Fatigue life forecasting method and system for dual-modulus sandwich composite material structure and electronic equipment

Abstract

The invention discloses a method and a system for forecasting the fatigue life of a dual-modulus sandwich composite material structure and electronic equipment. The structure fatigue life forecasting method comprises the following steps: establishing a finite element model for a sandwich composite material structure; applying material attributes under the condition of tension or compression to the model unit, performing static calculation to obtain an initial stress field, endowing corresponding tension or compression elasticity modulus according to positive and negative axial stress of each integral point in the initial stress field, updating the material attributes, performing static analysis again, and performing iterative calculation to obtain a final stress field; iterative calculation is repeated until convergence conditions are met, and then a dual-modulus structure stress strain field considering tension and compression elasticity modulus differences is obtained; and performing fatigue damage analysis on the fiber reinforced composite material laminated plate, the foam core material and the interlayer interface in a dual-modulus structure stress strain field. According to the life forecasting method provided by the invention, the dual-modulus characteristic of the material is considered, and the fatigue life of the dual-modulus sandwich composite material structure can be analyzed more accurately.

Description

Technical Field

[0001] This invention relates to the field of engineering structure calculation technology, and in particular to a method, system and electronic equipment for predicting the fatigue life of a dual-modulus sandwich composite material structure. Background Technology

[0002] Sandwich composite materials typically use high-strength glass fiber or carbon fiber for the face panels and lower-strength polyethylene or polyvinyl chloride foam for the core. These materials exhibit different tensile and compressive strains under the same tensile or compressive loads, and are thus called bimodal materials. The difference between the tensile modulus and compressive modulus of glass fiber can even exceed six times. Existing research has shown that bimodal characteristics significantly affect the mechanical response of engineering structures, leading to substantial differences in the location and magnitude of maximum stress. When the tensile modulus is greater than the compressive modulus, the maximum tensile stress calculated using the same modulus theory will be smaller than the actual value. Conversely, when the tensile modulus is less than the compressive modulus, the maximum compressive stress calculated using the same modulus theory will be smaller than the actual value. When the difference between tensile and compressive moduli is significant, the error between the single-modulus calculation results and the bimodulus calculation results can even exceed 100%. Because commercial finite element analysis software does not have the function of handling bimodulus problems, current calculations of sandwich composite structures usually consider the face panel as an orthotropic material with equal tension and compression, and the foam core as a homogeneous isotropic material, without considering the bimodulus characteristics of the materials.

[0003] Especially for ship structures, such as Figure 1 The diagram shows a schematic of a sandwich composite ship structure. Composite ship structures typically consist of outer hull panels and various forms of internal cap-shaped stiffeners, working together to resist external bending moments. Under the influence of total longitudinal bending moments and local loads, the hull structure is in a state of partial tension and partial compression for extended periods. The bimodal characteristics of the material significantly affect the calculated stress, strain, and displacement responses of the structure. For complex marine sandwich composite structures, it is necessary to conduct research on numerical calculation methods that consider the bimodal characteristics of the material, aiming to obtain accurate structural mechanical responses.

[0004] Currently, there are relatively mature direct calculation and fatigue assessment guidelines for metal ships. However, the strength assessment of composite material hull structures is still limited to small boats with larger safety margins, and there is a lack of complete and mature methods for assessing the strength and fatigue strength of composite material ship structures. Current research focuses mainly on the strength and fatigue assessment of laminated structures under localized working conditions, and it still does not consider the fatigue failure process of the core material or the coupling effect between failure modes of different component materials, making it difficult to develop an effective method for assessing the fatigue performance of sandwich composite material hull structures.

[0005] In summary, with the widespread application of sandwich composite structures in the engineering field, there is an urgent need for a method to predict the fatigue performance of sandwich composite structures that can take into account the dual modulus characteristics of materials, comprehensively consider the fatigue problems of sandwich composites under short-term high stress levels and long-term low stress levels, and clarify the failure mechanism of sandwich composite structures. Summary of the Invention

[0006] The main objective of this invention is to provide a method, system, and electronic device for predicting the fatigue life of dual-modulus sandwich composite structures. This method takes into account the dual-modulus characteristics of the material and can more accurately analyze the fatigue life of dual-modulus sandwich composite structures.

[0007] To achieve the above objectives, this invention proposes a method for predicting the fatigue life of a dual-modulus sandwich composite material structure, characterized by comprising the following steps: A finite element model was established for the sandwich composite material structure; Based on the finite element model, material properties under tension or compression are applied to the model elements and static calculations are performed to obtain the initial stress field. According to the sign of the axial stress at each integration point in the initial stress field, the corresponding tensile or compressive elastic modulus is assigned to the corresponding direction. After updating the material properties, static analysis is performed again for iterative calculation. The iterative calculation is repeated until the preset convergence condition is met, and then the stress-strain field of the dual-modulus structure considering the difference between the tensile and compressive elastic moduli is obtained. Based on the stress-strain field of the aforementioned bimodulus structure, fatigue damage analysis is performed on fiber-reinforced composite laminates, considering the material's bimodulus characteristics and laminate in-situ effect, and incorporating cumulative damage theory, based on the fatigue master curve method and failure criteria. Fatigue damage analysis is also performed on foam core materials, based on bimodulus iteration and referencing multiaxial fatigue assessment theory for ductile materials and linear cumulative damage theory. Finally, fatigue damage analysis is performed on interlaminar interfaces, considering the fatigue crack initiation stage, based on the cohesive zone model method, stress-life curve method, and crack propagation method.

[0008] Preferably, in the step of assigning corresponding tensile or compressive elastic modulus to the corresponding direction based on the sign of the axial stress at each integration point in the initial stress field, updating the material properties, and then performing static analysis and iterative calculation again, three field variable parameters are defined to correspond to the elastic modulus of the three principal axes of the material. When the axial stress is positive, the field variable parameter in the corresponding direction is 1; when the axial stress is negative, the field variable in the corresponding direction is 0. After replacing the material properties of each element, the next iterative calculation is performed.

[0009] Preferably, when the iterative calculation is repeated until the preset convergence condition is met, the stress components in the three principal axis directions corresponding to each element are extracted again, the field variable values ​​corresponding to each element are reassigned, and compared with the first...n The iteration relative to the first n-1 The convergence condition is that the total number of units whose field variables change during iteration is less than 1% of the total number of units in the overall structure. n The calculation result of the next iteration is the result when the tensile and compressive moduli are not equal.

[0010] Preferably, the specific steps for fatigue damage analysis of the fiber-reinforced composite laminate include: Based on the stress-strain field of the dual-modulus structure, each finite element mesh element of the fiber-reinforced composite laminate in the finite element model is given a tensile elastic modulus, loads and boundaries are applied, and the stress and strain at the integration points are initially calculated. Keeping constraints and loads constant, update the material properties of each element based on the preliminary calculated strain results, and update the stress and strain results. Perform multiple iterative calculations until the preset convergence conditions are met, and then complete the bimodal strength calculation of the structure. Keeping constraints and loads constant, read the stress in each direction at the integration point after iterative convergence, calculate the multiaxial ratio based on the stress results, and calculate the equivalent stress value and equivalent fatigue strength. Determine the multiaxial fatigue master curve based on the calculated equivalent fatigue strength, solve the fatigue life of each integration point under the stress state using the interval bisection method iterative method, calculate the damage degree and determine the failure using the cumulative damage theory, and reduce the stiffness of the failed element.

[0011] Preferably, when using the cumulative damage theory to calculate damage and determine failure, and when reducing the stiffness of the failed unit, it is assumed that each sub-step corresponds to n fatigue cycles. The damage degree of each sub-step and the cumulative damage degree of the entire fatigue calculation process are calculated. The unit is then determined to fail and its stiffness is reduced. When the cumulative damage degree of the unit is greater than or equal to 1, the elastic parameters of the unit material are degraded to a preset multiple of the original value.

[0012] Preferably, when performing fatigue damage analysis on the foam core material, based on bimodulus iteration and referencing the multiaxial fatigue assessment theory of plastic materials, the tensile, compressive, or shear failure of the foam core material is considered. The first principal stress, the third principal stress, and the Tresca stress are used as benchmarks, and the fatigue life is evaluated in conjunction with the linear cumulative damage theory. When performing fatigue damage analysis on the interlayer interface, the stress-life curve method and crack propagation method are combined, and the fatigue crack initiation stage of the interface is considered to form an interface fatigue life prediction method for sandwich composite material structures. Corresponding subroutines are written to realize the proceduralization of the interface fatigue life prediction method.

[0013] Preferably, the specific steps for fatigue damage analysis of the foam core material include: Based on the stress-strain field of the dual-modulus structure, each finite element mesh element of the foam core material in the finite element model is given a tensile elastic modulus, loads and boundaries are applied, and the stress and strain at the integration points are initially calculated. Keeping constraints and loads constant, update the material properties of each element based on the preliminary calculated strain results, and update the stress and strain results. Perform multiple iterative calculations until the preset convergence conditions are met, and then complete the bimodal strength calculation of the structure. After iterative convergence, the first principal stress, third principal stress, and Tresca stress of each integral point of the foam core material are read. The first principal stress, third principal stress, and Tresca stress are substituted into the corresponding stress-life curve to obtain the fatigue life value of each integral point, thereby calculating the final fatigue life. The linear cumulative damage theory is used to calculate the damage degree and determine the failure, and the performance of the failed element is reduced.

[0014] Preferably, the specific steps for performing fatigue damage analysis on the interlaminar interface include: Define the cohesive bilinear constitutive parameters, define the time-varying linear load, and calculate the cohesive bilinear constitutive matrix of the component. The structural response of the cohesive interface under the condition is analyzed, and the damage initiation displacement, final failure displacement and cohesive model damage parameters under the mixed mode are calculated. The cohesive model damage parameters are judged, the failure element deletion is updated, and it is determined whether the structure has entered the fatigue analysis process. Keeping the maximum load constant, each substep is defined as In the next cycle, combining the stress-life curve method and the crack propagation method, considering the fatigue crack initiation stage at the interface, we conduct fatigue life prediction of the cohesive interface.

[0015] This invention further proposes a system for implementing the above-mentioned method for predicting the fatigue life of dual-modulus sandwich composite structures, comprising: The modeling module is used to create finite element models of sandwich composite material structures. The dual-modulus static analysis module is used to apply material properties under tension or compression to the model unit based on the finite element model and perform static calculations to obtain the initial stress field. According to the sign of the axial stress at each integration point in the initial stress field, the corresponding tensile or compressive elastic modulus is assigned to the corresponding direction. After updating the material properties, the static analysis is performed again for iterative calculation. The iterative calculation is repeated until the preset convergence condition is met, and then the stress-strain field of the dual-modulus structure considering the difference between the tensile and compressive elastic moduli is obtained. The coupled fatigue analysis module is used to perform fatigue damage analysis on fiber-reinforced composite laminates based on the stress-strain field of the dual-modulus structure, using the fatigue master curve method and failure criteria, considering the dual-modulus characteristics of the material and the in-situ effect of the laminate, and combining the cumulative damage theory. Based on dual-modulus iteration, and referring to the multiaxial fatigue assessment theory of plastic materials and the linear cumulative damage theory, fatigue damage analysis is performed on foam core materials. Based on the cohesive zone model method, combined with the stress-life curve method and crack propagation method, fatigue crack initiation stage of the interface is considered to perform fatigue damage analysis on the interlaminar interface.

[0016] The present invention also proposes an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor, when executing the program, implements the above-mentioned fatigue life prediction method for dual-modulus sandwich composite material structures.

[0017] The fatigue life prediction method for dual-modulus sandwich composite material structures proposed in this invention has the following beneficial effects: 1. Forecast accuracy has been significantly improved, eliminating errors in single-modulus calculations. To address the stress calculation errors caused by neglecting the dual-modulus characteristics in existing technologies, this invention dynamically updates the tensile / compressive elastic modulus of the unit according to the positive and negative triaxial stress, and combines iterative convergence criteria to ensure that the structural mechanical response calculation closely matches the actual performance of the material. Targeted models such as the Hashin failure criterion and Tresca stress assessment are used for the fiber laminate and foam core material, respectively. The interlaminar interface is simulated through the cohesive zone model (CZM) method and the UMAT subroutine to simulate the entire crack initiation-propagation process, thereby improving the accuracy of fatigue life prediction from multiple dimensions. 2. Failure modes are clarified, covering coupling effects of all components. Breaking through the limitations of existing technologies that only focus on laminates and ignore core material and interface failures, this invention establishes fatigue assessment modules for fiber laminates, foam core materials, and interlaminar interfaces respectively; combining cumulative damage theory and stiffness reduction mechanism, it clarifies the failure sequence and coupling relationship of each component, and solves the problem that traditional methods cannot distinguish core failure paths such as "panel failure, core material damage, and interface peeling". 3. Expanded application scope, adaptable to complex engineering structures Overcoming the limitations of traditional theoretical methods that are only applicable to simple structures and loads, this invention is based on ABAQUS subroutines to achieve programmatic implementation, supporting long-term fatigue assessment of complex marine sandwich composite materials (including complex configurations such as outer wall panels and cap-shaped ribs) under total longitudinal bending moment and local loads; it takes into account both short-term high stress and long-term low stress conditions, meeting the needs of small structures such as small boats, and is also applicable to large composite material ships, filling the gap in existing ship structure fatigue assessment standards. 4. Improved computational efficiency and reliability, reducing reliance on experiments. To address the issues of poor convergence and low efficiency in existing numerical methods, this invention ensures stable convergence through a clear iterative process and a failure element performance reduction strategy. Using a framework of "theoretical analysis-numerical simulation-experimental verification," it reduces reliance on numerous fatigue tests by experimentally fitting key parameters such as stress-life curve coefficients, significantly saving experimental costs and R&D cycle time. Attached Figure Description

[0018] Figure 1 This is a structural diagram of a sandwich composite ship structure based on existing technology; Figure 2 This is a flowchart illustrating the fatigue life prediction method for dual-modulus sandwich composite material structures according to the present invention. Figure 3 This is a detailed flowchart of step S20 in the fatigue life prediction method for dual-modulus sandwich composite material structures of the present invention. Figure 4 This is a schematic diagram of the process for fatigue damage analysis of fiber-reinforced composite laminates in the fatigue life prediction method for dual-modulus sandwich composite structures of the present invention. Figure 5 This is a schematic diagram of the process for fatigue damage analysis of foam core material in the fatigue life prediction method for dual-modulus sandwich composite material structures of the present invention. Figure 6 This is a schematic diagram of the process for fatigue damage analysis of interlayer interfaces in the fatigue life prediction method for dual-modulus sandwich composite material structures of the present invention. Figure 7a This is a schematic diagram of the structure of the connection node when the fatigue life prediction method of the dual-modulus sandwich composite material structure of the present invention is applied to the real-scale "L-shaped" connection node; Figure 7b for Figure 7a A schematic diagram of the cross-sectional structure along the AA direction; Figure 8 This is a schematic diagram of the fatigue life PN curve of a real-scale "L-shaped" connection node according to an embodiment of the present invention; Figure 9 This is a schematic diagram of the finite element model of a real-scale "L-shaped" connection node according to an embodiment of the present invention; Figure 10a This is a schematic diagram of the finite element model of the fiber panel in the tensile state in the real-scale "L-shaped" connection node of this invention. Figure 10b This is a schematic diagram of the finite element model of the foam core material in the tensile state in a real-scale "L-shaped" connection node according to an embodiment of the present invention. Figure 11a This is a schematic diagram of the finite element model of the fiber panel in the "L-shaped" connection node under compression in a real-scale embodiment of the present invention; Figure 11b This is a schematic diagram of the finite element model of the foam core material in the "L-shaped" connection node under compression in a real-scale embodiment of the present invention. Figure 12 This is a schematic diagram of the fatigue life prediction results of a real-scale "L-shaped" connection node according to an embodiment of the present invention. Figure 13 This is a schematic diagram of the predicted fatigue life of a real-scale "L-shaped" connection node without considering the bimodulus, according to an embodiment of the present invention. Figure 14a This is a comparison chart of fatigue life prediction results for a real-scale "L-shaped" connection node in an embodiment of the present invention when core material failure begins to occur. Figure 14b This is a comparison chart of fatigue life prediction results for a real-scale "L-shaped" connection node in an embodiment of the present invention when adhesive layer failure begins. Figure 14c This is a comparison chart of fatigue life prediction results for a real-scale "L-shaped" connection node in an embodiment of the present invention when fiber failure begins to occur. Figure 14d This is a comparison chart of fatigue life prediction results for a real-scale "L-shaped" connection node at final failure according to an embodiment of the present invention. Figure 15a This is a schematic diagram of the finite element model of a real-scale "L-shaped" connection node according to an embodiment of the present invention after the core material fails and 100,000 cycles. Figure 15b This is a schematic diagram of the finite element model of a real-scale "L-shaped" connection node according to an embodiment of the present invention after the initial adhesive layer failure and 120,000 cycles. Figure 15c This is a schematic diagram of the finite element model of a real-scale "L-shaped" connection node in an embodiment of the present invention after fiber failure and 180,000 cycles. Figure 15d This is a schematic diagram of the finite element model of a real-scale "L-shaped" connection node in an embodiment of the present invention after final failure and 332,000 cycles. Figure 16 This is a physical diagram of a scaled-down "L-shaped" connection node according to an embodiment of the present invention.

[0019] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0020] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0021] This invention proposes a method for predicting the fatigue life of a dual-modulus sandwich composite material structure.

[0022] Reference Figure 2In this preferred embodiment, a method for predicting the fatigue life of a dual-modulus sandwich composite material structure includes the following steps: Step S10: Establish a finite element model for the sandwich composite material structure; Step S20: Based on the finite element model, apply material properties under tension or compression to the model unit and perform static calculation to obtain the initial stress field. According to the sign of the axial stress at each integration point in the initial stress field, assign the corresponding tensile or compressive elastic modulus to the corresponding direction. After updating the material properties, perform static analysis again for iterative calculation. Repeat the iterative calculation until the preset convergence condition is met, and obtain the stress-strain field of the dual-modulus structure considering the difference between tensile and compressive elastic moduli. Step S30: Based on the stress-strain field of the bimodal structure, and based on the fatigue master curve method and failure criteria, considering the bimodal characteristics of the material and the in-situ effect of the laminate, and combined with the cumulative damage theory, fatigue damage analysis is performed on the fiber-reinforced composite laminate. Based on bimodal iteration, and referring to the multiaxial fatigue assessment theory of plastic materials and the linear cumulative damage theory, fatigue damage analysis is performed on the foam core material. Based on the cohesive zone model method, combined with the stress-life curve method and crack propagation method, considering the fatigue crack initiation stage of the interface, fatigue damage analysis is performed on the interlaminar interface.

[0023] Specifically, in step S20, according to the sign of the axial stress at each integration point in the initial stress field, the corresponding tensile or compressive elastic modulus is assigned to the corresponding direction. After updating the material properties, the static analysis is re-performed for iterative calculation. Three field variable parameters are defined to correspond to the elastic modulus of the three principal axes of the material. When the axial stress is positive, the field variable parameter in the corresponding direction is 1. When the axial stress is negative, the field variable in the corresponding direction is 0. After replacing the material properties of each element, the next iterative calculation is performed.

[0024] Repeat the iterative calculation until the preset convergence condition is met. Then, extract the stress components in the three principal axis directions corresponding to each element, reassign the field variable values ​​to each element, and compare them with the first... n The iteration relative to the first n-1 The convergence condition is when the total number of units whose field variables change is less than 1% of the total number of units in the overall structure. The calculation result of the nth iteration is the calculation result considering the unequal tensile and compressive moduli.

[0025] Specifically, refer to Figure 3 Step S20 specifically includes the following steps.

[0026] Step S201: Calculate the material properties of each element under tension or compression at the initial stage, without considering its bimodulus problem. Step S202: Perform static calculations on the structure under its initial state, and extract the axial stresses in three directions at each integration point. ,in Represents the unit number, Representing the iteration The values ​​are 11, 22, and 33, representing the triaxial stresses of the element. Step S203, using the three field variable parameters respectively , and The elastic modulus of the material in three directions. Corresponding tensile modulus of elasticity Corresponding compressive modulus; Step S204: Determine the sign of the axial stress in the three directions. If the axial stress is positive, the field variable parameter in the corresponding direction is set to 1. If the axial stress is negative, the field variable in the corresponding direction is set to 0. Replace the material properties of each element and proceed to the next calculation. Step S205: After the calculation is completed, extract the stress components in the three directions corresponding to each element again, reassign the field variable values ​​to each element, and compare them with the previous step. The iteration relative to the first The number of units whose field variables change during iteration; when the total number of units whose field variables change is less than the total number of units in the overall structure. The calculation is considered to be convergent when the value is 1%, and the calculation result of the i-th iteration is the calculation result considering the unequal tensile and compressive moduli.

[0027] Specifically, in step S30, the specific steps for fatigue damage analysis of the fiber-reinforced composite laminate include: Step S31: Based on the stress-strain field of the dual-modulus structure, assign a tensile elastic modulus to each unit, apply loads and boundaries, and preliminarily calculate the stress and strain at the integration point; Step S32: Keeping the constraints and loads unchanged, update the material properties of each element based on the preliminary calculated strain results, and update the stress and strain results. Perform multiple iterative calculations until the preset convergence conditions are met, and then complete the structural bimodal strength calculation. Step S33: Keeping the constraints and loads unchanged, read the stress in each direction at the integration point after the iteration convergence, calculate the multiaxial ratio based on the stress results, and calculate the equivalent stress value and equivalent fatigue strength. Determine the multiaxial fatigue master curve based on the calculated equivalent fatigue strength, solve the fatigue life of each integration point under the stress state using the interval bisection method iterative method, calculate the damage degree and failure judgment using the cumulative damage theory, and reduce the stiffness of the failure element.

[0028] Fatigue damage analysis of fiber-reinforced composite laminates was performed using the USDFLD fatigue subroutine for fiber materials.

[0029] When using the cumulative damage theory to calculate damage and determine failure, and when reducing the stiffness of the failed element, it is assumed that each substep corresponds to n fatigue cycles. The damage degree of each substep and the cumulative damage degree of the entire fatigue calculation process are calculated. The element is then determined to fail and its stiffness is reduced. When the cumulative damage degree of the element is greater than or equal to 1, the elastic parameters of the element material are degraded to the original preset multiple.

[0030] Specifically, refer to Figure 4 The specific process for fatigue damage analysis of fiber-reinforced composite laminates is as follows.

[0031] (1) Preliminary calculation: Assign tensile elastic modulus to each finite element mesh element of the fiber-reinforced composite laminate in the finite element model, apply loads and boundaries, and complete the structural calculation; (2) Tension-compression inequality iteration: Keep the constraints and loads unchanged. Based on the element stress calculation results in step 1, determine the element state, assign tensile material properties to tensile elements and compressive material properties to compressive elements, update the element stress and strain results after replacement, and perform multiple iterations to complete the bimodal strength calculation of the structure; (3) Fatigue life prediction: 1) Keeping the constraints and loads constant, read the stress in each direction at the integration point after the second iteration convergence. Calculate the multiaxial ratio. , This is the first non-zero stress component. For the in-plane stress components of a single-layer plate, if ,but ; 2) Calculate the equivalent stress value according to equation (1) The equivalent fatigue strength is calculated according to equation (2). : (1) (2) In the formula, The stress component matrix of a single-layer plate. ; for The transpose of the matrix, To calculate the first non-zero stress component, The Hashin failure criterion under static load conditions; .

[0032] 3) Determine the multiaxial fatigue master curve according to equation (3): (3) In the formula, For fatigue life, n 1 n 2. All are curve fitting coefficients.

[0033] 4) Use the interval bisection method iteratively to solve for the fatigue life value at each integration point under this stress state. ; 5) Assuming each sub-step corresponds to n fatigue cycles, calculate the damage degree of each sub-step. Cumulative damage degree throughout the fatigue calculation process The element is judged for failure and stiffness is reduced by combining the Shokrieh-Hashin failure criterion and the Tserpes criterion. When the cumulative damage of the element is greater than or equal to 1, the elastic parameter of the element material is degraded to 0.01 times the original value (to avoid non-convergence of calculation).

[0034] Specifically, when performing fatigue damage analysis on the foam core material, based on bimodulus iteration and referencing the multiaxial fatigue assessment theory of plastic materials, the tensile, compressive, or shear failure of the foam core material is considered. The first principal stress, third principal stress, and Tresca stress are used as benchmarks, and the fatigue life is evaluated using linear cumulative damage theory. The USDFLD fatigue subroutine for foam core material is used for fatigue damage analysis. When performing fatigue damage analysis on the interlaminar interface, the stress-life curve method and crack propagation method are combined, considering the fatigue crack initiation stage at the interface, to form an interface fatigue life prediction method for sandwich composite structures. Corresponding subroutines are written to program the interface fatigue life prediction method. The UMAT fatigue subroutine for cohesive elements is used for fatigue damage analysis of the interlaminar interface.

[0035] The specific steps for fatigue damage analysis of foam core materials include: Step S301: Based on the stress-strain field of the dual-modulus structure, assign a tensile elastic modulus to each finite element mesh element of the foam core material in the finite element model, apply loads and boundaries, and preliminarily calculate the stress and strain at the integration points. Step S302: Keeping the constraints and loads unchanged, update the material properties of each element based on the preliminary calculated strain results, and update the stress and strain results. Perform multiple iterative calculations until the preset convergence conditions are met, and then complete the structural bimodal strength calculation. Step S303: Read the first principal stress, third principal stress and Tresca stress of each integration point of the foam core material after iterative convergence. Substitute the first principal stress, third principal stress and Tresca stress into the corresponding stress life curve to obtain the fatigue life value of each integration point, and then calculate the final fatigue life. Use the linear cumulative damage theory to calculate the damage degree and determine the failure, and reduce the performance of the failed unit.

[0036] Reference Figure 5 The specific process for fatigue damage analysis of foam core materials is as follows.

[0037] Its process is similar to the fatigue life prediction method of dual-modulus fiber reinforced composite laminate. It is assumed that the fatigue load of each sub-step is cycled n times. Combined with Miner's linear cumulative damage theory, after each sub-step, the stress components of the core material element are calculated. The fatigue life of the element under this stress state is calculated using Equations (4) and (5), and the corresponding damage degree is calculated. As the number of cycles increases, the element is judged to fail when the cumulative damage degree reaches 1, and the performance of the failed element is reduced.

[0038] (4) (5) In the formula , , The fatigue life of the foam core material is obtained based on the first principal stress, the third principal stress, and the Tresca stress prediction, respectively. , , These are the first principal stress, the third principal stress, and the Tresca stress of the foam core material, respectively. For the fatigue life of the core material; , These are the normalized stress-life curve coefficients, obtained through fatigue testing.

[0039] Specifically, the steps for fatigue damage analysis of interlaminar interfaces include: Step S3001: Define the cohesive bilinear constitutive parameters, define the time-varying linear load, and calculate the cohesive bilinear constitutive matrix of the component. Structural response of the cohesive interface under the given conditions The time increment is used to calculate the damage initiation displacement, final failure displacement, and cohesive model damage parameters under the mixed mode. The cohesive model damage parameters are judged, the failure element deletion is updated, and it is determined whether the structure has entered the fatigue analysis process. Step S3002, keeping the maximum load constant, each substep is defined as follows: In the next cycle, combining the stress-life curve method and the crack propagation method, considering the fatigue crack initiation stage at the interface, we conduct fatigue life prediction of the cohesive interface.

[0040] Reference Figure 6 The specific steps for fatigue damage analysis of interlayer interfaces are as follows.

[0041] The structural load application is carried out in two steps: (1) First step: 1) In the initial state t=0, Ds=0, Ds is..., define the cohesive bilinear constitutive parameters, and define the time-varying linear load; 2) According to equation (6), the bilinear constitutive matrix of the cohesion of the component ; (6) In the formula, For interface normal separation stress; These are two tangential separation stresses within the interface plane; The displacement is the interface normal, in mm. These represent two tangential separation displacements within the interface plane, in mm. For interface normal separation strain; These are the two tangential separation strains within the interface plane; These are the interface normal stiffness and the two tangential stiffnesses in the plane, respectively. These are the interface normal modulus and two in-plane tangential moduli, respectively.

[0042] 3) Calculation Structural responses such as stress, strain, and displacement at the lower cohesive interface; 4) Calculate the damage initiation displacement under the hybrid mode according to equations (7) and (8). and final failure displacement : (7) (8) In the formula For mixed mode ratio, . Displacement at the moment when interface damage begins ( ); Displacement when the interface eventually fails ( ); The maximum displacement that the interface can achieve during the loading process ( ); This represents the equivalent crack initiation displacement under hybrid mode. This represents the equivalent final failure displacement in the hybrid mode; This refers to the tangential composite stress generated in the hybrid mode; These represent the failure fracture energies of the cohesive model under a single mode in the normal and two tangential directions, respectively. This represents the total tangential displacement. This represents the total tangential fracture energy.

[0043] 5) Calculate the cohesive model damage parameters under the hybrid mode according to equation (9). , : (9) In the formula, For interface failure displacement in hybrid mode; This represents the equivalent crack initiation displacement under hybrid mode. This represents the equivalent final failure displacement in the hybrid mode; This represents the maximum calculated displacement in the hybrid mode.

[0044] 6) Determine the damage parameters of the cohesive model, update the deletion of failed elements, and determine whether the structure has entered the fatigue analysis process.

[0045] (2) Second step: Keeping the maximum load constant, each substep is defined as In the next iteration, the cohesive interface fatigue life prediction is performed. The UMAT materials subroutine is an implicit analysis program based on the Newton-Raphson algorithm. It iteratively solves for the unknowns in the next increment step, requiring the definition of the material's stress increment-strain increment matrix (Jacobi matrix). And update the stiffness matrix (Equation (11)), calculate the stress increment based on the strain increment, thereby achieving the force balance of the structural system. For the nonlinear problem caused by the softening of cohesive units, based on the bilinear cohesive force theory model, Turon proposed the corresponding Jacobian matrix as follows: (10) In the formula, ;function ; For Kronecker notation, when hour ,when hour .

[0046] (11) The following study uses a real-scale "L-shaped" connection node as the research object to illustrate the fatigue life prediction method of this dual-modulus sandwich composite material structure.

[0047] The geometric dimensions of the "L-shaped" connection node are as follows: Figure 7a and Figure 7b As shown, fatigue tests were conducted on the node at load levels of 20%, 25%, 28%, 30%, and 40% of the node's ultimate load. Furthermore, to further compare the effects of different load ratios on the fatigue life of the "L-shaped" node, fatigue tests were performed for load ratios of 0.1 (tension-tension) and 10 (compression-compression). The fatigue life results are shown in Table 1, and the fatigue life PN curves are shown in... Figure 8 As shown, the horizontal axis represents fatigue life, and the vertical axis represents the maximum fatigue load. Excluding specimens that did not fail after 1 million cycles, the fatigue test data at the load ratio were fitted to obtain the fatigue life PN curve corresponding to a 50% survival rate, as shown below. Figure 8 As shown by the solid black line in the middle. Since there are relatively few fatigue specimens corresponding to load ratios of 0.1 and 10, a method similar to... The same curve slope for and The experimental data for the two scenarios were fitted, as shown below. Figure 8 The red and blue dashed lines are shown in the figure. The PN curve function corresponding to a survival rate of 50% under the three load ratios is shown in Equation (12).

[0048] (12) In the formula, For fatigue life; This represents the maximum fatigue load.

[0049] Table 1. Test fatigue life results (unit: 10,000 cycles)

[0050] Based on the method proposed in this invention, and using the finite element software ABAQUS, a finite element model for fatigue testing of an "L-shaped" node is established as follows: Figure 9 As shown in the finite element model, the fiber-reinforced composite material is made by bonding layers of fiber cloth together, with the interlayer adhesive layer forming the bonding part: (1) The fiber-reinforced composite material was simulated using the continuous shell element SC8R, and the fiber material USDFLD fatigue subroutine was used to analyze and predict the damage, failure and life during the fatigue load application process. (2) The foam core material is simulated using the three-dimensional solid element C3D8 element, and the "dual-modulus foam core material fatigue life prediction method" established in the detailed technical solution is used to analyze and predict the damage failure and life of the foam core material during the fatigue load application process using the USDFLD fatigue subroutine. (3) Cohesive elements (COH3D8) are established at the interface between the cap profile and the sheet metal. Interface elements (Cohesive elements) are created between each fiber layer at the weak points observed in the experiment to simulate interlaminar delamination damage of the structure. The above interlaminar adhesive elements are based on the "interlaminar fatigue life prediction method" established in the detailed technical solution. The cohesive element UMAT fatigue subroutine is used to analyze and predict the damage failure and life during the fatigue load application process.

[0051] In the numerical analysis, the loading fixture, constraint fixture, and bolts were considered as isotropic materials, while the fiber panel and PVC foam core were considered as bimodal anisotropic materials. The material properties are shown in Tables 2 to 4. The fatigue master curve parameters for the fiber panel were determined with reference to Table 5, and the fatigue life stress-life curve for the foam core was taken as... The fatigue stress-life curve test data of the interlayer interface were obtained by fitting according to equation (13), where , .

[0052] Table 2 Material Properties of GFRP Single-Layer Sheets

[0053] Table 3 Properties of P100 Foam Material

[0054] Table 4 Interface Material Properties

[0055] Table 5. Master Curve Parameters for Glass Fiber / Epoxy Resin Single-Layer Boards

[0056] (13) In the formula, This represents the equivalent fatigue life coefficient under hybrid mode. .

[0057] 1. Dual Modulus Determination like Figures 10a to 11b As shown, where, Figure 10a and Figure 10b The node is in a tension state. Figure 10a and Figure 10bThe nodes are in tension, with red representing the tension region and blue representing the compression region. The calculation results show that the tension and compression regions of the "L-shaped" node are approximately opposite when under tension and compression. When subjected to tensile load, the corner region is mainly under tension, and when subjected to compressive load, the corner region is mainly under compression. The fatigue life prediction method for sandwich composite structures proposed in this paper can effectively consider the influence of the material's bimodulus characteristics.

[0058] 2. Fatigue life and failure mode prediction Table 6 shows the number of load cycles corresponding to each failure stage of the "L-shaped" node under different load conditions. By comparison, it is found that the logarithmic error between the fatigue life predicted by finite element method and the experimental mean is basically within 10%, which shows that the prediction accuracy is high.

[0059] Table 6. Fatigue life prediction results for “L-shaped” nodes (unit: 10,000 cycles)

[0060] Note: The logarithmic error in the last column of the table represents the error between the logarithmic value of the final failure life predicted by the finite element method and the logarithmic value of the average life measured in the experiment. .

[0061] Figure 12 The figure shows the fatigue life prediction results and experimental fatigue life curves. The comparison reveals that the predicted fatigue life of the "L-shaped" node under different loads and load ratios is generally within a 3-fold error band. Particularly for tensile and compressive fatigue specimens, the predicted life is closer to the experimental life. This paper's fatigue life assessment method for foam sandwich composite structures, considering the material's dual modulus characteristics, can accurately predict the fatigue life of foam sandwich structures. Furthermore, the prediction method considers the progressive failure process during load cycling, comprehensively taking into account the coupling effects of various failure modes such as interlaminar delamination, debonding, fiber failure, and core material failure, thus predicting the entire failure process of the sandwich composite structure.

[0062] Table 7 shows the number of load cycles for various types of damage at nodes when the material's bimodality is not considered. When the load ratio is -1, the fatigue life prediction results are similar. However, when the load ratios are 0.1 and 10, there are significant differences between the predicted and experimental results. Figure 13 The fatigue life prediction results also show that a large number of data points are outside the three-fold error band, resulting in poor prediction accuracy. Therefore, for sandwich composite material structures with bimodal characteristics, the influence of the material's bimodal characteristics needs to be considered when predicting fatigue life.

[0063] Table 7 Fatigue life prediction results without considering bimodulus (unit: 10,000 cycles)

[0064] For the load ratio of -1 Figures 14a to 14d The figure shows a comparison of fatigue life predictions considering and not considering the material's bimodulus characteristics. Since the load used in the fatigue life prediction is tensile, under tensile load, the tensile stress near the elbow corner is greater when considering the material's bimodulus than when not considering it. Therefore, considering the bimodulus leads to earlier core material failure and adhesive layer failure. Figure 14a and Figure 14b As shown. However, due to the different failure range of the core material, fiber failure will occur later when considering bimodulus.

[0065] The fatigue failure process of the "L-shaped" node is as follows: Figures 15a to 15d As shown, the failure mode of the "L-shaped" node predicted by the finite element model is... Figure 16 The final failure modes in the actual tests were consistent, which shows that the fatigue life prediction method proposed in this paper can not only accurately predict the fatigue life of foam sandwich composite structures, but also accurately predict their fatigue failure process and final failure mode.

[0066] The fatigue life prediction method for dual-modulus sandwich composite material structures proposed in this invention has the following beneficial effects: 1. Forecast accuracy has been significantly improved, eliminating errors in single-modulus calculations. To address the stress calculation errors caused by neglecting the dual-modulus characteristics in existing technologies, this invention dynamically updates the tensile / compressive elastic modulus of the unit according to the positive and negative triaxial stress, and combines iterative convergence criteria to ensure that the structural mechanical response calculation closely matches the actual performance of the material. Targeted models such as the Hashin failure criterion and Tresca stress assessment are used for the fiber laminate and foam core material, respectively. The interlaminar interface is simulated through the cohesive zone model (CZM) method and the UMAT subroutine to simulate the entire crack initiation-propagation process, thereby improving the accuracy of fatigue life prediction from multiple dimensions. 2. Failure modes are clarified, covering coupling effects of all components. Breaking through the limitations of existing technologies that only focus on laminates and ignore core material and interface failures, this invention establishes fatigue assessment modules for fiber laminates, foam core materials, and interlaminar interfaces respectively; combining cumulative damage theory and stiffness reduction mechanism, it clarifies the failure sequence and coupling relationship of each component, and solves the problem that traditional methods cannot distinguish core failure paths such as "panel failure, core material damage, and interface peeling". 3. Expanded application scope, adaptable to complex engineering structures Overcoming the limitations of traditional theoretical methods that are only applicable to simple structures and loads, this invention is based on ABAQUS subroutines to achieve programmatic implementation, supporting long-term fatigue assessment of complex marine sandwich composite materials (including complex configurations such as outer wall panels and cap-shaped ribs) under total longitudinal bending moment and local loads; it takes into account both short-term high stress and long-term low stress conditions, meeting the needs of small structures such as small boats, and is also applicable to large composite material ships, filling the gap in existing ship structure fatigue assessment standards. 4. Improved computational efficiency and reliability, reducing reliance on experiments. To address the issues of poor convergence and low efficiency in existing numerical methods, this invention ensures stable convergence through a clear iterative process and a failure element performance reduction strategy. Using a framework of "theoretical analysis-numerical simulation-experimental verification," it reduces reliance on numerous fatigue tests by experimentally fitting key parameters such as stress-life curve coefficients, significantly saving experimental costs and R&D cycle time.

[0067] This invention also proposes a system for predicting the fatigue life of dual-modulus sandwich composite material structures.

[0068] The present invention proposes a system for implementing the above-mentioned method for predicting the fatigue life of dual-modulus sandwich composite material structures, comprising: The modeling module is used to create finite element models of sandwich composite material structures. The dual-modulus static analysis module is used to apply material properties under tension or compression to the model unit based on the finite element model and perform static calculations to obtain the initial stress field. According to the sign of the axial stress at each integration point in the initial stress field, the corresponding tensile or compressive elastic modulus is assigned to the corresponding direction. After updating the material properties, the static analysis is performed again for iterative calculation. The iterative calculation is repeated until the preset convergence condition is met, and then the stress-strain field of the dual-modulus structure considering the difference between the tensile and compressive elastic moduli is obtained. The coupled fatigue analysis module is used to perform fatigue damage analysis on fiber-reinforced composite laminate, foam core material and interlaminar interface based on the stress-strain field of the dual-modulus structure, and to couple the damage evolution of each part to predict the fatigue life and failure mode of the overall structure.

[0069] The present invention also proposes an electronic device.

[0070] In this preferred embodiment, an electronic device includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, it implements the above-described method for predicting the fatigue life of a dual-modulus sandwich composite material structure.

Claims

1. A method for predicting the fatigue life of a dual-modulus sandwich composite material structure, characterized in that, Includes the following steps: A finite element model was established for the sandwich composite material structure; Based on the finite element model, material properties under tension or compression are applied to the model elements and static calculations are performed to obtain the initial stress field. According to the sign of the axial stress at each integration point in the initial stress field, the corresponding tensile or compressive elastic modulus is assigned to the corresponding direction. After updating the material properties, static analysis is performed again for iterative calculation. The iterative calculation is repeated until the preset convergence condition is met, and then the stress-strain field of the dual-modulus structure considering the difference between the tensile and compressive elastic moduli is obtained. Based on the stress-strain field of the aforementioned bimodulus structure, fatigue damage analysis is performed on fiber-reinforced composite laminates, considering the material's bimodulus characteristics and laminate in-situ effect, and incorporating cumulative damage theory, based on the fatigue master curve method and failure criteria. Fatigue damage analysis is also performed on foam core materials, based on bimodulus iteration and referencing multiaxial fatigue assessment theory for ductile materials and linear cumulative damage theory. Finally, fatigue damage analysis is performed on interlaminar interfaces, considering the fatigue crack initiation stage, based on the cohesive zone model method, stress-life curve method, and crack propagation method.

2. The fatigue life prediction method for dual-modulus sandwich composite material structures as described in claim 1, characterized in that, In the step of assigning corresponding tensile or compressive elastic modulus to the corresponding direction based on the sign of the axial stress at each integration point in the initial stress field, updating the material properties, and then performing static analysis and iterative calculation again, three field variable parameters are defined to correspond to the elastic modulus of the three principal axes of the material. When the axial stress is positive, the field variable parameter in the corresponding direction is 1, and when the axial stress is negative, the field variable in the corresponding direction is 0. After replacing the material properties of each element, the next iterative calculation is performed.

3. The fatigue life prediction method for dual-modulus sandwich composite material structures as described in claim 1, characterized in that, Repeat the iterative calculation until the preset convergence condition is met. Then, extract the stress components in the three principal axis directions corresponding to each element, reassign the field variable values ​​to each element, and compare them with the first... n The iteration relative to the first n-1 The convergence condition is that the total number of units whose field variables change during iteration is less than 1% of the total number of units in the overall structure. n The calculation result of the next iteration is the result when the tensile and compressive moduli are not equal.

4. The fatigue life prediction method for a dual-modulus sandwich composite material structure as described in claim 1, characterized in that, The specific steps for fatigue damage analysis of fiber-reinforced composite laminates include: Based on the stress-strain field of the dual-modulus structure, each finite element mesh element of the fiber-reinforced composite laminate in the finite element model is given a tensile elastic modulus, loads and boundaries are applied, and the stress and strain at the integration points are initially calculated. Keeping constraints and loads constant, update the material properties of each element based on the preliminary calculated strain results, and update the stress and strain results. Perform multiple iterative calculations until the preset convergence conditions are met, and then complete the bimodal strength calculation of the structure. Keeping constraints and loads constant, read the stress in each direction at the integration point after iterative convergence, calculate the multiaxial ratio based on the stress results, and calculate the equivalent stress value and equivalent fatigue strength. Determine the multiaxial fatigue master curve based on the calculated equivalent fatigue strength, solve the fatigue life of each integration point under the stress state using the interval bisection method iterative method, calculate the damage degree and determine the failure using the cumulative damage theory, and reduce the stiffness of the failed element.

5. The fatigue life prediction method for a dual-modulus sandwich composite material structure as described in claim 4, characterized in that, When using the cumulative damage theory to calculate damage and determine failure, and to reduce the stiffness of the failed unit, it is assumed that each sub-step corresponds to n fatigue cycles. The damage degree of each sub-step and the cumulative damage degree of the entire fatigue calculation process are calculated. The unit is then determined to fail and its stiffness is reduced. When the cumulative damage degree of the unit is greater than or equal to 1, the elastic parameters of the unit material are degraded to a preset multiple of the original value.

6. The fatigue life prediction method for a dual-modulus sandwich composite material structure as described in claim 1, characterized in that, When performing fatigue damage analysis on the foam core material, based on bimodulus iteration and referring to the multiaxial fatigue assessment theory of plastic materials, the tensile, compressive or shear failure of the foam core material is considered. The first principal stress, the third principal stress and the Tresca stress are used as benchmarks, and the fatigue life is evaluated in combination with the linear cumulative damage theory. When performing fatigue damage analysis on interlayer interfaces, the stress-life curve method and crack propagation method are combined to consider the fatigue crack initiation stage of the interface, forming an interface fatigue life prediction method for sandwich composite structures. Corresponding subroutines are written to realize the programming of the interface fatigue life prediction method.

7. The fatigue life prediction method for dual-modulus sandwich composite material structures as described in claim 1, characterized in that, The specific steps for fatigue damage analysis of the foam core material include: Based on the stress-strain field of the dual-modulus structure, each finite element mesh element of the foam core material in the finite element model is given a tensile elastic modulus, loads and boundaries are applied, and the stress and strain at the integration points are initially calculated. Keeping constraints and loads constant, update the material properties of each element based on the preliminary calculated strain results, and update the stress and strain results. Perform multiple iterative calculations until the preset convergence conditions are met, and then complete the bimodal strength calculation of the structure. After iterative convergence, the first principal stress, third principal stress, and Tresca stress of each integral point of the foam core material are read. The first principal stress, third principal stress, and Tresca stress are substituted into the corresponding stress-life curve to obtain the fatigue life value of each integral point, thereby calculating the final fatigue life. The linear cumulative damage theory is used to calculate the damage degree and determine the failure, and the performance of the failed element is reduced.

8. The fatigue life prediction method for a dual-modulus sandwich composite material structure as described in claim 1, characterized in that, The specific steps for fatigue damage analysis of the interlaminar interface include: Define the cohesive bilinear constitutive parameters, define the time-varying linear load, and calculate the cohesive bilinear constitutive matrix of the component. The structural response of the cohesive interface under the condition is analyzed, and the damage initiation displacement, final failure displacement and cohesive model damage parameters under the mixed mode are calculated. The cohesive model damage parameters are judged, the failure element deletion is updated, and it is determined whether the structure has entered the fatigue analysis process. Keeping the maximum load constant, each substep is defined as In the next cycle, combining the stress-life curve method and the crack propagation method, considering the fatigue crack initiation stage at the interface, we conduct fatigue life prediction of the cohesive interface.

9. A system for implementing the fatigue life prediction method for a dual-modulus sandwich composite structure as described in any one of claims 1 to 8, characterized in that, include: The modeling module is used to create finite element models of sandwich composite material structures. The dual-modulus static analysis module is used to apply material properties under tension or compression to the model unit based on the finite element model and perform static calculations to obtain the initial stress field. According to the sign of the axial stress at each integration point in the initial stress field, the corresponding tensile or compressive elastic modulus is assigned to the corresponding direction. After updating the material properties, the static analysis is performed again for iterative calculation. The iterative calculation is repeated until the preset convergence condition is met, and then the stress-strain field of the dual-modulus structure considering the difference between the tensile and compressive elastic moduli is obtained. The coupled fatigue analysis module is used to perform fatigue damage analysis on fiber-reinforced composite laminates based on the stress-strain field of the dual-modulus structure, using the fatigue master curve method and failure criteria, considering the dual-modulus characteristics of the material and the in-situ effect of the laminate, and combining the cumulative damage theory. Based on dual-modulus iteration, and referring to the multiaxial fatigue assessment theory of plastic materials and the linear cumulative damage theory, fatigue damage analysis is performed on foam core materials. Based on the cohesive zone model method, combined with the stress-life curve method and crack propagation method, fatigue crack initiation stage of the interface is considered to perform fatigue damage analysis on the interlaminar interface.

10. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the fatigue life prediction method for dual-modulus sandwich composite material structures as described in any one of claims 1 to 8.

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