STRENGTH DEGRADATION OF FILAMENT-WOUND GRAPHITE/EPOXY TUBES DUE TO EITHER IMPACT DAMAGE OR FABRICATION DEFECTS

摘要

Two test series were conducted to assess the strength degradation of filament-wound graph-ite/epoxy tubes caused by either impact damage or fabrication defects. The impact damage was made by a pendulum-type impactor. Fabrication defects were either a helical wrinkle (minor or severe) or a hoop wrinkle fabricated inside the tube walls. All tubes were filament wound from Toho G30-500-12K PAN fibers in Epon 828 epoxy resin. The baseline tube configuration had a nine-ply (+10°, -10°, 90°)_3 lay-up, which was used for all the impact and wrinkle defect specimens with the exception of one tube that had a (+15°, -15°, 90°, 90°)_3 stacking sequence. The latter lay-up configuration, which also had a fabricated helical defect, was designed to induce helical fiber-dominated failure. The tubes were subjected to either pneumatic or hydrostatic internal pressure, creating a biaxial stress state in the tube walls with a hoop stress equal to twice the axial stress. End-fittings were designed for sealing the ends of the tubes during pressurization. Each experiment was filmed with a Kodak high-speed video camera operating at 9000 to 13,500 frames/s to determine the location of fracture initiation and to observe the burst dynamics. The test results show that the burst pressure decreases significantly with increasing impact force for impact-damaged tubes. For impact loads of 823 to 1470 N (185 to 330 lb), the burst pressure dropped by as much as 24% to 32%, respectively, compared to the burst pressure of tubes with no impact damage. Most, but not all, failures initiated at the impact locations. Some of the failures originated from locations other than the impact sites. This suggests that different types of damage can occur in tubes that receive essentially identical impacts. For the wrinkle-defect specimens, minor helical wrinkles had no measurable effect on the burst pressure. More severe helical wrinkles caused an average reduction in burst pressure of 8%. Hoop wrinkles caused a significant degradation in strength (14% on average). All the failures initiated either in the membrane area (no wrinkles) or in the vicinity of the fabricated wrinkles.