Prepreg modulus testing device and method

Abstract

The invention discloses a prepreg modulus testing device which comprises a two-dimensional digital image correlation system (2D-DIC), a camera support and a mechanical testing machine. Wherein the 2D-DIC system comprises a CCD (Charge Coupled Device) camera, a lens, two monochromatic sources, a computer and a data line; the camera support is used for supporting a camera and a lens. The two monochromatic sources are used for emitting monochromatic light to a sample to be detected, and the monochromatic light is white light; the computer is used for processing the acquired image and calculating strain; the data line is used for transmitting images collected by the camera. The mechanical testing machine comprises a tensile fixture and a high-temperature environment box and is used for stretching a sample and collecting stress data. The testing method can be applied to the fields of unidirectional single-layer prepregs, mechanics and the like, solves the problem that the modulus of the unidirectional single-layer prepregs is difficult to test, accurately measures E1, E2 and G12, and provides reference suggestions for the molding process design of subsequent composite materials.

Description

Technical Field

[0001] The invention belongs to the field of materials science and mechanics, and in particular relates to a prepreg modulus measurement device and method. Background Art

[0002] Domestic carbon fiber reinforced resin-based composite materials have been widely used in aerospace, new energy, automobile and rail transportation, sports and leisure, construction and infrastructure, and the industry scale has expanded rapidly. Prepreg is an intermediate product for manufacturing composite materials, and its mechanical properties data will affect the subsequent molding design of composite materials. Therefore, accurate measurement of prepreg modulus is of great significance for guiding the design of material molding process. Due to its thin and soft characteristics, it is generally difficult to directly measure the mechanical properties of single-layer prepreg, such as tensile modulus E1 and E2, shear modulus G 12 At present, most prepreg tests are made into laminates. Laminate testing methods include uniaxial tension (E1), shear method (G 12 ), eccentric stretching method (E1, E2, G 12 ), among which the most critical part in modulus test is strain measurement, and its measurement methods include resistance strain gauge method, extensometer method, digital image correlation (DIC), etc. There are also many studies in related work.

[0003] De Baere proposed an improved design of the standard three-track shearing device and used the shear method to measure the interlaminar shear modulus of carbon fiber composite laminates. Martinc used strain gauges to collect strain data of composite laminates. Merzkirc used 10° eccentric stretching combined with DIC to test resin-reinforced carbon fiber composite laminates, and concluded that 10° eccentric stretching is an appropriate method to determine the interlaminar shear properties of resin-reinforced carbon fiber composites. Since the shear method requires holes or grooves to prepare samples, it is not suitable for unidirectional single-layer prepregs; while eccentric stretching is simple to prepare samples and has a wide range of applications. For the method of measuring strain, although contact strain measurement methods such as strain gauges and extensometers can accurately measure strain, they are not suitable for measuring the strain of soft materials such as unidirectional single-layer prepregs (such as silicone with a Shore A hardness of 10-70) due to their contact nature. The DIC method has the advantages of full-field non-contact, simple data collection, low measurement environment requirements, simple specimen surface treatment, high measurement accuracy, and a wide range of applications. It is a better method for testing the strain of unidirectional single-layer prepregs. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for measuring the modulus of a prepreg, so as to overcome the difficulty in testing the modulus of a unidirectional single-layer prepreg at different temperatures.

[0005] The technical solution adopted by the present invention to solve the technical problem is: a unidirectional single-layer prepreg modulus measurement method and device, including a two-dimensional digital image correlation system (2D-DIC) and a mechanical testing machine; wherein the 2D-DIC system includes a CCD camera, a lens, two monochromatic light sources, and a computer; the two monochromatic light sources are used to emit monochromatic light to the sample to be tested, and the monochromatic light is white light; the computer is used to process the collected image and calculate the strain. The mechanical testing machine includes a stretching fixture and a high-temperature environmental box, which are used to stretch the sample at different temperatures and collect stress data.

[0006] Another aspect of the present invention is a method for measuring the modulus of a prepreg, using the aforementioned prepreg modulus testing device, comprising the following steps:

[0007] 1) Cut the prepreg into the specified size, install the reinforcement sheet, use white paint as primer and black paint as speckle paint, randomly spray a layer of white primer on the surface of the sample that has been dipped and cured, wait for it to dry, and then spray black speckle. The diameter of the speckle is about 0.1mm, and the spacing is about 2 to 3 times the size of the speckle itself;

[0008] 2) clamping the tensile specimen obtained in step 1) on a mechanical testing machine, and setting tensile parameters such as temperature and strain rate;

[0009] 3) Actively illuminate the sample with a white light source, and adjust the position and height of the mechanical support adjustment platform so that the camera can be aligned with the test section of the tensile sample. During the test, the position and height of the CCD camera and lens are not changed to avoid errors in strain measurement. Adjust the camera's focal length, aperture size and other parameters so that the sample is located in the center of the test window and the speckle on its surface is clearly visible;

[0010] 4) Apply an initial load of 5% to the sample, and start the universal testing machine and CCD acquisition system at the same time, use the CCD to collect the image of the test piece in real time and transmit it to the computer until the sample is destroyed. During the test, do not move the position of the white light source or adjust its brightness to ensure that the brightness and contrast of the image in the test window are constant.

[0011] 5) Calculate the tensile strain data of the sample from the start of loading to the failure of the sample using the analysis software in the computer 11. The specific calculation process of tensile strain mainly includes setting the size of the calculation image sub-area, setting the step size, and calculating and storing the final strain data. Use the image acquisition frequency and the external load loading frequency for time correspondence, and process the strain data and stress data. When the prepreg sample has obvious internal defects or abnormal failure mode, it should be scrapped. When there are less than five valid samples in the same batch, repeat the test.

[0012] 6) Calculate the Young's modulus and in-plane shear modulus of the carbon fiber prepreg using the processed strain data. The tensile modulus in the fiber direction can be calculated according to formula (1):

[0013]

[0014] Where: E1 is the tensile modulus in the fiber direction, in GPa; σ L is the stress in the tensile direction of the fiber, in Pa; ε L is the strain calculated by DIC.

[0015] The shear modulus can be obtained by the 10° eccentric stretching method according to formula (2):

[0016]

[0017] G 12 is the in-plane shear modulus, in GPa; τ LT = -cosθ·sinθ·σ x , τ LT is the in-plane shear stress; LT =-2cosθ·sinθ·ε x +2cosθ·sinθ·ε y +(cos 2 θ-sin 2 θ)·γ xy , γ LT is the shear strain in the fiber direction, ε x The tensile strain in the DIC calculation is ε y The strain perpendicular to the tensile direction obtained by DIC calculation, γ xy The shear strain in the tensile direction is obtained for DIC calculation.

[0018] The tensile modulus of the fiber normal direction can be obtained by the 10° eccentric stretching method according to formula (3):

[0019]

[0020] E2 is the fiber normal tensile modulus, in GPa; σ T =sin 2 θ·σ x ,σ T is the fiber normal stress, σ x is the tensile stress, in Pa; ε T =(ε x -ε y )sin2α+γ xy cos2α,ε T is the fiber normal strain, ε xThe tensile strain in the DIC calculation is ε y The strain perpendicular to the tensile direction obtained by DIC calculation, γ xy The shear strain in the tensile direction is obtained for DIC calculation.

[0021] Poisson's ratio can be obtained according to formula (4):

[0022]

[0023] v is Poisson's ratio; ε is the strain in the tensile direction; l is the vertical strain.

[0024] The test value deviation of the above tensile performance index adopts the standard deviation, and the calculation formula is shown in (5):

[0025]

[0026] Where: S is the standard deviation; x i is a single sample data; is the sample arithmetic mean; N is the number of samples.

[0027] The present invention provides a prepreg modulus measurement device and method. Aiming at the current situation that the unidirectional single-layer prepreg is thin and soft, which makes the modulus test difficult, a DIC combined with an eccentric tensile test method is proposed to accurately measure the modulus E1, E2, G of the prepreg at different temperatures. 12 , providing reference opinions for the subsequent composite material molding process design and used to accurately measure the modulus of unidirectional single-layer prepreg.

[0028] The beneficial effects of the present invention are as follows:

[0029] 1. Use 2D-DIC to solve the problem of difficult strain measurement of unidirectional single-layer prepreg.

[0030] 2. Use eccentric stretching to solve the problem of unidirectional single-layer prepreg E2, G 12 Test difficult questions.

[0031] 3. Use left and right dual light sources instead of a single light source to ensure uniformity of lighting. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0033] Figure 1This is a prepreg modulus measuring device in Example 1.

[0034] Figure 2 This is a flow chart of the method for measuring the modulus of prepreg in Example 1.

[0035] Figure 3 is the sample size in Example 1.

[0036] Figure 4 Typical strain field diagram of the sample in Example 1 in the tensile direction of 10° eccentric stretching

[0037] In the figure: 1. High temperature environment box; 2. Tensile fixture; 3. Window (for observation); 4. White light source; 5. Lens; 6. CCD camera; 7. White light source; 8. Data cable; 9. Camera bracket; 10. Mechanical testing machine; 11. Computer DETAILED DESCRIPTION

[0038] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the experimental examples described are part of the experimental examples of the present invention, not all of the experimental examples. Based on the experimental examples in the present invention, all other experimental examples obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] like Figure 1 As shown, the present invention provides a prepreg modulus measurement device, including a two-dimensional digital image correlation system (2D-DIC), a camera bracket 9, and a mechanical testing machine 10; wherein the 2D-DIC system includes a CCD camera 6, a lens 5, two monochromatic light sources 4 and 7, a computer and a data line 8; the camera bracket 9 is used to support the camera 6 and the lens 5; the two monochromatic light sources 4 and 7 are used to emit monochromatic light to the sample to be tested, and the monochromatic light is white light; the computer is used to process the collected image and calculate the strain; the data line 8 is used to transmit the image collected by the camera 6. The mechanical testing machine 10 includes a stretching fixture 2 and a high temperature environment box 1, which are used to stretch the sample and collect stress data.

[0040] Example 1

[0041] 1) Press the prepreg Figure 3 That is, it is cut to 130mm in length and 25mm in width, and is installed on the prepreg with a cardboard reinforcement sheet, so that the size of the effective stretching part in the middle is 25mm in length and 15mm in width. After the reinforcement sheet is cured with a resin that cures quickly at room temperature, a layer of white primer is randomly sprayed on the surface of the sample that has been dipped in glue and cured, using white paint as a primer and black paint as a speckle paint. After drying, black speckles are sprayed. The diameter of the speckle is about 0.1mm, and the spacing is roughly 2 to 3 times the size of the speckle itself.

[0042] 2) clamping the reinforcing sheet of the tensile specimen obtained in step 1) on a mechanical testing machine 10, setting the temperature to 50° C. and then keeping the temperature for 5 minutes, and setting the strain rate to 0.5 mm / min;

[0043] 3) Actively illuminate the sample with white light sources 4 and 7, and adjust the position and height of the mechanical support adjustment platform of the camera bracket 9 so that the camera can be aligned with the test section of the tensile sample. During the test, the position and height of the CCD camera 6 and the lens 5 are not changed to avoid errors in strain measurement. Adjust the focal length, aperture size and other parameters of the camera 6 so that the sample is located in the center of the test window 3 and the speckle on its surface is clearly visible;

[0044] 4) Apply an initial load of 5% to the sample, and start the universal testing machine 10 and the CCD acquisition system at the same time, use the CCD to collect images of the test piece in real time and transmit them to the computer until the sample is destroyed, wherein the sampling frequency is 5fps. During the test, do not move the positions of the white light sources 4 and 7, and do not adjust their brightness, to ensure that the brightness and contrast of the image in the test window are constant.

[0045] 5) Calculate the tensile strain data of the sample from the start of loading to the failure of the sample using the analysis software in the computer 11. The specific calculation process of tensile strain mainly includes setting the calculation image sub-area size to 51, setting the step size to 5, and calculating and storing the final strain data. Use the image acquisition frequency and the external load loading frequency to correspond in time, process the strain data and stress data, and obtain the following Figure 4 The strain cloud diagram and data. When the prepreg sample has obvious internal defects or abnormal failure mode, it should be discarded. When there are less than five valid samples in the same batch, repeat the test.

[0046] 6) Calculate the Young's modulus and in-plane shear modulus of the carbon fiber prepreg using the processed strain data. The tensile modulus in the fiber direction can be calculated according to formula (1):

[0047]

[0048] Where: E1 is the tensile modulus in the fiber direction, in GPa; σ L is the stress in the tensile direction of the fiber, in Pa; ε L is the strain calculated by DIC.

[0049] The shear modulus can be obtained by the 10° eccentric stretching method according to formula (2):

[0050]

[0051] G 12 is the in-plane shear modulus, in GPa; τ LT= -cosθ·sinθ·σ x , τ LT is the in-plane shear stress; LT =-2cosθ·sinθ·ε x +2cosθ·sinθ·ε y +(cos 2 θ-sin 2 θ)·γ xy , γ LT is the shear strain in the fiber direction, ε x The tensile strain in the DIC calculation is ε y The strain perpendicular to the tensile direction obtained by DIC calculation, γ xy The shear strain in the tensile direction is obtained for DIC calculation.

[0052] The tensile modulus of the fiber normal direction can be obtained by the 10° eccentric stretching method according to formula (3):

[0053]

[0054] E2 is the fiber normal tensile modulus, in GPa; σ T =sin 2 θ·σ x ,σ T is the fiber normal stress, σ x is the tensile stress, in Pa; ε T =(ε x -ε y )sin2α+γ xy cos2α,ε T is the fiber normal strain, ε x The tensile strain in the DIC calculation is ε y The strain perpendicular to the tensile direction obtained by DIC calculation, γ xy The shear strain in the tensile direction is obtained for DIC calculation.

[0055] Poisson's ratio can be obtained according to formula (4):

[0056]

[0057] ν is Poisson's ratio; ε is the strain in the tensile direction; ε l is the vertical strain.

[0058] The test value deviation of the above tensile performance index adopts the standard deviation, and the calculation formula is shown in (5):

[0059]

[0060] Where: S is the standard deviation; x iis a single sample data; is the sample arithmetic mean; N is the number of samples.

[0061] The calculated test results are shown in Table 1.

[0062] T(℃) <![CDATA[G 12 (MPa)]]> <![CDATA[E2(MPa)]]> <![CDATA[E1(GPa)]]> v 50 886.21 120.64 16.102 0.25

[0063] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A unidirectional single-layer prepreg modulus testing device, comprising a two-dimensional digital image correlation system (2D-DIC) and a mechanical testing machine; wherein the 2D-DIC system comprises a CCD camera, a lens, two monochromatic light sources and a computer; the two monochromatic light sources are used to emit monochromatic light to the sample to be tested, and the monochromatic light is white light; the computer is used to process the collected image and calculate the strain; the mechanical testing machine comprises a stretching fixture and a high-temperature environmental chamber, which are used to stretch the sample and collect stress data.

2. The prepreg modulus testing device according to claim 1, characterized in that: The CCD camera and the lens are supported on a camera bracket.

3. A method for measuring the modulus of a unidirectional single-layer prepreg, characterized in that: Using the unidirectional single-layer prepreg modulus testing device according to any one of claims 1-2 comprises the following steps: 1) Make a tensile specimen, spray the speckle on the surface of the specimen, and place the tensile specimen on a mechanical testing machine; 2) When setting up the 2D-DIC test system, adjust it until the speckle image is clear; 3) Collect DIC data and mechanical testing machine data; 4) The acquired data is processed using 2D-DIC; 5) Use the off-axis stretching conversion principle to obtain E1, E2, G 12 .

4. The method for measuring the modulus of prepreg according to claim 4, characterized in that: The strain calculation method is to select the room temperature image as the reference image, use the 2-DIC algorithm to calculate the speckle pattern on the sample surface, and obtain the original strain data; combine the off-axis stretching conversion principle to obtain E1, E2, G 12 .

5. Use of the unidirectional single-layer prepreg modulus testing device according to any one of claims 1 to 2, characterized in that: For unidirectional single-layer prepreg E1, E2, G 12 measurement.

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