Structural Evaluation of an Advanced Composite Hydrofoil Foil Component.

An advanced composite foil test component tapered box beam was designed by the McDonnell Douglas Astronautics Company. Two identical box beams were fabricated for static and fatigue testing. Each consisted of two hybrid graphite epoxy skins bolted and bonded to three HY-130 steel spars. The box beams were tested to determine their structural behavior under a cantilever bending stress distribution due to a concentrated load, to establish a load versus stress relationship for the simulated sea load fatigue test, and to compare their response to cyclic loading with that of candidate metallic designs. In addition, initial flaws and subsequent damage as a result of static and cyclic loading were evaluated using various ultrasonic testing techniques. Initially, Box Beam 1 incurred delaminations to the inner surface of the tensile skin in the load application area at 80 percent of the preliminary maximum operating load foil broach of 60 kips. Attempted repairs and modifications in this area failed to fix this problem, and a subsequent proof test to 60 kips resulted in extensive delaminations to both surfaces of both skins however, there did not appear to be any significant through-the-thickness damage as evaluated by ultrasonic test techniques. Box Beam 2 was initially tested to a static load of 30 kips to aid in the evaluation of out-of-plane stresses which were felt to have been the cause of failure to Box Beam 1. The results of this test were inconclusive.