Notch Tensile Strength of Carbon Fiber/Epoxy Composite Plate with a Center Hole under Static and Cyclic Loading

The tensile and fatigue properties of epoxy reinforced with 68 volume percent carbon fiber, CFRP, have been fully investigated. The effect of the presence of an open circular hole on tensile, fatigue residual strength, and fatigue strength of CFRP plates was studied experimentally and numerically. The results of pin bearing, i.e. bolt-loaded holes, tests were also investigated and evaluated. A three-dimensional orthotropic-elastic finite element code was developed to calculate the stress concentration factors of open circular holes or bolt-loaded holes in composite materials. The modulus of elasticity, tensile strength, and maximum elongation of CFRP are approximately equal to the values found in the technical data sheet of the supplier. A rule of mixtures type relation is adequate to predict the modulus of elasticity of such CFRP plate. This may be attributed to the high mechanical properties and high volumetric content of the fibers compared with the matrix, i.e. there is a marginal contribution of the strength of the matrix or the fiber-matrix interfacial bond strength. Based on the net stress analysis, there is no difference between the mechanical behavior of smooth and open-holed specimens, i.e. notch insensitivity. This is due to the incapability of the matrix to transfer the load from the discontinuous part. The same behavior was also found under cyclic loading. Due to the weakness in the bond between the fiber and matrix or in the shear strength of the matrix compared with the tensile strength of the fiber, the failure mode of all tested bolt-loaded unidirectional CFRP plates is shear-out. Further, the strength of such bolted joints is mainly dependent on the size of the contact area between the hole and the bolt.