Understanding High-Modulus CFRP Compressive Strength Improvement

Abstract

There has been a strong demand in using high-modulus (HM) carbon-fiber composites potentially enabling lightweight aircraft structures with significant weight savings. However, extremely low fiber-direction compressive strength has been a well-recognized weakness of the HM carbon-fiber composites, prohibiting their implementation in aircraft platforms. Fiber-direction compressive strength of PANbased continuous HM and intermediate-modulus (IM) CFRP’s targeting structural applications is presumably governed by microstructural stability. However, strong decrease in fiber-direction compressive strength of HM carbon-fiber composites, compared to their intermediate modulus (IM) counterparts, contradicts predictions from available microstructural buckling theories. This work looks into weak fibermatrix interface as a major mechanism driving the fiber-direction compressive strength decrease of the HM CFRP’s. In-situ scanning electron microscopy (SEM) based experiments pushing out carbon fibers show approximately 40% decrease in the average values of the fiber-matrix interface shear strength for a selected HM composite compared to a legacy IM composite with the same toughened epoxy matrix. And such a strong reduction correlates with decreasing fiber-direction compression strength predicted by a microstructural buckling model which properly accounts for the fiber-matrix interface shear strength behavior. The results support the idea of integrating off-the-shelf IM carbon fibers with stronger fiber-matrix interface and higher shear modulus into a HM carbon-fiber composite to improve fiberdirection compressive strength of the HM composite material system.