Fracture control techniques for full-scale structural articles generally require laboratory test data, analysis of key parameters, and evaluation using fracture mechanics techniques. These parameters, usually fracture toughness or critical strain energy release rate, are often measured for metals and even thin-wall composite materials. However, data on thick-wall composites greater than 25 mm is virtually nonexistent.
The Space Shuttle solid rocket booster case is currently designed using D6AC steel. However, considerable weight savings are achieved by using conventional filament-wound composites to replace the steel case. The Filament Wound Case (FWC) program is aimed at developing graphite/epoxy composite segments for the Space Shuttle.
Each FWC segment is 5.9 to 8.2 m (19.5 to 27.0 ft) in length and 3.7 m (12 ft) in diameter. The thickness of the composite varies from 28 mm (1.12 in.) in the center, or membrane region, to 46 mm (1.8 in.) in the joint region. The layup in the membrane region consists of 11 layers of ±29° helical and 90° hoop fibers with the Hercules AS4-12K graphite and HBRF-55A resin system. In the joint region, 0° broadgoods are inserted to increase strength and stiffness. The joint helical and hoop layers are also Hercules AS4-12K graphite fiber with HBRF-55A resin, while the 0° broadgoods are AS4-12K graphite prepreg with 3501-5A resin.
As part of the fracture control program, emphasis was placed on the development of specific test methods to measure pertinent fracture parameters for the thick-wall, filament-wound graphite composite. This paper discusses several of the samples selected and analyzed to date as well as the experimental results derived from full-thickness samples removed from a dissected composite segment.
Test methods currently include both Mode I and Mode II fracture toughness measurements. Interlaminar fracture toughness and strain energy release rates were determined for both modes using a double cantilever beam and modified rail shear test, respectively. Mode I cross-ply fracture toughness was determined using a wide, double-edge notch specimen. Finite-element analyses were conducted on all of the specimens. A J-integral post processor was used to develop corresponding Mode I and Mode II strain energy release rates.
The values determined from the thick-wall, filament-wound composites were compared to other data from autoclaved systems reported in the open literature. Problems encountered in testing these thick-wall samples and existing analysis limitations are also discussed.