Filament Fracture and Postimpact Strength of Boron-Aluminum Composites

Abstract

A preceding study of the fracture behavior of shock-loaded boron-aluminum composites indicated that these materials can have a spall resistance substantially greater than that of unreinforced alu minum. In the present investigation, filament fracture and postimpact tensile strength of a similar composite system were studied. Filament damage was produced by the thin-flyer, plate-impact technique using momentum traps to suppress spall. The specimens contained approxi mately 50 volume percent boron filaments in a plasma-sprayed 6061- aluminum matrix. Filaments were in two mutually perpendicular di rections and the specimens were impacted normal to the plane of reinforcement. Filament fracture was shown to be an important impact damage mechanism in brittle-filament reinforced composites. Filaments were fractured during the initial passage of the loading wave by interactions between adjacent or contacting filaments and by focusing of stresses in isolated fibers. The tensile strength of specimens was reduced by two-thirds after severe impact. However, the impact velocity necessary to produce a substantial decrease of postimpact tensile strength was much higher than that required to produce spall in unreinforced 6061 aluminum. The impact velocity required to cause degradation in postimpact strength was independent of the tape-layup fabrication process, i.e., brazed versus diffusion bonded.