The strong need for high cycle fatigue mitigation has resulted in numerous techniques resulting in added weight, increased operational costs, and lower performance. The experimental investigation presented was a foundational effort towards mitigating HCF through the use of shape memory alloy in a composite system. The research objective was to quantify changes in eigenvalue, eigenvector, and amplitude of a vibrating cantilever beam with a thin SMA topical treatment; as quantified during SMA phase transformations and through comparison with a control. A composite beam consisting of a nitinol thin SMA foil adhered to an Aluminum Alloy 6061 substrate was designed and fabricated. The three configurations were utilized: (1) a full-span SMA treatment designed for maximum eigenvalue shift and maximum amplitude reduction, (2) a half-span SMA treatment designed for eigenvector shift, and (3) a full-span aluminum treatment for a control. Through a complete modal analysis, results illustrated that thin foil SMA treatments led to a significant shift in eigenvalue, up to 6.53%. Highlighting the reduction in amplitude was a 92% reduction in amplitude at second bending with constant excitation frequency with the full-span sample. Spanwise scans on the half-span sample with and without SMA actuation illustrated a 0.77% shift in node location.
Dynamic control of a vibrating beam is critical to the efforts of mitigation of high cycle fatigue, as it is a leading cause of component or engine failure. Recent advances in composite structures have afford the ability to control the eigenvalue, eigenvectors, and amplitude of vibration through the use of the shape memory effect of shape memory alloys (e.g. Nitinol). These "smart materials" are proven to create active damping and variable stiffness in engine components is an innovative concept. Research presented herein seeks to quantify the effectiveness of Nitinol as a HCF mitigation technique and map the frequency and damping performance. A composite beam consisting of a Nitinol topical actuator adhered to an aluminum alloy 6061 substrate was designed and fabricated. Test specimens comprised two configurations: (1) a composite beam with the topical treatment encompassing the full span of free length of the beam, and (2) a composite beam with the topical treatment encompassing half the span the free length of the beam. Benchtop tests allowed for the determination both modal frequency and damping of the aluminum-Nitinol composite beam with an 8 in. free length. Modal analyses were taken over a selected frequency range using a single point laser vibrometer, excited using a dynamic shaker. Experimental studies were completed over a frequency range which represented the second bending mode. Quality factor values, Q, of approximately 200 were observed. No correlation, however, with phase transformation was realized. The design space charts for temperature, modal frequency, and beam tip amplitude were compiled for both the full-span and half-span sample for second bending mode. These charts graphically depict how tip amplitude changes with varied temperature and illustrate the design capabilities created through the use of shape memory alloy components in dynamic applications.
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