Value Of Damping Ratio Research Articles

Dynamic Properties of EPS Geofoam: An Experimental Investigation

This paper presents the results of an experimental investigation of the dynamic properties of two expanded polystyrene (EPS) geofoam materials. Torsional resonant column tests and cyclic uniaxial tests were conducted on block-molded EPS geofoam specimens with mean densities of 12.4 and 17.1 kg/m3, under zero confining pressure. The estimated dynamic moduli values (shear modulus and modulus of elasticity) and damping ratio values are presented as functions of cyclic strain amplitude and loading frequency. The geofoam behavior for strains ranging from 5 × 10-4 to 8.0% was studied by combining the results of the two types of tests. It was found that EPS geofoam behaves linearly for strain values of up to 0.1%; however, a strong nonlinearity develops for strains greater than 1%. The geofoam density significantly affected the dynamic moduli values (the moduli increase with increasing density) whereas no appreciable effect on damping ratio values was found. The damping ratio values were very low (∼1%) for strains less than 0.1% and gradually increased to 10% for strains greater than 1%. The test results also indicated that frequency of loading does not significantly affect the moduli values, whereas the damping ratio values decreased with increasing frequency values. Based on the test results, shear modulus, G/G0, versus cyclic shear strain amplitude, ξc, and damping ratio, D versus ξc, curves are proposed to describe the EPS geofoam nonlinear properties.

Estimation of Modal Damping Ratios from Mode Indicator Function.

The authors propose the utilization of a MIF (mode indicator function) to estimate modal damping ratios approximately. The accuracy and the effect of this method are examined to determine its feasibility for use in experimental modal parameter identification. MIF is a straight-forward function derived from FRFs, and indicates the existence of normal modes. MIF is generally used to indicate undamped natural frequencies, but not for other purposes. However, MIF can be used to estimate modal damping ratios roughly as well as to estimate undamped natural frequencies. The authors propose using of MIF to set the initial values of modal damping ratios as well as the values of natural frequencies in any modal parameter identification method in the frequency domain.

Material damping studies on carbon-carbon composites

Experimental studies were performed to investigate the resonance frequency and damping behavior of carbon-carbon (C-C) composite specimens. Initially, the resonance frequency and damping-ratio values of the as-cured composite (graphite fabric impregnated with phenolic resin, referred to as the as-cured stage) specimens were measured. These parameters were measured again after subjecting the specimens to carbonization to a temperature of 800°C (referred to as the carbonized composite) in an inert atmosphere. It was observed that the specimens at the carbonized stage showed a reduction in resonance frequency and an increase in damping-ratio value. The microstructure of the specimens was studied using optical microscopy. The changes in resonance frequency and damping-ratio values were then related to the microstructure of the composite.

Resonant Column Testing of Agricultural Soils

ABSTRACT REMOLDED cylindrical specimens and minimally-disturbed cylindrical samples of three types of agricultural soils were tested in a torsional-type resonant column device. The soils were tested at confining stresses close to zero to simulate field conditions. Shear moduli and damping ratios were examined as functions of strain amplitudes ranging from 5 x 10~^ to 12 x 10~^ cm/cm. Computed shear modulus values, which decreased with increasing strain amplitude, ranged from 13 to 38 MPa. Calculated damping ratio values, which increased with increasing strain amplitude, ranged from 0.9 to 4.0%. Vibration responses of remolded specimens and minimally-disturbed samples were different, thereby demonstrating the effects of structure upon dynamic behavior of soil.

Energy Absorption of Structures Under Cyclic Loading

The results of an experimental and theoretical investigation of the energy absorption, or structural damping, characteristics of conventional plywood shear walls are described. The cyclic-loading test procedure, wherein the test specimen is subjected to a succession of reversed loading cycles of progressively increasing magnitude, was used for each test. Eight different tests were made, and the cyclic load-deflection curves were then constructed using the experimental data. From these curves, it was possible to derive back-bone curves, relating the maximum load per cycle to the average absolute value of the maximum deflections per cycle, and curves relating energy capacity to maximum deflection, energy absorption to maximum deflection, and residual deflection to maximum deflection. Values of an equivalent viscous damping ratio, ν\Deq\N, were found to average 0.10 for shear walls with plywood on both sides. Having ν\Deq\N, it is then possible to use the velocity response spectrum to determine the maximum relative velocity, and subsequently the earthquake energy output, for a particular plywood shear wall. The derived curves previously mentioned are used in conjunction with the energy method to obtain the maximum panel response.