Interlaminar crack propagation in thick composite shells

Delamination growth has been investigated as a potential failure mechanism for filament-wound composite cylinders used for offshore and underwater structures. Analysis and experiments on DCB, ENF, and MMB beam fracture specimens machined from angle-ply laminate panels and filament-wound composite cylinders are presented. Bending analysis of beam fracture specimens machined from flat panels and composite cylinders was derived from first order shear deformation theory and one-dimensional expressions obtained from laminated plate and shell theories. For the DCB specimens, elastic foundation effects were modeled. Experiments on flat, glass/polyester laminate beam specimens considered [0]6, [+/-30]5 and [+/-45] 5 lay-ups with mid-plane delaminations. Experiments on beam specimens machined from composite cylinders were conducted on [+/- q ]6 and [+/- q ]12 lay-ups with mid-surface delaminations where q = 30 degrees, 55 degrees and 85 degrees. For all lay-ups and specimen configurations, beam model predictions of compliance were in good agreement with experimental data over the range of laminate thicknesses, ply angles, and crack lengths examined. Fracture toughness for delamination propagation was examined for flat glass/polyester panels and glass/epoxy cylinders. The initiation value of mode II fracture toughness, GIIc, was much larger than the initiation value of mode I fracture toughness GIc. The initiation value of mixed mode fracture toughness, Gc, increased with decreased ratio GI/GII and increased ply angle q . Debonding of transversely oriented fiber bundles was observed as a major crack arrest and fracture resistance mechanism for the flat, glass/polyester angle-ply laminates. Bridging by interlaced fiber bundles and crack jumping to another interface contributed to crack arrest and limited the growth in the curved, glass/epoxy angle-ply laminates. For all lay-ups, the crack propagated in a non-uniform manner across the width of the specimen as explained by elastic coupling effects in the laminate beams of the cracked region.