In the present study, a new approach has been developed to characterize the low-velocity impact behavior of sandwich-structured composites reinforced by weft-knitted spacer fabric. This approach is to define various indexes to describe the impact resistance of spacer fabric composites. The experimental results of the drop-weight impact test were used to define different approaches in order to investigate different aspects of the impact behavior of sandwich composites. Novel indexes i.e. relative displacement index (RDI), energy per pile index (EPI), damaged volume index (DVI), damaged pile index (DPI), and damaged area index (DAI) were introduced to explain different features of the impact behavior of composites reinforced with 1×1 rib, 3×3 rib and 5×5 rib gaiting weft-knitted fabrics. As a conclusion, pile
orientation has a significant effect on the impact resistance of sandwich composites. Rib3 sample has the maximum impact stiffness due to the low RDI value and also has the minimum value of DVI value, which describes both the area and depth of damage. In addition to the regular damage pattern of Rib3 sample, it has minimum damage based on the DPI value. Rib5 sample has the maximum impact toughness because of the maximum values of the AEI value. Also, the highest damaged area according to the DAI value belongs to the Rib5 sample.