A method for evaluating the single-sided hot-press curing stress of composite patch-bonded structures.

Abstract

This invention relates to a method for evaluating the curing stress of a single-sided hot-pressed composite patch-bonded structure, comprising the following steps: calculating the exothermic curing reaction Q1 of the patch resin matrix and Q2 of the adhesive resin matrix in the heat conduction equations of the patch resin system and the adhesive resin system; using the Fortran language to compile subroutines USDFLD and HETVAL to describe the heat conduction behavior and curing kinetics of the two resin matrices, and distinguishing the two materials by adding strings; solving the stress-strain equation of the composite patch-bonded structure to obtain the corresponding stiffness coefficient. Based on the characteristics of single-sided asymmetric hot-pressing molding process, this invention achieves real-time monitoring of the temperature field, degree of cure field, and residual stress field during the curing process of the composite patch-bonded structure through secondary development of the ABAQUS finite element software subroutines, comprehensively evaluating the influence of curing rate, interface slope, and fiber orientation on the curing stress and repair quality of the composite patch-bonded structure.

Description

Technical Field

[0001] This invention relates to the fields of composite material repair technology and finite element simulation technology, specifically a method for evaluating the stress of single-sided hot-press curing of composite material patching and bonding structures. Background Technology

[0002] During service and maintenance, composite materials inevitably suffer damage such as pitting, delamination, partial penetration, and even penetrating damage due to factors like motor overload, wind and sand erosion, bird strikes, and improper maintenance. Directly replacing or scrapping damaged structures is not only costly but also significantly impacts the aircraft's service life due to prolonged replacement times. Typically, patching and bonding techniques are used to repair composite structures with partial / penetrating damage, effectively restoring the original load-bearing capacity. However, due to space constraints in aircraft structural maintenance, maintenance personnel can only perform in-situ repairs of damaged composite materials using single-sided hot-pressing curing. This asymmetric heating method creates a temperature gradient along the thickness of the composite material, and the coupling of exothermic curing reactions with external heat sources results in a highly complex internal temperature field. Furthermore, patching and bonding composite structures involves three material systems: raw materials, repair materials, and adhesives. Different material properties, interface parameters, and curing environments can cause varying degrees of curing deformation and residual stress, affecting curing quality. Evaluating the stress state during the curing process of patching and bonding composite structures can effectively guide the control and optimization of repair processes, further improving the repair quality of composite structures.

[0003] Currently, the main methods for measuring residual stress in cured composite structures in engineering fall into three categories: stress release, X-ray diffraction, and component embedding. Stress release primarily achieves this by releasing residual stress through drilling / cutting, and then using strain gauges attached around the notch to measure the strain during the stress release process, followed by calculation of the residual stress value. However, this method can cause delamination or even edge splitting damage when drilling into composite materials. Furthermore, the residual stress measured through stress release only reflects the final state of the composite material after curing, making it impossible to monitor the stress state of the component during curing in real time, thus hindering the optimization of repair parameters. While X-ray diffraction has long been widely used to measure residual stress in metallic crystalline materials, its application in measuring residual stress in resin-based composites requires embedding diffraction crystal particles in the fiber preform. The distribution and diameter of these crystal particles significantly influence diffraction; therefore, the use of X-ray diffraction for measuring residual stress in resin-based composites requires further optimization. The component embedding method uses temperature measuring elements such as resistance temperature detectors (RTDs), thermocouples, thermometers, and optical fiber gratings to accurately measure the temperature at various locations of the sample component, thereby obtaining the overall temperature field of the composite material structure. However, embedding the temperature measuring elements into the original damaged structure is not only difficult to achieve, but will also inevitably affect its performance.

[0004] Therefore, in order to reduce testing losses and secondary defects introduced by the original structure, it is essential to simulate the internal temperature field during the curing process of composite material patching and bonding structures using theoretical methods, analyze the influence mechanism of factors such as interface factors, curing rate, material parameters, and fiber orientation on curing deformation, and then reduce the residual stress of the structure by modifying the actual repair process. Domestically, there is also a growing focus on monitoring technology for the curing stress of composite materials. Chinese patent CN112149322A discloses a finite element analysis method for the curing deformation of reinforced composite material structures. This method, based on the characteristics of autoclave molding, uses finite element simulation software to simulate the entire curing process of composite materials, studies and summarizes the influence of ribs on the curing deformation of curved composite material structures. However, no related patent provides a method for evaluating the comprehensive influence of factors such as two or more resin systems, bonding interface parameters, and heat source asymmetry on the curing stress of composite repair structures. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a method for evaluating the single-sided hot-pressing curing stress of composite material patch-bonded structures. The technical problem to be solved by this invention is achieved through the following technical solution:

[0006] A method for evaluating the single-sided hot-press curing stress of a composite patch-bonded structure includes the following steps:

[0007] Step 1: Calculate the heat release Q1 of the curing reaction of the patch resin matrix and the heat release Q2 of the curing reaction of the adhesive resin matrix in the heat conduction equations of the patch resin system and the adhesive resin system;

[0008] Step 2: Based on the calculation results of Step 1, use the Fortran language to compile subroutines USDFLD and HETVAL to describe the thermal conduction behavior and curing kinetics of the two resin matrices, and distinguish the two materials by adding strings;

[0009] Step 3: Solve the stress-strain equation of the composite material patchwork structure to obtain the corresponding stiffness coefficient;

[0010] Step 4: Write the stiffness coefficient calculated in Step 3 into the UMAT subroutine to describe the stress and deformation behavior of the composite patch adhesive structure during the curing process. Compile the DISP program based on the curing process curve of the adhesive resin to describe the temperature boundary conditions of the structure.

[0011] Step 5: Establish a thermal conductivity analysis model for the composite material patch-bonded structure in ABAQUS software. The material properties of the mother plate are directly assigned in ABAQUS software; the material properties of the patch and adhesive layer are called and assigned by matching the component names with the strings in Step 2.

[0012] Step 6: Conduct on-site testing of the temperature field and residual stress field of the model, and synthesize the results;

[0013] Step 7: Use a multi-objective optimization method to arrange a scientific and reasonable simulation matrix, and change the DISP heating and cooling rate, model interface slope, and fiber direction in turn. Repeat the above steps to compare the distribution and magnitude of residual stress in each simulation.

[0014] Step 8: Optimize the curing parameters by setting the residual stress below the adhesive shear strength.

[0015] The exothermic reaction Q1 of the curing reaction of the patch resin matrix and the exothermic reaction Q2 of the adhesive resin matrix are calculated using the following formulas:

[0016]

[0017]

[0018] In the above equation, V f ρ represents the volume fraction of the patch fibers. r1 ρ r2 H represents the density of the patch resin and the density of the adhesive resin, respectively. r1 H r2These represent the enthalpy of reaction of the patch resin and the enthalpy of reaction of the adhesive resin, respectively. All of the above parameters are known.

[0019] α1 and α2 represent the curing degree of the patch resin and the curing degree of the adhesive resin, respectively, and dα / dt represents the curing rate, which is calculated using the following equation:

[0020]

[0021]

[0022] In the above equation, A represents the frequency factor, ΔE is the apparent activation energy of the reaction, and R is the universal gas constant, which is usually taken as R = 8.3143.

[0023] In the third step, the overall composite material patching and bonding structure is designed as a symmetrical structure where normal stress only causes normal strain and shear stress only causes shear strain. At this time, the stress-strain relationship of the structure under external force satisfies the following formula:

[0024]

[0025] The nine independent stiffness coefficients C in formula (5) can be calculated using the following formula:

[0026]

[0027]

[0028]

[0029]

[0030] C 44 =G 12 C 55 =G 13 C 66 =G 23 .

[0031] Based on the characteristics and dimensions of the original damaged composite material structure, corresponding mother plate, adhesive layer, and patch components are established. The component names correspond to the strings in the second step, and assembly is completed. Binding constraints are set between the patch and the adhesive layer, and between the adhesive layer and the mother plate. Then, following the ABAQUS modeling process for composite laminates, each component is segmented, material properties and layup are set, reference coordinates are defined, boundary conditions and loads are set, and meshing is performed.

[0032] The stress field distribution and magnitude of the assembled structure during the curing process, as well as the load application method, must simulate the actual temperature load while constraining the stiffness displacement of the structure to ensure calculation convergence.

[0033] The calculation of temperature field and cure degree field includes the following steps:

[0034] Step 1: Set up the heat transfer (transient) analysis step according to the actual curing temperature-time curve;

[0035] Step 2: Submit the calculation task by calling the USDFLD, HETVAL, and DISP subroutines, and view the calculation results of the temperature field and curing degree field;

[0036] Step 3: Record the start and end frame numbers of the temperature field and curing degree field history and the name of the result file in the results.

[0037] The calculation of residual stress includes the following steps:

[0038] Step 1: Copy the heat conduction analysis model and rename it to the thermal stress analysis model.

[0039] Step 2: Import the results file of the heat conduction analysis model into the thermal stress analysis model as the initial temperature boundary condition.

[0040] Step 3: Change the heat conduction analysis step in the thermal stress analysis model to a static analysis step, while keeping the time setting unchanged;

[0041] Step 4: Submit the calculation task by calling the UMAT subroutine and view the calculation results of residual stress.

[0042] The beneficial effects of this invention are as follows: Based on the characteristics of single-sided asymmetric hot pressing molding process, this invention achieves real-time monitoring of the temperature field, curing degree field and residual stress field during the curing process of composite material patching and bonding structure through secondary development of the finite element software ABAQUS subroutine. It comprehensively evaluates the influence of curing rate, interface slope and fiber direction on the curing stress and repair quality of composite material patching and bonding structure. Attached Figure Description

[0043] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0044] Figure 1 This is a schematic diagram of the composite material patching and bonding structure of the present invention. Detailed Implementation

[0045] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be described more clearly and completely below with reference to the accompanying drawings in the embodiments. Of course, the described embodiments are only a part of the present invention and not all of it. Based on this embodiment, other embodiments obtained by those skilled in the art without creative effort are all within the protection scope of the present invention.

[0046] A method for evaluating the single-sided hot-press curing stress of a composite patch-bonded structure includes the following steps:

[0047] Step 1: Calculate the heat release Q1 of the curing reaction of the patch resin matrix and the heat release Q2 of the curing reaction of the adhesive resin matrix in the heat conduction equations of the patch resin system and the adhesive resin system;

[0048] Step 2: Based on the calculation results of Step 1, use the Fortran language to compile subroutines USDFLD and HETVAL to describe the thermal conduction behavior and curing kinetics of the two resin matrices, and distinguish the two materials by adding strings;

[0049] Step 3: Solve the stress-strain equation of the composite material patchwork structure to obtain the corresponding stiffness coefficient;

[0050] Step 4: Write the stiffness coefficient calculated in Step 3 into the UMAT subroutine to describe the stress and deformation behavior of the composite patch adhesive structure during the curing process. Compile the DISP program based on the curing process curve of the adhesive resin to describe the temperature boundary conditions of the structure.

[0051] Step 5: Establish a thermal conductivity analysis model of the composite material patch-bonded structure in ABAQUS software. The material properties of the base plate are directly assigned in ABAQUS software; the material properties of the patch and adhesive layer are called and assigned by matching the component names with the strings in Step 2. The material properties of the base plate include density, specific heat, thermal conductivity, coefficient of thermal expansion, and various elastic constants. Density, specific heat, and thermal conductivity are necessary parameters for the software to calculate the heat conduction process and curing rate, while the coefficient of thermal expansion and various elastic constants are necessary parameters for the software to calculate thermal strain and mechanical strain. The assignment of material properties and temperature boundary conditions is directly selected from user-defined options. The load state should be set according to the stress state of the original damaged composite material structure and the single-sided hot-pressing curing process: the stiffness and displacement of the model are limited, the lower surface of the base plate is fixedly constrained, the upper surface is allowed to have longitudinal expansion deformation and a reasonable uniformly distributed pressure is applied, and symmetrical boundary conditions are set on both sides.

[0052] Step 6: Conduct on-site testing of the temperature field, curing degree field, and residual stress field of the model, and comprehensively calculate the results;

[0053] Step 7: Use a multi-objective optimization method to arrange a scientific and reasonable simulation matrix, and change the DISP heating and cooling rate, model interface slope, and fiber direction in turn. Repeat the above steps to compare the distribution and magnitude of residual stress in each simulation. The direction of the fiber layer that is in direct contact with the patch and the adhesive layer can be adjusted to optimize the thermal stress distribution of the adhesive layer.

[0054] Step 8: Optimize the curing parameters by setting the residual stress below the adhesive shear strength. These parameters include curing rate, interface slope, and fiber orientation. The influence of curing rate, interface slope, and fiber orientation on the curing quality of the composite patchwork structure is evaluated by assessing the magnitude and uniformity of the residual stress field at the adhesive interface. A larger or less uniform residual stress field will reduce the subsequent load-bearing capacity of the bonded structure. However, completely eliminating residual stress is impossible. Therefore, we continuously optimize the process to ensure that the residual stress does not exceed the shear strength, thus preventing premature failure of the bonded structure.

[0055] The exothermic reaction Q1 of the curing reaction of the patch resin matrix and the exothermic reaction Q2 of the adhesive resin matrix are calculated using the following formulas:

[0056]

[0057]

[0058] In the above equation, V f ρ represents the volume fraction of the patch fibers. r1 ρ r2 H represents the density of the patch resin and the density of the adhesive resin, respectively. r1 H r2 These represent the enthalpy of reaction of the patch resin and the enthalpy of reaction of the adhesive resin, respectively. All of the above parameters are known.

[0059] α1 and α2 represent the curing degree of the patch resin and the curing degree of the adhesive resin, respectively, and dα / dt represents the curing rate, which is calculated using the following equation:

[0060]

[0061]

[0062] In the above equation, A represents the frequency factor, ΔE is the apparent activation energy of the reaction, and R is the universal gas constant, usually taken as R = 8.3143. Taking the logarithm of both sides of the equation, it can be seen that ln(dα / dt) and T -1 Both showed a linear relationship. The kinetic parameters A and ΔE could be obtained by taking values ​​from the differential scanning calorimetry (DSC) curves and performing linear regression fitting. Then, based on the characteristics of the two resins, f1(α) and f2(α) that conform to their respective autocatalytic reaction laws were determined, and the reaction order values ​​were obtained by fitting the DSC data. Based on the calculation results of formulas 1 to 4, the Fortran language compilers USDFLD and HETVAL were used to describe the thermal conduction behavior and curing kinetics of the two resin matrices.

[0063] In the third step, the overall composite material patching and bonding structure is designed as a symmetrical structure where normal stress only causes normal strain and shear stress only causes shear strain to avoid coupling effects and curing deformation. At this time, the stress-strain relationship of the structure under external force satisfies the following formula:

[0064]

[0065] The nine independent stiffness coefficients C in formula (5) can be calculated using the following formula:

[0066]

[0067]

[0068]

[0069]

[0070] C 44 =G 12 C 55 =G 13 C 66 =G 23 ;

[0071] Where v ij E ij G ij (i, j = 1, 2, 3, 4, 5, 6) represent the Poisson's ratio, elastic modulus, and shear modulus of the unidirectional fiber-reinforced resin matrix composite material in this direction, respectively. During the entire curing simulation process, it is divided into three stages (liquid stage, rubber stage, and glass stage), and each stage is assigned a fixed constant.

[0072] Based on the characteristics and dimensions of the original damaged composite material structure, corresponding master plates, adhesive layers, and patch components are established. The component names correspond to the strings in the second step, and are then arranged according to... Figure 1 The assembly is completed, with binding constraints set between the patch and the adhesive layer, and between the adhesive layer and the mother plate. Then, following the ABAQUS modeling process for composite laminates, each component is segmented, material properties and layup are set, reference coordinates are defined, boundary conditions and loads are set, and meshing is performed. The above settings are the constraint process that ABAQUS software calculations must have. Segmentation and setting material properties and layup are the methods of this invention for assigning cross-sectional properties to adhesive composite material structures.

[0073] The stress field distribution and magnitude of the assembled structure during the curing process, as well as the load application method, must simulate the actual temperature load while constraining the stiffness displacement of the structure to ensure calculation convergence.

[0074] like Figure 1As shown: Master plate is 1, adhesive layer is 2, patch is 3, and additional layer is 4.

[0075] Figure 1 The motherboard shown is a solid model with a total thickness of 10mm and a single-layer thickness of 0.125mm. The damaged 5mm and the intact 5mm are created as set1 and set2 respectively. The ply of set1 is [0 / 90 / -45 / 0 / 45]8 and the ply of set2 is [45 / 0 / -45 / 90 / 0 / ]8. The adhesive layer is a solid model with a cohesive cross-section. The patch is a solid model with a thickness of 5mm, a single-layer thickness of 0.125mm, and a ply of [0 / 90 / -45 / 0 / 45][0 / 90 / -45 / 0 / 45]7.

[0076] The calculation of temperature field and cure degree field includes the following steps:

[0077] Step 1: Set up the heat transfer analysis step according to the actual curing temperature-time curve;

[0078] Step 2: Submit the calculation task by calling the USDFLD, HETVAL, and DISP subroutines, and view the calculation results of the temperature field and curing degree field; the curing degree field is calculated through the HETVAL subroutine.

[0079] Step 3: Record the start and end frame numbers and temperature field distribution results for the temperature field and cure degree field histories. Residual stress calculation includes the following steps:

[0080] Step 1: Copy the heat conduction analysis model and rename it to the thermal stress analysis model.

[0081] Step 2: Import the results file of the heat conduction analysis model into the thermal stress analysis model as the initial temperature boundary condition.

[0082] Step 3: Change the heat conduction analysis step in the thermal stress analysis model to a static analysis step, while keeping the time setting unchanged;

[0083] Step 4: Submit the calculation task using the UMAT subroutine and view the calculation results of residual stress. The temperature field affects the curing field, which in turn directly affects the fiber volume fraction. The fiber volume fraction is directly related to the elastic constant of the material, and the elastic constant determines the distribution of the structural stress field.

[0084] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely prisms of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claims. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for evaluating the single-sided hot-pressing curing stress of a composite material patch-bonded structure, characterized in that: Includes the following steps: first step: Calculate the exothermic reaction Q1 of the curing reaction of the patch resin matrix and the exothermic reaction Q2 of the curing reaction of the adhesive resin matrix in the heat conduction equations of the patch resin system and the adhesive resin system; Step 2: Based on the calculation results of Step 1, use the Fortran language to compile subroutines USDFLD and HETVAL to describe the thermal conduction behavior and curing kinetics of the two resin matrices, and distinguish the two materials by adding strings; Step 3: Solve the stress-strain equation of the composite material patchwork structure to obtain the corresponding stiffness coefficient; Step 4: Write the stiffness coefficient calculated in Step 3 into the UMAT subroutine to describe the stress and deformation behavior of the composite patch adhesive structure during the curing process. Compile the DISP program based on the curing process curve of the adhesive resin to describe the temperature boundary conditions of the structure. Step 5: Establish a thermal conductivity analysis model for the composite material patch-bonded structure in ABAQUS software. The material properties of the mother plate are directly assigned in ABAQUS software; the material properties of the patch and adhesive layer are called and assigned by matching the component names with the strings in Step 2. Step 6: Conduct on-site testing of the temperature field and residual stress field of the model, and synthesize the results; Step 7: Use a multi-objective optimization method to arrange a scientific and reasonable simulation matrix, and change the DISP heating and cooling rate, model interface slope, and fiber direction in turn. Repeat the above steps to compare the distribution and magnitude of residual stress in each simulation. Step 8: Optimize the curing parameters by setting the residual stress below the adhesive shear strength.

2. The method for evaluating the single-sided hot-press curing stress of a composite material patch-bonded structure according to claim 1, characterized in that: The exothermic reaction Q1 of the curing reaction of the patch resin matrix and the exothermic reaction Q2 of the adhesive resin matrix are calculated using the following formulas: ; ； In the above equation, V f Represents the volume fraction of the patch fibers. ρ r1 ρ r2 These represent the density of the patch resin and the density of the adhesive resin, respectively. H r1 , H r2 These represent the enthalpy of reaction of the patch resin and the enthalpy of reaction of the adhesive resin, respectively. All of the above parameters are known. α 1 , α 2 These represent the curing degree of the patch resin and the curing degree of the adhesive resin, respectively. dα / dt Represents the curing rate, and is calculated using the following equation: ； ； In the above equation A Represents frequency factor, ∆E The apparent activation energy, R As the universal gas constant, take R =8.3143.

3. The method for evaluating the single-sided hot-press curing stress of a composite material patch-bonded structure according to claim 1, characterized in that: In the third step, the overall composite material patching and bonding structure is designed as a symmetrical structure where normal stress only causes normal strain and shear stress only causes shear strain. At this time, the stress-strain relationship of the structure under external force satisfies the following formula: （5）。 4. The method for evaluating the single-sided hot-press curing stress of a composite material patch-bonded structure according to claim 3, characterized in that: The nine independent stiffness coefficients C in formula (5) can be calculated using the following formula: ； ； ； ； 。 5. The method for evaluating the single-sided hot-press curing stress of a composite material patch-bonded structure according to claim 1, characterized in that: Based on the characteristics and dimensions of the original damaged composite material structure, corresponding mother plate, adhesive layer, and patch components are established. The component names correspond to the strings in the second step, and assembly is completed. Binding constraints are set between the patch and the adhesive layer, and between the adhesive layer and the mother plate. Then, following the ABAQUS modeling process for composite laminates, each component is segmented, material properties and layup are set, reference coordinates are defined, boundary conditions and loads are set, and meshing is performed.

6. The method for evaluating the single-sided hot-press curing stress of a composite material patch-bonded structure according to claim 5, characterized in that: The stress field distribution and magnitude of the assembled structure during the curing process, as well as the load application method, must simulate the actual temperature load while constraining the stiffness displacement of the structure to ensure calculation convergence.

7. The method for evaluating the single-sided hot-press curing stress of a composite material patch-bonded structure according to claim 1, characterized in that: The calculation of temperature field and cure degree field includes the following steps: Step 1: Set up the transient thermal conduction analysis step according to the actual curing temperature-time curve; Step 2: Submit the calculation task by calling the USDFLD, HETVAL, and DISP subroutines, and view the calculation results of the temperature field and curing degree field; Step 3: Record the start and end frame numbers of the temperature field and curing degree field history, as well as the file name of the temperature field distribution results in the results.

8. The method for evaluating the single-sided hot-press curing stress of a composite material patch-bonded structure according to claim 1, characterized in that: The calculation of residual stress includes the following steps: Step 1: Copy the heat conduction analysis model and rename it to thermal stress analysis model; Step 2: Import the temperature field distribution result file of the heat conduction analysis model into the thermal stress analysis model as the initial temperature boundary condition; Step 3: Change the heat conduction analysis step in the thermal stress analysis model to a thermo-coupling analysis step, while keeping the time setting unchanged; Step 4: Submit the calculation task by calling the UMAT subroutine and view the calculation results of residual stress.

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