Composite material / metal glue joint structure II-type fracture performance evaluation method based on correction end load splitting

Abstract

The invention discloses a composite material / metal cementing structure II-type fracture performance evaluation method based on correction of end load splitting, and belongs to the field of cementing structure fracture mechanics. Comprising the following steps: 1) designing the thickness of a composite material and a metal cementing piece to obtain a cementing sample; 2) carrying out a clamp correction test on the glued sample to obtain clamp correction parameters; 3) calculating the equivalent bending modulus of the glued sample; (4) carrying out a cementing sample calibration end load splitting test to obtain II-type fracture data; and 5) based on the clamp correction parameters, the equivalent bending modulus, the II-type fracture data and the like, introducing an equivalent crack length, establishing a composite material / metal cementing structure II-type fracture toughness calculation model corrected by the coupling clamp, and solving the cementing sample II-type fracture toughness. According to the method, the thickness size design of the composite material / metal bonding sample, the difference between the material performance and the geometric size and the influence of a test fixture on the II-type fracture toughness calculation result are considered, and the accuracy of the test result is improved.

Description

Technical Field

[0001] The present invention relates to the field of fracture mechanics of bonded structures, and in particular to a method for evaluating mode II fracture performance of composite / metal bonded structures based on corrected end load splitting. Background Art

[0002] Evaluating the fracture performance of adhesively bonded structures is a crucial step in structural design. Compared to homogeneous bonded structures, dissimilar materials such as composites and metals are more complex to test due to differences in material properties and geometric discontinuities. Therefore, developing accurate fracture performance testing methods for dissimilar bonded structures is imperative.

[0003] However, unlike structures bonded together using the same material, due to the lack of fracture performance testing standards for structures bonded together using dissimilar materials, the current fracture performance testing of dissimilar material bonded structures such as composite materials / metals can usually only directly use the testing methods for structures bonded together using the same material. This can easily lead to large deviations and makes it difficult to achieve a unified quantitative evaluation, affecting the accuracy and reliability of the results. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for evaluating the type II fracture performance of composite / metal bonded structures based on corrected end load splitting. The method of the present invention not only takes into account the thickness size design of the composite / metal bonded specimens, but also takes into account the material properties and geometric size differences of the bonded specimens and the influence of the test fixture on the calculation results of the type II fracture toughness. Combined with the theory of fracture mechanics, a reliable testing method is provided for the evaluation of the type II fracture performance of composite / metal bonded structures.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for evaluating the mode II fracture performance of composite / metal bonded structures based on corrected end load splitting comprises the following steps:

[0007] 1) By matching the bonding surface strain of the bonded parts, the thickness of the composite material and the metal bonded parts is designed to obtain the bonded specimen, ensuring that the bonded specimen has a pure mode II fracture during the test;

[0008] 2) conducting a fixture calibration test based on reverse correction end load splitting on the bonded specimen to obtain fixture calibration parameters, wherein the fixture calibration parameters include a fixture calibration factor and a calibration curve slope;

[0009] 3) calculating the equivalent bending modulus of the bonded specimen based on the slope of the calibration curve in the fixture calibration parameters;

[0010] 4) Conducting a calibrated end-load split test on the bonded specimen to obtain Mode II fracture data, including crack length and corresponding load and displacement;

[0011] 5) Based on the fixture correction factor in the fixture correction parameters of step 2), the equivalent bending modulus of step 3), and the mode II fracture data of step 4), the equivalent crack length is introduced to establish a coupled fixture-corrected mode II fracture toughness calculation model for composite / metal bonded structures, and the mode II fracture toughness of the bonded specimens is obtained.

[0012] Compared with the prior art, the present invention has the following advantages:

[0013] 1) Dissimilar materials, such as composites and metals, exhibit significant differences in material properties and geometric dimensions, leading to significant deviations in the fracture performance test results obtained using methods for bonding structures made of the same material. This invention considers these differences in material properties between composite and metal bonded components and designs the thickness of the composite and metal bonded components by matching the bond surface strain, ensuring a pure Mode II fracture during bonded specimen testing.

[0014] 2) Differences in material properties and geometric dimensions between dissimilar materials, such as composites and metals, render fracture toughness calculation models for homogeneous materials unsuitable. This paper considers the influence of differences in material properties and geometric dimensions of bonded specimens, as well as the test fixture, on the calculated results of Mode II fracture toughness. A coupled fixture-influenced Mode II fracture toughness calculation model for composite / metal bonded structures is proposed, improving the accuracy of fracture toughness test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0016] Figure 1 is a flow chart of the calculation method of the present invention;

[0017] Figure 2 Compliance-free length curve of the calibrated fixture for carbon fiber composite / titanium alloy bonding specimens;

[0018] Figure 3 This is the mode II fracture toughness R curve of carbon fiber composite material / titanium alloy bonded structure. DETAILED DESCRIPTION

[0019] In order to further clarify the purpose, technical solutions and advantages of the present invention, specific implementation methods of the present invention are described in detail in conjunction with examples and drawings. The examples are only used to explain the present invention and are not intended to limit the present invention.

[0020] A method for evaluating the mode II fracture performance of composite / metal bonded structures based on corrected end load splitting, such as Figure 1 As shown, the following steps are included:

[0021] 1) The thickness of the composite material and metal bonded parts is designed by matching the bonded surface strain to ensure that the bonded layer has a pure type II fracture during the bonded specimen test. The calculation formula is as follows:

[0022]

[0023] Where E1 and E2 are the elastic moduli of the composite material and metal bonded parts, respectively; h1 and h2 are the thickness of the composite material and metal bonded parts, respectively. The recommended thickness range of h1 and h2 for the bonded parts is 2-4 mm.

[0024] 2) A fixture calibration test based on reverse-correction end-load splitting was conducted on composite / metal bonded structures. The specific method was as follows: Loads were applied to different free lengths (50 to 110 mm) on the uncracked side of the bonded specimen. Displacement-load curves for each free length were plotted, and the linear segments of the curves were fitted with a linear regression fit to obtain the slope of the curve, which is the specimen compliance. Based on the desired compliance values at different free lengths, a curve of the cube root of the specimen compliance versus free length was plotted. The slope of the curve is the fixture calibration curve slope, and the absolute value of the intercept of the curve with the displacement axis is the fixture calibration factor, ΔClamp. Here, the different free lengths refer to the span between the loaded end of the bonded specimen and the fixed end of the fixture.

[0025] 3) Based on the fixture calibration parameters in step 2), calculate the equivalent bending modulus E of the composite / metal bonded specimen f , the calculation formula is as follows:

[0026]

[0027] Where B is the width of the bonding specimen.

[0028] 4) Conduct a calibrated end-load splitting test on composite / metal bonded specimens. Load the prefabricated crack end of the specimen at a constant rate of 0.5 mm / min until the crack extends to within 10 mm of the fixture. Record the displacement δ and load P of the bonded specimen at different crack lengths a during the test to calculate the compliance C of the specimen at different crack lengths. The calculation formula is as follows:

[0029]

[0030] 5) Introducing equivalent crack length a e, establish a coupling fixture-corrected composite / metal bonded structure mode II fracture toughness calculation model, and according to the displacement and load corresponding to different crack lengths obtained in step 4), obtain the mode II fracture toughness of the composite / metal bonded specimen under different crack lengths, and take the average value as the evaluation result of the mode II fracture performance of the bonded specimen.

[0031] Among them, the fracture toughness calculation model is as follows:

[0032]

[0033] Where k is the ratio of the bending stiffness of the metal and composite bonded parts, and the calculation formula is k = E2I2 / E1I1, I1 and I2 are the moments of inertia of the composite and metal bonded parts, and the equivalent crack length a is e The calculation formula is as follows:

[0034]

[0035] Where C is the compliance corresponding to different crack lengths of the bonded specimen obtained in step 4), and l is the span between the loading end of the bonded specimen and the fixed end of the fixture, that is, the free length.

[0036] The parameters that need to be known in the present invention include: elastic moduli E1 and E2 of the composite material and the metal bonded component, thicknesses h1 and h2, and width B of the bonded sample.

[0037] In a specific implementation of the present invention, taking a carbon fiber composite material / titanium alloy bonded structure as an example, according to Figure 1 The calculation method flow shown in the figure mainly includes the following steps to evaluate the mode II fracture performance of carbon fiber composite material / titanium alloy bonded structure:

[0038] 1) By matching the bonding surface strain of the bonding parts, the thicknesses of the carbon fiber composite material and the titanium alloy bonding parts are designed to be h1 = 2.375 mm and h2 = 2.5 mm, respectively.

[0039] 2) Carry out reverse correction end load splitting test on carbon fiber composite material / titanium alloy bonded specimens to obtain Figure 2 The flexibility-free length curve (Cl curve) shown in the figure is used to obtain the clamp correction factor ΔClamp=32.35 and the correction curve slope Slope=0.002 based on the curve intercept and slope, respectively.

[0040] 3) Based on the fixture calibration parameters in step 2), calculate the equivalent bending modulus E of the carbon fiber composite material / titanium alloy bonded specimen f , the calculation formula is as follows:

[0041]

[0042] Where B is the width of the bonding specimen; Substitute the parameter Slope obtained in the previous steps and the known quantities h1, h2, and B into the above formula to obtain the equivalent bending modulus E f In this embodiment, E f =215.78GPa.

[0043] 4) Carry out the calibrated end load splitting test of carbon fiber composite material / titanium alloy bonded specimens, and record the load P and displacement δ corresponding to the crack length a of the bonded specimen in the range of 60 to 90 mm.

[0044] 5) Introducing equivalent crack length a e , the calculation model of mode II fracture toughness of carbon fiber composite material / titanium alloy bonded structure with coupling fixture correction is established as follows:

[0045]

[0046] Where k is the ratio of the bending stiffness of the titanium alloy and carbon fiber composite bonded parts, and the calculation formula is k = E2I2 / E1I1, E1 and E2 are the elastic modulus of the carbon fiber composite and titanium alloy bonded parts, I1 and I2 are the moments of inertia of the carbon fiber composite and titanium alloy bonded parts, and the equivalent crack length a is e The calculation formula is as follows:

[0047]

[0048] Wherein, C is the compliance corresponding to different crack lengths obtained in step 4) based on the load P and displacement δ corresponding to the crack length a of the adhesive specimen in the range of 60 to 90 mm obtained from the experiment, and l is the span between the loading end of the adhesive specimen and the fixed end of the fixture. In this embodiment, l = 100 mm.

[0049] By obtaining the mode II fracture toughness R curve of carbon fiber composite material / titanium alloy bonding structure under different crack lengths, Figure 3 As shown in the figure, the mean value of the mode II fracture toughness is 3.04 N / mm. Repeating the above calculation using five identical specimens shows that this performance evaluation method is stable and reliable.

[0050] The above examples are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above examples, and many variations are possible. All variations that can be directly derived or imagined by a person skilled in the art from the disclosure of the present invention should be considered to be within the scope of protection of the present invention.

Claims

1. A method for evaluating the mode II fracture performance of composite / metal bonded structures based on corrected end load splitting, characterized in that: The following steps are involved: 1) By matching the bonding surface strain of the bonded parts, the thickness of the composite material and the metal bonded parts is designed to obtain the bonded specimen, ensuring that the bonded specimen has a pure mode II fracture during the test; 2) conducting a fixture calibration test based on reverse correction end load splitting on the bonded specimen to obtain fixture calibration parameters, wherein the fixture calibration parameters include a fixture calibration factor and a calibration curve slope; 3) calculating the equivalent bending modulus of the bonded specimen based on the slope of the calibration curve in the fixture calibration parameters; 4) Conducting a calibrated end-load split test on the bonded specimen to obtain Mode II fracture data, including crack length and corresponding load and displacement; 5) Based on the fixture correction factor in the fixture correction parameters of step 2), the equivalent bending modulus of step 3), and the mode II fracture data of step 4), the equivalent crack length is introduced to establish a coupled fixture-corrected mode II fracture toughness calculation model for composite / metal bonded structures, and the mode II fracture toughness of the bonded specimens is obtained.

2. The method for evaluating the mode II fracture performance of composite / metal bonded structures based on corrected end load splitting according to claim 1, characterized in that: In step 1), the thickness of the composite material and the metal bonded parts is designed to be in the range of 2 to 4 mm, and the thickness design of the two parts follows the principle of strain matching on the bonding surface of the bonded parts as follows: Where E1 and E2 are the elastic moduli of the composite material and the metal bonded parts, respectively; h1 and h2 are the thickness of the composite material and the metal bonded parts, respectively.

3. The method for evaluating the mode II fracture performance of composite / metal bonded structures based on corrected end load splitting according to claim 1, characterized in that: The specific method of the fixture calibration test based on the reverse calibration end load splitting described in step 2) is as follows: By applying loads to different free lengths in the range of 50 to 110 mm on the side of the bonded specimen without pre-cracks, the displacement-load curve of the specimen at each free length is plotted, and the linear regression fitting of the linear segment of the curve is performed to obtain the slope of the curve, which is the specimen flexibility; according to the flexibility values of the specimen at different free lengths, the cube root of the specimen flexibility-free length curve is plotted, and the slope of the curve is the slope of the fixture correction curve, and the absolute value of the intercept of the curve with the displacement axis is the fixture correction factor.

4. The method for evaluating the mode II fracture performance of composite / metal bonded structures based on corrected end load splitting according to claim 1, characterized in that: In step 3), based on the slope of the calibration curve in the fixture calibration parameters, the equivalent bending modulus is calculated as follows: Where B is the width of the bonding specimen, h1 and h2 are the thickness of the composite material and the metal bonding part, Slope is the slope of the calibration curve, and E f is the equivalent bending modulus.

5. The method for evaluating the mode II fracture performance of composite / metal bonded structures based on corrected end load splitting according to claim 1, characterized in that: In step 4), the bonded specimens were subjected to a load splitting test at a constant rate of 0.5 mm / min until the crack expanded to within 10 mm of the fixture. The displacement and load at different crack lengths recorded in the test were used to calculate the compliance of the specimens at different crack lengths.

6. The method for evaluating the mode II fracture performance of composite / metal bonded structures based on corrected end load splitting according to claim 1, characterized in that: The calculation formula of the equivalent crack length in step 5) is as follows: Where a e is the equivalent crack length, C is the compliance corresponding to a certain crack length, l is the free length between the loading end of the adhesive specimen and the fixed end of the fixture, E f is the equivalent bending modulus, B is the width of the bonded specimen, h1 and h2 are the thickness of the composite and metal bonded parts, ΔClamp is the clamp correction factor, k is the ratio of the bending stiffness of the metal and composite bonded parts, and E1 and E2 are the elastic moduli of the composite and metal bonded parts, respectively.

7. The method for evaluating mode II fracture performance of composite / metal bonded structures based on corrected end load splitting according to claim 6, characterized in that: The calculation model for the mode II fracture toughness of the composite / metal bonded structure corrected by the coupling fixture described in step 5) is as follows: Where G Ⅱ is the fracture toughness, and P is the load at the loading end of the bonded specimen.

8. The method for evaluating mode II fracture performance of composite / metal bonded structures based on corrected end load splitting according to claim 1, characterized in that: In step 5), the average of the mode II fracture toughness values calculated for the bonded specimens at different crack lengths is taken as the final mode II fracture performance evaluation result.

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