Multiscale characterization of thermoacoustic response and fatigue failure of aerospace structures

[摘要] Sustained hypersonic flight has presented a complex problem to researchers and structural designers in recent decades as it has been seen to induce failure of thin aerospace panels in modes that had not been previously accounted for. These new and unaccounted for failure modes are attributed to the extreme and coupled loading conditions of the thermoacoustic environment prevalent in hypersonic flight. Prior research has highlighted the resonant behavior of simple structures in a combined loading environment of vibration and heating. The effects of various heating distributions on pre-thermally and post-thermally buckled plates have been evaluated in theoretical and experimental work. However, this understanding has not yet found its way into advanced thermomechanical coupled simulations, in part because fatigue failure caused by in-plane thermal gradients from localized heating, vibration, and mechanical boundary conditions has not been sufficiently addressed in the laboratory setting to validate such complex simulations. The present work seeks to add to our current understanding of this topic with a series of experiments which investigate structural response and failure at multiple length scales. Non-contact optical methods for displacement and strain measurement were used to study the resonance, broadband excitation response, and thermal loading response of structures with varying boundary conditions. Thin aerospace-type beams and plates made of a nickel super-alloy, Hastelloy X, Al 1100-O, and Al 1100-H14 were subjected to forced vibration initially at room temperature and subsequently with localized heating to examine the effects of thermal stress gradients on structural response. Coarse-grained specimens were then produced by annealing aluminum Al 1100-O (commercially pure Al) to explore the role of microstructural phenomena in the thermoacoustic environment and their influences on global behavior. Using oligocrystal samples in this fashion made the grain scale effects occur at the same scale as the sample size and thus both effects could be investigated simultaneously. The microstructural heterogeneity of coarse-grained beams was shown to have significant effect on plastic hinging behavior at the beam root. Finally, fatigue experiments were performed in a combined loading environment to assess behavior beyond the linear elastic regime and promote plasticity and failure. Although fatigue failure was suppressed in thin beams and panels, adding a stress concentrator, such as a notch near the beam root, promoted fatigue crack nucleation.

[关键词] Fatigue, Failure, Aerospace