Abstract:

Objective Carbon fiber warp-knitted biaxial fabrics have increasingly captured attention and gained widespread use in aerospace, automotive and other industries due to exceptional mechanical properties. Such fabrics consist of two layers of carbon fiber bundles arranged at ±45°, with the warp threads bundling together. This design ensures that the fiber alignment is similar to that of unidirectional fabric. However, the inherent low toughness of carbon fiber necessitates enhancements in its impact resistance. While current research predominantly focuses on unidirectional fabrics, it is essential to also assess the impact protection performance of carbon fiber warp-knitted biaxial fabrics.
Method This study is aimed at elucidating the damage characteristics of warp-knitted biaxial carbon fiber reinforced composites subjected to low-velocity impact, while also analyzing their impact response and energy absorption mechanisms. The laminates were fabricated using warp-knitted biaxial carbon fiber fabric through the vacuum-assisted resin transfer bag molding, and progressive low-velocity impact tests were conducted at impact energies of 9, 15, 21 and 27 J to analyze their features of impact response curves. After the low-velocity impact tests, the damage morphology of the samples was characterized. This paper also examined the influence of varying fabric surface densities on laminates' impact resistance.
Results The results show that during the progressive impact process, the compression failure of the resin, fiber extraction and fracture, and hierarchical damage to the fabric occurred sequentially. As the impact energy increased, the severity of laminate damage also escalated, resulting in a greater energy absorption, which was demonstrated by the energy absorption rate rising from 50.6% to 75.2% and specific energy absorption increasing from 51.01 J/kg to 231.14 J/kg. The peak load initially rose and then fell as the impact energy increased, reaching its maximum of approximately 6 001.08 N at 21 J impact. The stiffness of the laminates started to diminish under 15 J impact, dropping from 1 579 N/mm to 952.8 N/mm at 27 J impact. During the progressive impact process, damage accumulated and expanded through the thickness of the samples, causing the front to develop white pits due to stress whitening. After the 27 J impact, the pit depth was approximately 0.55 mm. On the back, T-shaped or X-shaped cracks appeared, characterized by clean fractures at the fiber break points and fiber protrusions, clearly illustrating tensile fracture and layered damage. After the final impact of 27 J, when the cumulative energy absorption reached 521.89 J/kg, the specific energy absorption still increased to 231.14 J/kg and the back convex height measured approximately 2 mm. When the laminate's surface density was fixed, the influence of fabric surface density on the laminate's impact resistance became complex and multifaceted. When the resin sustained primary damage, a lower fabric surface density-indicating a greater number of layers-resulted in decreased specific energy absorption and energy absorption rate. As impact energy increased, interface damage became more pronounced. Laminates with more fabric layers absorbed greater amounts of energy because of the presence of additional interfaces. Meanwhile, laminates with fewer layers started to rely on fiber destruction to dissipate energy. When fiber destruction was the primary failure mechanism, laminates with more layers exhibited better elastic recovery and lower energy absorption. In contrast, laminates with fewer layers tended to absorb more energy and sustained more severe damage due to the accumulation of fiber damage.
Conclusion In conclusion, this study has analyzed the impact response and energy absorption mechanisms of warp-knitted biaxial carbon fiber laminates under progressive impact conditions. Warp-knitted biaxial carbon fiber reinforced composites demonstrates outstanding impact protection performance and enhanced resistance to delamination. Additionally, reducing the fabric surface density can effectively raise the upper limit of energy absorption. In comparison to the laminate made from 300 g/m² fabric, the laminate composed of 150 g/m² fabric exhibits a 19.8% reduction in specific energy absorption and an 11% decrease in energy absorption rate.

Key words: warp-knitted biaxial fabric, carbon fiber reinforced composite, low-velocity impact response, progressive impact, mechanism of impact damage, vacuum assisted resin transfer molding with bagging process