Analysis of automobile frame member strain fracture end

In the test process, four working conditions are adopted, and working condition 1 is the test field. The actual vehicle road test is conducted according to the specifications to reflect the fatigue life of the shock absorber bearing under the conditions of normal use of the user. The working condition 2 is in the shock absorber. The area where the bearing cracks is more often used for road test according to the normal use requirements of the vehicle to reflect the impact of the road surface on its life. The condition 3 is the use area where the shock absorber bearing is cracked more frequently, and the tire exceeds the standard pressure to run. Reflect the impact of tire pressure on its life; Case 4 is to conduct a road test in the case of a shock absorber leakage (test area with B) to reflect the impact of the shock absorber on its life. Finally, the time history signals of 4 kinds of working conditions and 3 measuring points are obtained.

Test data processing and analysis Rain flow count is performed on the strain signal of the shock absorber bearing, combined with the estimation of its fatigue properties, and the fatigue life under different working conditions is predicted; the vertical load of the shock absorber and its strain on the bearing A comparative analysis was performed to analyze the relationship between the load on the shock absorber and its support.

Calculation of fatigue life Material St13 for shock absorber bearings and its basic properties (tensile strength) are 270-370 MPa, where the average value σb = 320 MPa is used. Due to the low alloy steel used, the strain fatigue properties are The methods given in the literature are estimated as follows.

Εta = εea + εpa (1) where εta is the total strain amplitude, εea is the elastic strain amplitude, and εpa is the plastic strain amplitude.

In formula (1), εea=σf′(2Nf)b/Eεpa=ef′(2Nf)cE=210000MPa Fatigue strength coefficient σf′=1.5σb Fatigue strength index b=-0.087 Fatigue ductility coefficient ef=0.59F Fatigue ductility index c=-0.58 cycle enhancement coefficient K'=1.65σb cycle enhancement index n'=0.15Nf is the fatigue life, then 2Nf is the reverse number.

When σb/E≤0.003, F=1; when σb/E>0.003, F=1.375-125σb/E. Here σb/E ≤ 0.003, so F=1.0.

According to formula (1), the strain life relationship shown in FIG. 3 can be obtained.

According to the measured load spectra of the four working conditions, combined with the fatigue characteristics of the material shown in Figure 3, for the measuring point B, the local stress-strain method is applied to the fatigue life Li (i = 1, 2, 3, 4), and relative Condition 1 (test field conditions) The fatigue life L1 is calculated as shown in 1.

Comparative analysis of shock absorber vertical load and its bearing strain From the previous study, it was found that the damper damage caused the damper bearing's fatigue life to drop by more than 99%. For a bidirectional drum shock absorber, it not only acts to reduce the amplitude of the vertical impact load of the vehicle during the stretching stroke, but also works in the compression stroke. If the failure of the compression stroke causes the bearing's fatigue life to decrease, the fatigue life of the bearing can be improved by increasing the stiffness of the spring; if it is caused by the tension stroke, the above problem can be solved by adjusting the damping coefficient of the shock absorber. Theoretically speaking, it should be the main reason for the reduction of fatigue life caused by the tensile stroke, because the compression stroke plays a major role in the spring rather than the shock absorber. In order to further confirm this problem, it is necessary to support the Strain and shock absorbers are compared for vertical loads.

Conclusion Through the above analysis, the following conclusions can be drawn: (1) The reliability test of the automobile based on the specification at the test site, the fatigue damage to the shock absorber bearing is less than the fatigue damage of the shock absorber bearing under the actual use conditions. . This shows that, in some cases, although the reliability of the vehicle meets the requirements of the national test specification, it does not necessarily satisfy the user's actual use requirements. Therefore, in the automotive design process, the user's actual use needs need to be investigated and tested. Meet the use of norms as the highest criteria. (2) During the running of the vehicle, the tire pressure and the condition of the shock absorber will have a great influence on the fatigue life of the shock absorber bearing, and the vehicle should be driven under the specified air pressure of the tire. (3) The damage to the shock absorber will have a fatal effect on the fatigue life of the bearing, and its fatigue life will be more than 99% lower than the normal use condition. Therefore, the damper should be replaced in time after being damaged.

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