A foil test component (tapered box beam) consisting of hybrid graphite/epoxy skins and HY-130 steel spars was designed and fabricated. This design is representative of a hydrofoil strut/foil system having an approximate 25 percent weight savings compared to current metallic strut/foil systems fabricated entirely of steel with a yield strength of 900 MPa. Two identical box beams will be tested in the laboratory (one in air, the other in salt water) to assess the fatigue behavior of a typical graphite/epoxy structure under simulated sea loads and to compare its behavior with current metal designs.

The graphite/epoxy hybrid (T-300, GY-70 fibers) laminate selected for the skin material has been characterized; the elastic constants and ultimate strengths were in excellent agreement with the theoretical results predicted during the preliminary design phase. An in-depth joint evaluation was conducted as the next step of design verification. Each type of joint was modeled and tested statically and in fatigue. Although a maximum joint efficiency of 98 percent was developed in the I-beam tests, the scarf joint tests led to a 33 percent reduction in the design ultimate stress allowable for the composite material. One box beam has been successfully fabricated and assembled. A comprehensive nondestructive evaluation of the components and finished assembly reveal no detectable defects as a result of laminate fabrication or box beam assembly.