Today, we will analyze the working conditions, use environment, fatigue analysis, etc. of the front axle disc brake of the car. A reasonable engineering structure needs to meet the requirements of strength, stiffness and fatigue life. Among them, the common cause of failure is fatigue damage. However, the cyclic load applied during the fatigue analysis of the automobile front axle disc brake is generally more The load applied by force analysis is much smaller, so the problem of fatigue failure cannot be solved by static strength analysis.
In the process of driving, if some effective measures are not taken to prevent fatigue failure, there will be great loss and waste of manpower and material resources. There are no signs of fatigue failure, and it is too late to prevent it. It has caused a large number of serious traffic accidents. Even the advanced technical detection methods in scientific research cannot accurately predict it. Therefore, before studying the crack failure problem of the front axle brake disc of a heavy-duty truck bridge, the fatigue damage of the brake disc should be studied first.
There are several characteristics of fatigue damage of automobile front axle disc brakes. The optimization of the local structure is enough to improve the fatigue life of the structure for the entire structure; fatigue damage must be accumulated over a period of time; through the study of fatigue damage in the macro and The characteristic mechanism of the crack at the microscopic level can be summarized as the factors that cause fatigue fracture.
Stress-fatigue analysis method
Because the structural parts must be under the action of alternating loads, after a period of time, after a certain time history, fatigue failure occurs, and the index used to describe the time history of the delivery period is the fatigue life. Fatigue life refers to the number or time of cyclic loading experienced by a part from the beginning of operation to failure.
As fatigue analysis needs to consider many factors, such as load form, material mechanical properties, etc. When the load is more complicated, the calculation of fatigue life will be more difficult. At present, neither theoretical calculation nor finite element analysis can achieve high accuracy, only estimation.
At present, there are several methods to calculate fatigue life, namely PS1 such as nominal stress method, local stress-strain method, fracture mechanics method, etc.
Nominal stress method P9: The SN curve is used as the starting point, and various factors such as part geometry and processing methods are fully considered to carry out fatigue design. Sending is a commonly used method. This chapter uses the distribution law and test of nominal fatigue life In the analysis of fatigue life, this probability is usually defined as the survival rate P, also known as reliability. Based on this, the life value N can be determined. Each survival rate value corresponds to a stress-life curve, so, Call it PSN curve.
The S-N curve obtained through the estimation of the P-S-N curve is only suitable for standard samples, which is very different from the actual size and geometric structure of the brake disc, so it needs to be corrected in terms of life and strength. When revising the S-N curve, the following factors are mainly considered:
a) The influence of stress concentration
For the processing of mechanical parts, due to the shape and processing requirements, it is inevitable that there will be certain discontinuous parts such as variable cross-sections, holes, shoulders, etc., and stress concentration is likely to occur in these parts, which seriously affects the fatigue performance of the components. At this time, this influence factor is called the effective stress concentration factor. The stress concentration is already considered in the results calculated by the finite element software. Therefore, when revising the S-N curve, there is no need to consider the influence of the theoretical concentration factor.
b) The influence of component size
The test shows that the fatigue strength of the component is inversely proportional to the size of the size. The larger the size, the lower the fatigue strength. Generally, the size factor E is used to describe the size effect. According to the size of the brake disc and the manual of mechanical design, select £=0.78.
C) The influence of surface condition
Regardless of the roughness of the surface processing and the strengthening method, set the surface condition coefficient = 1.
d) The influence of loading method
The same component has different fatigue strengths under different loading methods (bending, tension and compression, and torsion). Generally, load factors are used to characterize the effect of loading methods on fatigue strength. According to the actual force of the front axle disc brake of the car, the load factor is taken.
e) The influence of average stress
In the process of fatigue analysis using ANSYS, the average stress influence coefficient needs to be carried out by the average correction method.
