A review on aircraft spectra simplification techniques for composite structures

Variable amplitude fatigue testing is a time consuming process and a necessary part of certification for aircraft structures. While the frequency of loading can be increased for testing metals, for composites this approach is limited due to heating effects. Hence it is more pertinent to develop effective methods for reducing/compressing fatigue test spectra for composite structures. Effective spectrum reduction techniques mainly include a spectrum truncation method which removes load cycles that negligibly effect fatigue damage initiation/growth, and a cycle merging method that combines cycles to form a cycle with the equivalent effect on fatigue damage initiation/growth. Application of these methods requires understanding of the fatigue mechanisms. These techniques have seen wide use for metallic structures, with comparatively small amounts of literature existing for composite materials. For composites this is more complex due to their non-isotropic, non-homogeneous, multi-failure mode characters. Due to the lack of research concerning spectrum reduction for composites, there are no widely accepted simplification approaches with the exception of omitting low stress levels. Research is therefore required to develop and test methods for accelerating variable amplitude fatigue tests for composite aircraft structures. On the basis of review of literature, new research directions in this area are identified