Abstract: The reasons affecting fatigue damage of metal parts were analyzed. Through experiments, it was found that the damage to metal parts subjected to dynamic loads is caused by two conditions: wear and fatigue. On this basis, six measures were proposed to improve the fatigue strength.
Keywords: metal parts; Stress; Fatigue; measure
1. Reasons affecting fatigue damage of metal parts
The fatigue damage of parts mainly exists at high stress concentration points. Generally, under the action of alternating stress, micro cracks are generated after multiple plastic deformations of the material. These micro cracks are the starting causes of fatigue damage of parts. Generally speaking, there are two causes of damage to metal parts subjected to dynamic loads: wear and fatigue. The mixing arms, open gears, bearings, and lifting drums of concrete mixers are subjected to various alternating stresses, which can cause fatigue and wear to varying degrees. According to statistical data, about 80% of component fractures are caused by material fatigue, and the fatigue resistance of components is of great significance to their service life. The causes of fatigue vary greatly. Therefore, it is necessary to understand the occurrence of fatigue and carry out timely and reasonable scientific treatment to further improve the fatigue resistance of parts.
1.1 Working conditions
1.1.1 Effects of stress alternating frequency
According to the experiment, if the frequency of alternating stress is higher than 104 times/min, the fatigue limit of the part material increases with the increase of frequency. For parts that typically operate with alternating stress frequencies within the range of 3000-10000 cycles/min, the alternating frequency has no significant effect on the fatigue limit of the material. However, when the alternating frequency is less than 60 times/min, the fatigue limit of the material actually decreases.
Under the action of alternating stress, the part must undergo a certain number of stress cycles before fatigue failure occurs. The larger the maximum alternating stress value in the stress cycle under the same cyclic characteristics, the fewer cycles the part will undergo before damage occurs; On the contrary, the smaller the maximum alternating stress value in the stress cycle under the same cyclic characteristics, the more cycles the part will undergo before damage occurs. Below a critical value below the maximum stress, the part can undergo multiple stress cycles without fatigue failure, which is often referred to as the material's endurance limit or fatigue limit.
1.1.2 The impact of overload and secondary load
In engineering machinery, many parts often work in a state higher than the fatigue limit for a short period of time, such as sudden start of heavy-duty drums, sudden loading of mixers, sudden loading of gears, and occasional overloading. The overload limit is generally used to measure the impact of exceeding the fatigue limit on fatigue damage.
The overload damage limit of materials is determined by experiments. Firstly, obtain the complete fatigue curve and identify the fatigue limit σ- 1. Then use the sample to measure any value higher than σ- Perform fatigue tests under a stress of 1. After a certain number of cycles, operate at the fatigue limit stress and observe whether it affects the fatigue life N0. If it does not affect the lifespan, it indicates that the alternating load has not caused damage. As shown in Figure 1, under each overload stress, the overload damage boundary is obtained by searching for points a, b, c, etc. that begin to show damage after different cycles N, and connecting points a, b, c, etc.
The overload damage zone refers to the number of cycles under overload that fall into this zone, which can cause a reduction in the fatigue life of the part. Therefore, the narrower the zone, the stronger the material's ability to resist overload.
The diagonal line on the fatigue curve is called the overload endurance value, which represents the maximum number of stress cycles that can be endured until fracture under stress exceeding the fatigue limit. The steeper the line, the more stress cycles it can withstand under the same overload, indicating a higher ability to resist overload. Through experiments, it has been found that the fatigue limit of metals increases after operating below or near the fatigue limit for a certain number of times, and this phenomenon is called sub load exercise. So, when the assembled components or assemblies run for a period of time under no load or no full load, on the one hand, it can improve the running in of various moving parts, and at the same time, it can improve the fatigue resistance of the parts and extend their service life.
Figure 1 Overload fatigue damage boundary
1.1.3 The impact of usage temperature
If the temperature increases, the ability of the metal to resist deformation decreases, making it prone to fatigue cracks and leading to a decrease in the fatigue limit of the material. On the contrary, if the temperature decreases, the fatigue limit of the material increases.
1.1.4 Impact of environmental media
Generally speaking, the working environment that affects mechanical components can be roughly divided into two categories: corrosive media (aqueous solutions containing acids, alkalis, salts, etc., humid, etc., such as the working environment of mixers) and active media (active media that have adsorption effects on the surface but do not have corrosive effects, such as grease and engine oil containing a small amount of fatty acids). When parts work in corrosive media environments, metal corrosive substances are easily embedded in the parts, causing stress concentration and reducing fatigue limit.
1.2 The influence of surface and size factors on parts
From the analysis of the fatigue process, it can be seen that the uneven sliding of metals under alternating loads is mainly concentrated on the metal surface, where fatigue cracks are prone to occur, affecting the fatigue limit. The damage (knife marks, markings, etc.) and roughness on the surface of the parts can cause stress concentration, leading to a decrease in fatigue limit. The higher the surface roughness of the same material, the higher the surface processing quality, the smaller the stress concentration on the metal surface, and the higher the fatigue limit.
1.3 Impact of surface strengthening treatment effect
Due to the fact that the surface of the part is a place where fatigue cracks are prone to occur, and it is also a place where stress concentration occurs when the part is subjected to alternating bending or torsional loads. Therefore, surface treatment of parts is of great significance in improving the surface fatigue limit. The commonly used surface treatment methods include surface cold deformation (shot peening, rolling, rolling polishing, etc.), surface heat treatment (surface carburization, nitriding, cyanide, surface high-frequency or flame quenching, etc.), surface coating and coating, etc. By surface treatment of parts, the strength of the surface layer can be improved, the distribution of surface stress can be changed, residual stress can be generated on the surface, thereby reducing the tensile stress on the surface under alternating loads, preventing the propagation of fatigue cracks, and ultimately improving its ability to resist fatigue limits.
1.4 Effect of Alloy Structure
(1) When carbon is present in a solid solution of metal (steel), its fatigue limit decreases with the increase of carbon content, such as high and medium carbon quenched martensite or low-temperature tempered martensite.
(2) Metallographic structure.
A. Grain size. Refining grain size can reduce the degree of uneven slip under alternating stress, delay the generation of fatigue crack cores, that is, grain boundaries are an obstacle to fatigue crack propagation. Therefore, refining grain size can extend fatigue life.
B. Organizational type. The fatigue limit of different materials varies, especially after heat treatment, such as the fatigue limit of tempered martensite σ- 1. Highest, quenched martensite is not as good as martensite, and high-temperature tempered martensite σ- 1 is lower than martensite and martensite.
(3) The presence of debris. Non metallic impurities in metal materials are prone to fatigue cracks, resulting in a decrease in fatigue limit.
Direction of metal fibers. The direction relationship between component streamline and principal stress has a significant impact on fatigue limit. When the streamline direction of the formed part is parallel to the direction of the main stress, its fatigue limit is higher than when they are perpendicular. As the material strength increases, this difference also becomes greater.
2. Measures to improve the fatigue strength of metal parts
In order to improve the fatigue resistance of mechanical components and scientifically deal with fatigue imagination in metal parts in a timely and reasonable manner, it is of great benefit to improve the service life of mechanical products and the productivity of construction equipment. Therefore, the following measures are taken to improve the fatigue resistance of metal components.
(1) Select appropriate materials and external dimensions based on the working environment and frequency of the parts.
(2) Minimize the operation of parts in overload areas as much as possible.
(3) Parts should undergo secondary load training to improve their fatigue limit.
(4) Damage to the surface of the parts should be avoided, the roughness of the surface of the parts should be improved, and stress concentration should be reduced.
(5) If necessary, strengthen the surface of the parts to reduce the possibility of fatigue cracks.
(6) Reasonable selection of materials and appropriate metallographic treatment can improve the overall fatigue resistance limit of the parts.
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