Abstract:

Objective Fiber reinforced composites are widely used in automotive, petroleum, aerospace, and wind energy fields attributed on their high specific strength and modulus. In order to overcome the out-of-plane delamination defects of laminated composites, three-dimensional (3-D) woven preforms are adopted to add reinforcing fibers along thickness direction which effectively prevents delamination cracks. Fabric reinforcement plays an important role in enhancing impact resistance especially under low-velocity impact load. Previous studies have shown that the weaving process can significantly affect the fiber structure. More studies on compression after impact (CAI) of fiber reinforced composites had been carried out, but few studies referred to the influence of weaving tension on the impact resistance of 3-D woven composites. The goal of this research is to investigate the influence of tow tension during weaving on the impact and compression after impact (CAI) performance of 3-D orthogonal woven composite.
Method 3-D woven fabrics were fabricated using self-made loom. Toray T700-24K and T700-12K carbon fibers were used as warp and weft tows, respectively. Aramid yarn from DuPont was used as the z-binding yarn. Warp density and weft density were 50 ends/(10 cm) and 33 picks/(10 cm), respectively. Three tow tension levels (25, 50, 100 cN) were adopted based on the multi-rapier weaving process to obtain 3-D orthogonal woven fabric samples 1-3, respectively. Vacuum-assisted resin transfer technology was employed to fabricate the composite samples. Low-velocity impact test and post-impact compression test were carried out according to ASTM D7136/D7136M-2005 and ASTM D7137 /D7137M-2012.
Results Under the impact energy of 30 J, the 3-D orthogonal woven composites with different weaving tension levels showed different bending stiffnesses and impact strengths, which were specifically notable for the impact force increase rate and impact peak force at the initial stage. The impact resistance of 3-D orthogonal woven composites was expressed as flexural stiffness. Sample 1 had a larger flexural stiffness, and its maximum impact reaction force was 6.3% higher than that of sample 3. The rising stage of the impact force-deformation curves showed different slopes indicating different flexural stiffnesses of the samples. The recovery displacement of sample 3 was the smallest suggesting that a large amount of impact kinetic energy was rebounded. Conversely, the large recovery displacement of sample 1 implying that the impact kinetic energy was absorbed in various forms of damage.
The force-displacement response curves of the composite samples were significantly different, especially in the peak values of the compression load. When the peak values were reached, samples 1 and 2 underwent instantaneous fracture failure, and the bearing capacity decreased rapidly. In contrast, sample 3 had a plateau curve after the peak, which could still bear a certain compression load. The front and back of impact face showed different damage morphologies, including resin fracture and fiber fracture. The z-binding yarns tension caused weft yarn crimping and resin-rich pocket. The samples with high tow tension had a large resin crack and delamination area resulting in the exposure of the weft reinforced tows. In addition, the weft crimping created by high tension of z-binding yarns resulted in buckling failure under compression loading in weft direction. The CAI performance suffered almost 50% decreases with high z-binding yarns tension. Therefore, the increase of z-binding yarns tension reduces the bending stiffness and post-impact compression performance of 3-D orthogonal woven composites.
Conclusion 3-D orthogonal woven fabrics were woven with three tow tension levels. The compression properties of the composite samples were evaluated after impacting on the samples. It was found that when the tension of the z-binding yarns is increased, the crimp of the surface weft tow is increased. The composite material with high z-binding yarns tension formed a large area of delamination on the impact front face, which resulted in the surface weft exposed. The crimping weft tows caused the post-impact compression failure behavior to change from fiber breakage to local buckling, which reduced the compression resistance capacity of the composite. Therefore, the bending stiffness and post-impact compression properties of the 3-D orthogonal woven composites decrease with the increase of the z-binding yarns tension applied during weaving.

Key words: 3-D orthogonal fabric, yarn tension, composite material, compression after impact, yarn crimp