ANALYTICAL EXPERIMENTAL EVALUATION OF HYBRID COMMINGLED CARBON GLASS EPOXY THICK-SECTION COMPOSITES UNDER COMPRESSION

This paper outlines a current effort at improving the compressive stre ngth of carbon fiber reinforced composites (CFRC) by using a hybrid ca rbon/glass commingling concept. Prior investigations into the hydrosta tic strength of thick-section carbon fiber reinforced composite cylind ers resulted in failures which were significantly lower (50 to 70% of design pressure) than anticipated. The formation and propagation of fi ber kink bands at the microscopic level, triggered by the fiber misali gnment defects formed during the manufacturing process, leading to a s hear crippling failure at the macroscopic level is the dominant compre ssive failure mode in the presence of fiber misalignment or waviness. It is theorized that one way to improve compressive strength is throug h the use of a commingled (at the tow level) hybrid fiber system. A no vel fracture mechanics based concept for commingling a small percentag e of glass with carbon fibers to suppress compressive failures due to inelastic micro-buckling of fibers and kink band propagation is develo ped. A Griffith type fracture criterion for a mode II crack growth, ba sed on the principle of energy balance, is introduced to derive the hi therto unknown concept of kink toughness (i.e., resistance to kink ban d propagation), and to determine the required glass-carbon ratio for p ossible enhancement of toughness against the kink band propagation. Th e width of a kink, band normalized with respect to a characteristic le ngth scale, such as thickness of a matrix impregnated fiber tow can be used as a practical measure for kink toughness. Preliminary experimen tal 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, inspectio n of the failed compression test coupons clearly demonstrates that int roduction of this small percentage of glass fibers is effective in cha nging the failure mode away from the catastrophic kink band failure mo de. Furthermore, it may be noted that the flexural hybrid commingled g lass/carbon/epoxy composite specimens have failed in tension, which is in sharp contrast to what has been observed in their baseline carbon/ epoxy counterparts.