Method for finely evaluating strength of small-size large or full-opening CFRP (carbon fiber reinforced plastic) shell

Abstract

The invention relates to the field of carbon fiber composite material mechanics, in particular to a fine evaluation method for the strength of a small-size large or full-opening CFRP (carbon fiber reinforced plastic) shell, which comprises the following steps: establishing a microscopic damage model containing material anisotropy, defining damage variables and evolution equations, uniformly quantifying multi-scale damage behaviors such as fiber fracture, matrix cracking and layered expansion, and evaluating the strength of the small-size large or full-opening CFRP shell. The problem that a traditional single-scale model cannot represent the multiphase coupling effect is solved; through cross-scale transmission of damage variables, whole-process prediction from microscopic damage initiation to macroscopic failure is realized, and the precision of strength evaluation of the complex opening structure is significantly improved. The method realizes refinement, reliability and engineering practicability of strength evaluation of the small-size large-opening or full-opening CFRP shell, is particularly suitable for the fields of aerospace and the like with extremely high requirements on structural weight and safety, and has remarkable technical leading and application value.

Description

Technical Field

[0001] The present invention relates to the field of carbon fiber composite material mechanics, and in particular to a method for finely evaluating the strength of small-sized, large or fully open CFRP shells. Background Art

[0002] Carbon fiber reinforced composite (CFRP) shells have become the preferred material for key components such as aircraft engine nacelles and satellite brackets due to their excellent specific strength and designability. However, carbon fiber composite laminates with notches will experience a decrease in mechanical properties under different working conditions or loads. Holes will reduce the bearing area of the material, resulting in reduced tensile strength, compressive strength and shear strength. When subjected to tension, stress concentration is likely to occur near the opening. When the external force reaches a certain level, cracks will begin to appear from the edge of the hole, causing the material to be damaged. Fatigue performance deteriorates. Under the action of cyclic loads, stress concentration around the opening will make cracks more likely to occur and expand. Since carbon fiber composite laminates are subjected to repeated forces in many application scenarios, this fatigue problem may pose a safety hazard. When affected by external forces or environmental factors, separation and delamination are likely to occur between layers, thus affecting the overall performance of the material.

[0003] The failure modes and damage evolution of CFRP with holes and notches are extremely complex, and strength assessment methods still face challenges in the following areas: 1. Fiber fracture (microscale), matrix cracking (mesoscale), and delamination extension (macroscale) occur simultaneously at the edges of openings under load, making it difficult to accurately characterize with traditional single-scale models; 2. Temperature cycling can exacerbate interfacial debonding between the resin matrix and the fibers, making it environmentally sensitive; 3. Conventional detection methods such as industrial CT have insufficient spatial resolution to capture the submicron-level crack initiation process in the opening area, limiting damage monitoring. Although Chinese patent CN111209704A proposes a strength testing method, it does not establish a quantitative mapping relationship between microscopic damage parameters and macroscopic strength, resulting in the need to rely on a high safety factor in engineering applications, causing material waste. Summary of the Invention

[0004] The purpose of the present invention is to provide a refined strength assessment method for small-sized, large, or fully open CFRP shells. This assessment method combines a microscopic damage model of small-sized, large, or fully open CFRP shells with the establishment of a shell strength calculation formula, etc., which can evaluate the safety and reliability of its structure in advance, provide theoretical guidance for design optimization, and effectively reduce the cost and time of actual experiments. It is of great significance to promote the widespread application of CFRP in the military and automotive fields.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: A refined strength assessment method for a small-sized, large- or fully-opened CFRP shell comprises the following steps: S1. Establish a microscopic damage model that takes into account material anisotropy, ply layup method, geometry, opening size, boundary effects, and interface interactions. By defining damage variables and damage evolution equations, construct a shell strength calculation formula that considers multiphase coupling effects. S2. Use multimodal in-situ experimental technology to obtain three-dimensional distribution data of hole edge damage, combine the strain field and crack propagation path analysis under dynamic load, and establish a damage evolution database; S3. Combining finite element analysis with multi-scale simulation methods, simulate the microscopic damage evolution process of a small-sized, large / full-opening CFRP shell under typical loads, and perform parameter calibration on the strength calculation formula established in step S1; S4. Conduct environmental-mechanical coupling tests to obtain the interface strength degradation law under the combined action of temperature gradient cycling and fatigue loading, and establish an environmental factor correction model; S5. Design temperature-fatigue combined test and establish empirical formula for environmental factor (temperature) and interface strength degradation; S6. The revised strength calculation formula is verified through damage tests, and after iterative parameter optimization, a refined strength assessment method system for small-sized and large / full-opening CFRP shells is formed.

[0006] Furthermore, the construction of the microscopic damage model includes geometric modeling and material parameter setting, as follows: Material system: T800SC carbon fiber / 5228 epoxy resin; Single layer mechanical parameters: E1 = 210 GPa, E2 = 8.7 GPa, G 12 =4.5GPa,ν 12 =0.28, X T =3200MPa, Y T =45MPa,S L =75MPa, G IIc =0.82kJ / m 2 ; Among them, E1: longitudinal elastic modulus, E2: transverse elastic modulus, G 12 : in-plane shear modulus, ν 12 : Poisson's ratio, X T : Longitudinal tensile strength, Y T : Transverse tensile strength, S L : In-plane shear strength, G IIc : Type II critical strain energy release rate. Shell layer modeling: A parametric model was established with an opening edge chamfer radius of 0.5 mm and a layer sequence of [0 / 90 / ±45 / 90 / ±45]. Interface characteristics: The fiber-matrix interface strength τ = 62 MPa was obtained through nanoindentation testing.

[0007] Furthermore, the definition of damage variables and the establishment of coupling equations include the following: Multi-scale damage characterization: Fiber breakage (microscopic): Among them, d f : Fiber fracture damage variable, used to measure the degree of fiber fracture, ε1: strain in the fiber direction. Matrix cracking (mesoscopic): Among them, d m : matrix cracking damage variable, measuring the degree of matrix cracking, σ2: normal stress perpendicular to the fiber direction, Y T : Transverse tensile strength of the matrix, τ 12 : In-plane shear stress, S L : In-plane shear strength. Hierarchical expansion (macro): Among them, d d : Layered expansion damage variable, measuring the degree of layered expansion, G II : Type II strain energy release rate, G IIc : Type II critical strain energy release rate, δ: displacement, δ c : critical displacement, δ c =0.02mm. Coupling strength equation: σ eff =σ0·(1-d f )·[1-min(1,d m +0.2d d )]·f(T,N), where σ eff : effective stress, σ0: initial stress, d f : fiber fracture damage variable, d m : matrix cracking damage variable, d d : Delamination expansion damage variable, f(T,N): Environmental factor, considering the influence of temperature T and number of cycles N on material properties.

[0008] Furthermore, the damage evolution database includes: performing a dynamic damage observation experiment: A tensile-compressive cyclic load (frequency 1 Hz) with an amplitude of ±250 MPa was applied to the specimens on an MTS-810 testing machine. High-resolution μCT (0.5 μm) was used simultaneously: scanning was performed every 10 cycles to reconstruct the hole edge damage in three dimensions; Digital image spectroscopy (DIC): collects surface strain fields and calculates crack growth rates; Generate a damage database.

[0009] Furthermore, by extracting the fiber fracture density, matrix crack length and delamination area from the CT image, a mapping relationship between damage variables and the number of load cycles was established for model calibration; a finite element model was established and the model was calibrated.

[0010] Furthermore, a macro shell model was constructed in ABAQUS software, and the hole edges were encrypted using adaptive meshes (minimum size 0.05 mm).

[0011] Furthermore, when the parameters were iteratively optimized, the load-displacement curves of the simulation and the experiment were compared, and the initial error was 18.5%. After three rounds of iterations, the interface strength τ and fracture toughness G were corrected. IIc , the final error ≤ 3% (after correction τ = 58MPa, G IIc =0.78kJ / m 2 ).

[0012] Furthermore, the temperature-fatigue combined test includes the following contents: The specimen is tested in an environmental chamber: Temperature Cycle: (rate 15°C / min), each cycle lasts 30 minutes; Mechanical load: stress ratio R = 0.1, maximum stress 200 MPa, frequency 5 Hz; The machine was stopped after every 500 cycles and ultrasonic C-scan was used to detect delamination expansion.

[0013] The beneficial effects of the present invention are as follows: the refined strength assessment method for small-sized, large or fully open CFRP shells provided by the present invention has the following advantages when used in practice: 1. By establishing a microscopic damage model that incorporates material anisotropy and defining damage variables and evolution equations, the multi-scale damage behaviors of fiber fracture, matrix cracking, and delamination extension are uniformly quantified, resolving the difficulty of traditional single-scale models in characterizing multiphase coupling effects. By transferring damage variables across scales, the entire process from microscopic damage initiation to macroscopic failure can be predicted, significantly improving the accuracy of strength assessments of complex open structures. 2. Using multimodal in-situ observation methods such as industrial CT and digital imaging (DIC), the three-dimensional distribution of hole edge damage, strain field evolution, and crack propagation path under dynamic loads are obtained in real time. This overcomes the limitations of insufficient spatial resolution of conventional CT detection and provides high-precision experimental data support for damage models. 3. By designing temperature gradient cycle and fatigue load coupling tests, an environmental factor correction model is established to clarify the accelerating effect of temperature changes on the debonding of the resin matrix and fiber interface, as well as the degradation law of interface strength under cyclic load; this model fills the gap in traditional methods for evaluating the impact of environmental loads, avoiding large strength deviations caused by ignoring temperature-mechanical coupling; 4. The technical route of this method systematically solves the shortcomings of traditional methods in multi-scale damage coupling, environmental sensitivity, monitoring accuracy and quantitative mapping through a full-process closed loop of microscopic modeling-in-situ observation-multi-scale simulation-environmental correction-experimental verification, and realizes the refinement, reliability and engineering practicality of the strength assessment of small-size large-opening / full-opening CFRP shells. It is especially suitable for fields such as aerospace that have extremely high requirements on structural weight and safety, and has significant technological leadership and application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic flow chart of the method of the present invention; Figure 2 1 is a CFRP shell sheet material opening design dimension drawing in an embodiment of the present invention; Figure 3 is a three-dimensional diagram of a small-opening CFRP shell in an embodiment of the present invention; Figure 4 This is a cloud diagram of the strength simulation of a small-opening CFRP shell in an embodiment of the present invention; Figure 5 This is an electron microscope observation image of the damage of the small-opening CFRP shell in an embodiment of the present invention. DETAILED DESCRIPTION

[0015] Specific embodiment 1: The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the embodiments described are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. It should be noted that: in the present invention, unless otherwise specified, all the implementation methods and preferred implementation methods mentioned herein can be combined with each other to form a new technical solution. In the present invention, unless otherwise specified, all the technical features and preferred features mentioned herein can be combined with each other to form a new technical solution. The "scope" disclosed in the present invention can be in the form of lower limit and upper limit, which can be one or more lower limits, and one or more upper limits respectively. Unless otherwise specified, the professional and scientific terms used in this article have the same meaning as those familiar to those skilled in the art. In addition, any method or material similar or equal to the recorded content can also be applied to the present invention.

[0016] As the specification of the present invention Figure 1As shown, in order to solve the problems existing in the existing CFRP shell strength detection methods, the present invention proposes a small-sized large or fully open CFRP shell strength refinement assessment method, which mainly includes: combining the small-sized large / fully open CFRP shell micro-damage model, considering the material anisotropy, ply method, geometric shape, opening size, boundary effect and interface interaction of the CFRP shell, comprehensively considering the multi-phase coupling effect of micro-damage, introducing damage variables and damage evolution equations, and establishing a shell strength calculation formula. Experimental design analysis is adopted, combined with finite element analysis and multi-scale simulation calculation methods, to simulate the micro-damage evolution process of the CFRP shell under typical loads, and verify and calibrate the shell strength calculation formula. Combined with the damage test of the shell under typical loads, the parameters of the shell strength formula are adjusted and compared with the test results to construct a small-sized large / fully open CFRP shell strength refinement assessment method, and the assessment method is experimentally verified and optimized; specifically, the present invention proposes a small-sized large or fully open CFRP shell strength refinement assessment method, which mainly includes the following steps: S1. Establish a microscopic damage model that takes into account material anisotropy, ply layup method, geometry, opening size, boundary effects, and interface interactions. By defining damage variables and damage evolution equations, construct a shell strength calculation formula that considers multiphase coupling effects. S2. Use multimodal in-situ experimental technology to obtain three-dimensional distribution data of hole edge damage, combine the strain field and crack propagation path analysis under dynamic load, and establish a damage evolution database; S3. Combining finite element analysis with multi-scale simulation methods, simulate the microscopic damage evolution process of a small-sized, large / full-opening CFRP shell under typical loads, and perform parameter calibration on the strength calculation formula established in step S1; S4. Conduct environmental-mechanical coupling tests to obtain the interface strength degradation law under the combined action of temperature gradient cycling and fatigue loading, and establish an environmental factor correction model; S5. Design temperature-fatigue combined test and establish empirical formula for environmental factor (temperature) and interface strength degradation; S6. The revised strength calculation formula is verified through damage tests, and after iterative parameter optimization, a refined strength assessment method system for small-sized and large / full-opening CFRP shells is formed.

[0017] Furthermore, a microscopic damage model and a multi-scale strength calculation formula are established: Geometric modeling and material parameter setting; Material system: T800SC carbon fiber / 5228 epoxy resin; Single layer mechanical parameters: E1 = 210 GPa, E2 = 8.7 GPa, G 12 =4.5GPa,ν 12=0.28, X T =3200MPa, Y T =45MPa,S L =75MPa, G IIc =0.82kJ / m 2 ; Among them, E1: longitudinal elastic modulus, E2: transverse elastic modulus, G 12 : in-plane shear modulus, ν 12 : Poisson's ratio, X T : Longitudinal tensile strength, Y T : Transverse tensile strength, S L : In-plane shear strength, G IIc : Type II critical strain energy release rate. Shell layer modeling: A parametric model was established with an opening edge chamfer radius of 0.5 mm and a layer sequence of [0 / 90 / ±45 / 90 / ±45]. Interface characteristics: The fiber-matrix interface strength τ = 62 MPa was obtained through nanoindentation testing.

[0018] Define damage variables and establish coupling equations.

[0019] The multi-scale damage characterization is as follows: Fiber breakage (microscopic): Among them, d f : Fiber fracture damage variable, used to measure the degree of fiber fracture, ε1: strain in the fiber direction. Matrix cracking (mesoscopic): Among them, d m : matrix cracking damage variable, measuring the degree of matrix cracking, σ2: normal stress perpendicular to the fiber direction, Y T : Transverse tensile strength of the matrix, τ 12 : In-plane shear stress, S L : In-plane shear strength. Hierarchical expansion (macro): Among them, d d : Layered expansion damage variable, measuring the degree of layered expansion, G II : Type II strain energy release rate, G IIc : Type II critical strain energy release rate, δ: displacement, δ c : critical displacement, δ c =0.02mm. Coupling strength equation: σ eff =σ0·(1-d f )·[1-min(1,d m +0.2d d )]·f(T,N), where σ eff : effective stress, σ0: initial stress, d f : fiber fracture damage variable, dm : matrix cracking damage variable, d d : Delamination expansion damage variable, f(T,N): Environmental factor, considering the influence of temperature T and number of cycles N on material properties.

[0020] Constructing a multimodal in-situ experiment and damage evolution database: Conduct dynamic damage observation experiments: A tensile-compressive cyclic load (frequency 1 Hz) with an amplitude of ±250 MPa was applied to the specimens on an MTS-810 testing machine. High-resolution μCT (0.5 μm) was used simultaneously: scanning was performed every 10 cycles to reconstruct the hole edge damage in three dimensions; Digital image spectroscopy (DIC): collects surface strain fields and calculates crack growth rates.

[0021] Generate damage database; Extract fiber fracture density, matrix crack length and delamination area from CT images; A mapping relationship between damage variables and the number of loading cycles is established for model calibration.

[0022] Create a finite element model and perform model calibration: A macro shell model was constructed in ABAQUS, and the hole edges were refined using adaptive meshes (minimum size 0.05 mm); Parameter iterative optimization; Comparing the load-displacement curves of simulation and experiment, the initial error is 18.5%; The interface strength τ and fracture toughness G were modified by three rounds of iteration IIc , the final error ≤ 3% (after correction τ = 58MPa, G IIc =0.78kJ / m 2 ); Correction of environmental-mechanical coupling effects.

[0023] Temperature-fatigue combined test; The specimen is tested in an environmental chamber: Temperature Cycle: (rate 15°C / min), each cycle lasts 30 minutes; Mechanical load: stress ratio R = 0.1, maximum stress 200 MPa, frequency 5 Hz; The machine was stopped after every 500 cycles and ultrasonic C-scan was used to detect delamination expansion; Environmental factor modeling: Statistical analysis shows that: for every 10°C increase in temperature amplitude ΔT, the interface strength τ decreases by 1.8%; Establishing environmental correction function; Validation of experimental design; Three groups of specimens (opening diameters 50 / 60 / 70 mm) were prepared and subjected to static tension until failure. The predicted and experimental values were compared, and the predicted failure loads were compared with the measured values and the errors were compared.

[0024] Through the above-mentioned integrated implementation, the present invention achieves a closed-loop process from microscopic modeling, multi-source data fusion, environmental effect correction, to engineering verification, providing a standardized solution for refined strength assessment of small-sized, large-opening / full-opening CFRP shells. The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation on the present invention. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can, without departing from the scope of the present invention, make slight changes or modifications to the above-disclosed technical content into equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention are still within the scope of the present invention.

Claims

1. A method for fine-grained strength assessment of small-sized, large or fully open CFRP shells, characterized in that: The steps include: S1. Establish a microscopic damage model that takes into account material anisotropy, ply layup method, geometry, opening size, boundary effects, and interface interactions. By defining damage variables and damage evolution equations, construct a shell strength calculation formula that considers multiphase coupling effects. S2. Use multimodal in-situ experimental technology to obtain three-dimensional distribution data of hole edge damage, combine the strain field and crack propagation path analysis under dynamic load, and establish a damage evolution database; S3. Combining finite element analysis with multi-scale simulation methods, simulate the microscopic damage evolution process of a small-sized, large / full-opening CFRP shell under typical loads, and perform parameter calibration on the strength calculation formula established in step S1; S4. Conduct environmental-mechanical coupling tests to obtain the interface strength degradation law under the combined action of temperature gradient cycling and fatigue loading, and establish an environmental factor correction model; S5. Design temperature-fatigue combined test and establish empirical formula for environmental factor (temperature) and interface strength degradation; S6. The revised strength calculation formula is verified through damage tests, and after iterative parameter optimization, a refined strength assessment method system for small-sized and large / full-opening CFRP shells is formed.

2. A refined strength assessment method for a small-sized, large or fully open CFRP shell according to claim 1, characterized in that: The construction of the microscopic damage model includes geometric modeling and material parameter setting, as follows: Material system: T800SC carbon fiber / 5228 epoxy resin; Single layer mechanical parameters: E1 = 210 GPa, E2 = 8.7 GPa, G 12 =4.5GPa,ν 12 =0.28, X T =3200MPa, Y T =45MPa,S L =75MPa, G IIc =0.82kJ / m 2 ; Among them, E1: longitudinal elastic modulus, E2: transverse elastic modulus, G 12 : in-plane shear modulus, ν 12 : Poisson's ratio, X T : Longitudinal tensile strength, Y T : Transverse tensile strength, S L : In-plane shear strength, G IIc : Type II critical strain energy release rate. Shell layer modeling: A parametric model was established with an opening edge chamfer radius of 0.5 mm and a layer sequence of [0 / 90 / ±45 / 90 / ±45]. Interface characteristics: The fiber-matrix interface strength τ = 62 MPa was obtained through nanoindentation testing.

3. A refined strength assessment method for a small-sized, large-or fully-opened CFRP shell according to claim 2, characterized in that: Defining damage variables and establishing coupling equations include the following: Multi-scale damage characterization: Fiber breakage (microscopic): Matrix cracking (mesoscopic): Hierarchical expansion (macro): Among them, δc=0.02mmδc=0.02mm is the critical displacement; Coupling strength equation: σ eff =σ0·(1-d f )·[1-min(1,d m +0.2d d )]·f(T,N), where f(T,N) is the environmental factor.

4. A refined strength assessment method for a small-sized, large-or fully-opened CFRP shell according to claim 3, characterized in that: The damage evolution database includes: performing dynamic damage observation experiments: A tensile-compressive cyclic load (frequency 1 Hz) with an amplitude of ±250 MPa was applied to the specimens on an MTS-810 testing machine. High-resolution μCT (0.5 μm) was used simultaneously: scanning was performed every 10 cycles to reconstruct the hole edge damage in three dimensions; Digital image spectroscopy (DIC): collects surface strain fields and calculates crack growth rates; Generate a damage database.

5. A refined strength assessment method for a small-sized, large or fully open CFRP shell according to claim 4, characterized in that: By extracting the fiber fracture density, matrix crack length and delamination area from CT images; Establish a mapping relationship between damage variables and load cycle numbers for model calibration; establish a finite element model and perform model calibration.

6. A refined strength assessment method for a small-sized, large or fully open CFRP shell according to claim 5, characterized in that: A macro shell model was constructed in ABAQUS, and the hole edges were encrypted using adaptive meshes (minimum size 0.05 mm).

7. A refined strength assessment method for a small-sized, large or fully open CFRP shell according to claim 3, characterized in that: During the parameter iterative optimization, the load-displacement curves of the simulation and the experiment were compared, and the initial error was 18.5%. After three rounds of iteration, the interface strength τ and fracture toughness G were corrected. IIc , the final error ≤ 3% (after correction τ = 58MPa, G IIc =0.78kJ / m 2 ).

8. A refined strength assessment method for a small-sized, large or fully open CFRP shell according to claim 1, characterized in that: The temperature-fatigue combined test includes the following: The specimen is tested in an environmental chamber: Temperature Cycle: (rate 15°C / min), each cycle lasts 30 minutes; Mechanical load: stress ratio R = 0.1, maximum stress 200 MPa, frequency 5 Hz; The machine was stopped after every 500 cycles and ultrasonic C-scan was used to detect delamination expansion.

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