Tensile properties of CFRP fabricated by resin transfer molding and correlation with low velocity impact test and simulation

Abstract

This thesis is concerned with tensile properties of Carbon Fiber Reinforced Polymer (CFRP) fabricated by Resin Transfer Molding (RTM) and correlation of low velocity impact test and FE analysis. CFRP is new rising material for auto-body structures for its high strength, light weight, and decreasing price. Auto-body structure experiences the strain rates up to several hundreds per second during car crash. In order to apply the CFRP panel into auto-body structures, it is critical to acquire the material properties of CFRP at various strain rates considering stacking sequences. The tensile test method is improved for acceptable test results. Test specimens are modified for high speed tensile tests of CFRP in order to achieve designated strain rates and eliminate the effect from unfavorable conditions of inhomogeneity of deformation. Various types of grip tab materials are employed for acceptable failure modes. Tensile tests have been carried out with Non-Crimp Fabric made by 50K high strength carbon fiber (NCF) and woven fabric made by a 2/2 twill pattern of 3K high strength carbon fiber (TWILL) with different stacking sequences of 0° and 90° unidirectional cases as well as [0$^\circ$C/90$^\circ$C] and [45$^\circ$C/-45$^\circ$C] symmetric cases. Digital Image Correlation (DIC) method and Force Equilibrium Grid Method (FEGM) are adopted for strain and stress measurement of a CFRP specimen during a tensile test. The material properties acquired indicate that the carbon fiber has little rate dependency while the epoxy matrix has remarkable rate hardening at strain rates from 0.001 $s^-1$ to 100 $s^-1$. In order to verify acquired material properties from tensile tests, low velocity impact test is conducted in order to figure out specific conditions of CFRP panels. FE analysis of CFRP is conducted in order to correlate material model of CFRP. Material models of NCF, TWILL, and MIXED type of CFRP specimen are optimized for successful FE anlysis.