An indirect test method for the interfacial shear strength of composite fiber-matrix based on brazilian splitting method

Abstract

The application discloses a composite material fiber-matrix interface shear strength indirect testing method based on a Brazilian splitting method, which comprises the following steps: step 1, determining the fiber length and the Brazilian disc sample size according to preset conditions, and preparing a single fiber Brazilian sample; step 2, carrying out a compression Brazilian destruction test, obtaining a load-displacement curve and a sample section; step 3, analyzing the sample section, if the section shows a fiber pull-out state, the test data is valid, if the section shows that the fiber is not pulled out, the test data is invalid, jumping to step 1, repeating steps 1-3 until the test data is valid; step 4, obtaining a load value Fmax before a load sudden drop through the load-displacement curve; step 5, establishing a fiber Brazilian test simulation model through a finite element software, inputting the load value Fmax, and obtaining a fiber tensile stress σ f ; and step 6, calculating and obtaining a fiber / matrix interface shear strength.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of composite materials, and more particularly relates to an indirect testing method for the interfacial shear strength of fibers-matrix of composite materials based on the Brazilian splitting method. BACKGROUND

[0002] Carbon fibers and other materials have been widely used in composite materials due to their excellent tensile strength to significantly improve the load-bearing performance of structural components. The interfacial shear strength between fibers and matrix is a key factor restricting the load-bearing performance of composite materials, and therefore, the accurate testing of the interfacial shear strength is very important. The commonly used testing methods for the interfacial strength of fibers-matrix generally include single fiber pull-out method, critical length method, indentation method, and ejection method, etc. However, due to the brittle characteristics of carbon fibers and other materials and the small diameter (um~mm) of the fibers, the fibers have poor shear resistance and are prone to unstable fracture under pressure,

[0003] Therefore, the single fiber pull-out method has the problems of difficult sample preparation and the inability to clamp the fiber end, the indentation method has the problem of unstable fracture of single fibers, the ejection method has the problem of inaccurate loading, and the critical length method (single fiber composite material tensile method) requires the constraints of transparent matrix material, matrix failure strain greater than fiber, and known critical length fiber tensile strength. Therefore, there is still a lack of effective testing method for the interfacial shear strength of brittle fiber / matrix. SUMMARY

[0004] In view of the deficiencies in the prior art, the present application provides a testing method for the interfacial shear strength of fiber / matrix based on the single fiber Brazilian test method. The method solves the problem of difficult testing of the interfacial strength of brittle fiber / matrix, and also solves the problem of difficult sample preparation. The method can be applied to both cast and press-formed composite materials, and can also be applied to the testing of the interfacial shear strength of other non-brittle fiber / matrix.

[0005] To achieve the above-mentioned application purposes, the present application adopts the following technical solutions:

[0006] An indirect testing method for the interfacial shear strength of fibers-matrix of composite materials based on the Brazilian splitting method, comprising the following steps:

[0007] Step 1: preliminarily determine the fiber length and the size of the Brazilian disc sample according to the preset conditions, and prepare a single fiber Brazilian sample;

[0008] Step 2: perform a compression Brazilian failure test to obtain a load-displacement curve and a sample fracture surface;

[0009] Step 3: analyze the sample section, if the section shows that the fiber is pulled out, the test data is valid, if the section shows that the fiber is not pulled out, the test data is invalid, jump to step 1, repeat steps 1-3 until the test data is valid;

[0010] Step 4: obtain the load value Fmax before the load drop through the load-displacement curve;

[0011] Step 5: establish a fiber brazil test simulation model through finite element software, input the load value Fmax, and calculate the fiber tensile stress σ f ;

[0012] Step 6: calculate the fiber / matrix interface shear strength.

[0013] In step 1, the preset condition is the interface shear strength estimate value and formula (1)

[0014]

[0015] In the formula, σ f0 is the tensile strength of the fiber, σ f is the axial tensile stress borne by the fiber, d is the fiber diameter, b is the fiber length, and τ f is the shear strength.

[0016] In step 3, jump to step 1, repeat steps 1-3 until the test data is valid; including jumping to step 1, reducing the fiber length according to formula (1), and repeating steps 1-3 until the test data is valid.

[0017] In step 5, the fiber tensile stress σ f is obtained by formula (2) or formula (2).

[0018]

[0019] In the formula, F max is the failure load, D is the diameter of the brazil sample, h is the thickness of the brazil sample, E b is the elastic modulus of the matrix, and E f is the elastic modulus of the fiber.

[0020] In step 6, the fiber / matrix interface shear strength is obtained by formula (3);

[0021]

[0022] In the formula, d is the fiber diameter, b is the fiber length, and σ f is the tensile stress borne by the fiber center position at failure.

[0023] Compared with the prior art, the method has the advantages of simple sample preparation, convenient testing process and wide applicability, and is suitable for testing the interfacial shear strength of brittle fibers such as carbon fibers and matrices, and is also suitable for testing the interfacial shear strength of other fiber composites, and is suitable for fields such as compression molding composites and casting molding composites. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a schematic diagram of a single fiber Brazil sample;

[0025] Figure 2 It is a schematic diagram of a single fiber Brazil loading direction;

[0026] Figure 3 It is a typical load-displacement / time curve of a Brazil test;

[0027] Figure 4 It is a typical single fiber Brazil cross-sectional view. DETAILED DESCRIPTION

[0028] The present application will be further described below in conjunction with examples, and the described examples are only a part of the examples of the present application, and are not all the examples. Based on the examples in the present application, other examples obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0029] The object of the present application is to provide a brittle fiber / matrix interfacial shear strength testing method with simple sample preparation, convenient testing process and wide application range, which can be applied to the fields of compression / casting explosives, ceramics and other materials.

[0030] To realize the above testing method, the basic idea of the present application is as follows: a single fiber Brazil sample as shown in Figure 1 is prepared by compression / casting, a force load is applied perpendicular to the fiber direction (such as Figure 2 ), the tensile stress at the center position of the Brazil sample is the largest, and as the load increases, the tensile stress at the center position of the Brazil sample will pull out the fiber, and the load borne by the instantaneous sample after pulling out will instantaneously drop, and the interfacial shear strength of the fiber / matrix can be converted according to the sudden drop load F max . f The calculation formula is as follows:

[0031]

[0032] In the formula, d is the fiber diameter, b is the fiber length, σ f is the tensile stress borne by the fiber center position at failure

[0033] The determination method of σ f can be divided into two kinds:

[0034] ① Formula method (convenient, relatively large error, when the interface strength precision requirement is not high or only horizontal comparison is used): assuming that the presence of the fiber has little effect on the overall strain distribution, at this time according to the strain compatibility condition, the fiber bears stress:

[0035]

[0036] In the formula, F max is the failure load, D is the diameter of the Brazilian specimen, h is the thickness of the Brazilian specimen, E b is the elastic modulus of the matrix, E f is the elastic modulus of the fiber.

[0037] ② Simulation method (accurate, relatively poor convenience): a numerical simulation model of the single fiber Brazilian loading process is established by the finite element method, and the matrix and fiber material parameters are respectively assigned, and the load F max is applied, and the axial tensile stress σ f borne by the fiber is calculated and obtained.

[0038] Content to be explained: the indirect test method of the fiber-matrix interface shear strength of the composite material based on the Brazilian split method requires the fiber to be pulled out from the matrix, so the matrix and the fiber will not fail before the fiber is pulled out. Therefore, the application limitation of this method is: ① the fiber failure strain is higher than the matrix material ε f0 > ε b0 ; ② the stress borne by the fiber during pulling out is lower than the fiber tensile strength, that is,

[0039]

[0040] In the formula, σ f0 is the fiber tensile strength, σ f is the axial tensile stress borne by the fiber, d is the fiber diameter, b is the fiber length, τ f is the shear strength.

[0041] Specific implementation:

[0042] 1) According to the estimated value of the interface shear strength and formula (1), the fiber length and the Brazilian disc specimen size are initially determined, and a single fiber Brazilian specimen is prepared;

[0043] 2) Develop a compression Brazilian failure test to obtain the load-displacement curve and the specimen fracture surface;

[0044] 3) Analyze the specimen fracture surface. If the fracture surface shows that the fiber is pulled out (such as Figure 4 a), the test data is valid, and if the fracture surface shows that the fiber has not been pulled out (the fiber is broken or the matrix produces cracks, such as Figure 4b), the test data is invalid, the fiber length is reduced according to formula (1), and step 1 is jumped to;

[0045] 4) Obtain the load value Fmax before the load drop through the load-displacement curve (such as Figure 3 );

[0046] 5) Establish a fiber Brazilian test simulation model through a finite element software, input the load value Fmax, and calculate to obtain the fiber tensile stress σ f ; or calculate the fiber tensile stress through formula (2);

[0047] 6) Calculate the fiber / matrix interface shear strength through formula (3).

[0048] Example 1

[0049] The invention is successfully applied to explosive materials of the Institute of Chemical and Material Engineering of the China Academy of Engineering Physics, realizes indirect testing of the brittle fiber / matrix interface shear strength, and achieves good application effect. The method of the invention can also be applied to testing of the interface shear strength of other non-brittle fiber / matrix.

[0050] The selected Brazilian sample has a diameter D = 20 mm and a thickness h = 6 mm, the elastic modulus of the matrix material is E b = 6 GPa, the elastic modulus of the selected fiber is E f = 300 GPa, the fiber diameter is d = 0.218 mm, and the fiber tensile strength is σ f0 = 2600 MPa.

[0051] 1) The interface shear strength is estimated as τ = 3 MPa, according to formula (1), the fiber length needs to satisfy

[0052]

[0053] Since the diameter of the Brazilian sample is only 20 mm, the fiber length is selected as b = 10 mm to prepare a single fiber Brazilian sample;

[0054] 2) Perform a compression Brazilian destruction test to obtain a load-displacement curve and a sample cross section;

[0055] 3) Analyze the sample cross section, and the cross section shows a fiber pull-out state (such as Figure 4 a), the test data is valid;

[0056] 4) Obtain the load value Fmax = 850 N before the load drop through the load-displacement curve (such as Figure 3 );

[0057] 5) Calculate the fiber tensile stress through formula (2);

[0058]

[0059] 6) Calculate the fiber / matrix interfacial shear strength by equation (3).

[0060]

[0061] The interfacial shear strength in this case is 2.45 MPa.

[0062] The above merely provides the preferred embodiments of the present application, and is not used to limit the present application, and any modification, equivalent replacement, and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. An indirect method for testing the shear strength of the fiber-matrix interface in composite materials based on the Brazilian splitting method, characterized in that, Includes the following steps: Step 1: Determine the fiber length and Brazilian disc sample size according to the preset conditions, and prepare a single fiber Brazilian sample; Step 2: Conduct a compression failure test to obtain the load-displacement curve and the cross-section of the specimen; Step 3: Analyze the cross-section of the sample. If the cross-section shows that the fiber has been pulled out, the test data is valid. If the cross-section shows that the fiber has not been pulled out, the test data is invalid. Go back to step 1 and repeat steps 1-3 until the test data is valid. Step 4: Obtain the load value Fmax before the sudden load drop using the load-displacement curve; Step 5: Establish a fiber Brazilian test simulation model using finite element software, input the load value Fmax, and calculate the fiber tensile stress σ. f ; Step 6: Calculate the fiber / matrix interface shear strength.

2. The indirect testing method for the fiber-matrix interface shear strength of composite materials based on the Brazilian splitting method according to claim 1, characterized in that, In step 1, the preset conditions are the estimated value of the interfacial shear strength and equation (1). In the formula, σ f0 σ represents the tensile strength of the fiber. f τ is the axial tensile stress borne by the fiber, d is the fiber diameter, b is the fiber length, and τ is the axial tensile stress borne by the fiber. f This represents the shear strength.

3. The indirect testing method for the fiber-matrix interface shear strength of composite materials based on the Brazilian splitting method according to claim 2, characterized in that, In step 3, jump to step 1 and repeat steps 1-3 until the test data is valid; including jumping to step 1, reducing the fiber length according to formula (1), and repeating steps 1-3 until the test data is valid.

4. The indirect testing method for the fiber-matrix interface shear strength of composite materials based on the Brazilian splitting method according to claim 2, characterized in that, In step 5, the fiber tensile stress σ f Alternatively, it can be calculated using formula (2); In the formula, F max The failure load is given by D, where D is the diameter of the Brazilian specimen, h is the thickness of the Brazilian specimen, and E is the thickness of the specimen. b E represents the elastic modulus of the matrix. f This represents the fiber's elastic modulus.

5. The indirect testing method for the fiber-matrix interface shear strength of composite materials based on the Brazilian splitting method according to claim 2, characterized in that, In step 6, the fiber / matrix interface shear strength is calculated using equation (3); In the formula, d is the fiber diameter, b is the fiber length, and σ f This represents the tensile stress at the center of the fiber at the time of failure.

Images