VOID EFFECTS ON THE INTERLAMINAR SHEAR-STRENGTH OF UNIDIRECTIONAL GRAPHITE-FIBER-REINFORCED COMPOSITES

A study was conducted to evaluate the effect of voids on the interlaminar shear strength (ILSS) of a polyimide matrix composite system. The graphite/PMR-15 composite was chosen for study because of the extensive amount of experience that has been amassed in the processing of this material. Composite densities and fiber contents of more than 30 different laminates were measured along with interlaminar shear strengths. Void contents were calculated and the void geometry and distribution were noted using microscope techniques such as those used in metallography. A good empirical correlation between ILSS and composite density was found. However, the most acceptable relationship between the ILSS and density was found to be a power equation that closely resembles theoretically derived expressions. As the void content increased, an increase in scatter in the strength data was observed. In laminates with low void content, the voids appeared to be more segregated in one area of the laminate. We found that void-free composites could be processed in matched metal die molds at pressures greater than 1.4 MPa and less than 6.9 MPa.