Structure application risk evaluation method for composite material connection node

Abstract

The invention provides a structure application risk evaluation method for a composite material connection node. Comprising the steps that a target fiber reinforced composite material typical plate static test and a target foam core material typical plate static test are carried out, target overall structure analysis is carried out, composite material lap joint node design is carried out, a composite material lap joint node test piece is prepared, and a composite material lap joint node strength test is carried out. And test data processing and application security margin calculation of the connection nodes in the overall structure. Based on strength test data of the composite material lap joint connection node, the application safety margin of the connection node under the design static load condition of the overall structure is judged. The safety margin is improved or reduced to a certain extent by changing the proportion of the thickness of the surface layer to the thickness of the core material of the composite material of the lap joint node.

Description

Technical Field

[0001] The present invention relates to the field of composite material structure design and performance testing and evaluation, and in particular to a structural application risk assessment method for composite material connection nodes. Background Art

[0002] Composite structures are characterized by large overall size and numerous and complex connection nodes. Given these characteristics, when verifying the overall stiffness and strength of composite structures, finite element simulation is generally used to establish an overall structural model. Due to the complexity of the connection nodes, in order to minimize the time cost of the finite element model calculation, the connection nodes are usually ignored, and the connection parts are treated as completely continuous structures. For connection nodes, strength tests are conducted by selecting full-scale local structures to obtain the connection node's ultimate load and strain data. Based on the connection node test data, it is necessary to determine the safety margin of the connection node when it is used in the overall structure.

[0003] Research on composite connection nodes typically focuses on the nodes themselves. This includes connection node design, including application number "CN201720350468.2," titled "A Connection Structure Between Marine Composite Materials and Steel Plates." The design involves a bolted connection structure between a composite sandwich panel and a steel plate, which improves the mechanical properties of the node by avoiding secondary molding of the composite material. Regarding connection node parameter optimization, literature such as "Bearing Performance Analysis and Optimization Design of Single-Lap Adhesive Joints of Composite Materials" published by Shang Xinlong, "Optimization of Composite Bolted Connection Structure Considering Layup and Size Parameters" published by Yang Zhilong et al., and "Multi-Parameter Optimization Design of Composite Material Double-Nail, Rubber-Screw Hybrid Connection Joints" published by Li Mingkun et al. primarily focuses on the impact of changes in the connection node's parameters on its bearing performance. Regarding connection node testing, since structural connection nodes are typically tested using non-standard methods, there are also literature studies in this area, such as "Experimental Research and Numerical Simulation of Single-Lap Adhesive Joints of Composite Materials." In addition, after the design and testing of the connection nodes themselves are completed, there is also the need to evaluate their safety risks in structural applications. Although the overall structural test can cover the assessment of the connection nodes, the overall test often does not focus on some connection nodes and cannot quantify the safety margin. Summary of the Invention

[0004] In view of this, the present invention aims to propose a structural application risk assessment method for composite material connection nodes to solve the problem in the prior art that overall testing often does not focus on some connection nodes and cannot quantify the safety margin.

[0005] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0006] A structural application risk assessment method for composite material connection nodes comprises the following steps:

[0007] (1) Conduct static tests on typical laminates of target fiber-reinforced composite materials and typical panels of target foam core materials;

[0008] (2) Carry out target overall structural analysis, with the nominal size of the overall structure being 1 to 5 m; based on finite element analysis software, according to the specified target overall structural design scheme, use the material performance data obtained in step (1) to establish a finite element simulation model, the finite element model uses shell elements, and the fiber reinforced composite material surface layer and core material are subjected to performance stratification settings; mesh the overall structure finite element model, with the nominal mesh size being 20 to 50 mm; apply the specified design static load conditions, obtain the overall structure strain cloud map, and read the strain value data of each element in each direction;

[0009] (3) Design of composite material lap joints;

[0010] (4) Preparation of composite material lap joint test pieces;

[0011] (5) Carry out strength testing of composite material lap joints;

[0012] (6) Experimental data processing;

[0013] (7) Safety margin calculation is applied to connection nodes in the overall structure.

[0014] Furthermore, the static test of the typical plate of the target fiber reinforced composite material in step (1) includes: unidirectional tensile test, unidirectional compression test, interlaminar shear test, to obtain the tensile, compression and shear properties of the fiber reinforced composite material.

[0015] Furthermore, the static test of the target foam core material typical panel in step (1) includes a unidirectional tensile test and a unidirectional compression test to obtain the tensile and compressive properties of the foam core material.

[0016] Furthermore, a simplified finite element simulation model of the target overall structure without connection details is established.

[0017] Furthermore, in step (3), the composite material lap connection node is composed of two upper and lower lapped composite material sandwich panels, and the thickness of the two sandwich panels in the connection area is halved accordingly. The composite material sandwich panel is composed of a fiber reinforced composite material surface layer + a foam core material + a fiber reinforced composite material surface layer, with a total thickness of 20 to 50 mm, wherein the thickness of the single-side composite material surface layer is 1.5 to 4.0 mm, and the width of the connection area is 150 to 250 mm.

[0018] Furthermore, in step (3), the bolt arrangement design of the connection node is as follows: the two upper and lower overlapped composite sandwich panels are connected by bolts, the bolt row spacing is 1 / 2 of the width of the connection area, and the bolt row spacing is 150~300mm.

[0019] Furthermore, in step (4), based on the mature fiber reinforced composite material forming process and machining means, the preparation of the lap joint node test piece is completed, the total cross-sectional width of the test piece is selected as 3 times the width of the connection area plus 50~60mm margin on the left and right sides, and the span length of the test piece is 2~4 times the bolt row spacing, where the bolt row margin is 1 / 2 of the row spacing.

[0020] Furthermore, the step (5) includes a quasi-static test process, which is performed as follows:

[0021] a. Place the test piece on the tooling according to the loading diagram;

[0022] b. Connect and debug strain testing equipment;

[0023] c. When measuring, apply an initial load of 20~50N, check and adjust the specimen and instrument to ensure they are in normal working condition;

[0024] d. Clear the strain gauge data and start the strain acquisition system at the same time as the testing machine starts loading. Then continue to apply load and automatically record the corresponding strain.

[0025] e. Apply pressure to the specimen at a constant crosshead displacement rate of ≤2mm / min until failure;

[0026] f. During the test, apply the load evenly and continuously until the specimen fails, and record the maximum load value and the form of specimen failure.

[0027] Furthermore, the step (6) includes: integrating the obtained load data and strain-time data into load-strain data of each strain channel with time as the intermediate correlation term.

[0028] Furthermore, the step (7) includes:

[0029] a. Based on the overall structural calculation results obtained in step (2), determine the absolute maximum principal strain ε0 of the connection position interval in the overall structural calculation;

[0030] b. Based on the load-strain curve obtained in step (4), determine the test load value F0 corresponding to the strain gauge channel data with the largest absolute value reaching ε0;

[0031] c. Calculate the application safety margin η of this connection node based on F0 and the limit load F1 in the load-time data.

[0032] .

[0033] Compared with the prior art, the structural application risk assessment method for composite material connection nodes described in the present invention has the following advantages:

[0034] Based on the strength and strain test data of the connection nodes, the safety margin of the connection node is determined under the design load conditions of the overall structure. This overcomes the shortcoming of the difficulty in considering connection details during the overall structure verification calculation and improves the content of structural design verification. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a schematic diagram of a composite material overlap connection node according to an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of a composite material overlap connection test piece according to an embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of loading the overlapped test piece according to an embodiment of the present invention;

[0038] Figure 4 Schematic diagram of the arrangement of strain gauges of the overlapped test piece according to an embodiment of the present invention;

[0039] Figure 5 This is a schematic diagram of the overall structure of a carbon fiber reinforced composite material-PVC core sandwich panel according to an embodiment of the present invention;

[0040] Figure 6 The finite element model of the overall structure described in the embodiment of the present invention;

[0041] Figure 7 A connection node in the overall structure described in the embodiment of the present invention;

[0042] Figure 8 The load and boundary conditions of the finite element model of the overall structure described in the embodiment of the present invention;

[0043] Figure 9 This is a cloud diagram of the strain distribution of the overall structure according to an embodiment of the present invention;

[0044] Figure 10 This is a cross-sectional view of a lap joint connection node according to an embodiment of the present invention;

[0045] Figure 11 This is a bolt arrangement diagram for a lap joint connection node according to an embodiment of the present invention;

[0046] Figure 12 This is a diagram showing the arrangement of strain gauges according to an embodiment of the present invention;

[0047] Figure 13 Testing the test piece described in the embodiment of the present invention;

[0048] Figure 14 This is the test load-displacement curve described in the embodiment of the present invention;

[0049] Figure 15 This is the experimental strain-time curve described in the embodiment of the present invention;

[0050] Figure 16 Schematic diagram of the strain in the connection position interval according to an embodiment of the present invention.

[0051] Description of reference numerals:

[0052] 1-composite sandwich panel, 11-foam core material, 12-surface layer, 2-connection area, 3-support axis, 4-loading end, 5-connection position interval. DETAILED DESCRIPTION

[0053] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0054] A structural application risk assessment method for composite material connection nodes, comprising:

[0055] (1) Conduct static tests on typical laminates of target fiber reinforced composite materials, including unidirectional tensile tests, unidirectional compression tests, and interlaminar shear tests, to obtain the tensile, compressive, and shear properties of fiber reinforced composite materials; conduct static tests on typical panels of target foam core materials, including unidirectional tensile tests and unidirectional compression tests, to obtain the tensile and compressive properties of foam core materials; and obtain relevant performance data of the materials.

[0056] (2) Carry out target overall structure analysis (the overall structure is a wall panel structure including connection parts), and the nominal size of the overall structure is generally 1~5m; based on mature commercial finite element analysis software, in accordance with the specified target overall structure design scheme, use the material performance data obtained in step (1) to establish a simplified target overall structure finite element simulation model that does not include connection details. The finite element model should generally use shell elements, and the fiber reinforced composite material surface layer and core material should be set up for performance stratification; the overall structure finite element model is meshed, and the nominal mesh size is 20~50mm; apply the specified design static load conditions to obtain the overall structure strain cloud map, and the strain value data of each unit in each direction can be read.

[0057] (3) Design of composite material lap joints.

[0058] Connection node profile design: The composite lap connection node is composed of two composite sandwich panels 1 overlapped at the top and bottom, such as Figure 1As shown, the thickness of the two sandwich panels in the connection area is halved. The composite sandwich panel 1 is constructed of a fiber-reinforced composite surface layer 12, a foam core 11, and a fiber-reinforced composite surface layer 12. The total thickness is 20-50 mm, with the composite surface layer on one side being 1.5-4.0 mm thick. The width of the connection area is 150-250 mm.

[0059] Bolt arrangement design of connection nodes: The two upper and lower overlapped composite sandwich panels are connected by bolts. The bolt row spacing is 1 / 2 of the width of the connection area, and the bolt row spacing is 150~300mm. Figure 2 shown.

[0060] (4) Preparation of composite material lap joint node test pieces

[0061] Based on the mature fiber reinforced composite material forming process and machining methods, the lap joint node test piece was prepared. The total cross-sectional width of the test piece was selected as 3 times the width of the connection area plus a 50~60mm margin on the left and right sides. The span length of the test piece was 2~4 times the bolt row spacing, where the bolt row edge distance was 1 / 2 of the row spacing, such as Figure 2 shown.

[0062] (5) Conduct strength testing of composite material lap joints

[0063] Tests are performed using static load testing machines, dynamic strain testing systems, and strain gauges.

[0064] Auxiliary tooling for overlapping test pieces Figure 3 As shown, the loading end 4 of the testing machine applies downward displacement loading to the center of the upper surface of the test piece, and support shafts 3 are provided at both ends of the lower surface of the test piece. The span of the auxiliary tooling is 3 times the width of the connection area.

[0065] The strain gauge arrangement of the test piece is as follows Figure 4 As shown, strain gauges are marked with a ■. Strain gauges can be unidirectional or bidirectional. If unidirectional, they are arranged in the x-direction as shown in the figure below. If bidirectional, they are arranged parallel to the x and y directions, respectively. Strain gauges should be attached according to specifications.

[0066] Carry out preparatory work before testing, including inspection of test piece appearance, numbering, measurement, and recording.

[0067] The quasi-static test process is performed as follows:

[0068] a. Place the test piece on the tooling according to the loading diagram;

[0069] b. Connect and debug strain testing equipment;

[0070] c. When measuring, apply the initial load (20~50N load), check and adjust the sample and instrument to ensure they are in normal working condition;

[0071] d. Clear the strain gauge data and start the strain acquisition system at the same time as the testing machine starts loading. Then continue to apply load and automatically record the corresponding strain.

[0072] e. Apply pressure to the specimen at a constant crosshead displacement rate of ≤2mm / min until failure;

[0073] f. During the test, apply the load evenly and continuously until the specimen fails, and record the maximum load value and the form of specimen failure.

[0074] In quasi-static experiments, the failure of composite components is a multi-mode failure. The occurrence of each failure mode will show a staged decrease in the load-displacement curve. A load decrease of 20% or a load decrease in a longer displacement range is considered to be structural failure. The location and failure mode of the specimen at each stage of failure are recorded and photographed.

[0075] The original test data that need to be obtained include the load data set collected by the static testing machine and the strain-time data set of each strain channel collected by the dynamic strain testing system.

[0076] (6) Experimental data processing

[0077] The obtained load data and strain-time data are integrated into the load-strain data of each strain channel with time as the intermediate correlation term.

[0078] (7) Apply safety margin calculation to connection nodes in the overall structure

[0079] The safety margin calculation for connection nodes in the overall structure is performed according to the following method.

[0080] a. Based on the overall structural calculation results obtained in step (2), determine the absolute maximum principal strain ε0 of the connection position interval in the overall structural calculation;

[0081] b. Based on the load-strain curve obtained in step (4), determine the test load value F0 corresponding to the strain gauge channel data with the largest absolute value reaching ε0;

[0082] c. Calculate the application safety margin η of the connection node based on F0 and the limit load F1 in the load-time data. .

[0083] Example 1

[0084] Taking a carbon fiber reinforced composite material-PVC core sandwich panel as an example, the structural application static load safety margin evaluation method for composite lap joints includes the following steps:

[0085] (1) Static tests on typical carbon fiber reinforced composite panels were conducted, including uniaxial tensile tests, uniaxial compression tests, and interlaminar shear tests, to obtain the tensile, compressive, and shear properties of the fiber reinforced composite materials, as shown in Table 1. Static tests on typical target PVC core panels were conducted, including uniaxial tensile tests and uniaxial compression tests, to obtain the tensile and compressive properties of the foam core materials, as shown in Table 2.

[0086] Table 1 Mechanical properties of carbon fiber reinforced composites

[0087] Table 2 Mechanical properties of PVC core material

[0088] (2) Carry out the overall structural analysis of a sandwich panel using carbon fiber reinforced composite materials and PVC core materials. Figure 5 The wall panel in the figure is the target sandwich panel, with an overall length of 4200mm and a width of 3300mm, meeting the nominal size requirement of 1 to 5m. The sandwich panel is composed of a 2.5mm carbon fiber reinforced composite surface layer, a 25mm PVC core material, and a 2.5mm carbon fiber reinforced composite surface layer.

[0089] The finite element analysis software ABAQUS is used to establish the overall structural finite element simulation model, such as Figure 6 In the process of establishing the finite element model, in order to save calculation time, it is necessary to simplify the connection position interval 5 in the actual structural scheme, as shown in Figure 7 As shown in the figure, in the actual structural scheme, the sandwich panels are connected by upper and lower overlap. This detail is ignored in the finite element model, and each wall is set as a sandwich panel with uniform geometry and material properties.

[0090] This finite element model uses S4R shell elements, and sets the performance of fiber reinforced composite surface layer and core material in layers; the nominal size of the finite element grid is 50mm; the design load and boundary conditions of the overall structure are as follows Figure 8 As shown in the figure, the design load is a uniformly distributed pressure of 4.16 kPa on one side, and the bottom clamped boundary condition is adopted.

[0091] The overall structural strain distribution cloud diagram is calculated, such as Figure 9 shown.

[0092] (3) Design of composite material lap joints.

[0093] The cross section of the composite material lap joint is as follows Figure 10 As shown, the basic dimensions of the sandwich panel are 2.5mm carbon fiber reinforced composite facings, 25mm PVC core, and 2.5mm carbon fiber reinforced composite facings, for a total thickness of 30mm. The upper and lower panels in the overlapped joint area are halved in thickness and filled with fiber reinforced composite preforms of the same material. The joint area is 200mm wide, and the cross-sectional width of the test specimen to be prepared is 700mm.

[0094] Bolt arrangement of composite material lap joints Figure 11 As shown, the bolt row spacing is 100 mm, the column spacing is 250 mm, and the span length of the test piece is 500 mm, which is twice the column spacing.

[0095] (4) The preparation of the lap joint node test piece is completed according to the mature fiber reinforced composite material molding process and machining methods.

[0096] (5) Conduct strength testing of composite material lap joints

[0097] The test used an S311-500kN electro-hydraulic servo fatigue testing machine, a DH5921-1 dynamic strain testing system, and a BE120-3CA bidirectional strain gauge.

[0098] Strain gauge arrangement as follows Figure 12 As shown, the position marked with ■ in the figure.

[0099] The test process was as described above, with a loading rate of 2 mm / min.

[0100] (6) After the strength test of the lap joint, the load and strain data are obtained, such as Figure 14 and Figure 15 shown.

[0101] (7) Apply safety margin calculation to connection nodes in the overall structure

[0102] According to the strain calculation results in step (2), the absolute maximum principal strain of the overlap connection position interval under the design static load condition is ε0 = 496με, as shown in Figure 16 shown.

[0103] According to the load and strain test results obtained in step (6), when the strain channel with the largest absolute value reaches the value, the corresponding loading load is F0 = 6.9kN. In addition, the ultimate load of the test piece is F1 = 28.94kN. The safety margin applied to the composite lap joint in the overall structure is

[0104] η=F1 / F0=4.19.

[0105] That is, the safety margin of the overlap connection node under the material and size conditions of the present invention used in the target structure is 4.19 times.

[0106] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the scope defined by the claims.

Claims

1. A structural application risk assessment method for composite material connection nodes, characterized in that: The following steps are involved: (1) Conduct static tests on typical target fiber-reinforced composite laminates and typical target foam core panels to obtain relevant material performance data; (2) Conduct target overall structural analysis, with the nominal size of the overall structure being 1 to 5 m; Based on finite element analysis software, according to the specified target overall structural design scheme, the material performance data obtained in step (1) are used to establish a finite element simulation model, the finite element model uses shell elements, and the performance layering setting is performed on the fiber reinforced composite material surface layer and core material; the overall structural finite element model is meshed, and the mesh nominal size is 20~50mm; the specified design static load conditions are applied to obtain the overall structural strain cloud map, and the strain value data of each unit in each direction are read; wherein the overall structure is a wall panel structure including a connection part; (3) Design of composite material lap joints; (4) Preparation of composite material lap joint test pieces; (5) Carry out strength testing of composite material lap joints; (6) Experimental data processing; (7) Safety margin calculation is applied to connection nodes in the overall structure.

2. The method according to claim 1, characterized in that The static test of the typical plate of the target fiber reinforced composite material in step (1) includes: uniaxial tensile test, uniaxial compression test, and interlaminar shear test, so as to obtain the tensile, compressive and shear properties of the fiber reinforced composite material.

3. The method according to claim 1, characterized in that The static test of the typical panel of the target foam core material in step (1) includes a unidirectional tensile test and a unidirectional compression test to obtain the tensile and compressive properties of the foam core material.

4. The method according to claim 1, wherein A simplified finite element simulation model of the target overall structure without connection details is established.

5. The method according to claim 1, wherein In the step (3), the composite material lap connection node is composed of two upper and lower lapped composite material sandwich panels, and the thickness of the two sandwich panels in the connection area is halved accordingly. The composite material sandwich panel is composed of a fiber reinforced composite material surface layer + a foam core material + a fiber reinforced composite material surface layer, with a total thickness of 20 to 50 mm, wherein the thickness of the single-side composite material surface layer is 1.5 to 4.0 mm, and the width of the connection area is 150 to 250 mm.

6. The method according to claim 1, characterized in that The bolt arrangement design of the connection node in step (3) is as follows: the two upper and lower overlapped composite sandwich panels are connected by bolts, the bolt row spacing is 1 / 2 of the width of the connection area, and the bolt row spacing is 150~300mm.

7. The method according to claim 1, characterized in that In step (4), the preparation of the lap joint node test piece is completed based on the mature fiber reinforced composite material forming process method and machining means. The total cross-sectional width of the test piece is selected as 3 times the width of the connection area plus 50~60mm margin on the left and right sides. The span length of the test piece is 2~4 times the bolt row spacing, where the bolt row margin is 1 / 2 of the row spacing.

8. The method according to claim 1, characterized in that The step (5) includes a quasi-static test process, which is performed as follows: a. Place the test piece on the tooling according to the loading diagram; b. Connect and debug strain testing equipment; c. When measuring, apply an initial load of 20~50N, check and adjust the specimen and instrument to ensure they are in normal working condition; d. Clear the strain gauge data and start the strain acquisition system at the same time as the testing machine starts loading. Then continue to apply load and automatically record the corresponding strain. e. Apply pressure to the specimen at a constant crosshead displacement rate of ≤2mm / min until failure; f. During the test, apply the load evenly and continuously until the specimen fails, and record the maximum load value and the form of specimen failure.

9. The method according to claim 1, characterized in that The step (6) includes: integrating the obtained load data and strain-time data into load-strain data of each strain channel with time as the intermediate correlation item.

10. The method according to claim 1, characterized in that The step (7) comprises: a. Based on the overall structural calculation results obtained in step (2), determine the absolute maximum principal strain ε0 of the connection position interval in the overall structural calculation; b. Based on the load-strain curve obtained in step (4), determine the test load value F0 corresponding to the strain gauge channel data with the largest absolute value reaching ε0; c. Calculate the application safety margin η of this connection node based on F0 and the limit load F1 in the load-time data. 。

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