The paper deals with development and characterization of 3D sandwich composite structures reinforced with newly-designed multi-cell flat-knitted spacer fabrics in terms of compressive behaviour and Poisson’s ratio. Multi-cell spacer knitted preforms was produced on a computerized flat knitting machine. 3D composite samples with three different cross-sectional geometries were prepared via vacuum assisted resin transfer moulding method. Quasi-static compressive experiments were carried out on the prepared 3D composite samples. The Poisson’s ratio of re-entrant 3D knitted composite varied between -6 and -1, which clearly points to existence of auxetic behaviour of the samples. The re-entrant 3D composites also demonstrated the highest initial slope and area under the compression force-displacement curve than spear-head or hexagonal composite structures which refer to higher energy absorbing capacity. The Poisson’s ratio of 3D regular hexagonal knitted composites at small strain was usually 4 which gradually decreased to 1.6 as the exerted compressive strain increased. Additionally, 3D spear-head knitted composite having zero Poisson’s ratio was also developed.

3D knitted fabrics are regarded as a viable option for advanced composite materials. Flat-knitted spacer fabrics (FKSF) which are in the category of 3D knitted structures have attracted many attentions due to outstanding characterizations such as high formability and good impact behavior. These structures consist of two surface layers which are linked together by multiple knitted connecting layers. Despite the merits of 3D-FKSFs as composite reinforcements, they have some structural restrictions such as limited thickness. This study aims to develop and characterize bi-directional 3D knitted spacer structures which could be replaced with conventional FKSFs. In the developed structures, the upper and lower surface layers are connected together by two truncated pyramids which can be configurated in any dimensions using an innovative knitting technique. For providing a report regarding their compression behavior, these 3D structures were produced in two different thicknesses on an electronic flat knitting machine. Then, they were impregnated with epoxy resin via vacuum resin transfer molding and the cured composites were subjected to compression force. The results revealed that their compression behavior is similar to the behavior of conventional honey-comb sandwich structures.