Analytical/Experimental Evaluation of Hybrid Commingled Carbon/Glass/Epoxy Thick-Section Composites under Compression

Abstract

This paper outlines a current effort at improving the compressive strength of carbon fiber reinforced composites (CFRC) by using a hybrid carbon/glass commingling concept. Prior investigations into the hydrostatic strength of thick-section carbon fiber reinforced composite cylinders resulted in failures which were significantly lower (50 to 70% of design pressure) than anticipated. The formation and propagation of fiber kink bands at the microscopic level, triggered by the fiber misalignment defects formed during the manufacturing process, leading to a shear crippling failure at the macroscopic level is the dominant compressive failure mode in the presence of fiber misalignment or waviness. It is theorized that one way to improve compressive strength is through the use of a commingled (at the tow level) hybrid fiber system. A novel fracture mechanics based concept for commingling a small percentage of glass with carbon fibers to suppress compressive failures due to inelastic micro-buckling of fibers and kink band propagation is developed. A Griffith type fracture criterion for a mode II crack growth, based on the principle of energy balance, is introduced to derive the hitherto unknown concept of kink toughness (i.e., resistance to kink band propagation), and to determine the required glass-carbon ratio for possible enhancement of toughness against the kink band propagation. The width of a kink band normalized with respect to a characteristic length scale, such as thickness of a matrix impregnated fiber tow can be used as a practical measure for kink toughness. Preliminary experimental data suggest enhancement of the compressive and flexural strengths of the composite material even with a small percentage (15%) of glass fibers in commingled hybrid composites. Most significantly, inspection of the failed compression test coupons clearly demonstrates that introduction of this small percentage of glass fibers is effective in changing the failure mode away from the catastrophic kink band failure mode. Furthermore, it may be noted that the flexural hybrid commingled glass/carbon/epoxy composite specimens have failed in tension, which is in sharp contrast to what has been observed in their baseline carbon/epoxy counterparts.