Burst Tests of Filament-Wound Graphite-Epoxy Tubes: Pathfinder Test Series for Delta II GEM Motor Cases

A test program was conducted to investigate the failure modes of undamaged and damaged graphite-epoxy cylindrical tubes subject to internal pressure, and to simulate the failure mode of the Delta graphite epoxy motor GEM case K404. There were two phases of this test program the mini-test series and the pathfinder series. The mini-test series, using 1.5-in. diameter tubes, included five tests. The pathfinder series, using 4.00-in. diameter tubes, included 41 tests. Two types of intentional damage were made to some of the tubes either a longitudinal-line cut or a single-point impact. A Kodak High Speed Motion Analyzer model 4540 was used to record images of the fracture process. Testing was conducted under three conditions hydraulic pressurization, pneumatic pressurization with solid inserts, and pneumatic pressurization with inert propellant inserts. Several failure modes were observed. A tube with a longitudinal cut can fail in three different modes a local leakage mode, a bursting mode with a fracture initiating from the cut, or a bursting mode with complete tube disintegration. In testing of impact-damaged tubes, the results show that the burst pressure decreases with increasing impact load. At an impact load of 335 Ib, the burst pressure decreases by 33 compared to the undamaged condition. The failure initiation point of the impact-damaged tubes was not necessarily at the impact location. Apparently, impact can cause localized damage to other areas of tubes. However, thermography and ultrasound inspections failed to detect this damage. This is possibly because these nondestructive examination NDE techniques can detect delamination but not fiber breakage in composite materials.