Computational Investigation on the Crack in Kevlar Based Composite Materials at Cryogenic Temperatures to be used in Superconducting Motors

Abstract

The compatibility of Kevlar based composites at cryogenic temperatures made it possible to be used in various superconducting applications including superconducting motors. The torque tube used in the superconducting motors is made up of Kevlar due its capability to hinder the heat transfer between driving to the driven end of the motor. However, at cryogenic temperatures brittle fracture is highly pronounced due to contraction of the Kevlar. Hence, in the present work, the fracture behaviour of Kevlar-49/Epoxy with 60% fibre volume fraction at cryogenic temperatures is investigated using a commercial code namely ABAQUS. This code uses extended finite element method (XFEM) of formulation in solving the stiffness matrix with realistic boundary conditions. A uniaxial tensile load of 700MPa is considered with one end as fixed. Further, Kevlar-49/Epoxy is considered to be isotropic with the magnitude of Young’s modulus to be 99GPa and the poison’s ratio to be 0.34 at 75K. The consideration of isotropic properties would be valid as the XFEM assumes linear elastic fracture mechanics (LEFM) approach. Further, the accuracy of XFEM is superior over contour integral method due to the enrichment of elements in the entire computational domain including the crack tip. A Kevlar specimen of 50X30 mm is modeled and meshed (quadrilateral) in ABAQUS. The mesh size is considered to be 2 mm to capture the results at maximum number of nodes. Three cases of edge crack, one being towards the loading end, the second being at the middle and the last being towards the fixed end of the specimen are considered. It is found that significant changes in Von Mises stress, strain and strain energy density exist according to the initiation of crack location. Further, the computational results are validated with the obtained analytical results.