Understanding compressive strength improvement of high modulus carbon-fiber reinforced polymeric composites through fiber-matrix interface characterization

Abstract

Low fiber-direction compressive strength of high-modulus (HM) carbon fiber-reinforced polymers (CFRPs) has been their major weakness prohibiting implementation of such materials in aircraft primary structures despite improving mechanical stiffness at a lower weight. A new HM CFRP achieving fiber-direction compressive strength of intermediate-modulus (IM) CFRPs but with more than 30% higher axial modulus has been recently developed. This work looks into fiber-matrix interface shear strength as a potential mechanism driving the compression strength improvement of the new material system. In-situ scanning electron microscopy (SEM) based single fiber push-out experiments addressing standing challenges associated with manufacturing high-quality samples as well as distinguishing same-diameter HM and IM carbon fibers in the hybrid composite system are used to measure fiber-matrix interface shear strength. The experiments show a 29% lower average value of the fiber-matrix interface shear strength for the HM carbon fibers compared to the IM carbon fibers in the new material system. Such a significant reduction corresponds to a 22% lower fiber-direction compressive strength of the HM CFRP without the integrated IM fibers. The results support the idea of integrating off-the-shelf IM carbon fibers with a stronger fiber-matrix interface and a higher shear modulus into HM CFRPs to improve their compressive strength.