ABSTRACT The propagation of very long fatigue cracks in a redundant and non-redundant stiffened-
panel structure is assessed. In ship structures, cracks are known to propagate to a length greater than 24m without sudden fracture or instability. To investigate this phenomenon,
large stiffened single panels and cellular box beams were fatigue tested and the crack length versus number of cycles monitored. The panels failed by ductile tearing at service
temperatures due to net section plastic collapse. In addition, probabilistic numerical finite
element models were developed and the solution for the stress intensity factor (K) obtained and used with the Paris Law to characterize the crack propagation rate. The results can be used to specify inspection and maintenance intervals to avoid complete
system failure.