The Preparation And Property Of Warp-knitted Spacer Fabrics With Auxetic

Poisson's ratio is defined as the negative ratio of the strain vertical to the load direction and the strain along the load direction.Materials with positive Poisson's ratio usually compact perpendicularly to load under stretch or expand under compression.Most of our daily engineering materials have a positive Poisson's ratio.Materials with negative Poisson's ratio,also named as auxetic materials,show a completely different performance with expansion perpendicularly to load under stretch or contraction under compression,such as our elbow or knee skin.Researches on auxetic materials keep coming while few are actually able to be applied in engineering.Compared to conventional materials,auxetic materials are endowed with much more enhanced mechanical properties such as indentation resistance,fracture toughness and energy absorption,leading to wider application in protective clothing or equipment,smart fabrics,composite reinforcement,and vibration-absorbing mitten etc.Warp-knitted spacer fabrics are three-dimensional fabrics composed of two separate face fabrics and spacer yarns connecting and supporting two faces,usually knitted on Raschel warp knitting machines with two needle bars.They have plenty of unique performances,such as air and moisture permeability,anti-seismic property,filtrability,acoustical insulation,energy absorption,etc.Therefore,they are widely used in garment,automotive interior,home furnishing,architecture reinforcement,etc.With the rapid growth of researches on auxetic structures and principles,fabric materials with negative Poisson's ratio are gaining more and more attention.In order to combine the outstanding properties of auxetic structures and warp-knitted spacer fabrics to meet the growing needs of the market,cutting-edge auxetic warp-knitted spacer fabrics with normal yarns are prepared and investigated in this paper.Moreover,properties such as shape memory ability and electromagnetic effect can also be integrated with auxetic effects so that fabric materials with negative Poisson's ratio could have a broader prospect in the industrial field.Firstly,different structural models with negative Poisson's ratio including re-entrant structures,rotating structures,nodal-fibril structures,chiral structures,liquid-crystal models and helical structures are generalized in this paper.The structure of rotating hexagon is selected as the structural model of auxetic warp-knitted spacer fabrics.Numerical characterization and analysis of influential factors in x-y plane and x-z plane are conducted respectively.It is concluded that Poisson's ratio of the structure in x-y plane is determined by the rotation angle of hexagons and the length ratio of different sides with constant length and width of six sides,and the auxetic effect is more easily achieved with a higher degree of rotational deformation.In x-z plane,higher rotational degree of spacer filaments leads to a more dramatic slant,thus bringing a better auxetic performance in the thickness direction.Secondly,unidirectional stretch with fixed maximum elongation is conducted on warp-knitted spacer fabric samples based on the definition of Poisson's ratio.A high-speed camera is used for picture collection during the stretch,after which pictures acquired are processed to measure and calculate strains of fabric samples in both longitudinal and horizontal directions.Thus,Poisson's ratio is calculated to characterize the auxeticperformance of warp-knitted spacer fabric samples and help to analyze the influence of fabric structure on its auxetic performance.Results show that miss-lapping chain stitches in the surface fabric structures lead to weft-directional contraction of hexagonal meshes and the laying direction of spacer yarns would affect its rotational deformation.In a pattern cycle of the hexagon mesh,successive laying in the same direction for the first six courses and successive laying in the reversed direction for the last six courses could enhance the rotational deformation of surface hexagon meshes.The more dramatic warp-directional rotation and weft-directional contraction of warp-knitted spacer fabrics are,the better auxetic performance could be achieved.Thirdly,energy absorption of warp-knitted spacer fabrics with different Poisson's ratios under quasi-static stretch are further investigated.Different fabric samples are stretched until it fractures,and stress-strain curves are obtained to calculate its energy absorption.The fracture morphology of fabric samples under quasi-static stretch is also analyzed.It is shown that the energy absorption of warp-knitted spacer fabrics under quasi-static stretch mainly relies on its structural deformation capacity and yarn loading capacity,between which the latter matters much more than the former.The auxetic performance of warp-knitted spacer fabrics helps increase its structural deformation capacity thus increasing its energy absorption.The fracture sections of fabric samples under warp-directional and weft-directional quasi-static stretches are both irregular zigzag-shaped.Under warp-directional stretch,cracks tend to propagate along stretch direction forming a fracture section with a relatively higher slope.Under weft-directional stretch,crack propagation also tends to be vertical to the stretch direction forming a fracture section with a lower slope.The weakest part which is mostly easily fractured lies in warp sateen of the hexagonal warp-knitted spacer fabrics.Lastly,heat-setting process is adopted to enhance and stabilize the rotational deformation of warp-knitted spacer fabrics and low-velocity impact tests are conducted on samples to investigate the energy absorption under low-velocity impact.Results indicate that under low-velocity impact fabrics with better auxetic performance have higher energy absorption and better impact resistance with the same initial impact energy.Fabrics with auxetic turns compact under impact leading to higher energy absorption.However,initial impact energy can hardly affect its load limit,impact displacement limitation or energy absorption.In this research,influence of knitting parameters on the fabric structural deformation and its auxetic performance is investigated.Also,quasi-static stretch tests as well as low-velocity impact tests are both conducted on warp-knitted spacer fabrics with different auxetic performances so as to study the energy absorbing abilities.This research is intended to lay the foundation for the industrialization of three-dimensional warp-knitted spacer fabrics with auxetic and provide theoretical guidance for their broader application.