THERMOOXIDATIVE STABILITY AND FIBER SURFACE MODIFICATION EFFECTS ON THE INPLANE SHEAR PROPERTIES OF GRAPHITE PMR-15 COMPOSITES/

Experiments were conducted to study the effects of thermo-oxidative st ability (weight loss) and fiber surface modification on the inplane sh ear properties of graphite/PMR-15 unidirectional composites. The isoth ermal aging was conducted at 316 degrees C and up to 1000 hours of agi ng times. The role of fiber surface treatment on the composite degrada tion during the thermo-oxidative aging was investigated by using A-4 g raphite fibers having three different surface modifications, namely un treated (AU-4), surface treated (AS-4), and surface treated and sized with epoxy-compatible sizing (AS-4G). Weight loss of matrix, fibers, a nd composites was determined during the aging. The effect of thermal a ging was seen in all the fiber samples in terms of their weight loss a nd reduction in fiber diameter. Calculated values of weight loss fluxe s for different surfaces of rectangular unidirectional composite plate s showed that the largest weight loss occurs at those cut surfaces whe re fibers are perpendicular to the surface. Consequently, the largest amount of damage was also noted on these cut surfaces. Optical observa tion of neat matrix and composites plates subjected to the different a ging times revealed that the degradation (such as matrix microcracking , void growth, etc.) occurred within a thin surface layer near specime n edges. The inplane shear modulus of the composites was unaffected by the fiber surface treatment and the thermal aging. The shear strength of the composites having the untreated fibers was the lowest and it d ecreased with aging. Fracture surface examination of the composites ha ving untreated fibers suggests that the weak interface allows the oxid ation reaction to proceed along the interface and thus expose the inne r material to further oxidation. The results indicate that fiber-matri x interface affects the composite degradation process during its therm al aging and that the weak interface accelerates the composite degrada tion.