Comparison of out-of-plane tensile moduli of CFRP laminates obtained by 3-point bending and direct loading tests

The out-of-plane tensile modulus of laminated carbon fiber reinforced plastics (CFRPs) was evaluated through 3-point bending tests using laminated UD-CFRP and quasi-isotropic (QI) CFRP samples with out-of-plane span directions. Direct loading tests of QI-CFRP samples were performed for comparison. It was shown that the bending modulus measured for a UD-CFRP specimen with L/t > 25 is consistent with the tensile modulus if the bending modulus is less than 15 GPa. An analytical description of the out-of-plane modulus of thin QI-CFRP samples was derived by modifying the three-dimensional laminate theory and introducing conditions similar to plane-stress. Subsequently, it was demonstrated that this modified out-of-plane modulus is the lower bound of the apparent bending moduli and that the one predicted by the three-dimensional laminate theory is the upper bound. Finally, it was demonstrated that the 3-point bending test method is more practical than a direct loading test when comparing both methods.