What are the laws of fatigue crack initiation and propagation and life prediction methods of steel parts under alternating loads?
Release Time : 2025-05-06
In engineering practice, many steel parts are subjected to alternating loads, such as engine crankshafts, bridge structures, etc. Under alternating loads, steel parts will suffer fatigue damage, which is a very dangerous form of failure. Therefore, it is of great engineering significance to study the fatigue crack initiation and propagation laws of steel parts under alternating loads and the life prediction method.
Under alternating loads, stress concentration areas on the surface or inside of steel parts are the main locations for fatigue crack initiation. These areas include notches, holes, surface processing defects, etc. of parts. When the alternating stress exceeds the fatigue limit of the material, local plastic deformation will occur at the stress concentration point. As the number of cycles increases, dislocation movement forms dislocation pileups and slip bands in these local areas, and then forms microcracks. In addition, inclusions and second phase particles inside the material may also become the source of crack initiation, because they will cause local stress unevenness and accelerate the formation of cracks.
After the fatigue crack initiates, it will gradually expand under the action of alternating loads. Crack expansion can be divided into two stages. In the first stage, the crack propagates in a shearing manner along the direction of the maximum shear stress, the propagation rate is slow, and the crack propagation path is tortuous. When the crack propagates to a certain extent, it enters the second stage. At this time, the crack propagates in an open manner perpendicular to the direction of the maximum tensile stress, the propagation rate is significantly accelerated, and the crack propagation path is relatively straight. During the crack propagation process, the stress concentration degree at the crack tip and the microstructure of the material have an important influence on the propagation rate. The stress intensity factor amplitude is an important parameter to describe the driving force of crack propagation. When the stress intensity factor amplitude reaches a certain threshold, the crack begins to propagate rapidly until the part breaks.
There are many factors that affect the initiation and propagation of fatigue cracks in steel parts. The composition, microstructure and heat treatment state of the material are internal factors. For example, grain refinement can improve the strength and toughness of the material, thereby delaying the initiation and propagation of fatigue cracks; while impurities and defects in the material will reduce the fatigue performance of the material. External factors include the size, frequency, waveform and working environment of the alternating load. The greater the load and the lower the frequency, the faster the initiation and expansion of fatigue cracks; high temperature, corrosive environment, etc. will accelerate the damage of materials and promote the formation and expansion of fatigue cracks.
The main life prediction methods include empirical formula method, stress-life curve method and fracture mechanics method. The empirical formula method is to establish an empirical relationship between the fatigue life of parts and parameters such as stress and material properties based on a large amount of test data and engineering experience. For example, Miner's law assumes that fatigue damage is linearly accumulated. When the accumulated damage reaches 1, the part is fatigued. This method is simple and easy to implement, but the accuracy is relatively low. It is suitable for preliminary design and estimation.
The stress-life curve method (S-N curve method) is to obtain the fatigue life of the material at different stress levels through experiments and draw a stress-life curve. According to the alternating stress borne by the part, the corresponding life is found on the S-N curve. This method takes into account the fatigue characteristics of the material, but has poor adaptability to complex loads and actual working conditions. In order to improve the accuracy, the modified S-N curve method can be used to consider the influence of factors such as average stress, size effect, and surface processing quality on fatigue life.
The fracture mechanics method is based on the crack propagation theory. By calculating the stress intensity factor amplitude at the crack tip, the relationship between the crack propagation rate and the stress intensity factor amplitude is established to predict the remaining life of the part. This method can take into account factors such as the initial size of the crack, the propagation law, and the fracture toughness of the material. It is suitable for the life prediction of parts with initial defects or cracks and has high accuracy. However, this method requires accurate measurement of parameters such as crack size and fracture toughness of the material, which is difficult to apply in practice.
The initiation and propagation of fatigue cracks in steel parts under alternating loads is a complex process affected by many factors. By studying its laws, different life prediction methods can be used to evaluate the fatigue life of parts. In actual engineering, the appropriate life prediction method should be selected according to the specific situation, and combined with test verification and actual operation data, the accuracy of the prediction should be continuously improved to ensure the safe and reliable operation of steel parts.