Abstract
A novel application of magnetostrictive actuators in underplatform dampers of bladed disks is proposed for adaptive control of the normal load at the friction interface to achieve the desired friction damping in the structure. Friction damping in a bladed disk depends on operating parameters, such as rotational speed, engine excitation order, nodal diameter normal contact load, and contact interface parameters, such as contact stiffness and friction coefficient. The operating parameters have a fixed value, whereas the contact interface parameters vary in an unpredictable way at an operating point. However, the ability to vary some of these parameters such as the normal contact load in a controlled manner is desirable to attain an optimum damping in the bladed disk at different operating conditions. Under the influence of an external magnetic field, magnetostrictive materials develop an internal strain that can be exploited to vary the normal contact load at the friction interface, which makes them a potentially good candidate for this application. A commercially available magnetostrictive alloy, Terfenol-D is considered in this analysis that is capable of providing magnetostrain up to 2 × 10-3 under prestress and a blocked force over 1500 N. A linearized model of the magnetostrictive material, which is accurate enough for a direct current application, is employed to compute the output force of the actuator. A nonlinear finite element contact analysis is performed to compute the normal contact load between the blade platform and the underplatform damper as a result of magnetostrictive actuation. The nonlinear contact analysis is performed for different actuator mounting configurations and the obtained results are discussed. The proposed solution is potentially applicable to adaptively control vibratory stresses in bladed disks and consequently to reduce failure due to high-cycle fatigue. Finally, the practical challenges in employing magnetostrictive actuators in underplatform dampers are discussed.
Highlights
Dry friction damping is widely used to reduce vibratory stresses in bladed disks to avoid failure due to highcycle fatigue (Griffin, 1980; Srinivasan and Cutts, 1983)
A typical variation of the nonlinear forced response curve with normal contact load, is shown in Appendix 1, indicates that the optimum damping in the system occurs at a particular normal load
The computed results reveal that a change in normal load as high as 750 N can be obtained by properly designing the actuator mounting
Summary
Dry friction damping is widely used to reduce vibratory stresses in bladed disks to avoid failure due to highcycle fatigue (Griffin, 1980; Srinivasan and Cutts, 1983). Terfenol-D is an often used commercially available magnetostrictive alloy for actuator application due to its high magnetostrain capacity, fast response, and high magneto-elastic transduction efficiency (.50%) and known as giant magnetostrictive material (GMM; Zhang et al, 2012) This means that actuators driven by Terfenol-D have larger displacement and higher output force capacity compared to other common transducer materials. The nonlinear static contact analysis of the bladed disk and the UPD reveals that a change in normal load more than 700 N can be achieved using a reasonable actuator size. This will give a very good control on friction damping once applied in practice. Terfenol-D drive element has a very high yield strength in compression (700 MPa) compared to the tension (28 MPa) and exhibits enhanced magnetostriction under moderate compression, and as a consequence, Terfenol-D actuators are operated almost exclusively under a compressive load (Calkins et al, 1997; Grunwald and Olabi, 2008)
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