1. Introduction
The locomotive coupler is a crucial component in railway transportation, playing a vital role in connecting train carriages and ensuring the safe operation of trains. It endures complex loads during train operation, such as tensile and compressive forces during starting and braking. Insufficient strength or fatigue failure of the coupler can lead to serious accidents, causing significant losses to life and property. Therefore, accurately evaluating the working stress and fatigue life of locomotive couplers is of great importance for ensuring railway safety.
In recent years, with the development of computer technology and numerical simulation methods, finite element analysis has become an effective tool for studying the mechanical properties of components. By establishing a finite – element model of the locomotive coupler and simulating its working conditions, we can obtain detailed information about stress distribution and fatigue life, which provides a basis for the design, optimization, and maintenance of couplers. This paper focuses on the investment – casting locomotive coupler, conducts material performance tests, and uses ANSYS Workbench to perform static and fatigue finite – element analyses under different working conditions and load spectra, aiming to provide a reference for the rapid evaluation of the stress distribution state and fatigue life of the coupler.
2. Structure and Material of the Investment – Casting Locomotive Coupler
2.1 Structure of the Coupler
The investment – casting locomotive coupler studied in this paper has a complex structure. Its external dimensions are 594mm×370mm×350mm, belonging to a large – scale complex investment – casting. The minimum cross – sectional area in the longitudinal axis cross – section of the coupler is \(0.008m^{2}\). The coupler mainly consists of parts such as the hook head, hook tongue, hook body, and connection port. When two couplers are coupled, one hook head inserts into the hook head hole of the other. The inner side of the convex cone presses the hook tongue of the opposite coupler to rotate during the approaching process, and after the connection surface contacts, the hook tongue returns to the locked position under the action of the spring in the uncoupling cylinder.
2.2 Material of the Coupler
The coupler is made of E – grade steel ZG25MnCrNiMo. This material has excellent mechanical properties, which can meet the requirements of high – strength and high – toughness for locomotive couplers. Table 1 shows the mechanical property parameters of ZG25MnCrNiMo before and after heat treatment.
| Material | Elongation after fracture δ / % | Reduction of area ψ / % | Tensile strength σb / MPa | Yield strength σs / MPa | Elastic modulus E / MPa | Hardness / HBW |
|---|---|---|---|---|---|---|
| Before heat treatment | 2.1 | 3.9 | 626 | – | – | 235 – 241 |
| After heat treatment | 14.5 | 34.5 | 1020 | 920 | – | 315 – 320 |
| Industry standard (after heat treatment) | ≥14 | ≥30 | ≥830 | ≥690 | – | 241 – 311 |
| Table 1: Mechanical property parameters of ZG25MnCrNiMo before and after heat treatment |
As can be seen from Table 1, after heat treatment (normalizing + quenching at 910℃ + tempering at 590℃ for 2h), the mechanical properties of the coupler material are significantly improved. The tensile strength increases from 626MPa in the as – cast state to 1020MPa, the elongation after fracture increases from 2.1% to 14.5%, and the yield strength reaches 920MPa, meeting the requirements of the industry standard.
The microstructure of the coupler in the as – cast state mainly consists of pearlite and ferrite. Compared with the 1 – level standard in TB/T2942.2 – 2018 “Cast Steel Parts for Railway Vehicles – Part 2: Metallographic Structure Inspection Atlas”, the grain size and distribution morphology of the as – cast structure exceed the standard requirements. After heat treatment, the microstructure becomes fine and uniform tempered sorbite. Under high – magnification SEM observation, many fine and uniform carbide particles are distributed on the ferrite matrix. Figure 1 shows the microstructure of the investment – casting locomotive coupler in the as – cast state and after heat treatment.
3. Working Principle and Load Conditions of the Coupler
3.1 Working Principle
The connection process of the coupler is as follows: when two couplers are about to be connected, the hook head of one coupler inserts into the hook head hole of the other. The inner side of the convex cone presses the hook tongue of the opposite coupler to rotate counter – clockwise by 40°, and the spring in the uncoupling cylinder is compressed. When the connection surfaces of the two couplers contact, the pressure of the convex cone on the hook tongue disappears, and the hook tongue rotates clockwise and returns to the locked position under the action of the spring, completing the connection.
During train operation, the coupler is mainly subjected to longitudinal forces. When the train starts, the coupler is under tension, and the main stressed part is the arc surface of the hook tongue. When the train brakes, the coupler is under compression, and the arc surface of the hook tongue and the connection port are the main stressed parts.
3.2 Load Conditions
The coupler is subjected to different loads during train operation. The maximum loads occur during starting and braking. According to the “Technical Specifications for Metro Vehicle Couplers”, the coupler and its seat should be able to withstand a static pressure of not less than 1000kN and a tensile force of not less than 800kN. The maximum pulling force during the stable running of the vehicle is 600kN.
In fatigue analysis, the load – time history of the coupler on the Daqin Line is used to compile an 8 – level load spectrum. However, since the Shibata – type close – coupled coupler is used for urban rail passenger lines, which is different from the maximum load of freight trains on the Daqin Line, the maximum amplitude of the Daqin Line load is corrected. The amplitude exceeding 800kN is uniformly taken as 800kN. In addition, taking Chongqing Metro Line 3 as an example, the load spectrum is simplified into a sine – wave – shaped load for simulation calculation, and its amplitude is 800kN, which is in line with the requirements of the metro coupler technical specifications. Table 2 summarizes the load conditions for different analysis purposes.
| Analysis Purpose | Load Conditions |
|---|---|
| Static strength simulation | Tensile loads of 600kN, 800kN, and 1000kN; Compressive loads of 600kN, 800kN, and 1000kN |
| Fatigue analysis (Daqin Line) | Modified Daqin Line 8 – level load spectrum with a maximum amplitude of 800kN |
| Fatigue analysis (Chongqing Metro Line 3) | Sine – wave – shaped load with an amplitude of 800kN |
| Table 2: Load conditions for different analysis purposes |
4. Finite – Element Model Establishment of the Coupler
4.1 Meshing
Due to the complex structure and uneven wall thickness of the coupler, tetrahedral elements are used for meshing. In order to obtain more accurate calculation results, the global mesh size starts from 10mm, and the mesh size is gradually reduced for calculation until the change in the stress result is less than 3%. At this time, the global mesh size of the coupler is reduced to 1.75mm. Then, the stress – concentration area is locally refined, and the minimum mesh size is 0.5mm. After meshing, the total number of nodes of the assembled model is 3369299, and the number of elements is 1924534. Figure 2 shows the meshing effect of the coupler.
4.2 Material Property Assignment
The material property parameters obtained from the material performance test are input into the finite – element model. For ZG25MnCrNiMo steel, parameters such as elastic modulus, Poisson’s ratio, tensile strength, and yield strength are set according to the test results. In addition, the fatigue strength index \(b=-0.05927\), fatigue strength coefficient \(\sigma_{f}’ = 1051\), fatigue ductility index \(c=-1.03023\), and fatigue ductility coefficient \(\varepsilon_{f}’ = 13.2987\) are used in the fatigue analysis.
4.3 Boundary Conditions
In the static strength simulation, appropriate constraints are applied according to the actual working conditions of the coupler. For example, the connection end of the coupler is fixed, and tensile or compressive loads are applied at the other end. In the fatigue analysis, the load spectrum is applied as the boundary condition, and the surface of the hook tongue is set as the casting surface with \(-40\mu m < Ra\leq75\mu m\) to make the fatigue calculation results more conservative and safe.
5. Simulation Results and Analysis of the Coupler
5.1 Static Strength Simulation Results
The coupler is simulated under three working conditions: stable running (600kN tensile load), starting (800kN tensile load), and emergency braking (1000kN compressive load). The stress distribution and deformation results are shown in Figure 3.
| Working Condition | Load | Maximum Stress / MPa | Maximum Deformation / mm | Stress – Concentrated Position |
|---|---|---|---|---|
| Stable running | 600kN tensile | 362.04 | 2.41 | Traction end |
| Starting | 800kN tensile | 482.72 | 3.21 | Bottom of the hook head |
| Emergency braking | 1000kN compressive | 603.4 | 4.01 | Near the connection port and the hook tongue boss |
| Table 3: Stress and deformation results of the coupler under different working conditions |
From the material mechanical test results, the average tensile limit of the coupler material after heat treatment is 1020MPa, and the elongation after fracture is 14.5%. The total length of the coupler excluding the hook head is 430mm, and the maximum allowable deformation is 62.35mm. The stresses and deformations in the normal working conditions are much smaller than the allowable values of the coupler’s yield strength and deformation, indicating that the coupler can be safely used in normal working conditions.
5.2 Fatigue Analysis Results
5.2.1 Fatigue Analysis Method
The fatigue simulation mainly studies the structural damage of the coupler under tensile – compressive cyclic stress. The strain – life (\(E – N\)) method is used in this paper. According to the fatigue theory, the relationship between stress – fatigue life is \(\frac{\Delta\sigma}{2}=\sigma_{f}'(2N_{f})^{b}\), and the relationship between plastic strain – fatigue life is \(\frac{\Delta\varepsilon_{p}}{2}=\varepsilon_{f}'(2N_{f})^{c}\). The strain – fatigue life formula is \(\frac{\Delta\varepsilon}{2}=\frac{\Delta\varepsilon_{e}}{2}+\frac{\Delta\varepsilon_{p}}{2}=\frac{1}{E}\sigma_{f}'(2N_{f})^{b}+\varepsilon_{f}'(2N_{f})^{c}\). By substituting the fatigue strength index b, fatigue strength coefficient \(\sigma_{f}’\), fatigue ductility index c, and fatigue ductility coefficient \(\varepsilon_{f}’\) of the E – grade steel coupler material into the formula, the Manson – Coffin equation of the E – grade steel is obtained: \(\frac{\Delta\varepsilon}{2}=\frac{1051}{2.1\times10^{5}}(2N_{f})^{-0.05927}+13.2987(2N_{f})^{-1.03023}\). By substituting the damage amount of a single – stress cycle into this equation, the fatigue life of the coupler can be estimated.
5.2.2 Fatigue Simulation Results
Daqin Line Load Spectrum Fatigue Simulation: By importing the measured load spectrum data of the Daqin Line into the ANSYS ncode module and using the strain – life (\(E – N\)) curve for multi – axis fatigue analysis and Goodman mean – stress correction, the fatigue cycle times of the coupler under the load spectrum are calculated. The results show that the most fatigue – prone position of the coupler appears at the circled position in Figure 4, and the minimum fatigue life is 105.2 times. The fatigue – prone positions are consistent with the stress – concentrated positions in the static stress analysis, mainly including the bottom of the hook head, the connection port, and the connection between the hook body and the hook head cavity.
6. Influence of Different Factors on the Coupler’s Performance
6.1 Influence of Material Microstructure
The microstructure of the coupler material has a significant impact on its mechanical properties. The as – cast microstructure of ZG25MnCrNiMo steel mainly consists of pearlite and ferrite, with relatively large grain size and uneven distribution. After heat treatment, the microstructure transforms into fine – grained tempered sorbite, and the carbide particles are evenly distributed on the ferrite matrix. This refined microstructure improves the strength, toughness, and fatigue resistance of the material. For example, the increase in elongation after fracture from 2.1% in the as – cast state to 14.5% after heat treatment indicates a significant improvement in the material’s ductility, which is beneficial for the coupler to withstand complex loads during operation. Table 4 shows the comparison of mechanical properties corresponding to different microstructures.
| Microstructure State | Tensile Strength / MPa | Elongation after Fracture / % | Fatigue Resistance (Qualitative) |
|---|---|---|---|
| As – cast | 550 – 675 | 1.5 – 2.7 | Poor |
| Heat – treated | 900 – 1450 | 14.5 | Good |
| Table 4: Comparison of mechanical properties corresponding to different microstructures |
6.2 Influence of Load Spectrum Characteristics
The characteristics of the load spectrum, such as load amplitude, frequency, and waveform, have a major impact on the fatigue life of the coupler. The Daqin Line load spectrum has complex load conditions due to freight transportation and complex road conditions. Although the maximum amplitude is corrected to 800kN, the load – time history still contains various load levels. In contrast, the load spectrum of Chongqing Metro Line 3 is simplified into a sine – wave – shaped load with an amplitude of 800kN, representing the typical operation process of urban rail between stations (starting acceleration, uniform running, and braking deceleration). The fatigue life calculated based on the Daqin Line load spectrum is 105.2 cycles at the most severe position, while the fatigue life calculated based on the sine – wave – shaped load of Chongqing Metro Line 3 can reach 31.5 years under an 800kN amplitude. This shows that different load spectrum characteristics can lead to significant differences in the calculated fatigue life of the coupler. Table 5 summarizes the influence of different load spectra on the coupler’s fatigue life.
| Load Spectrum | Load Characteristics | Fatigue Life (Typical Value) |
|---|---|---|
| Daqin Line | Complex load levels, large – amplitude fluctuations (corrected maximum amplitude 800kN) | 105.2 cycles at the most fatigue – prone position |
| Chongqing Metro Line 3 (sine – wave – shaped) | Amplitude 800kN, representing urban rail operation between stations | 31.5 years (800kN amplitude) |
| Table 5: Influence of different load spectra on the coupler’s fatigue life |
6.3 Influence of Geometric Structure
The geometric structure of the coupler, especially the areas with complex shapes such as the hook head, connection port, and hook tongue, is prone to stress concentration. In the static strength simulation, it is found that the bottom of the hook head and the connection between the connection port and the hook tongue boss are the positions with the maximum stress. In the fatigue analysis, these stress – concentrated positions are also the areas with the shortest fatigue life. For example, when the coupler is under tension, the non – central position of the arc – shaped stress – bearing area of the hook tongue causes uneven stress in the hook head, resulting in stress concentration at the bottom of the hook head. The complex shape at the connection between the connection port and the hook tongue boss also leads to stress concentration during compression. Optimizing the geometric structure of these areas can effectively reduce stress concentration and improve the fatigue life of the coupler. Figure 6 shows the stress – concentration areas in the coupler’s geometric structure.
7. Optimization Strategies for the Coupler
7.1 Material – Level Optimization
Based on the current material ZG25MnCrNiMo, further alloying optimization can be carried out. For example, adding trace elements such as vanadium (V) or niobium (Nb) can refine the grains and improve the strength and toughness of the material. These elements can form stable carbides during heat treatment, which can hinder the growth of grains and improve the material’s performance. Another option is to explore new materials with better fatigue resistance, such as high – strength low – alloy steels with excellent fatigue performance. Table 6 shows the potential improvements of adding trace elements to the material.
| Trace Element | Effect on Material | Impact on Coupler Performance |
|---|---|---|
| Vanadium (V) | Refines grains, improves strength and toughness | Increases fatigue life, enhances load – bearing capacity |
| Niobium (Nb) | Forms stable carbides, hinders grain growth | Improves material uniformity, reduces stress concentration |
| Table 6: Potential improvements of adding trace elements to the material |
7.2 Structural Optimization
The geometric structure of the coupler can be optimized to reduce stress concentration. For the bottom of the hook head, a more rounded transition can be designed to change the stress distribution. At the connection between the connection port and the hook tongue boss, the shape can be optimized to make the stress transfer more uniform. Finite – element analysis can be used to simulate different structural designs and evaluate their stress – reduction effects. Figure 7 shows an example of structural optimization of the hook head.
8. Maintenance and Inspection Considerations Based on Simulation Results
8.1 Determination of Maintenance Intervals
According to the fatigue life simulation results, the maintenance intervals of the coupler can be reasonably determined. For example, if the coupler is used on Chongqing Metro Line 3 and operates under normal load conditions, with a fatigue life of 31.5 years at an 800kN amplitude, a relatively long – interval maintenance plan can be formulated. However, if the train frequently experiences overloads, the actual fatigue life will be shortened. In this case, the maintenance interval needs to be adjusted accordingly. Table 7 shows the relationship between load conditions and maintenance intervals.
| Load Conditions | Fatigue Life Estimate | Suggested Maintenance Interval |
|---|---|---|
| Normal load (600kN – 800kN) | 31.5 – 740.7 years | Long – interval (e.g., 5 – 10 years) |
| Frequent overloads (above 800kN) | Significantly shortened | Short – interval (e.g., 1 – 2 years) |
| Table 7: Relationship between load conditions and maintenance intervals |
8.2 Inspection Focus Areas
The simulation results show that the stress – concentrated areas and fatigue – prone areas of the coupler are the key inspection areas. These areas mainly include the bottom of the hook head, the connection port, and the connection between the hook body and the hook head cavity. In the inspection process, non – destructive testing methods such as ultrasonic testing and magnetic particle testing can be used to detect potential cracks in these areas. Regular inspection of these key areas can help detect problems in time and prevent fatigue failures. Figure 8 shows the key inspection areas of the coupler.
9. Conclusion
In this paper, through material performance tests and finite – element simulations of investment – casting locomotive couplers, a comprehensive analysis of the working stress and fatigue life of couplers is carried out. The main conclusions are as follows:
- The microstructure of the investment – casting locomotive coupler material is uniform and fine – grained after heat treatment, and its mechanical properties meet the requirements of coupler use. The tensile strength can reach 1020MPa after heat treatment, and the elongation after fracture is 14.5%, which is beneficial for the coupler to withstand complex loads.
- Under different working conditions such as starting, stable running, and emergency braking, the stress and deformation of the coupler are far less than the allowable yield strength and deformation of the coupler, indicating that the coupler can meet the service requirements during normal operation.
- By simulating different load spectra, the fatigue life curves of the coupler are obtained. The fatigue life of the coupler under the Daqin Line load spectrum and the sine – wave – shaped load spectrum of Chongqing Metro Line 3 can meet the use requirements. The fatigue life of the coupler on Chongqing Metro Line 3 can reach 31.5 years under an 800kN amplitude, which exceeds the service life requirements of the specifications.
- The material microstructure, load spectrum characteristics, and geometric structure all have important influences on the performance of the coupler. Optimizing materials, structures, and manufacturing processes can effectively improve the fatigue life of the coupler.
- Based on the simulation results, reasonable maintenance intervals and inspection focus areas can be determined, which is of great significance for ensuring the safe operation of the coupler.
This research provides a reliable basis for the design, optimization, maintenance, and inspection of investment – casting locomotive couplers, and also provides a reference for similar research on railway components. Future research can further explore the influence of more complex factors on the coupler’s performance and develop more accurate fatigue life prediction methods.
