Research On The Multi-scale Mechanical Behavior And Interlaminar Shear Performance Of Kevlar Fabric Reinforced Composites

Kevlar fabrics have excellent performances such as high strength-to-weight ratio, high modulus and high impact resistance, which are widely used in ballistic systems, aerospace structures, protective materials and other fields. In addition, Kevlar fabrics are also popular as reinforcement in composite called AFRP (Aramid Fibre Reinforced Polymers). AFRP is valuable alternative to conventional materials due to its high strength to weight ratio, corrosion resistance and fatigue tolerance, and these superior properties enable its applications in national defense industry, automobile industry, subway tunnel and marine structures. Applications in strengthening, retrofit and rehabilitation of structural members of civil engineering infrastructure have also been gradually adopted as well. Present paper shows a series of study about the mechanical performance of Kevlar and AFRP, and the results of these investigations will be valuable for theoretical research and practical application.The mechanical parameters obtained in the tensile tests of Kevlar 29/49 in the forms of fiber and yarn, and distribution parameters obtained by Weibull analysis on the tensile strengths of fiber and yarn are implemented into user-defined subroutines (USERMAT) of ANSYS to establish multi-scale constitutive models. This new model can present the flaws introduced during the handling process, which offsets the deficiency of classical deterministic approach. The constitutive models can obtain identical results with tensile tests, and shows the the development of internal damage of multi-scale material subjected to uniaxial tensile test.Aramid fabric (Kevlar 29/49) reinforced polymers (AFRP-K29/49) samples are tested utilizing a MTS (Manager Testing Solution) servo-hydraulic high rate testing machine in order to investigate their mechanical properties at four different strain rates (25,50,100 and 200 s-1) and five different temperatures (-25,0,25,50 and 100 ?). Weibull statistics are also used to quantify the degree of variability in strength at different strain rates and temperatures. The results show that with the increasing strain rate, Young's modulus and tensile strength increase firstly and then decrease, and the toughness presents an opposite changing trend. The tensile strength of AFRP-K29 and the Young's modulus of both kinds of AFRP are sensitive to temperature, while the tensile strength of AFRP-K49 and the toughness of both kinds of AFRP are less affected by temperature changes. The Young's modulus of AFRP drops markedly as experimental temperature exceeds the glass transition temperature of the epoxy resin. The failure patterns of AFRP samples are similar at the strain rates and temperatures investigated, and the fracture surfaces are relatively straight.Kevlar double-layer and Kevlar/Steel single-lap shear joint specimens are manufactured and tested in present paper. For Kevlar double-layer of single-lap shear joint specimens with the width of 25 mm, the effective bond length is 25 mm, while for Kevlar/Steel single-lap shear joints specimens with the width of 25 mm, the effective bond length is 50 mm. Within the effective bond length, bond force increases with increasing bond length, while the increasing amplitude decreases, and the bond stress decreases with increasing bond length.