PSI - Issue 17

Michał Kwietniewski et al. / Procedia Structural Integrity 17 (2019) 58–63 Michał Kwietniewski / Structural Integrity Procedia 00 (2019) 000 – 000

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3 Boundary conditions. The model has been subjected to a tensile that is carried along the core axis. A displacement value of 1.1 mm was applied to the nodes on the ground plane of one of the ends of the model. And the other end of the model has been fixed by taking away the degrees of freedom responsible for the longitudinal movement. At both ends of the model, the degrees of freedom responsible for transverse displacement to the load direction were not received. This treatment allows the diameters to be narrowed along the entire length of the model during tensile, also in the fixed and load application plane. This makes it possible to treat the model as a fragment from an tensioned auxetic thread in the elastomer matrix. It also eliminates the analysis of complicated thread fixing conditions in testing machine during tensile test. The material of the wrap is Kevlar24, which fibers come from the family of aramid polyamides. The fibers spun from this material are characterized by a unique combination of high strength and stability under increased temperature. Kevlar does not react with organic solvents. It dissolves in concentrated sulphuric acid. Its low density makes it five times lighter than steel while maintaining similar strength. Kevlar found application in the production of bulletproof vests and helmets, but also sport equipment as skis and kayaks. But there are many other uses of this material, mainly due to the very good strength-mass ratio (Kevlar Aramid Fiber, Technical Guide). Kevlar24 mechanical properties tested by the authors were included in Tab.1 The elastomer core was made of TPEE (Thermoplastic Polyester Ether Elastomer). This material exhibits elastomer features. It has elasticity of rubber, durability of plastics and easy moulding as in thermoplastic materials. TPEE is very well suited for the production of parts that require very good flexibility and a work in a wide temperature range. It shows high heat, tearing, deformation as a result of cutting and attrition resistance as reported in work of Zhang et al. (2014). This material includes additives to protect against UV radiation, resistant to hydrolysis and flame retardants, as evidenced by Zhong et al. (2015). The TPEE mechanical properties tested for this paper purposes are included in Tab. 1. The matrix of the composite should have a lower stiffness than the fibers of the auxetic strand. The aim of the analysis was to find the appropriate Young's modulus to ensure correct work of auxetic fibers surrounded by the matrix. As proper work, it was meant that the thread passes freely into the auxetic during stretching by appropriate bending of the thread. The aramid wrap, elastomeric core and matrix were defined as the material model 001-ELASTIC available in the LS-Dyna software as given by LS-Dyna keyword user's manual volume II (2002). The 001_ELASIC model treats the material as isotropic, elastic and can be used to analyse models built of beam, shell and solid elements. The specification of this material model allows it to be used for modelling liquids as given by LS-Dyna keyword user's manual volume II (2002). This model may not exhibit stability for large strains, therefore the calculations were interrupted at the time of numerical instability. The reason why this material is used is its simplicity, where only the parameter - Young's modulus - is used to express the elasticity of the material. Considering the very large differences in Young's aramid braid and elastomer matrix values, it was decided to use a simple model to minimize the probability of numerical errors. The linear characteristic of the material model 001_ELASTIC is sufficient for stretching considerations in the elastic range, which does not destroy any of the elements of the model.