A method for calculating the fatigue life of a power battery pack

Abstract

The application discloses a kind of power battery pack fatigue life calculation methods, including multi-axis vibration fatigue analysis and two working conditions of torsional fatigue, the load input of two kinds of calculation conditions is from test measured data, while considering the influence of the key area of power battery pack weld and weld on fatigue life, according to the power battery pack fatigue life calculation method proposed in the application, the stress condition of the power battery pack with flexible connection structure is also considered in vibration fatigue analysis, the stress load of the power battery pack on various pavements is accurately and comprehensively considered, this method can quickly and effectively evaluate the performance of power battery pack structure, help engineering and technical personnel to predict the risk of power battery pack, so as to optimize the power battery pack, improve the performance of power battery pack.

Description

Technical fields:

[0001] This invention relates to the field of new energy vehicle technology, specifically a method for calculating the fatigue life of a power battery pack. Background technology:

[0002] The power battery pack is a core component of new energy vehicles, serving as their "heart." Its performance affects not only driving comfort and reliability but also safety. The structural mechanical properties of the power battery pack are fundamental to reliability and safety. Predicting the fatigue life of the power battery pack through simulation analysis and optimizing the pack is a crucial part of battery pack structural design. Simulation analysis can improve the structural mechanical properties of the power battery pack, thereby enhancing its performance and quality. Conventional structural mechanical analyses of battery packs include fixed-frequency, swept-frequency, and random vibration methods. These methods primarily rely on relevant standards, such as GB / T 31467 "Lithium-ion Power Battery Packs and Systems for Electric Vehicles," ISO 12405 "Electric Road Vehicles - Test Specifications for Lithium-ion Traction Battery Packs and Systems," and SAE J2380, the standard for vibration testing of electric vehicle batteries. However, these standards are not uniform. Furthermore, they only consider vibrations in the X, Y, and Z directions, neglecting complex vibrations and often deviating from actual operating conditions. Furthermore, when a vehicle travels on uneven surfaces, it experiences a certain torsional angle, which in turn causes the battery pack to twist. However, conventional analyses do not include torsional strength analysis. Failure analysis of a commercial vehicle battery pack revealed that torsional fatigue failure is one of the causes of battery pack failure. Therefore, accurately calculating the fatigue life of the battery pack is a pressing issue that needs to be addressed. Summary of the Invention:

[0003] The purpose of this invention is to provide a method for calculating the fatigue life of a power battery pack, comprising the following steps:

[0004] a. Acceleration load acquisition: Three triaxial acceleration sensors are arranged on the crossbeam near the power battery pack to acquire the acceleration load spectrum of the power battery pack on a typical road surface in the test field.

[0005] b. Acceleration load processing: Check the data rationality of the measured acceleration load spectrum, select relevant channels for filtering, and remove low-frequency and high-frequency parts that have little impact on fatigue strength;

[0006] c. Establish a vibration fatigue finite element model: Establish a finite element model of the vehicle frame section with power battery pack, use shell elements to simulate the weld points and welds in the key areas of the power battery pack, and perform constrained modal analysis to determine whether the model meets the calculation requirements so as to apply the acceleration load spectrum.

[0007] d. Calculate modal stress and modal coordinates: The acceleration load spectrum obtained in step b is applied to the finite element model established in step c. Forced acceleration excitation is applied at the acceleration measurement point. Modal stress and modal coordinates are calculated using the modal stress recovery method.

[0008] e. Transient vibration fatigue analysis: Substitute the modal stress and modal coordinate files obtained in step d into the fatigue analysis program to calculate the multiaxial vibration fatigue life of the power battery pack shell, weld points, and welds on each road surface, and sum the fatigue damage values ​​of each road surface to obtain the total fatigue damage value of all units of the power battery pack.

[0009] f. Frame torsion angle measurement: Measure the torsion angle of the vehicle frame under extreme torsion conditions;

[0010] g. Establish a torsional fatigue finite element model: Establish a finite element model of the vehicle frame assembly with the power battery pack, wherein the modeling method of the power battery pack is the same as in step c.

[0011] h. Calculate the torsional stress under unit load: Calculate the stress of the power battery pack under a unit torsional angle;

[0012] i. Torsional fatigue analysis: Substitute the torsional stress of the battery pack obtained in step h into the fatigue analysis program to calculate the torsional fatigue life of the power battery pack shell, key weld points, and key welds.

[0013] j. Result judgment and structural design optimization: Based on the fatigue damage calculation results of the power battery pack shell, key weld points and key welds in e and i, optimize the structure of the power battery pack so that the fatigue life of the power battery pack meets the requirements.

[0014] Furthermore, in step a, two triaxial acceleration sensors are arranged on the left and right sides of the front axle in front of the power battery pack, and one triaxial acceleration sensor is arranged in the middle of the crossbeam of the vehicle frame behind the power battery pack. The sampling frequency of the acceleration load spectrum is 512Hz. The three triaxial acceleration sensors can realistically reflect the actual vibration of the power battery pack itself, which is more accurate than traditional single-axis fixed frequency, sweep frequency and random vibration simulation.

[0015] Furthermore, in step a, the typical road surface includes Belgian roads and shortwave roads, but does not include twisted roads. Since twisted roads are low-frequency signals, the acceleration signal has a relatively large error compared to the acquired low-frequency signal, and the distortion phenomenon is serious. Therefore, twisted roads are not used as the object of acceleration signal acquisition.

[0016] Furthermore, in step b, the relevant channels are the left front X and Z direction channels, the right front point is the X and Z direction channel, and the right middle point is the Y and Z direction channel.

[0017] Furthermore, in step b, the acceleration load spectrum is filtered to remove frequency components in the signal from 0 to 5 Hz and above 35 Hz.

[0018] Furthermore, the condition satisfied by the model in step c is that the first-order modal frequency value of the frame truncated portion under the constraint conditions is not less than 40Hz.

[0019] Furthermore, in step c, the key areas of the weld are the four corners of the upper and lower shells of the power battery pack, and the key areas of the weld points are the connection between the longitudinal beams of the power battery pack and the lower shell.

[0020] In step c, the finite element model uses hexahedral elements to simulate the core of the power battery pack, and the contact and adhesive parts between the core of the power battery pack and the lower shell are connected by TIE bonding contact pairs.

[0021] Furthermore, in step c, the upper and lower sheet metal weld nuggets in the finite element model each consist of 12 quadrilateral shell elements. The upper and lower weld nuggets are connected by BAR beam elements, the length of which is 0.5 times the sum of the thicknesses of the welded parts. The elastic modulus of the inner weld nugget material is 8.4 × 10⁻⁶. 6 MPa, the elastic modulus of the outer welding core material is 2.1×10 MPa. 5 MPa.

[0022] Furthermore, in step c, the nodes on one side of the quadrilateral unit in the splicing welding of the power battery pack in the finite element model must be on the weld line, and the two sides of the weld line are quadrilateral units, the size of which is 2 to 4 times the material thickness.

[0023] Furthermore, in step c, the power battery pack is connected to the vehicle frame via a rubber bushing. The rubber bushing is simulated using a CBUSH unit. The stiffness of the CBUSH in six directions is consistent with the design value, and the damping parameters of the CBUSH are set according to the following formula.

[0024]

[0025] B—damping parameter;

[0026] λ — the ratio of the dynamic stiffness to the static stiffness of the rubber bushing;

[0027] k—Stiffness of CBUSH in all directions;

[0028] ω is the angular frequency, which is equal to 2πf, where f is the vibration frequency.

[0029] Furthermore, in step c, the global structural damping coefficient G of the entire finite element model is set to 0.02.

[0030] Furthermore, in step d, the finite element model is constrained in the X and Z directions at the left front point, constrained in the X and Z directions at the right front point, and constrained in the Y and Z directions at the rear midpoint.

[0031] Furthermore, in step e, the fatigue life calculation of weld points, weld seams, and structural components takes into account the survival rate, which is set to 99.9%. The higher the survival rate is set, the greater the safety margin. The fatigue life of weld points is calculated using the structural stress method.

[0032] Furthermore, in step e, the total fatigue damage value D on all cells of the power battery pack v for:

[0033] D v =C1D1+C2D2+…+C n D n

[0034] D v Total damage value on each cell of the power battery pack;

[0035] C n : The number of cycles on road surface n;

[0036] D n Fatigue damage value after one cycle on road surface n.

[0037] Furthermore, in step f, the vehicle's limit torsional angle is measured under quasi-static conditions, with the left front wheel raised by 150mm and the right rear wheel raised by 150mm. The torsional angle of the front and rear axles of the frame is measured as θ.

[0038] Furthermore, the finite element model described in step g includes the vehicle frame, front axle, power battery pack, and dummy leaf springs.

[0039] Furthermore, the operating condition for calculating the torsional fatigue life in step i is that the torsional angle of the front and rear axles of the frame is -1.25θ to 1.25θ, and the number of cycles is 150,000.

[0040] Furthermore, the fatigue life requirement for the power battery pack in step j is as follows:

[0041] D vmax ≤0.2,D Tmax ≤1.0;

[0042] D vmax —The maximum total fatigue life of each cell in the power battery pack under acceleration load excitation;

[0043] D Tmax —The maximum fatigue life of each cell in the power battery pack under torsional conditions.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] 1. The present invention relates to a method for calculating the fatigue life of a power battery pack, which includes not only testing the fatigue strength of multiaxial vibration, but also torsional fatigue strength. It accurately and comprehensively considers the stress load of the power battery pack on various road surfaces. This method can quickly and effectively evaluate the performance of the power battery pack structure, help engineers predict the risks of the power battery pack, and thus optimize the power battery pack in a targeted manner to improve its performance.

[0046] 2. The present invention relates to a method for calculating the fatigue life of a power battery pack. In the multi-axis vibration fatigue analysis, the translational motion of the power battery pack in the X, Y, and Z directions and the rotation in the X, Y, and Z spatial directions are considered. The load signals of these vibrations are obtained by measuring and converting the accelerometers placed on the actual vehicle, which truly reflects the actual vibration of the power battery pack itself. It is more accurate than traditional single-axis fixed-frequency, sweep-frequency and random vibration simulations.

[0047] 3. The present invention relates to a method for calculating the fatigue life of a power battery pack, which considers the power battery pack with flexible connection structure in vibration fatigue analysis, and proposes a modeling method for rubber bushings, thereby improving modeling accuracy and reducing simulation errors;

[0048] 4. The present invention relates to a method for calculating the fatigue life of a power battery pack. In the multiaxial vibration fatigue analysis, the method considers the weld points and welds of key parts, proposes a modeling method for welds, and defines key areas, thereby improving the modeling efficiency of engineers and improving the accuracy of simulation. Attached image description:

[0049] Figure 1 This is a flowchart of a method for calculating the fatigue life of a power battery pack according to the present invention;

[0050] Figure 2 This is a diagram showing the distribution of acceleration sensors in the multiaxial vibration fatigue analysis described in this invention.

[0051] Figure 3 This is a finite element model diagram for multiaxial vibration fatigue analysis as described in this invention;

[0052] Figure 4 This refers to the key weld seam on the upper shell of the power battery pack described in this invention;

[0053] Figure 5 This refers to the key weld seam on the lower housing of the power battery pack described in this invention;

[0054] Figure 6 This refers to the key welding point connecting the longitudinal beam and the lower housing of the power battery pack described in this invention;

[0055] Figure 7 This is a schematic diagram of the solder joint modeling described in this invention;

[0056] Figure 8 This is a schematic diagram of the modeling of the splicing weld seam described in this invention;

[0057] Figure 9 This is a schematic diagram of the finite element model for torsional fatigue analysis described in this invention. Detailed implementation method:

[0058] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0059] Example 1: As Figure 1 As shown, a method for calculating the fatigue life of a power battery pack includes the following steps:

[0060] a. Acceleration load acquisition: Three triaxial acceleration sensors, model PCB 356A26, are arranged on the crossbeam near the power battery pack to collect the acceleration load spectrum of the power battery pack on a typical road surface in the test field.

[0061] b. Acceleration load processing: Check the data rationality of the measured acceleration load spectrum, select relevant channels for filtering, and remove low-frequency and high-frequency parts that have little impact on fatigue strength;

[0062] c. Establish a vibration fatigue finite element model: Establish a finite element model of the vehicle frame section with power battery pack, use shell elements to simulate the weld points and welds in the key areas of the power battery pack, and perform constrained modal analysis to determine whether the model meets the calculation requirements so as to apply the acceleration load spectrum.

[0063] d. Calculate modal stress and modal coordinates: The acceleration load spectrum obtained by step b is applied to the finite element model established in step c. Forced acceleration excitation is applied at the acceleration measurement point, and acceleration load consistent with the experiment is applied at the test point. Modal stress and modal coordinates are calculated using the modal stress recovery method.

[0064] e. Transient vibration fatigue analysis: Substitute the modal stress file, modal coordinate file, and SN curves of sheet metal material, weld points, and welds obtained in step d into the fatigue analysis program (such as FEMFAT) to calculate the multiaxial vibration fatigue life of each road surface power battery pack shell, weld point, and weld, and sum the fatigue damage values ​​of each road surface to obtain the total vibration fatigue damage value of all units of the power battery pack.

[0065] f. Frame torsion angle measurement: Measure the torsion angle of the vehicle frame under extreme torsion conditions;

[0066] g. Establish a torsional fatigue finite element model: Establish a finite element model of the vehicle frame assembly with the power battery pack, wherein the modeling method of the power battery pack is the same as in step c.

[0067] h. Calculate the torsional stress under unit load: Calculate the stress of the power battery pack under a unit torsional angle. Simulate the working condition where the rear axle is fixed and the front axle is torsional at an angle of 1 degree around the vehicle's X-axis, and calculate the stress of the power battery pack.

[0068] i. Torsional fatigue analysis: Substitute the torsional stress, cycle number, and SN curves of the sheet metal material, weld points, and weld seams obtained in step h into the fatigue analysis program (such as FEMFAT) to calculate the torsional fatigue life of the power battery pack shell, key weld points, and key weld seams.

[0069] j. Result judgment and structural design optimization: Based on the fatigue damage calculation results of the power battery pack shell, key weld points and key welds in e and i, optimize the structure of the power battery pack so that the fatigue life of the power battery pack meets the requirements.

[0070] like Figure 2 As shown, in step a, two three-dimensional acceleration sensors are arranged on the left and right sides of the front axle in front of the power battery pack, and one three-dimensional acceleration sensor is arranged in the middle of the crossbeam of the vehicle frame behind the power battery pack. The sampling frequency of the acceleration load spectrum is 512Hz. The multi-axis vibration fatigue frequency range studied by the power battery pack is 5 to 35Hz. The sampling frequency is set to be higher than 10 times the study frequency range. The sampling frequency is higher than 350Hz. The sampling frequency is higher than 10 times the signal study frequency and will not cause obvious amplitude distortion. The higher the sampling frequency, the closer the acquired digital signal is to the real signal.

[0071] In step a, typical road surfaces include Belgian roads and shortwave roads, but not twisted roads. Since twisted roads are low-frequency signals, the acceleration signals have a large error compared to the acquired low-frequency signals, resulting in severe distortion. Therefore, twisted roads are not used as the object of acceleration signal acquisition.

[0072] In step b, the relevant channels are the left front X and Z direction channels, the right front point is the X and Z direction channel, and the right middle point is the Y and Z direction channel. The selection of the number of channels ensures that the frame cut-off in the finite element model cannot be over-constrained. The simulated frame cut-off has 6 degrees of freedom in three directions, namely translation in the X, Y, and Z directions and rotation in the X, Y, and Z directions.

[0073] In step b, the acceleration load spectrum is filtered to remove frequency components in the signal from 0 to 5 Hz and above 35 Hz. The frequency components in the 0 to 5 Hz are frequency signals, and it is inaccurate to directly input the acceleration signal for simulation. Moreover, the influence of low-frequency signals on fatigue strength is negligible, and the influence of high-frequency signals above 35 Hz on the fatigue of the power battery pack structure is also negligible.

[0074] The condition satisfied by the model in step c is that the first-order modal frequency of the frame truncated section under constraints is not less than 40Hz. The finite element model established in step c is as follows: Figure 3 As shown, the frame truncation includes acceleration measurement points A, B, and C in step a, constraining the X and Y translational degrees of freedom at positions A and B, and constraining the Y and Z degrees of freedom at position C. The calculated first-order modal frequency value of the frame truncation is 48.3Hz, which meets the requirement that the first-order modal frequency value is not less than 40Hz. That is, the frame truncation can only generate 3 translational degrees of freedom and 3 rotational degrees of freedom. Relative to the power battery pack, the frame is a rigid body.

[0075] like Figure 4 As shown in Figures 5 and 6, the critical areas of the weld in step c are the four corners of the upper and lower shells of the power battery pack, and the critical areas of the weld points are the connection between the longitudinal beams of the power battery pack and the lower shell.

[0076] In step c, the power battery pack core is simulated using hexahedral elements in the finite element model. The contact and adhesive parts between the power battery pack core and the lower shell are connected by TIE bonding, which is equivalent to a rigid connection between the contact surfaces of the power battery pack core, the lower shell, and the adhesive parts. The bonding area does not undergo relative movement or deformation.

[0077] like Figure 7 As shown, in step c, the upper and lower sheet metal weld nuggets in the finite element model each consist of 12 quadrilateral shell elements. The upper and lower weld nuggets are connected by BAR beam elements. The upper weld nugget is weld nugget 1, and the lower weld nugget is weld nugget 2. The length of the BAR beam element is 0.5 times the sum of the thicknesses of the welded parts. The elastic modulus of the inner weld nugget material is 8.4 × 10⁻⁶. 6 MPa, the elastic modulus of the outer welding core material is 2.1×10 MPa. 5 MPa.

[0078] like Figure 8As shown, in step c, the nodes on one side of the quadrilateral element in the splicing welding of the power battery pack in the finite element model must be on the weld line. The two sides of the weld line are quadrilateral elements. The size of the quadrilateral element is 2 to 4 times the material thickness. The thickness of the upper and lower shells of the power battery pack is 1 mm, and the size of the quadrilateral elements on both sides of the weld line is 4 mm.

[0079] In step c, the power battery pack is connected to the vehicle frame through a rubber bushing. The rubber bushing is simulated using a CBUSH unit. The stiffness of the CBUSH in six directions is consistent with the design value. The damping parameters of the CBUSH are set according to the following formula. The CBUSH is a connection unit, a general-purpose six-way spring-damper unit, which can define the stiffness and damping in up to six directions (three translational and three rotational).

[0080]

[0081] B—damping parameter;

[0082] λ — the ratio of dynamic stiffness to static stiffness of the rubber bushing, with a selected value of 1.025;

[0083] k—Stiffness of CBUSH in all directions;

[0084] ω is the angular frequency, which is equal to 2πf, where f is the vibration frequency.

[0085] In step c, the global structural damping coefficient G of the entire finite element model is set to 0.02.

[0086] In step d, the left front point constrains the X and Z directions of freedom, the right front point constrains the X and Z directions of freedom, and the rear midpoint constrains the Y and Z directions of freedom. This ensures that the frame stage in the finite element model is not over-constrained, and guarantees that the simulated frame has truncated the translational and rotational degrees of freedom in the X, Y, and Z directions, totaling six directions.

[0087] In step e, the fatigue life calculation of weld points, weld seams, and structural components considers the survival rate, which is set to 99.9%. The fatigue life of weld points is calculated using the structural stress method. The structural stress method provides a fine weld point mesh and high calculation accuracy, facilitating subsequent processing to determine which weld point has failed. A higher survival rate setting results in a larger safety margin. The relationship between stress and time on the battery pack can be represented by the product of modal stress and modal coordinates. The greater the stress amplitude and the more cycles, the higher the total damage value D of the battery pack. v The larger the battery pack, the more easily it is damaged.

[0088] In step e, the total fatigue damage value D on all cells of the power battery pack v for:

[0089] D v=C1D1+C2D2+…+C n D n

[0090] D v Total damage value on each cell of the power battery pack;

[0091] C n : The number of cycles on road surface n;

[0092] D n Fatigue damage value after one cycle on road surface n.

[0093] Example 1: The loss value of node number 403826 on the weld of a power battery pack after one cycle on Belgian road No. 1 is 1.6e. -5 The loss value for one cycle on shortwave radio path 2 is 2.2e. -6 Therefore, the total damage value for node number 403826 is:

[0094] D v =C1D1+C2D2;

[0095] D v Total damage value on each cell of the power battery pack;

[0096] C1: Number of cycles on Belgian road No. 1, C1 = 20440;

[0097] C2: Number of cycles on shortwave radio path 2, C2 = 4088;

[0098] D v =20440 × 1.6e -5 +4088×2.2e -6 =0.33>0.2 does not meet the requirements.

[0099] Example 2: The loss value of node number 1905295 on the weld of a power battery pack after one cycle on Belgian road No. 1 is 2.81e. -6 The loss value for one cycle on shortwave radio path No. 2 is 1.05e. -6 Therefore, the total damage value for node number 1905295 is:

[0100] D v =C1D1+C2D2;

[0101] D v Total damage value on each cell of the power battery pack;

[0102] C1: Number of cycles on Belgian road No. 1, C1 = 20440;

[0103] C2: Number of cycles on shortwave radio path 2, C2 = 4088;

[0104] D v =20440 × 2.81e -6 +4088×1.05e -6 =0.062 < 0.2, which meets the requirements.

[0105] In step f, the vehicle's limit torsional angle was measured under quasi-static conditions, with the left front wheel raised by 150mm and the right rear wheel raised by 150mm. The torsional angle of the front and rear axles of the frame was measured to be θ = 0.8°.

[0106] like Figure 9 As shown, the finite element model in step g includes the vehicle frame, front axle, power battery pack, and dummy leaf springs.

[0107] The operating conditions for calculating torsional fatigue life in step i are: the torsional angle of the front and rear axles of the frame is -1° to 1°, and the number of cycles is 150,000.

[0108] The fatigue life requirement for the power battery pack in step j is as follows:

[0109] D vmax ≤0.2,D Tmax ≤1.0;

[0110] D vmax —The maximum total fatigue life of each cell in the power battery pack under acceleration load excitation;

[0111] D Tmax —The maximum fatigue life of each cell in the power battery pack under torsional conditions.

[0112] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit and essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0113] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for calculating the fatigue life of a power battery pack, characterized in that, Includes the following steps: a. Acceleration load acquisition: Three triaxial acceleration sensors are arranged on the crossbeam near the power battery pack to acquire the acceleration load spectrum of the power battery pack on a typical road surface in the test field. b. Acceleration load processing: Check the data rationality of the measured acceleration load spectrum, select relevant channels for filtering, and remove low-frequency and high-frequency parts that have little impact on fatigue strength; c. Establish a vibration fatigue finite element model: Establish a finite element model of the vehicle frame section with power battery pack, use shell elements to simulate the weld points and welds in the key areas of the power battery pack, and perform constrained modal analysis to determine whether the model meets the calculation requirements so as to apply the acceleration load spectrum. d. Calculate modal stress and modal coordinates: The acceleration load spectrum obtained in step b is applied to the finite element model established in step c. Forced acceleration excitation is applied at the acceleration measurement point. Modal stress and modal coordinates are calculated using the modal stress recovery method. e. Transient vibration fatigue analysis: Substitute the modal stress and modal coordinate files obtained in step d into the fatigue analysis program to calculate the multiaxial vibration fatigue life of the power battery pack shell, weld points, and welds on each road surface, and sum the fatigue damage values ​​of each road surface to obtain the total fatigue damage value of all units of the power battery pack. f. Frame torsion angle measurement: Measure the torsion angle of the vehicle frame under extreme torsion conditions; g. Establish a torsional fatigue finite element model: Establish a finite element model of the vehicle frame assembly with the power battery pack, wherein the modeling method of the power battery pack is the same as in step c. h. Calculate the torsional stress under unit load: Calculate the stress of the power battery pack under a unit torsional angle; i. Torsional fatigue analysis: Substitute the torsional stress of the battery pack obtained in step h into the fatigue analysis program to calculate the torsional fatigue life of the power battery pack shell, key weld points, and key welds. j. Result judgment and structural design optimization: Based on the fatigue damage calculation results of the power battery pack shell, key weld points and key welds in e and i, optimize the structure of the power battery pack so that the fatigue life of the power battery pack meets the requirements.

2. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step a, two triaxial acceleration sensors are arranged on the left and right sides of the front axle in front of the power battery pack, and a triaxial acceleration sensor is arranged in the middle of the crossbeam of the vehicle frame behind the power battery pack. The sampling frequency of the acceleration load spectrum is 512Hz.

3. A method for calculating the fatigue life of a power battery pack according to claim 1 or 2, characterized in that, Typical road surfaces in step a include Belgian roads and shortwave roads.

4. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step b, the relevant channels are the X and Z direction channels at the left front point, the X and Z direction channels at the right front point, and the Y and Z direction channels at the rear center point.

5. A method for calculating the fatigue life of a power battery pack according to claim 1 or 4, characterized in that, In step b, the acceleration load spectrum is filtered to remove frequency components in the signal from 0 to 5 Hz and above 35 Hz.

6. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, The condition that the model satisfies in step c is that the first-order modal frequency value of the frame truncated part under the constraint is not less than 40Hz.

7. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step c, the key areas of the weld are the four corners of the upper and lower shells of the power battery pack, and the key areas of the weld points are the connection between the longitudinal beams of the power battery pack and the lower shell.

8. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step c, the finite element model uses hexahedral elements to simulate the core of the power battery pack, and the contact parts between the core of the power battery pack and the lower shell, as well as the adhesive parts, are connected by bonded contact pairs.

9. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step c, the finite element model of the weld joint consists of 12 quadrilateral shell elements for both the upper and lower sheet metal weld nuggets. The upper and lower weld nuggets are connected by BAR beam elements, with the length of each beam element being 0.5 times the sum of the thicknesses of the welded parts. The elastic modulus of the inner weld nugget material is 8.4 × 10⁻⁶. 6 MPa, the elastic modulus of the outer welding core material is 2.1×10 MPa. 5 MPa.

10. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step c, the nodes on one side of the quadrilateral element in the splicing welding of the power battery pack in the finite element model must be on the weld line, and the two sides of the weld line are quadrilateral elements. The size of the quadrilateral element is 2 to 4 times the material thickness.

11. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step c, the power battery pack is connected to the vehicle frame through a rubber bushing. The rubber bushing is simulated using a CBUSH unit. The stiffness of the CBUSH in six directions is consistent with the design value. The damping parameters of the CBUSH are set according to the following formula. B—damping parameter; λ — the ratio of the dynamic stiffness to the static stiffness of the rubber bushing; k—Stiffness of CBUSH in all directions; ω is the angular frequency, which is equal to 2πf, where f is the vibration frequency.

12. A method for calculating the fatigue life of a power battery pack according to any one of claims 1, 6, 7, 8, 9, 10, and 11, characterized in that, In step c, the global structural damping coefficient G of the entire finite element model is set to 0.

02.

13. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step d, the finite element model is constrained in the X and Z directions at the left front point, in the X and Z directions at the right front point, and in the Y and Z directions at the rear midpoint.

14. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step e, the fatigue life calculation of weld points, weld seams, and structural components takes into account the survival rate, which is set to 99.9%. The fatigue life of weld points is calculated using the structural stress method.

15. A method for calculating the fatigue life of a power battery pack according to claim 1 or 14, characterized in that, In step e, the total fatigue damage value D of all units in the power battery pack v for: D v =C1D1+C2D2+…+C n D n ; D v Total damage value on each cell of the power battery pack; C n : The number of cycles on road surface n; D n Fatigue damage value after one cycle on road surface n.

16. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, In step f, the vehicle's limit torsional angle is measured under quasi-static conditions, with the left front wheel raised by 150mm and the right rear wheel raised by 150mm. The torsional angle of the front and rear axles of the frame is measured as θ.

17. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, The finite element model described in step g includes the vehicle frame, front axle, power battery pack, and dummy leaf springs.

18. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, The operating condition for calculating torsional fatigue life in step i is that the torsional angle of the front and rear axles of the frame is -1.25θ to 1.25θ, and the number of cycles is 150,000.

19. The method for calculating the fatigue life of a power battery pack according to claim 1, characterized in that, The fatigue life requirement for the power battery pack in step j is as follows: D v max ≤0.2,D T max ≤1.0; D v max —The maximum total fatigue life of each cell in the power battery pack under acceleration load excitation; D T max —The maximum fatigue life of each cell in the power battery pack under torsional conditions.

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