Impact characteristics of composite panel stitched by I-fiber process
Introduction
Impact damage to composite materials occurs from low-velocity or high-velocity impacts, such as tool dropping during maintenance or high-speed object strikes, resulting in a significant loss in the stiffness and compressive strength of the composite material. Impact damage generated during a manufacturing process or operation is undetectable to the naked eye, hard to repair, and decreases the lifetime of a product. The damage area can be reduced by reinforcing the z-directional strength. Z-pinning and stitching are typical z-directional reinforcement methods, however there is an in-plane property degradation, for this reason a Compression after impact (CAI) test has been performed [1], [2].
Park et al. [3] conducted a parametric study on the effects of carbon Z-pinning on the pull-off strengths of composite T-joints. Francesconi et al. [4] conducted an experimental investigation into the effect of Z-pinning on the impact resistance of composite laminates with different multi-directional layups, and Knaupp et al. [5] investigated the effect of rectangular shaped z-pins on the impact performance of reinforced composite laminates, with carbon composites being reinforced by a rectangular Z-pin. However, the Z-pin can only be used in prepreg materials, and there is a limitation in that the Z-pinning method cannot be applied to a dry preform, as the inserted pins are not fixed.
Stickler et al. [6] proved that through-thickness-stitched T-joints under bending load reduce the susceptibility to delamination and increase joint strength. Hosur et al. [7] studied the performance of stitched/unstitched woven carbon/epoxy composites under high-velocity impact loading. Several investigations have shown that z-pins and stitches could be useful in limiting the damage/delamination in thin composite components [8]. Dransfield et al. and Lopresto et al. concluded that the thick laminates showed the better damage/delamination resistances than the thin laminates [9], [10]. In addition, many researchers have proven that stitching improves the impact damage resistance of composites, both in high-velocity [11], [12] and low-velocity [13], [14], [15], [16] impact cases. In general, most of the stitching cases can be used in a dry preform, and there is a limitation in that upper and lower devices are required for weaving the upper and lower threads, respectively.
The I-fiber stitching process is a z-directional reinforcement method that combines the advantages of Z-pinning and a conventional stitching process. The I-fiber stitching process can be applied to both prepreg and dry preforms, can be performed with one-side access, and requires a device in only one direction. Kim et al. [17] evaluated the strength of a T-joint stitched by the I-fiber stitching process, and proved that its pull-off failure load was improved. An et al. [18] studied the strength of a single lap joint reinforced by the I-fiber stitching process, and proved that its joint strength was increased by more than 60%. Tapullima et al. [19] tested double cantilever beam (DCB) specimens with different stitched patterns, and analyzed a DCB test model using a cohesive zone method (CZM).
In this paper, composite panels stitched by an I-fiber process were manufactured, and impact tests were performed. The damage areas and compressive strengths after impact were measured and compared with an unstitched panel.
Section snippets
Manufacture of specimen and impact test
The I-fiber stitching process was applied to a composite prepreg panel, for reinforcing the z-directional strength and reducing the impact damage area. Fig. 1 shows a schematic diagram of the I-fiber process. The I-fiber stitching process starts with the insertion of a long cylinder-type needle (the needle diameter depend on the yarn type) in the prepreg, and insertion of the carbon yarn through it. After insertion of the needle, the carbon yarn is ejected by air pressure, the needle moves up,
Measurement of damage area
An ultrasonic C-Scan was performed to measure the damage area of each composite panel after the impact test. An FS-28 ultrasonic scanner from Acoulab Co. was used. In this regard, 10 images were acquired through thickness, and one image which included every crack was selected. From the selected C-Scan image, the damage area was evaluated using the Image-J program. Fig. 12 shows the C-Scan image of the pure composite panel without the 3-D reinforcing I-fiber stitching process. As shown in Fig. 12
Compression after impact tests
Referring to ASTM D7137, a compression test fixture was fabricated, and compression tests were performed after the impact tests. The E45 Universal Testing Machine from MTS Co. was used for testing, and its crosshead speed was set to 1.25 mm/min. Fig. 16 shows a photograph of the compression test setup.
Fig. 17 shows the compression test results after impact. As shown in Fig. 17, the compressive strengths of the composite panels with 0.5% reinforcing area density were increased by 2–3.5% as
Conclusion
From the compression after impact and impact tests of the composite panels stitched by the I-fiber stitching process, the following conclusions can be made:
- 1.
The stitching fiber absorbs the impact energy, and interferes with the delamination of the composite panel.
- 2.
The narrower the interval between the stitching fibers at the same stitching density, the smaller the damaged area.
- 3.
The damage areas of the composite panels with the 3-D reinforcing I-fiber stitching process were 18–48% smaller than
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
This work was partly supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (NRF-2017R1A5A1015311) and Civil & Military Technology Cooperation Program through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (15-CA-MA-15-MKE).
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2023, International Journal of Mechanical SciencesCitation Excerpt :Song et al. [28] also found that selecting high-performance fibers with better shear resistance as stitch yarns can significantly improve the bending properties of the 3DSC. Kim et al. [29] found that the impact damage areas of 12.5 mm × 12.5 mm and 5.56 mm × 5.56 mm were 18.4 and 31.9% smaller than that of the 2DLC, respectively. This was because the tight stitching could inhibit the growth of cracks caused by the impact of the 3DSC.