Damping of layered porous composites and an application in machinery

The structured composite can involve the porous materials providing the important internal damping that is usable in mechanical engineering applications. The stiff base material is equipped by layer/s of softer porous material/s covered by constrained layer. Such layered structure is characterized by increased internal damping in vibration process. Paper analyses some either measured or simulated mechanical properties of designed layered porous composites. Novelty provided in paper is a specific application of structured composite for damping the high frequency vibrations in resonance frequency obtaining the beneficial results, e.g. 40-48% of maximum amplitude reduction.

STABILITY AND VIBRATION ANALYSES OF AN INTERNALLY DAMPED TAPERED COMPOSITE DRIVESHAFT USING THE FINITE ELEMENT METHOD

The present study considers the linear vibration and stability analyses of an internally damped rotating tapered composite shaft supported on rolling bearings. The Timoshenko beam theory is utilized to model the tapered drive-shaft based on Equivalent Single Layer Theory (ESLT). The ESLT considers a laminated driveshaft that consists of several lamina with different fiber orientations. Since the bearings are considered as rolling element bearings, the bearings stiffnesses are modeled using linear translational springs and dampers. The equations of motion are derived by applying Lagrange’s equation, including the hysteretic internal damping of composite material, and then finite element formulation is utilized to solve the equations. The effects of various system parameters on the natural frequencies and instability threshold are investigated. An extensive parametric study has been carried out to determine the effects of various system parameters including hysteresis internal damping, fiber orientation, stacking sequence, taper angle, rotational velocity, and bearings stiffness and damping on the natural frequencies, critical speeds, and instability thresholds of internally damped tapered composite drive-shafts. Furthermore, Campbell and critical speed map diagrams are depicted to present the effects of rotational velocity and bearings stiffness on natural frequencies and critical speeds. It is shown that the stability of the driveshaft is enhanced by increasing the damping of the bearings, whereas increasing the internal damping of the composite driveshaft may reduce the instability threshold.

Phase difference and stability of a shaft mounted a dry friction damper: Effects of viscous internal damping and gyroscopic moment

This study investigates the stability and phase difference of a shaft mounted a dry friction damper with effects of viscous internal damping and gyroscopic moment. The equations of the system with the vibration reduction effect of the dry friction damper on the shaft are derived in the form of the rectangular coordinate and polar coordinate in the vicinity of critical speed. The phase difference characteristics in the rub-impact process and its physical mechanism are analyzed by mathematical derivation. The characteristic equation is studied to investigate the stability of the periodic solution. Effects of different parameters of the system, especially viscous internal damping of the composite shaft and gyroscopic moment on the phase difference and stability regions are presented in detail by analytical and numerical simulation based on a helicopter tailrotor driveline. The experimental investigation is conducted in a test rig to validate theoretical formulas and simulation analysis. The analysis results show that rub impact delays the change of phase difference, viscous internal damping improves the stability of synchronous full annual rub solution, and gyroscopic moment affects the increase of the phase difference.

Dimensional analysis of spring-wing systems reveals performance metrics for resonant flapping-wing flight

Flapping-wing insects, birds and robots are thought to offset the high power cost of oscillatory wing motion by using elastic elements for energy storage and return. Insects possess highly resilient elastic regions in their flight anatomy that may enable high dynamic efficiency. However, recent experiments highlight losses due to damping in the insect thorax that could reduce the benefit of those elastic elements. We performed experiments on, and simulations of, a dynamically scaled robophysical flapping model with an elastic element and biologically relevant structural damping to elucidate the roles of body mechanics, aerodynamics and actuation in spring-wing energetics. We measured oscillatory flapping-wing dynamics and energetics subject to a range of actuation parameters, system inertia and spring elasticity. To generalize these results, we derive the non-dimensional spring-wing equation of motion and present variables that describe the resonance properties of flapping systems: N , a measure of the relative influence of inertia and aerodynamics, and K ^ , the reduced stiffness. We show that internal damping scales with N , revealing that dynamic efficiency monotonically decreases with increasing N . Based on these results, we introduce a general framework for understanding the roles of internal damping, aerodynamic and inertial forces, and elastic structures within all spring-wing systems.