Aviation composite material structure allowable value determination method

Abstract

The invention belongs to the technical field of structural material design, and particularly relates to a method for determining an allowable value of an aviation composite material structure. The method mainly comprises the following steps: correcting a tension-compression strength allowable value according to a first ratio of the breaking strength of a single-side lap joint tension-compression test to the breaking strength of a single composite material tension-compression test; a plurality of second ratios of the shear failure strength of the composite materials with different thicknesses to the shear failure strength of a single composite material are formed through bonding, and the shear strength allowable value is corrected; correcting the allowable value of the tensile strength according to a plurality of third ratios of the tensile breaking strength of the composite materials with different thicknesses formed by screw connection to the tensile breaking strength of a single composite material; and correcting the allowable value of the compression strength according to a plurality of fourth ratios of the compression breaking strength of the composite materials with different thicknesses after impact breaking to the compression breaking strength of a single composite material. A basis is provided for relatively reasonable, safe and effective design of the composite material structure.

Description

Technical Field

[0001] This application belongs to the field of structural material design technology, and specifically relates to a method for determining allowable values ​​of aerospace composite material structures. Background Technology

[0002] Composite materials, due to their high specific strength, high specific modulus, and excellent designability, have been widely used in modern aerospace structures, gradually expanding from secondary load-bearing structures to primary load-bearing structures such as wings and fuselages. With the increasing criticality of these applications, the size and thickness of composite material structures are constantly increasing, resulting in a large number of thick composite structures. These structures typically cannot be co-cured in a single process and must be assembled and integrated using various mechanical connections (such as bolted connections) and adhesive bonding. However, the presence of these connection areas significantly alters the force transmission path and local stress state of the structure, introducing complex problems such as stress concentration, compression, bypass loads, and deformation incompatibility. This leads to a significant degradation in their failure modes and strength properties compared to standard smooth specimens, making them a weak link and a key constraint in the overall structural design.

[0003] Currently, the field of aerospace composite materials has established methods for determining allowable values ​​(such as tensile, compressive, and shear strength) of standard specimens based on extensive experimental statistics. This system is well-suited for simple, smooth laminate structures. However, the industry has yet to develop a mature and engineering-applicable method for determining allowable values ​​for thick composite structures (especially those involving connections). The core challenge lies in:

[0004] 1. The failure mechanism of thick composite materials is not a simple amplification of standard small specimens. Increased thickness leads to a more significant three-dimensional stress state, and its failure mode and strength properties exhibit obvious nonlinear scaling effects, making it impossible to directly apply allowable values ​​established based on thin plate specimen data.

[0005] 2. Under the influence of deformation-incompatible loads, significant peak stresses are generated at the lap joints, resulting in tensile and compressive strengths far below standard values. The shear strength of adhesive joints strongly depends on the properties of the adhesive layer and the thickness and stiffness of the bonded materials; its allowable value is difficult to predict accurately using theoretical models. Bolted connections exhibit complex failure modes, and their strength is closely related to factors such as plate thickness, hole diameter, and bolt preload. The morphology, depth, and impact on residual compressive strength of impact damage in thick structures differ significantly from those in thin plates, lacking a unified evaluation benchmark.

[0006] For the reasons mentioned above, current engineering practices often employ extremely conservative design strategies for such complex structures, or rely on limited full-scale tests for verification. This not only leads to excessive structural weight, limiting the weight reduction advantages of composite materials, but also significantly increases design time and cost. Therefore, developing a systematic, efficient, and engineering-practical method for accurately determining the allowable values ​​of thick composite material structures containing interconnections under various operating conditions has become a key technological bottleneck that urgently needs to be overcome in the field of aerospace composite material structure design. Summary of the Invention

[0007] To address the aforementioned issues, this application provides a method for determining allowable values ​​for aerospace composite material structures, filling the gap in methods for determining allowable values ​​for thick composite materials applicable to aerospace structures. This method guides the control of allowable values ​​for aerospace structures with typical thick composite material application areas, enabling relatively reasonable, safe, and effective design of related composite material structures.

[0008] The method for determining allowable values ​​for aerospace composite material structures provided in this application mainly includes:

[0009] (1) Determine a first allowable value for the tensile and compressive strength of the composite material structure under a first working condition, wherein the first working condition refers to the working condition under the influence of deformation incoordination load on a composite material structure of a specified thickness formed by end overlap. Determining the first allowable value includes:

[0010] Step S1: Obtain the first ratio of the failure strength of the single-sided lap tensile-compression test to the failure strength of a single composite material tensile-compression test;

[0011] Step S2: Correct the given allowable value of tensile and compressive strength of aerospace composite material structure according to the first ratio to obtain the first allowable value;

[0012] (2) Determine a second allowable value for the shear strength of the composite material structure under the second working condition, wherein the second working condition refers to the working condition under shear load when a composite material structure of a specified thickness is formed by bonding. Determining the second allowable value includes:

[0013] Step S3: Obtain multiple second ratios between the shear failure strength of composite materials of different thicknesses formed by bonding and the shear failure strength of a single composite material;

[0014] Step S4: Correct the given allowable shear strength value of the aerospace composite material structure according to the second ratio to obtain multiple second allowable values;

[0015] (3) Determine the third allowable value of the tensile strength of the composite material structure under the third working condition, wherein the third working condition refers to the working condition under tensile load when a composite material structure of a specified thickness is formed by screw connection. Determining the third allowable value includes:

[0016] Step S5: Obtain multiple third ratios between the tensile breaking strength of composite materials of different thicknesses formed by screw connection and the tensile breaking strength of a single composite material.

[0017] Step S6: Correct the allowable tensile strength value of the given aerospace composite material structure according to the third ratio to obtain multiple third allowable values;

[0018] (4) Determine the fourth allowable value of the tensile strength of the composite material structure under the fourth working condition, wherein the fourth working condition refers to the working condition of a composite material structure of a specified thickness after impact failure under the influence of compressive load, and determining the fourth allowable value includes:

[0019] Step S7: Obtain multiple fourth ratios of the compressive strength of composite materials of different thicknesses after impact failure to the compressive strength of a single composite material.

[0020] Step S8: Correct the allowable compressive strength of the given aerospace composite material structure according to the fourth ratio to obtain multiple fourth allowable values.

[0021] Preferably, in step S1, the first ratio is determined by the average value of multiple batches of tests or the B benchmark value of multiple batches of tests.

[0022] Preferably, in step S3, the multiple second ratios at any thickness are processed using the average value of multiple batch tests or the B benchmark value of multiple batch tests.

[0023] Preferably, step S4 further includes:

[0024] Step S41: Determine the thickness ratio of the given aerospace composite material structure to a reference thickness;

[0025] Step S42: Interpolate among a plurality of second ratios based on the thickness ratio to obtain a second ratio that matches the given aerospace composite material structure;

[0026] Step S43: Calculate the second allowable value based on the interpolated second ratio.

[0027] Preferably, in step S5, the multiple third ratios at any thickness are processed using the average value of multiple batches of tests or the B benchmark value of multiple batches of tests.

[0028] Preferably, step S6 further includes:

[0029] Step S61: Determine the thickness ratio of the given aerospace composite material structure to a reference thickness;

[0030] Step S62: Based on the thickness ratio, interpolate among a plurality of third ratios to obtain a third ratio that matches the given aerospace composite material structure;

[0031] Step S63: Calculate the third allowable value based on the interpolated third ratio.

[0032] Preferably, in step S7, the multiple fourth ratios at any thickness are processed using the average value of multiple batch tests or the B benchmark value of multiple batch tests.

[0033] Preferably, in step S7, a constant energy or a constant dent depth is used to ensure that the impact damage degree of composite materials of different thicknesses is consistent.

[0034] Preferably, step S8 further includes:

[0035] Step S81: Determine the thickness ratio of the given aerospace composite material structure to a reference thickness;

[0036] Step S82: Based on the thickness ratio, interpolate among a plurality of fourth ratio values ​​to obtain a fourth ratio value that matches the given aerospace composite material structure;

[0037] Step S83: Calculate the fourth allowable value based on the interpolated fourth ratio.

[0038] This application can accurately assess the allowable values ​​of thick aerospace composite material structures, providing a basis for the relatively reasonable, safe, and effective design of composite material structures. Attached Figure Description

[0039] Figure 1 This is a flowchart of a preferred embodiment of the method for determining allowable values ​​of aerospace composite material structures according to this application. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be described in more detail below with reference to the accompanying drawings. In the drawings, the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The described embodiments are only some, not all, of the embodiments of this application. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application. The embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0041] This application provides a method for determining allowable values ​​for aerospace composite material structures, such as... Figure 1 As shown, it mainly includes:

[0042] (1) Determine a first allowable value for the tensile and compressive strength of the composite material structure under a first working condition, wherein the first working condition refers to the working condition under the influence of deformation incoordination load on a composite material structure of a specified thickness formed by end overlap. Determining the first allowable value includes:

[0043] Step S1: Obtain the first ratio of the failure strength of the single-sided lap tensile-compression test to the failure strength of a single composite material tensile-compression test;

[0044] Step S2: Correct the given allowable tensile and compressive strength values ​​of aerospace composite material structures according to the first ratio to obtain the first allowable value;

[0045] (2) Determine a second allowable value for the shear strength of the composite material structure under the second working condition, wherein the second working condition refers to the working condition under shear load when a composite material structure of a specified thickness is formed by bonding. Determining the second allowable value includes:

[0046] Step S3: Obtain multiple second ratios between the shear failure strength of composite materials of different thicknesses formed by bonding and the shear failure strength of a single composite material;

[0047] Step S4: Correct the given allowable shear strength value of the aerospace composite material structure according to the second ratio to obtain multiple second allowable values;

[0048] (3) Determine the third allowable value of the tensile strength of the composite material structure under the third working condition, wherein the third working condition refers to the working condition under tensile load when a composite material structure of a specified thickness is formed by screw connection. Determining the third allowable value includes:

[0049] Step S5: Obtain multiple third ratios between the tensile breaking strength of composite materials of different thicknesses formed by screw connection and the tensile breaking strength of a single composite material.

[0050] Step S6: Correct the allowable tensile strength value of the given aerospace composite material structure according to the third ratio to obtain multiple third allowable values;

[0051] (4) Determine the fourth allowable value of the tensile strength of the composite material structure under the fourth working condition, wherein the fourth working condition refers to the working condition of a composite material structure of a specified thickness after impact failure under the influence of compressive load, and determining the fourth allowable value includes:

[0052] Step S7: Obtain multiple fourth ratios of the compressive strength of composite materials of different thicknesses after impact failure to the compressive strength of a single composite material.

[0053] Step S8: Correct the allowable compressive strength of the given aerospace composite material structure according to the fourth ratio to obtain multiple fourth allowable values.

[0054] The allowable values ​​for composite material structures that take into account the influence of thickness in this application mainly include the following four aspects:

[0055] 1) Control of stress state and allowable value in the thickness direction;

[0056] 2) Control of interlayer stress state and allowable values ​​along the thickness direction;

[0057] 3) Stress distribution and allowable value control in the height direction of mechanical connection holes;

[0058] 4) Control of local stress state and allowable values ​​in structures with damage / process defects.

[0059] Therefore, this application determines the allowable values ​​under the above four conditions through four types of operating condition tests.

[0060] The first type of working condition mainly considers the deformation incompatibility effect in the thickness direction, such as the load effect or the ply effect, and is divided into tensile test and compression test, as shown in Table 1.

[0061] Table 1 Tensile and Compression Test Planning Table

[0062]

[0063]

[0064] It should be noted that conventional tensile / compression tests are based on standards such as ASTM or GB, and it is recommended to conduct tests including holes. Single-sided lap tensile / compression tests are conducted on the basis of conventional tensile / compression tests, while maintaining consistent specimen dimensions, specimen treatment, and test environment. The relevant tests are carried out after mechanical or adhesive bonding at the ends of the specimens.

[0065] In some alternative implementations, in step S1, the first ratio is determined using the average value of multiple batches of tests or the B benchmark value of multiple batches of tests.

[0066] Finally, in step S2, for composite material structures with typical overlap areas, the first ratio × the original design allowable value can be used to control the design allowable value of the corresponding part.

[0067] The second type of working condition mainly considers the influence of bonding on interlayer stress in the thickness direction. The short beam shear test is used, which means that the bonded parts are fixed at both ends and a pressure load is applied in the middle. The test plan is shown in Table 2.

[0068] Table 2. Short Beam Shear Test Planning Table

[0069]

[0070] It should be noted that Thickness 1 refers to the standard thickness, and the test is conducted based on standards such as ASTM or GB. Other thickness tests are conducted on the basis of Thickness 1 tests, while maintaining the same specimen treatment and test environment.

[0071] In some alternative implementations, in step S3, the multiple second ratios at any thickness are processed using the average value of multiple batch tests or the B benchmark value of multiple batch tests.

[0072] In this embodiment, step S3 involves conducting multiple tests for each thickness (compared to the reference thickness) to obtain multiple second ratios for that thickness, and then calculating the average value or the B reference value as the second ratio corresponding to that thickness.

[0073] In some alternative implementations, step S4 further includes:

[0074] Step S41: Determine the thickness ratio of the given aerospace composite material structure to a reference thickness;

[0075] Step S42: Interpolate among a plurality of second ratios based on the thickness ratio to obtain a second ratio that matches the given aerospace composite material structure;

[0076] Step S43: Calculate the second allowable value based on the interpolated second ratio.

[0077] In this embodiment, for a given thickness of an aerospace composite material structure, the original design allowable value needs to be corrected using a different second ratio.

[0078] The third type of working condition mainly considers the stress distribution in the height direction of the mechanical connection hole, such as the distribution of compressive stress, the eccentric effect of fasteners, and the superposition of bypass loads. Here, the nail hole extrusion test is used, that is, two test pieces connected by screws are stretched at both ends. The test plan is shown in Table 3.

[0079] Table 3. Nail Hole Extrusion Test Planning Table

[0080]

[0081] It should be noted that the Thickness 1 test is based on standards such as ASTM or GB and can be conducted using single or double shear, depending on the actual application. Other thickness tests are conducted on the basis of the Thickness 1 test, while maintaining consistency in the specimen treatment and test environment.

[0082] In some alternative implementations, in step S5, multiple third ratios at any thickness are processed using the average value of multiple batch tests or the B benchmark value of multiple batch tests.

[0083] In this embodiment, step S5 involves conducting multiple tests for each thickness (compared to the reference thickness) to obtain multiple third ratios for that thickness, and then calculating the average value or the B reference value as the third ratio corresponding to that thickness.

[0084] In some alternative implementations, step S6 further includes:

[0085] Step S61: Determine the thickness ratio of the given aerospace composite material structure to a reference thickness;

[0086] Step S62: Based on the thickness ratio, interpolate among a plurality of third ratios to obtain a third ratio that matches the given aerospace composite material structure;

[0087] Step S63: Calculate the third allowable value based on the interpolated third ratio.

[0088] In this embodiment, for a given thickness of an aerospace composite material structure, the original design allowable value needs to be corrected using different third ratios.

[0089] The fourth type of working condition mainly considers the local stress state and allowable value control of the structure with damage / process defects. The test adopted is the impact compression test, that is, the test piece is subjected to a drop hammer impact in the middle and then the test piece is compressed at both ends. The test plan is shown in Table 4.

[0090] Table 4 Post-impact compression test planning table

[0091]

[0092] In some alternative implementations, in step S7, the multiple fourth ratios at any thickness are processed using the average value of multiple batch tests or the B benchmark value of multiple batch tests.

[0093] In this embodiment, step S7 performs multiple tests for each thickness (compared to the reference thickness) to obtain multiple fourth ratios for that thickness, and then calculates the average value or the B reference value as the fourth ratio corresponding to that thickness.

[0094] In some alternative implementations, in step S7, a constant energy or a constant dent depth is used to ensure that the impact damage degree of composite materials of different thicknesses is consistent.

[0095] In this embodiment, the test for thickness 1 is first carried out based on standards such as ASTM or GB. Then, for the test for other thicknesses, a constant energy or constant pit depth method is selected according to the specific application of the part to ensure the consistency of the test conditions. In addition, it is also necessary to keep the test piece processing and test environment consistent.

[0096] In some alternative implementations, step S8 further includes:

[0097] Step S81: Determine the thickness ratio of the given aerospace composite material structure to a reference thickness;

[0098] Step S82: Based on the thickness ratio, interpolate among a plurality of fourth ratio values ​​to obtain a fourth ratio value that matches the given aerospace composite material structure;

[0099] Step S83: Calculate the fourth allowable value based on the interpolated fourth ratio.

[0100] In this embodiment, for a given thickness of an aerospace composite material structure, different fourth ratios are needed to correct the original design allowable value.

[0101] This application conducts experiments to determine the allowable values ​​of composite material structures based on the above four project conditions, plans the relevant allowable value test matrix for composite materials, and, based on the obtained test results, realizes the control of the allowable values ​​of composite material structures considering the influence of thickness, thereby achieving a reasonable, safe, and effective design of the relevant composite material structures.

[0102] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for determining an allowable value of an aerospace composite structure, characterized by, The method comprises: (1) determining a first allowable value of tensile and compressive strength of a composite structure under a first working condition, the first working condition being a working condition of a specified thickness composite structure formed by end lap under the influence of deformation incompatibility load, and the determination of the first allowable value comprising: Step S1, obtaining a first ratio of the failure strength of a single-sided lap tensile test to the failure strength of a single composite tensile test; Step S2, correcting the given allowable value of the tensile and compressive strength of the aviation composite structure according to the first ratio to obtain the first allowable value; (2) determining a second allowable value of shear strength of a composite structure under a second working condition, the second working condition being a working condition of a specified thickness composite structure formed by bonding under the influence of shear load, and the determination of the second allowable value comprising: Step S3, obtaining a plurality of second ratios of the shear failure strength of a composite material of different thicknesses formed by bonding to the shear failure strength of a single composite material; Step S4, correcting the given allowable value of the shear strength of the aviation composite structure according to the second ratio to obtain a plurality of second allowable values; (3) determining a third allowable value of tensile strength of a composite structure under a third working condition, the third working condition being a working condition of a specified thickness composite structure formed by screw connection under the influence of tensile load, and the determination of the third allowable value comprising: Step S5, obtaining a plurality of third ratios of the tensile failure strength of a composite material of different thicknesses formed by screw connection to the tensile failure strength of a single composite material; Step S6, correcting the given allowable value of the tensile strength of the aviation composite structure according to the third ratio to obtain a plurality of third allowable values; (4) determining a fourth allowable value of tensile strength of a composite structure under a fourth working condition, the fourth working condition being a working condition of a specified thickness composite structure after impact failure under the influence of compression load, and the determination of the fourth allowable value comprising: Step S7, obtaining a plurality of fourth ratios of the compression failure strength of a composite material of different thicknesses after impact failure to the compression failure strength of a single composite material; Step S8, correcting the given allowable value of the compression strength of the aviation composite structure according to the fourth ratio to obtain a plurality of fourth allowable values.

2. The method of determining an aerospace composite material structure allowable according to claim 1, wherein, In step S1, the first ratio is determined by using the average value of multiple batches of tests or the B reference value of multiple batches of tests.

3. The method of determining an aerospace composite material structure allowable according to claim 1, wherein, In step S3, for a plurality of second ratios under any thickness, the average value of multiple batches of tests or the B reference value of multiple batches of tests is used for processing.

4. The method of determining an aerospace composite material structure allowable according to claim 1, wherein, Step S41, determining a thickness ratio of the thickness of the given aviation composite structure compared to the reference thickness; Step S42, obtaining a second ratio matching the given aviation composite structure from the plurality of second ratios according to the thickness ratio; Step S43, calculating the second allowable value based on the interpolated second ratio. In step S5, for a plurality of third ratios under any thickness, the average value of multiple batches of tests or the B reference value of multiple batches of tests is used for processing.

5. The method of determining an aerospace composite material structure allowable according to claim 1, wherein, In step S6, further comprising:

6. The method of determining an aerospace composite material structure allowable according to claim 1, wherein, ​ Step S61, determining a thickness ratio of the thickness of the given aviation composite structure compared to the reference thickness; Step S62, interpolating a third ratio value matching the given aviation composite structure from the plurality of third ratio values according to the thickness ratio; Step S63, calculating a third allowable value based on the interpolated third ratio value.

7. The method of determining an aerospace composite material structure allowable according to claim 1, wherein, In step S7, the average of multiple batches of tests or the B reference value of multiple batches of tests is used to process the plurality of fourth ratio values at any thickness.

8. The method of determining an aerospace composite material structure allowable according to claim 1, wherein, In step S7, the impact damage degree of the composite material of different thicknesses is ensured to be consistent in a constant energy or constant pit depth manner.

9. The method of determining an aerospace composite material structure allowable according to claim 1, wherein, Step S8 further comprises: Step S81, determining a thickness ratio of the thickness of the given aviation composite structure compared to the reference thickness; Step S82, interpolating a fourth ratio value matching the given aviation composite structure from the plurality of fourth ratio values according to the thickness ratio; Step S83, calculating a fourth allowable value based on the interpolated fourth ratio value.

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