Failure Mechanisms of Interply Hybrid Carbon Fiber/Epoxy Unidirectional Composites with Improved Flexural Impact Properties

The flexural impact properties of a PAN-based carbon fiber/epoxy unidirectional composite are improved by applying an outer reinforcing layer of pitch-based, low modulus carbon fibers. This increase in the impact performance is due to a larger compressive failure strain of the outer layer, which delays the initiation of the fiber microbuckling failure of the PAN-based fiber layers on the compression side. This paper examined the failure mechanisms of interply hybrid laminates in order to determine the influence of suppressed fiber microbuckling on the flexural impact properties. PAN-based carbon fiber with fiber modulus of 230 GPa was used as the core, and an outer reinforcing layer of low modulus carbon fiber with fiber modulus of 55 GPa was applied on the compression side of the core. A drop-weight, flexural impact test suggested that the low modulus fiber layer increased the compressive failure strength of the PAN-based fiber core in the hybrid laminates. A damage mechanics analysis using a finite element (FE) method was performed to simulate the flexural failure. When the compressive strength of the PAN-based fiber core was increased to reflect the influence of hybridization, the FE analysis showed good agreement with the experimental results. It was indicated that the compressive strength of the PAN-based fiber core was increased from 1 470 MPa in a monolithic PAN-based carbon fiber unidirectional laminate to a maximum of 2 355 MPa in the hybrid laminates. The FE results then suggested that the compressive stress in the thickness direction around the loading point had an effect similar to that of a hydrostatic stress state, which restrains fiber microbuckling failure. The improvement in the flexural impact properties of the hybrid laminates was thus demonstrated by the FE analysis from the increased compressive failure strength of the PAN-based fiber core.