TEST METHOD FOR TENSILE PROPERTIES OF CARBON FIBER
The single-filament method has a lot of work to test the mechanical properties of carbon fibers, with no less than 40 fibers per batch, but only more than 40 cm multi-filament can be tested. The tensile property test of carbon fiber monofilament is to stick the monofilament to the paper frame. In the tensile test, the paper frame is fixed with fixture, the paper frames on both sides are cut, and then the fibers are drawn. High strength and high modulus fiber strength tester is often used to test carbon fibers.
Measurement of cross-sectional area of monofilament
The diameter of carbon fibers can be measured according to GB/T3364-2008 "Test Method for Diameter and Root Number of Carbon Fibers", and then the cross-sectional area of carbon fibers can be calculated. The linear density and bulk density of carbon fibers can be measured first according to GB/T 3362-2005, and the cross-sectional area of carbon fibers can be calculated. Then the cross-sectional area of carbon fibers can be divided by the root number of single fibers, that is the cross-sectional area of single fibers
For sample preparation, paper with certain strength, such as coordinate paper, is carved out as shown in Figure 3. The spacing of the paper frame is (25 +0.5) mm. The width and total length of the paper frame are determined according to the size of the tensioner gripper, and the paper frame is not deformed when clamped with tweezers. In Figure 3, A is the bond between the fibre and the paper frame, and B is the clamp of the tensioner.
Fiber bundles about 4 cm in length were randomly sampled from the samples to be tested and separated from each other without damaging the fibers. The separated single fibers are placed one by one on the central line of the paper frame. As shown in Figure 3, the fibers are bonded with a binder. In order to obtain accurate data, the fibers should not be damaged during operation. Fast epoxy resin binder at room temperature should be used to fix the fibers, and the single fibers should be kept straight and in the center.
When clamping the sample, the sample should not deviate from the fixture. Fix the fixture first, then fix the fixture after the position is correct. Fixtures should not be touched, let alone clamped to the bonding point between the fibers and the paper frame, so as to avoid damaging the fibers. As shown in Figure 4, it is necessary to ensure that the single filament axis, the center line of the fixture and the paper frame coincide with the drawing direction. The data of tensile strength and elongation at break can be obtained directly by single-wire tensile test, and the tensile modulus of the sample can be obtained by corrected calculation. Similarly, preloading should be set in the single-wire tensile test to overcome the relaxation of the sample, and the setting value should be set at 0.05 cn/dtex [12-13]. The influence of setting conditions on tensile strength and elongation is the same as that of multifilament, that is, whether or not preloading setting and its value are not related to tensile strength, but the setting values of preloading affect the test results of elongation and tensile modulus.
Accurate testing of tensile properties is an important part of the development, production and application of carbon fibers. Many units at home and abroad are researching and developing new software and equipment in order to obtain test data more conveniently and accurately. The key in the test is not to damage the sample when making the sample. Sampling conditions and test parameters of the same sample should be the same and constant. In order to reduce the fluctuation of data caused by manual operation, the automation of sample preparation is a difficult point in the testing of fiber tensile properties, but it is also an important direction of testing research.
In the tensile test, it is necessary to ensure that the axial direction of the specimen, the center line of the upper and lower fixture and the tensile direction coincide, so as to avoid the phenomenon of cable-stayed, so as to reduce the phenomenon of decreasing the tensile performance data and increasing the discrete coefficient.
