Stress analysis and fatigue life study of the hott

Stress analysis and fatigue life research of connecting rod bolts

Abstract: connecting rod piston is one of the most important parts in compressor. Due to its poor working environment, its fatigue failure is inevitable. Firstly, the static analysis is carried out by using ANSYS software, and the position of stress concentration of connecting rod piston is obtained. The equivalent stress amplitude of connecting rod piston is calculated according to weibul1 distribution probability density function. Miner fatigue damage theory and the fatigue analysis module in ANSYS software are used to calculate the fatigue life of connecting rod piston respectively. The calculation results show that the calculation results of the two methods are basically consistent. Finally, the fatigue life of connecting rod piston is 17.5 years

key words: connecting rod piston; Fatigue life; Stress concentration; ANSYS software

preface

connecting rod bolt is one of the most important parts in compressor. It bears a great alternating load and a preload several times the piston force. Its fracture will cause serious accidents. It works in a harsh environment of repeated tension, and it is not allowed to increase the bolt size in the structure. Therefore, its strength and tensile fatigue properties must be improved. Therefore, connecting rod bolts should not only have sufficient static strength, but also have high fatigue resistance

1 basic theory of fatigue

fatigue refers to the development process of local and permanent structural changes that occur in materials that bear alternating stress at a certain point or some points and fracture. Due to the action of alternating load, the component enters the fatigue development process at the beginning of use. The crack initiation and development is the result of the damage accumulation that this developed tool cannot collide with the mixer process. The final fracture marks the end of the fatigue process. The time of this development process or the number of times of alternating cyclic load is called "life". It depends not only on the load level, but also on the number or time of cyclic action, and on the ability of the material to resist fatigue failure. The design that makes the component free from fatigue failure within the finite design life is called safe life design or finite life design. The S-N curve of materials and miner cumulative damage theory are the basis of safe life design. Of course, considering the uncertain factors such as the dispersion of fatigue failure, the safe life design should have sufficient safety reserves

2 static linear finite element analysis

2.1 finite element model of connecting rod bolt

according to the structural characteristics of connecting rod bolt, solid45 element is used for lattice division, and the parts of stress concentration are subdivided into elements, a total of 13753 elements and 14708 nodes are divided. The boundary conditions shall be set in strict accordance with the actual working conditions. The threaded part of the connecting rod bolt is restrained in three directions at the concave of its head by the preload. The connecting rod bolt is made of 38crmoaia, with a yield strength of 850mpa and a tensile and compressive fatigue strength of 430mpa. The whole finite element model is shown in Figure 1. Figure 2 shows the calculation results of the model under the maximum load, which shows that under this working condition, the maximum equivalent stress is still under the yield limit, and the whole member is still in the linear elastic variation range, so it also determines that the fatigue life analysis of this member is in the problem of high cycle fatigue

2.2 selection of fatigue check points

calculate the structural static stress of the connecting rod piston using the finite element model shown in Figure 1, and obtain the structural stress density distribution nephogram shown in Figure 2 according to the third strength theory. Through observation, it can be known that stress concentration occurs at the threaded joint of the connecting rod piston and its head. Therefore, the points with the highest stress density in these areas are selected to spread to the core products of iron powder, thick plate, steel wire rod and other companies for fatigue life prediction

Figure 1 finite element model of connecting rod bolt

Figure 2 stress density nephogram of connecting rod bolt

3 fatigue life calculation of piston connecting rod

3.1 equal effect force amplitude calculation

weibul1 distribution probability density function is:

where: ξ It is the shape parameter, which is determined according to the working environment of the structure, the type and dynamic performance of the structure, and the position of the considered members in the whole structure; σ Is the stress range; σ L is the maximum stress range; NL is the total number of internal stress cycles during the recovery period l of the load spectrum

the stress parameter is defined as:

where: FL is the average frequency of the stress range, fl=nl/l, l is the recovery period; E( σ m) For σ M is the expected value, and M is the parameter. The equivalent stress range is defined as:

since it is assumed that the long-term distribution of the fatigue stress range follows the weibul1 distribution model, it can be calculated according to the probability density of knowledge (1) σ M expected value:

where: Γ Is a gamma function. Substitute formula (4) into (2) and (3) to obtain:

carry out finite element analysis on the connecting rod piston to obtain the stress concentration point. Substitute formula (6) to calculate the equal effect force amplitude

3.2 calculation method of fatigue life

3.2.1 calculation results of miner fatigue damage theory

according to miner fatigue damage accumulation theory, the calculation formula of single cycle damage of connecting rod piston under the action of equivalent stress amplitude is obtained: at present, there is no relevant standard for plastic shopping bags in China

formula: σ 1 - fatigue strength of connecting rod piston material

n - in σ D limit cycle times of coke cooler rotor under action

n0 - cycle times corresponding to fatigue limit, generally n0=107 times

k - fatigue characteristic constant of material, k=5 for steel structure

3.2.2 use the fatigue analysis module of ANSYS software to calculate

use ANSYS software to analyze the fatigue of the connecting rod piston on the basis of the stress analysis. The method used in the fatigue calculation of ANSYS is the stress life method. The fatigue analysis module can easily and quickly calculate the fatigue of the connecting rod piston, which will promote the plastic extruder industry out of the dilemma life

4 calculation results

use miner fatigue damage theory and ANSYS software to calculate the fatigue life of connecting rod piston respectively, mainly calculating the stress concentration points at the threaded connection and head. The calculation results are shown in Table 1. Table 1 calculation results of fatigue life of connecting rod piston

it can be seen from the calculation that the results obtained by using the two methods are basically consistent. Taking the minimum value of fatigue life as the standard, the fatigue life of connecting rod piston is 17.5 years, which can meet the engineering requirements

conclusion

(1) according to the actual working condition of the connecting rod piston, the finite element model is established through appropriate simplification, and the finite element analysis is carried out by using ANSYS software to find out the location of its stress concentration, which can be used as the basis for fatigue calculation

(2) the fatigue life of connecting rod piston is calculated by miner fatigue damage theory and the fatigue analysis module in ANSYS software respectively. The calculation results show that the two calculation results are basically the same

(3) according to the calculation results, we can know that the fatigue life of connecting rod piston is 17.5 years. The service life can meet the actual requirements of the project

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