Marcelo Braga dos Santos

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

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.© 2012 ASME

Design of a New Test Rig to Evaluate Under-Platform Damper Performance

This work presents the development of a test rig capable of measuring the forces transferred between the blade platforms through the under-platform damper. This test rig is composed of two distinct parts each one representing a platform. The static part contains the load cells, which measure the forces in two perpendicular directions; the moving part controlled using two piezoelectric actuators reproduces any in-plane relative displacement between two adjacent platforms. In this scheme, the damper is placed between these two platforms and loaded by dead weights that reproduce the effects of centrifugal force. The hysteresis cycle, of the damper system, is obtained using the measured forces and the imposed displacement. In addition, two laser beams can be used to measure the damper displacement and its tilt angle, which allows validating dynamic models of the damper. Moreover, the test rig is designed to allow heating the specimens up to temperatures which are normally found in real operation. Finally, the test rig provides necessary variables to study the damper performance and to evaluate some contact models used to simulate under-platform dampers.Copyright

A Procedure for the Parametric Identification of Viscoelastic Dampers Accounting for Preload

Passive vibration isolators are usually made of viscoelastic materials. These materials have non-linear characteristics that change their dynamical properties with temperature, frequency and strain level. The vibration isolator’s mathematical modeling and optimal design require the prior knowledge of the stiffness and damping of the applied viscoelastic material. This work presents a dynamical characterization methodology to identify the stiffness and damping of three samples of viscoelastic rubber with hardness of 25, 33 and 48 SHORE A. The experimental apparatus is a one-degree of freedom vibratory mechanical system coupled to the viscoelastic damper. Sweep sine excitations are applied to the system and the resulting forces and vibration levels are measured. The amplitude of the excitation is controlled to achieve a constant RMS level of strain in the viscoelastic samples. The experimental results are obtained for conditions of no pre-strain and with a 10% of pre-strain. The time domain data is post-processed to generate frequency response functions that are used to identify the damping and stiffness properties of the viscoelastic damper. Keywords: viscoelastic, damping and complex stiffness

Non-Linear Signal Analysis Applied to Surface Wear Condition Monitoring in Reciprocating Sliding Testing Machines

When the surfaces of two elastic bodies present relative motions under certain amount of contact pressure the mechanical system can be unstable. Experiments conducted on elastic bodies in contact shown that the dynamic system is self-excited by the non-linear behavior of the friction forces. The main objective of this paper is to estimate the friction force using the vibrations signals, measured on a reciprocating wear testing machine, by the proposed non-linear signal analysis formulation. In the proposed formulation the system global output is the sum of two outputs produced by a linear path associated in parallel with a non-linear path. This last path is a non-linear model that represents the friction force. Since the linear path can be identified by traditional signal analysis, the non-linear function can be evaluated by the global input/output relationships. Validation tests are conducted in a tribological system composed by a sphere in contact with and a prismatic body, which has an imposed harmonic motion. The global output force is simultaneously measured by a piezoelectric and by a piezoresistive load cells. The sphere and prismatic body vibrations are measured by a laser Doppler vibrometer and by an accelerometer respectively. All signals are digitalized at the same time base and the data is transferred to a microcomputer. The non-linear signal analysis technique uses this data to identify the friction force.

Tunable Auxiliary Mass Damper with Friction Joint: Numerical Assessment

Auxiliary Mass Dampers (AMD) are often used to reduce excessive vibration amplitude in mechanical systems. It is also known that their performances are susceptible to changes in the frequency or in the nature of the excitation force. Therefore, to improve the robustness of the AMD it is necessary to design new systems which are adaptable to the excitation, i.e., tunable devices that could be used over large frequency range. In this work a friction damper, which is the association of an elastic element and a scratcher, is used to tune the AMD by changing the normal force in the scratcher at the same time that it dissipates the mechanical energy of the principal mass. This AMD is named Tunable Auxiliary Mass Damper (TAMD). Three normal force control strategies, and two combinations of them, are studied: (i) The normal force is assumed constant; (ii) The normal force is obtained from the solution of the equation of motion assuming null displacement for the principal mass; (iii) The normal force is obtained based on the vibratory system’s movement, warranting that the direction of the friction force promotes the movement of the principal mass toward its static equilibrium position. The effectiveness of the proposed TAMD is numerically evaluated based on mass and frequency ratios variations for each strategy. Therefore, a multi-degree-of-freedom (MDOF) system analysis is made in order to verify the TAMD’s robustness and efficiency.