Carbon fiber composite porosity detection reference block and preparation method thereof

Abstract

The invention belongs to the technical field of nondestructive testing, and particularly relates to a carbon fiber composite porosity detection reference block and a preparation method thereof. Comprising the steps that carbon fiber prepreg is blanked according to the laying layer angle and the layer number of a detected part, and the allowance not smaller than 20 mm is reserved; carbon fiber prepreg is laid, pre-compaction is not needed during laying, hot compaction is conducted once every 2-8 layers, and after laying is completed, a test panel is obtained; auxiliary materials including a non-porous isolating membrane and a breathable felt are sequentially laid on the surface of the test plate, and then packaging is conducted; and finally, curing and demolding to obtain the reference block. The reference block with no macroscopic defect and uniform pore distribution is prepared by multiple paving, hot compacting and controlled curing processes, and the process is simple and controllable without introducing foreign substances.

Description

Technical Field

[0001] This invention belongs to the field of nondestructive testing technology, and particularly relates to a comparative test block for detecting the porosity of carbon fiber composite materials and its preparation method. Background Technology

[0002] Porosity is one of the most common internal quality defects in carbon fiber composites. The presence of porosity in composites reduces the mechanical properties of the material, such as interlaminar shear strength, longitudinal and transverse flexural strength and modulus, longitudinal and transverse tensile strength and modulus, compressive strength and modulus, impact toughness, and fatigue life. Therefore, porosity testing of carbon fiber composites is essential.

[0003] Porosity is an indicator that directly reflects the density of pore defects in composite materials. Currently, quantitative detection of composite material porosity generally falls into two categories: destructive metallographic testing and ultrasonic non-destructive testing. According to GB / T 3365 "Test Methods for Pore Content and Fiber Volume Content of Carbon Fiber Reinforced Plastics," the porosity of a sample is determined by measuring the percentage of the total pore area to the sample's cross-sectional area using an optical microscope, image analyzer, or transparent graph paper. This method is a destructive testing method.

[0004] Ultrasonic nondestructive testing (NDT) utilizes the influence of the acoustic properties and internal structure of the material on the propagation of ultrasonic waves. By detecting the degree and extent of this influence, the material's properties and structural changes can be understood. It is a nondestructive testing method that is economical, practical, and highly efficient. To determine porosity using ultrasonic NDT, comparative test blocks with different porosities need to be fabricated. By comparing the ultrasonic signal attenuation characteristics of the comparative test blocks and the composite material, the porosity of the composite material can be determined.

[0005] Invention patent CN104407060B, "A Method for Manufacturing a Composite Material Porosity Comparison Specimen," introduces glass microspheres as pre-embedded artificial defects during the laying process to simulate the manufacturing of the porosity comparison specimen. Invention patent CN107379576A, "A Method for Manufacturing a Carbon Fiber Composite Material Porosity Detection Comparison Specimen," introduces solvents during the laying process to create pores. Invention patent CN110658122B, "A Method for Manufacturing a Porosity Comparison Specimen and a Porosity Comparison Specimen," introduces soluble pre-embedded fibers during the laying process, which dissolve from the specimen to form pores. Invention patent CN104833728A, "A Composite Material Porosity Detection Standard and Its Preparation Method," introduces hollow fibers during the laying process, using the hollowness of the fibers as the pores of the composite material. All of these inventions introduce foreign substances during the laying process, either through non-removable foreign substances remaining in the specimen to form pores, or through removable foreign substances volatilizing or dissolving to form pores. The above methods have the following defects: (1) The target porosity of the porosity comparison test block is usually (0.5-2.0)%, and the porosity is relatively small. In the method of introducing foreign materials, the amount of foreign materials used is small. During the manufacturing and laying process, it is difficult to accurately place the amount of foreign materials; (2) If the placement of foreign materials is not properly controlled, it is easy to form large pores and become macroscopic defects, making the test board unusable; (3) When the ply of the comparison test block is large, such as when the number of ply layers is greater than 20, it is difficult to obtain a comparison test block with uniform pore distribution, and even macroscopic defects may occur, causing the test board to be scrapped.

[0006] Patent CN117091920A employs a fiberglass cloth resin absorption process, where the fiberglass cloth absorbs resin during curing, reducing the resin content in the control sample and creating porosity. This patent uses only multiple layers of fiberglass cloth placed on the upper and lower surfaces of the preform for resin absorption; however, the fiberglass cloth can only absorb resin from adjacent layers. When the control sample is thick or a control sample with higher porosity is desired, firstly, the multiple layers of fiberglass cloth cannot fully absorb resin, failing to achieve the desired result; secondly, the fiberglass cloth can only absorb resin from adjacent layers, and excessive absorption of surface resin can lead to macroscopic defects such as looseness or even delamination in the control sample, making it impossible to produce control samples with uniform porosity using this method. Summary of the Invention

[0007] To address the aforementioned problems in the existing technology, this invention provides a carbon fiber composite material porosity detection comparison test block and its preparation method, which does not require the introduction of foreign materials, has uniform pores, and is simple and highly versatile.

[0008] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0009] In a first aspect, the present invention provides a method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials, comprising:

[0010] The carbon fiber prepreg is cut according to the layup angle and number of layers of the part being inspected, with a margin of not less than 20mm on all sides.

[0011] Carbon fiber prepreg is laid without pre-compaction. Hot compaction is performed every 2-8 layers, and no hot compaction is performed after the final layer is laid. After the laying is completed, a test panel is obtained.

[0012] Auxiliary materials, namely a non-porous isolation film and a breathable felt, were sequentially laid on the surface of the test plate, and then sealed. Finally, the plate was cured and demolded to obtain a control test block.

[0013] Optionally, the hot compaction includes: heating to a preset heat preservation temperature under a vacuum of not less than 0.075 MPa, then heat preservation at the preset heat preservation temperature, and finally cooling to below 60°C after heat preservation.

[0014] Optionally, the preset insulation temperature is: for carbon fiber prepreg with a curing temperature ≤130℃, the preset insulation temperature range is (60-100)℃; for carbon fiber prepreg with a curing temperature of 130-180℃, the preset insulation temperature is (100-120)℃; for carbon fiber prepreg with a curing temperature >180℃, the preset insulation temperature range is (130-150)℃; and / or, the insulation temperature is maintained for 20-40 minutes at the preset insulation temperature.

[0015] Optionally, the heating and cooling rate during the heating and cooling process is 0.5-3℃ / min.

[0016] Optionally, the sample to be hot-compacted is bagged and sealed, and the vacuum degree inside the bag is controlled to be no less than 0.075 MPa during the hot-compacting process.

[0017] Optionally, hot compaction can be performed every 4-6 layers.

[0018] Optionally, the curing process includes: evacuating the entire cavity under vacuum to maintain a vacuum level of no less than 0.075 MPa, then heating to a certain temperature and curing at a pressure of 0-0.25 MPa for 4-8 hours, and finally cooling down to below 60°C to end the curing process. During the curing process, for carbon fiber prepregs with a curing temperature ≤130°C, the insulation temperature range is set to 60-100°C; for carbon fiber prepregs with a curing temperature of 130-180°C, the insulation temperature range is set to 100-120°C; and for carbon fiber prepregs with a curing temperature >180°C, the insulation temperature range is set to 130-150°C.

[0019] Optionally, during the curing process, to obtain a comparative test block with a porosity range of (0-0.5)%, the curing pressure is controlled at (0.2-0.25) MPa; to obtain a comparative test block with a porosity range of (0.5-1.5)%, the curing pressure is controlled at (0.1-0.20) MPa; and to obtain a comparative test block with a porosity range of >1.5%, the curing pressure is controlled at (0-0.1) MPa.

[0020] Optionally, the encapsulation includes: arranging two thermocouples diagonally on the test plate, sealing it with a vacuum bag film, and connecting a vacuum system inside the vacuum bag film.

[0021] Optionally, it also includes using an ultrasonic tester to determine the porosity uniformity of the comparison test block after demolding and to calibrate the porosity of the comparison test block.

[0022] Secondly, this application also provides a comparative test block for detecting the porosity of carbon fiber composite materials, characterized in that it is prepared by the method described in any one of claims 1-9.

[0023] Compared with the prior art, the present invention has at least the following beneficial effects:

[0024] This invention prepares a comparative test block with no macroscopic defects and uniform pore distribution by using a multiple-layer hot-compacting and controlled curing process. The process does not require the introduction of foreign materials, is simple and controllable, has a high success rate, and can obtain a comparative test block with a large effective area.

[0025] This invention addresses the preparation of comparative test blocks of varying thicknesses, particularly comparative test blocks with a thickness of 20 layers or more, achieving uniform porosity.

[0026] This invention allows for the acquisition of comparative test blocks with different porosity ranges by adjusting curing parameters, exhibiting strong controllability and versatility. Attached Figure Description

[0027] Figure 1 Metallographic image of the porosity of the comparative specimen obtained in Example 1;

[0028] Figure 2 Metallographic image of porosity of the comparative specimen obtained in Example 2;

[0029] Figure 3 Metallographic image of porosity of the comparative specimen obtained in Example 3;

[0030] Figure 4 Metallographic image of the porosity of the comparative specimen obtained in Example 4;

[0031] Figure 5 Metallographic image of porosity of the comparative specimen obtained in Example 5;

[0032] Figure 6 Metallographic image of the porosity of the comparative specimen obtained in Comparative Example 1. Detailed Implementation

[0033] The present invention will now be described in further detail with reference to the accompanying drawings:

[0034] Unless otherwise specified, the experimental methods used in the embodiments of this invention are all conventional methods.

[0035] All reagents and materials used in this example can be purchased routinely. The quantitative experiments involved in the examples were all repeated at least three times, and the results were averaged.

[0036] Raw material source:

[0037] The carbon fiber prepreg used in Examples 1-4 of this invention is a high-temperature curing carbon fiber unidirectional prepreg with a curing temperature of 180℃, grade EH919-34%-12KHF40C-U-194gsm-1000HLU; the carbon fiber prepreg used in Example 5 and Comparative Example 1 is a medium-temperature curing carbon fiber unidirectional prepreg with a curing temperature of 125℃, both from Jiangsu Hengshen Co., Ltd., grade EM817B-33%-12KHF30F-U-133gsm.

[0038] Example 1

[0039] A method for preparing a carbon fiber composite porosity comparison test includes the following steps:

[0040] The ply design is [45 / 0 / -45 / 90]. 3s (This means that the angles of the four adjacent layers are 45°, 0°, -45° and 90° respectively, and the layers are repeated three times and then mirrored three times, for a total of 24 layers.) The material size is 200mm×200mm.

[0041] Prepreg is laid, and hot compaction is performed every 6 layers until a total of 24 layers are laid. After the last layer is laid, no hot compaction is performed to obtain the test panel. That is, layers P06, P12, and P18 are hot compacted. During hot compaction, the sample to be hot compacted is bagged and sealed, and the vacuum degree is controlled to reach not less than 0.075MPa. The temperature is increased to 115℃ at 2℃ / min, held for 20min, and then cooled to below 60℃ at 2℃ / min to end the curing.

[0042] After the lining was completed, a layer of non-porous release film and a layer of breathable felt were sequentially laid on the surface of the test panel. Then, a curing oven was used for curing. The curing regime was as follows: full vacuum was applied, with a vacuum degree not lower than 0.075 MPa; the temperature was increased to 110℃ at a rate of 2℃ / min and held for 6 hours; then the temperature was decreased to below 60℃ at a rate of 2℃ / min. After removal and demolding, a carbon fiber composite porosity testing comparison test block was obtained. The results are as follows: Figure 1 As shown.

[0043] The porosity uniformity of the control specimens was determined using an ultrasonic testing instrument to check for macroscopic defects. Then, the porosity of the control specimens was calibrated using the GB / T3365 method, with three samples taken from each fiber axis for calibration. The results are shown in Table 1.

[0044] Example 2

[0045] A method for preparing a carbon fiber composite porosity comparison test includes the following steps:

[0046] The ply design is [45 / 0 / -45 / 90]. 3s The blank size is 200mm×200mm;

[0047] Prepreg is laid, and hot compaction is performed every 6 layers until a total of 24 layers are laid. After the last layer is laid, hot compaction is not performed to obtain a test panel. That is, layers P06, P12, and P18 are hot compacted. During hot compaction, the sample to be hot compacted is bagged and sealed, and the vacuum degree is controlled to reach not less than 0.075MPa. The temperature is increased to 115℃ at 2℃ / min, held for 20min, and then cooled to below 60℃ at 2℃ / min to end the curing process.

[0048] After the lining was completed, a layer of non-porous release film and a layer of breathable felt were sequentially laid on the surface of the test panel. Then, a thermostatic press was used for curing. The curing process was as follows: a vacuum was applied throughout, with a vacuum level not lower than 0.075 MPa; the temperature was increased at 2℃ / min while the pressure was increased to 0.1 MPa; after reaching 110℃, the temperature was maintained for 6 hours; then, the temperature was reduced at 2℃ / min to below 60℃, and the panel was removed and demolded to obtain a comparative test block for carbon fiber composite porosity testing. The results are as follows: Figure 2 As shown.

[0049] The porosity uniformity of the control specimens was determined using an ultrasonic testing instrument to check for macroscopic defects. Then, the porosity of the control specimens was calibrated using the GB / T3365 method, with three samples taken from each fiber axis for calibration. The results are shown in Table 1.

[0050] Example 3

[0051] A method for preparing a carbon fiber composite porosity comparison test includes the following steps:

[0052] The ply design is [45 / 0 / -45 / 90]. 3s The blank size is 200mm×200mm;

[0053] Prepreg is laid, and hot compaction is performed every 6 layers until a total of 24 layers are laid. After the last layer is laid, hot compaction is not performed to obtain a test panel. That is, layers P06, P12, and P18 are hot compacted. During hot compaction, the sample to be hot compacted is bagged and sealed, and the vacuum degree is controlled to reach not less than 0.075MPa. The temperature is increased to 115℃ at 2℃ / min, held for 20min, and then cooled to below 60℃ at 2℃ / min to end the curing process.

[0054] After the lining was completed, a layer of non-porous release film and a layer of breathable felt were sequentially laid on the surface of the test panel. Then, a thermostatic press was used for curing. The curing process was as follows: a vacuum was maintained throughout, with a vacuum level not lower than 0.075 MPa; the temperature was increased at 2℃ / min while the pressure was increased to 0.2 MPa; after reaching 110℃, the temperature was held for 6 hours; then, the temperature was decreased at 2℃ / min to below 60℃, and the panel was removed and demolded to obtain a comparative test block for carbon fiber composite porosity testing. The results are as follows: Figure 3 As shown.

[0055] The porosity uniformity of the control specimens was determined using an ultrasonic testing instrument to check for macroscopic defects. Then, the porosity of the control specimens was calibrated using the GB / T3365 method, with three samples taken from each fiber axis for calibration. The results are shown in Table 1.

[0056] Example 4

[0057] A method for preparing a carbon fiber composite porosity comparison test includes the following steps:

[0058] The ply design is [45 / -45 / 90 / 45 / -45 / 45 / -45 / 0 / 45 / -45] 2s The blank size is 200mm×200mm;

[0059] Prepreg is laid, and hot-compacted every 6 layers until a total of 40 layers are laid. The last layer is not hot-compacted, resulting in a test panel. Specifically, layers P06, P12, P18, P24, P30, and P36 are hot-compacted. During hot-compacting, the sample to be hot-compacted is bagged and sealed, and the vacuum degree is controlled to reach no less than 0.075 MPa. The temperature is increased to 115℃ at 2℃ / min, held for 20min, and then cooled to below 60℃ at 2℃ / min to end the curing process.

[0060] After the lining was completed, a layer of non-porous release film and a layer of breathable felt were sequentially laid on the surface of the test panel. Then, a thermostatic press was used for curing. The curing process was as follows: a vacuum was applied throughout, with a vacuum level not lower than 0.075 MPa; the temperature was increased at 2℃ / min while the pressure was increased to 0.1 MPa; after reaching 110℃, the temperature was maintained for 6 hours; then, the temperature was reduced at 2℃ / min to below 60℃, and the panel was removed and demolded to obtain a comparative test block for carbon fiber composite porosity testing. The results are as follows: Figure 4 As shown in Table 1.

[0061] Example 5

[0062] A method for preparing a carbon fiber composite porosity comparison test includes the following steps:

[0063] The ply design is [45 / 0 / -45 / 90]. 2s The material size is 200mm×200mm; prepreg is laid, and hot compaction is performed every 6 layers until a total of 16 layers are laid. The last layer is not hot compacted, resulting in a test panel; that is, layers P06 and P12 are hot compacted. During hot compaction: the sample to be hot compacted is bagged and sealed, and the vacuum degree is controlled to reach not less than 0.075MPa. The temperature is increased to 90℃ at 2℃ / min, held for 20min, and then cooled to below 60℃ at 2℃ / min to end the curing.

[0064] After the lining was completed, a layer of non-porous release film and a layer of breathable felt were sequentially laid on the surface of the test panel. Then, a thermostatic press was used for curing. The curing process was as follows: a vacuum was maintained throughout, with a vacuum level not lower than 0.075 MPa; the temperature was increased at 2℃ / min while the pressure was increased to 0.1 MPa; after reaching 90℃, the temperature was held for 6 hours; then, the temperature was reduced to below 60℃ at 2℃ / min, and the panel was removed and demolded to obtain a comparative test block for carbon fiber composite porosity testing. The results are as follows: Figure 5 As shown in Table 1.

[0065] Comparative Example 1

[0066] The ply design is [45 / 0 / -45 / 90]. 2s The material size is 200mm×200mm; prepreg is laid, and cold compaction is performed every 6 layers until a total of 16 layers are laid. The final layer is not cold compacted to obtain a test plate; that is, layers P06 and P12 are cold compacted. During cold compaction: the sample to be cold compacted is bagged and sealed, and the vacuum degree is controlled to reach not less than 0.075MPa for 15 minutes.

[0067] After the lining was completed, a layer of non-porous release film and a layer of breathable felt were sequentially laid on the surface of the test panel. Then, a thermostatic press was used for curing. The curing process was as follows: a vacuum was maintained throughout, with a vacuum level not lower than 0.075 MPa; the temperature was increased at 2℃ / min while the pressure was increased to 0.1 MPa; after reaching 90℃, the temperature was held for 6 hours; then, the temperature was reduced to below 60℃ at 2℃ / min, and the panel was removed and demolded to obtain a comparative test block for carbon fiber composite porosity testing. The results are as follows: Figure 6 As shown in Table 1.

[0068] Table 1. Test results of the comparative test blocks

[0069] Example Number of layers of comparative test pieces Macroscopic defects Porosity calibration Example 1 24 No 2.54% Example 2 24 No 0.67% Example 3 24 No 0.14% Example 4 40 No 1.22% Example 5 16 No 0.70% Comparative Example 1 16 Yes /

Claims

1. A method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials, characterized in that, include: The carbon fiber prepreg is cut according to the layup angle and number of layers of the part being inspected, with a margin of not less than 20mm on all sides. Carbon fiber prepreg is laid without pre-compaction. Hot compaction is performed every 2-8 layers, and no hot compaction is performed after the final layer is laid. After the laying is completed, a test panel is obtained. Auxiliary materials, namely a non-porous isolation film and a breathable felt, were sequentially laid on the surface of the test plate, and then sealed. Finally, the plate was cured and demolded to obtain a control test block.

2. The method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials according to claim 1, characterized in that, The hot compaction includes: heating to a preset heat preservation temperature under a vacuum of not less than 0.075 MPa, then heat preservation at the preset heat preservation temperature, and finally cooling to below 60°C after heat preservation.

3. The method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials according to claim 2, characterized in that, The preset insulation temperature is as follows: for carbon fiber prepreg with a curing temperature ≤130℃, the preset insulation temperature range is (60-100)℃; for carbon fiber prepreg with a curing temperature of 130-180℃, the preset insulation temperature is (100-120)℃; for carbon fiber prepreg with a curing temperature >180℃, the preset insulation temperature range is (130-150)℃; and / or, the insulation temperature is maintained for 20-40 minutes at the preset insulation temperature.

4. The method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials according to claim 2, characterized in that, The heating and cooling rates during the heating and cooling process are 0.5-3℃ / min.

5. The method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials according to claim 2, characterized in that, The sample to be hot-compacted is bagged and sealed, and the vacuum degree inside the bag is controlled to be no less than 0.075 MPa during the hot compaction process.

6. The method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials according to claim 1, characterized in that, Hot compaction is performed every 4-6 layers.

7. The method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials according to claim 1, characterized in that, The curing process includes: evacuating the entire cavity under vacuum to maintain a vacuum level of no less than 0.075 MPa, then heating to a certain temperature and curing at a pressure of 0-0.25 MPa for 4-8 hours, and finally cooling down to below 60°C to end the curing process. During the curing process, for carbon fiber prepregs with a curing temperature ≤130°C, the insulation temperature range is set to 60-100°C; for carbon fiber prepregs with a curing temperature of 130-180°C, the insulation temperature range is set to 100-120°C; and for carbon fiber prepregs with a curing temperature >180°C, the insulation temperature range is set to 130-150°C.

8. The method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials according to claim 7, characterized in that, During the curing process, to obtain a control block with a porosity range of (0-0.5)%, the curing pressure was controlled at (0.2-0.25) MPa; to obtain a control block with a porosity range of (0.5-1.5)%, the curing pressure was controlled at (0.1-0.20) MPa; and to obtain a control block with a porosity range of >1.5%, the curing pressure was controlled at (0-0.1) MPa.

9. The method for preparing a comparative test block for detecting the porosity of carbon fiber composite materials according to claim 1, characterized in that, It also includes using an ultrasonic testing instrument to determine the porosity uniformity of the comparison test block after demolding and to calibrate the porosity of the comparison test block.

10. A comparative test block for detecting the porosity of carbon fiber composite materials, characterized in that, It is prepared by the method described in any one of claims 1-9.

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