Experimental study of low-velocity impact behavior and damage characteristics of flat-knitted spacer fabrics reinforced composites

Flat-knitted spacer fabrics are three-dimensional fabrics with good formability and impact resistance, which can be rapidly produced complicated shape preforms. In this work, low-velocity impact behavior and damage characteristics of flat-knitted spacer fabrics reinforced composites were studied and the mechanism of the effects of spacer yarn structure on the impact properties of the composites were explored. Flat-knitted spacer fabrics with three spacer structures were produced using ultrahigh molecular weight polyethylene (UHMWPE) filaments, and composite plates were prepared by vacuum-assisted resin infusion. Low-velocity impact tests were carried out using a drop-weight impact testing machine equipped with a hemispherical impactor. It was revealed that the impact process of the flat-knitted spacer fabrics reinforced composites could be divided into three areas. The effect of initial impact energy on the impact properties of the sample was mainly reflected in the damage speed and initial damage threshold in the first areas. With the increase of spacer yarn span, the damage tolerance, stiffness, and impact resistance of the composite were also increased. Besides, the main damage mechanism of flat-knitted spacer fabric reinforced composites was plastic deformation, matrix fracture, fiber-matrix debonding, and tow splitting.