EFFECT OF FIBER-MATRIX INTERFACE ON FIBER-DIRECTION COMPRESSIVE STRENGTH OF CARBON FIBER COMPOSITES

Abstract

The rotorcraft industry has shown a strong interest in High-modulus (HM) carbon fiber-reinforced polymers (CFRPs) due to their potential to create lightweight airframes and rotor components, resulting in significant weight reduction. However, a major drawback of HM CFRPs has been their very low compressive strength in the fiber direction compared to the currently used intermediate-modulus (IM) CFRPs in primary structures. This weakness has been delaying the implementation of HM CFRPs in aircraft structures. Microstructural tailoring may provide an innovative means for breaking through the fiber-direction compressive strength barrier of the HM CFRPs. The primary failure mechanism in both HM and IM CFRPs under fiber-direction compression is shear microbucking, which is significantly influenced by fiber-matrix interface strength. In-depth analysis using in-situ scanning electron microscopy (SEM) experiments has revealed significant differences in the surface characteristics of the carbon fibers, leading to a much stronger interface for IM fibers compared to HM fibers. These findings have prompted a microstructural tailoring strategy involving the reinforcement of HM fibers with IM fibers, which enhances the overall stability of the microstructure governing the fiber-direction compressive strength performance of the material. A laboratory-scale production-quality manufacturing system has been delivered, and promising experimental results enabling HM CFRPs with adequate fiber-direction compressive strength have been achieved through hybridization of IM and HM fibers at the filament level in HM CFRP toughened with nano-silica. This new material solution not only approaches the compressive strength of IM CFRPs but also provides more than 30% higher axial modulus.