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Auxetic materials have negative Poisson ratio which has a multiple ranges of functional applications. The helical auxetic yarn was successfully developed through direct twist system by using core and wrap yarn or filament, which shows Auxeticity when the HAY is stretched in longitudinal direction in response it expand in transverse direction, Helical Auxetic Yarns were developed using various parameters of the core and warp filament, these parameters are wrapping angle (Twist per meter), diameter ratio, and modulus ratio. The strength of Helical yarn was characterized using single yarn strength and Image J software was used for the calculation of poisson’s ratio. According to test results, it is concluded that the core filament of helical auxetic yarn increased its thickness in transverse direction under stress, and a considerable negative poisson’s ratio was calculated. The values of negative poisson’s ratio described that the auxeticity had a direct relation with core filament thickness or diameter and inversely proportional to the linear density of wrap filament, in case of the wrap angle the auxeticity of HAY yarn had an inverse effect with wrap angle. Kevlar/polypropylene combination sample showed maximum auxeticity at a 15-degree angle while Kevlar/nylon combination sample showed minimum auxeticity at a 25-degree wrap angle.

Inducing auxetic nature is one of the emerging techniques to enhance impact tolerance in 3D woven composites due to their unique bidimensional-energy dissipation capability. The conventional 3D orthogonal structure has inherent auxetic nature; however, such auxeticity is completely restricted due to brittle thermoset resins-based composites. To overcome this problem, three types of 3D woven auxetic structures were developed and used as reinforcement with thermoplastic resins, that is, polycarbonate (PC) and polyvinyl butyral (PVB). The results revealed that the warp interlock structure showed the highest auxeticity, while the bidirectional interlock structure showed the least auxeticity. Charpy and low-velocity impact tests were performed to evaluate the effect of auxeticity on the impact properties of corresponding composites. Warp interlock with PC showed 49% and 47% higher Charpy impact strength and force, respectively, than bidirectional interlock with PC resin. Similarly, the low-velocity impact test results of warp interlock showed 32% and 32.5% higher impact force and absorbed energy, respectively, in warp direction than bidirectional interlock with PC resin.