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This paper presents a computational model for simulating axial compressor stall inception and development via disturbances with length scales on the order of several (typically about three) blade pitches. The model was designed for multistage compressors in which stall is initiated by these “short-wavelength” disturbances, also referred to as spikes. The inception process described is fundamentally nonlinear, in contrast to the essentially linear behavior seen in so-called “modal stall inception”. The model was able to capture the following experimentally observed phenomena: (1) development of rotating stall via short-wavelength disturbances, (2) formation and evolution of localized short-wavelength stall cells in the first-stage of a mismatched compressor, (3) the switch from long to short-wavelength stall inception resulting from the re-staggering of the inlet guide vane, (4) the occurrence of rotating stall inception on the negatively sloped portion of the compressor characteristic. Parametric investigations indicated that: (1) short-wavelength disturbances were supported by the rotor blade row, (2) the disturbance strength was attenuated within the stators, and (3) the reduction of inter-blade row gaps can suppress the growth of short-wavelength disturbances. It is argued that each local component group (rotor plus neighboring stators) has its own instability point (i.e., conditions at which disturbances are sustained) for short-wavelength disturbances, with the instability point for the compressor set by the most unstable component group.

Delamination migration is particularly critical in multi-directional composite laminates and is often observed in different loading scenarios and components. Further understanding on the migration mechanism, especially the similarities and differences in the quasi-static and fatigue delamination migrations, is important for the design of composite structures. In this study, the process of delamination migration under mode I quasi-static and fatigue loadings was experimentally investigated for specimens with a +θ/-θ centreline interface. Specimens, with a specially designed stacking sequence, which allows migration events using a simple Double Cantilever Beam setup , were tested for θ=75 o and 60 o. Delamination migration via intralaminar ply splitting has been observed and this was confirmed by the X-ray computed tomography scan results. All the specimens from both quasi-static and fatigue loadings had a fairly similar sequence of damage events; delamination grows through the-θ and +θ ply block successively until it reaches the 0 o ply that prevents further migration. The delamination paths and shape of fracture surfaces were observed to be the same, while the Scanning Electron Microscope fractography results showed that the quasi-static fracture surface was rougher in comparison with that of fatigued specimens. In addition, the distances of migration points from the pre-crack tip were slightly smaller in the fatigue specimens, which may indicate a greater propensity for migration under fatigue loading. This study provides important guidelines to the damage tolerance design of multidirectional composite

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