Reasons for fatigue fracture failure of metal components

Thread Content

The fatigue fracture failure of metal components has various underlying causes. In summary, these mainly include unreasonable design, inappropriate material selection, manufacturing defects, the influence of the environment, as well as changes in load frequency and pattern. 1. Structural shape of components: Unreasonable design in terms of structure refers to the absence of arcs at points where the cross-section changes, which leads to stress concentration. The design and placement of features such as holes, grooves, and threads at points of structural change are also unreasonable, for example when they are located in weaker areas. Figure 1 shows fracture at the root of the bolt, located at a stress concentration area. Figure 12: Surface condition. Surface processing such as turning, milling, planing, and grinding creates various-sized notches, pits, and other defects on the surface of the component. These small notches also cause stress concentration, which has an effect similar to that of cracks. 3. Material and microstructural condition: The use of inappropriate materials, or unreasonable heat treatment processes, can also lead to fatigue fracture failure. Fatigue fracture caused by replacing quenched and tempered materials with normalized materials is a common occurrence in failure analysis. Generally speaking, tempered martensite structure has higher fatigue resistance compared to other structures. If, in addition to martensite, other types of structures such as pearlite or bainite are present, the fatigue strength will decrease. If the microstructure consists of ferrite plus pearlite, then the fatigue strength of the material indeed increases as the pearlite content increases. Heat treatment processes that increase material strength often enhance the material’s fatigue resistance. Surface heat treatments such as surface quenching and surface carburizing can enhance the fatigue resistance of materials. However, if the process is improper, it will instead reduce fatigue strength. If the material structure is uneven or contains defects, stress concentration will occur in certain areas of the material, which in turn reduces its fatigue strength. Defects such as intermetallic compounds, inclusions, porosity, and segregation can all have an adverse effect on the fatigue resistance of materials. Figure 2 shows fatigue fracture caused by inclusions. Figure 24: Assembly and bonding effects. The assembly and bonding effects have a significant impact on the fatigue life of components. For example, using an appropriate tightening force when tightening bolts can extend their service life by more than 5 years. It is people’s intuition that the tighter they are tightened, the greater their fatigue life will be, but experiments show this to be not the case. 5. Environmental factors such as temperature and corrosive media also have a significant impact on fatigue strength. Changes in temperature and a corrosive environment can both reduce the fatigue strength of materials.