Effect of Shape of Armoring Fibers on Strength of Composite Materials

In this work, experimental studies of the effect of fiber curvature on the strength and ultimate deformations of epoxy carbon plastic samples under loading are carried out. On the basis of the experimental studies carried out, the static characteristics of the composite layered material, which are promising for use in the structures under consideration, have been determined. Based on the test results, it was demonstrated that in the design calculations for the products under consideration, it is possible to use static characteristics, since an increase in the deformation rate of the material leads to an increase in strength and, therefore, the calculation results will provide an additional margin of safety.


Study of the strength of composite materials
Samples of the composite material were made on the basis of ED-20 epoxy resin with Torey T800 reinforcing fibers. The volumetric content of fibers was 60%, the diameter of the fibers was 5 μm. Samples of three types were considered: 1) a quasi-homogeneous layered composite with a unidirectional reinforcement scheme, 2) a unidirectional composite with a wavelike layered structure, and 3) a pure matrix.
Mechanical tests were carried out on a universal electrical installation Instron 5969 (50kN) and Instron 5982 (100kN) (UK) with Bluehill 3 software. Compression tests were carried out. Test speed was 1 mm/min.
Photos of the samples after testing are shown in Figure 1. According to the test results, characteristic loaddisplacement and stress-strain diagrams were obtained, Fig. 1a for straight installation, Fig. 1b for wavy styling. For each batch, the resulting diagram was obtained as a result of averaging the experimental data for three samples of the same type ( Figure 2). During the tests, the maximum load, ultimate strength and ultimate deformation were determined for each specimen. The results of static tests are presented in the Figure 2. The strength of the matrix was also tested. The tests were carried out on an Instron 5969 installation, with the maximum possible speed for this installation equal to 600 mm/min. The stress-strain graph is shown in Figure 3. From static tests it can be seen that the ultimate strength in unidirectional laying is higher than in wavy laying (in the direction A-5%, in the direction B-32%, in the direction C-23%), and the ultimate deformations in unidirectional laying are higher than in wavy styling.
From static tests it can be seen that the ultimate strength in direction A for unidirectional laying (335 MPa) is higher than for wavy (318 MPa), in direction B for unidirectional laying (168 MPa) it is higher than for wavy (114 MPa), in direction C for unidirectional laying (177 MPa) is higher than for wavy (137 MPa).
It is also seen from static tests that the ultimate deformations in the A direction for unidirectional laying (3,47%) are lower than for wavy (6,78%), in the B direction for unidirectional laying (4,34%) is lower than for wavy (5,16%), in the C direction for unidirectional styling (5,12%) is lower than for wavy (6,04 %).

Conclusions
Static tests of the strength of unidirectional CFRP specimens with rectilinear and wavy structure have been carried out. For the first time, a detailed study of the effect of fiber curvature on the properties of CFRP under static deformation has been carried out. As a result of static tests, it was found that the ultimate strength in unidirectional laying is higher than in wavy laying. The effect of increasing the ultimate deformations of specimens with bent fibers is established, which was noted earlier for the case of tensile tests.

Acknowledgements
This work was carried out with the financial support of RFBR, project No. 20-38-90043.