue formed by impact load on the surface, contact load, pits and pits formed on the surface to relieve fatigue, micro-motion wear fatigue, such as when the surfaces of two parts are in contact, the contact surface undergoes a small reciprocating relative motion
- Movement, and then the surface of the parts will produce wear, oxidation, fatigue peeling and other forms of micro-motion wear fatigue, etc., which can be divided into high temperature, low temperature, high and low temperature cycle, corrosion fatigue, etc. according to the ambient temperature.
- Under high temperature conditions (above the melting point of the metal or above the recrystallization temperature), the plasticity of the metal increases and the hardness decreases, which makes it easier to deform. Under low temperature conditions, the plasticity of the metal decreases, the brittleness increases, and the metal is more likely to have brittle fracture and other problems.
- Due to the characteristics of thermal expansion and contraction, the metal will produce internal stress under the conditions of high and low temperature cycles, which will cause fatigue damage to the metal. Corrosion fatigue refers to the formation of oxides on the metal surface under the action of water vapor in the air, which will destroy the surface strength of the metal and make the corrosion area more susceptible to damage. According to the stress state, it can be divided into single stress fatigue and multi-directional stress fatigue. Under the action of single stress cycle, the parts will have a life slightly lower than the static load strength limit, while under the action of multi-directional stress, the parts are more likely to be fatigued due to deformation.
After understanding the conditions for the formation of metal fatigue, we will explore how to discover hidden metal fatigue. Since the discovery of metal fatigue in the early 19th century, people have been exploring the causes of fatigue. In the process of exploration, people have mastered a variety of flaw detection methods.
There are five common flaw detection methods: X-ray detection, ultrasonic detection, eddy current detection, magnetic particle detection, and penetration detection. Taking X-ray detection as an example, the metal is penetrated by x-rays. The defective parts inside the metal can penetrate more rays, while the parts with uniform density will reflect back more rays.
Therefore, when imaging, the defects are darker, and the parts with uniform density are brighter. In this way, we can more intuitively and quickly determine the distribution of defects inside the metal, so that we can avoid the occurrence of fatigue damage to a certain extent by avoiding defects as working areas and strengthening the strength of defects.
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