Measurement of the Scatter of Underplatform Damper Hysteresis Cycle: Experimental Approach

Abstract

This work presents the design and the calibration of a test rig specially developed to measure the in-plane forces transferred between the blade platforms through the under-platform damper and their relative displacement. This device is composed of two distinct parts each one representing a platform. One is static and accommodates the load cells which measure the forces in two perpendicular directions; the other produces the in-plane motion, actuated by two piezoelectric stacks. The device reproduces any in-plane relative displacement between two adjacent platforms and measures both the relative motion between platforms and the forces they reciprocally transmit. The damper, placed between the two platform simulators, is loaded by thin wires pulled by dead weights, a way to apply the equivalent of the centrifugal force. The mechanical features of the rig are described and discussed with their influence on the measurements. An example application is given. Tests aim at assessing the role of “outer” measured parameters (such as frequency and amplitude of platform-to-platform relative displacement, damper external load (simulating the in-service centrifugal load), damper geometry) on the shape and area of the hysteresis cycle and therefore the damper real and imaginary stiffness components. It is found that equal values for the supposedly governing “outer” parameters may lead to a multiplicity of markedly different hysteresis cycles. The same happens if platform-to-platform force is considered rather than displacement. It is shown how the system evolves through the many possible equilibrium conditions. It is also shown how the forces between damper and underplatforms are calculated. It is suggested that the measurement of platform-to-platform hysteresis cycles is an effective way to synthetically approach the problem of elastic coupling and energy dissipation between adjacent blades, while detailed knowledge of forces exchanged between the underplatform and damper contact surfaces will be a valuable tool toward the better knowledge of damper micromechanics, perhaps opening a better way to finding damper geometries capable of reducing the scatter of hysteresis cycle shape and area. Two dampers are investigated, at this stage, in order to assess the dependence of the above said behavior on the damper geometry. Results show that dampers exhibit multiple behaviors under the same input conditions. They may be alarming because they show that the damper-platforms system always converges to the solution with the lowest hysteresis area, a fact which deserves of course deeper investigations.