Energy dissipation prediction of particle dampers

This paper presents initial work on developing models for predicting particle dampers (PDs) behaviour using the Discrete Element Method (DEM). In the DEM approach, individual particles are typically represented as elements with mass and rotational inertia. Contacts between particles and with walls are represented using springs, dampers and sliding friction interfaces. In order to use DEM to predict damper behaviour adequately, it is important to identify representative models of the contact conditions. It is particularly important to get the appropriate trade-off between accuracy and computational efficiency as PDs have so many individual elements. In order to understand appropriate models, experimental work was carried out to understand interactions between the typically small (1.5–3 mm diameter) particles used. Measurements were made of coefficient of restitution and interface friction. These were used to give an indication of the level of uncertainty that the simplest (linear) models might assume. These data were used to predict energy dissipation in a PD via a DEM simulation. The results were compared with that of an experiment

Model structure detection and system identification of metal rubber devices

Metal rubber (MR) devices, a new wire mesh material, have been extensively used in recent years due to several unique properties especially in adverse environments. Although many practical studies have been completed, the related theoretical research on metal rubber is still in its infancy. In this paper, a semi-constitutive dynamic model that involves nonlinear elastic stiffness, nonlinear viscous damping and bilinear hysteresis Coulomb damping is adopted to model MR devices. After approximating the bilinear hysteresis damping using Chebyshev polynomials of the first kind, a very efficient procedure based on the orthogonal least squares (OLS) algorithm and the adjustable prediction error sum of squares (APRESS) criterion is proposed for model structure detection and parameter estimation of an MR device for the first time. The OLS algorithm provides a powerful tool to effectively select the significant model terms step by step, one at a time, by orthogonalizing the associated terms and maximizing the error reduction ratio, in a forward stepwise procedure. The APRESS statistic regularizes the OLS algorithm to facilitate the determination of the optimal number of model terms that should be included into the dynamic model. Because of the orthogonal property of the OLS algorithm, the approach leads to a parsimonious model. Numerical ill-conditioning problems confronted by the conventional least squares algorithm can also be avoided by the new approach. Finally by utilising the transient response of a MR specimen, it is shown how the model structure can be detected in a practical application. The identified model agrees with the experimental measurements very well

In situ observation of NiTi transformation behaviour: A micro-macro approach

A novel experimental investigation is presented of thermally and stress induced transformation behaviour of a Polycrystalline NiTi Shape Memory Alloy (SMA) plate for flexural-type applications: In situ techniques are employed to allow simultaneous macroscopic and microstructural observation of the SMA in a 4-point flexural test. Forming part of a wider research towards realising a NiTi SMA Variable Stator Vane assembly for the gas turbine engine, the study explores variables critical to flexural-type morphing NiTi structures: (1) temperature; (2) strain; and (3) cyclic loading. It builds a relationship between the macro and micro response of the SMA under these key variables and lends critical implications for the future understanding and modelling of shape memory alloy behaviour for all morphing applications. This paper presents the methodological aspects of this study

The dynamic characterisation of disk geometry particle dampers

Particle dampers (PDs) have the advantages of being simple in geometry, small in volume and applicable in extreme temperature environments. Experimental studies have shown that PDs can offer considerable potential for suppressing structural resonant conditions over a wide frequency range. In this paper, the nonlinear characteristics of PDs are studied experimentally in a series of response-level-controlled tests. The effect of the geometry is studied and a method is developed to model the nonlinear damping of PDs as equivalent viscous dampers that can be applied directly to engineering structures at the design stage

Computer modelling of the dynamic response of viscoelastic vibroisolators

The paper investigates ways to model the response of vibro-isolation mounts that utilise viscoelastic materials. Simple models based on linear and nonlinear static stiffness are developed. Dynamic response is approximated through appropriate scaling of the viscoelastic Young’s modulus and use of the measured material loss factor. The approach is validated using cylindrical mounts made of polyurethane. The response of a 68 kg mass supported by two mounts and subjected to two different high-amplitude shock loads is predicted. Measured and predicted behaviour correlate closely of the nonlinear model while the linear model gives a reasonable representation. It is noted that the sensitivity of such mounts to temperature is high: the change in response associated with a temperature excursion of 10 C is significantly greater than the inaccuracy involved with using the linear model

A Tangential Microslip Model for Circularly and Elliptically Loaded Structures

It is well known that the noise and vibration control of structures is a continuing challenge. With many structures this is commonly achieved through passive vibration damping; often using viscoelastic materials. Many structures however do not permit the use of such materials as their properties change over time, or when they are subjected to low and high temperature environments. For these structures, it is often that clamping or applied boundary conditions are critical for providing the energy dissipation through microslip. This makes it necessary to understand the level of damping that arises from the clamping zones. In general, the estimation of damping is complicated in that most structures are not uni-directionally loaded and can have a planar path of motion (e.g. gas turbine blades, circular motion valves). Although it is typical during experiments and simulations to reduce a structure into a single axis of excitation, this can often be an over simplification which does not describe the dynamics of the system; but should be included. This paper presents a biaxial planar motion tangential microslip model that accounts for the vibratory loads arising from circular and elliptical motion. This model vectorially decouples and reduces the planar vibratory circular and elliptical motion into two separate independent tangential microslip models. The models account for tip loading and for centroid loading within the microslip region of the clamping zone. Each analytical microslip model is presented and is compared to numerical simulations using finite elements. The analytical models are then coupled to demonstrate the net effect that various eccentricities have on the overall energy dissipation within the structure. The coupled models are then compared to numerical simulations using finite elements through ANSYS

Strategies for using cellular automata to locate constrained layer damping on vibrating structures

It is often hard to optimise constrained layer damping (CLD) for structures more complicated than simple beams and plates as its performance depends on its location, the shape of the applied patch, the mode shapes of the structure and the material properties. This paper considers the use of cellular automata (CA) in conjunction with finite element analysis to obtain an efficient coverage of CLD on structures. The effectiveness of several different sets of local rules governing the CA are compared against each other for a structure with known optimum coverage-namely a plate. The algorithm which attempts to replicate most closely known optimal configurations is considered the most successful. This algorithm is then used to generate an efficient CLD treatment that targets several modes of a curved composite panel. To validate the modelling approaches used, results are also presented of a comparison between theoretical and experimentally obtained modal properties of the damped curved panel

The dissipative characteristics of oblate particles in granular dampers

In numerical models for granular dampers, particles are generally considered to be perfect spheres. However, in practical engineering applications these particles can slightly deviate from being true spheres. It has been observed experimentally that sphericity, which defines the proximity degree of a shape to a sphere, plays an important role in the amplitude dependent behaviour of granular dampers. This paper mainly examines the significance of the sphericity level for slightly oblate particles in a granular damper that are subjected to sinusoidal vibrations in the same direction as standard gravity. This investigation is carried out by evaluating the dissipated power from the granular medium by utilizing three-dimensional discrete element method simulations. Apart from the effect of amplitude of vibrations in the dissipated power, the relative contributions of frictional and inelastic collisional damping mechanisms in the overall power dissipation, are also investigated for varying sphericity levels of the oblate particles

A new methodology for measuring the vibration transmission from handle to finger whilst gripping

© 2017The transmission of vibration from hand-held tools via work gloves and into the operators' hands can be affected by several factors such as glove material properties, tool vibration conditions, grip force, and temperature. The primary aim of this study is to develop a new methodology to measure and evaluate vibration transmissibility for a human finger in contact with different materials, whilst measuring and controlling the grip force. The study presented here used a new bespoke lab-based apparatus for assessing vibration transmissibility that includes a generic handle instrumented for vibration and grip force measurements. The handle is freely suspended and can be excited at a range of real-world vibration conditions whilst being gripped by a human subject. The study conducted a frequency response function (FRF) of the handle using an instrumented hammer to ensure that the handle system was resonance free at the important frequency range for glove research, as outlined in ISO 10819: 1996: 2013, and also investigated how glove material properties and design affect the tool vibration transmission into the index finger (Almagirby et al. 2015). The FRF results obtained at each of six positions shows that the dynamic system of the handle has three resonance frequencies in the low frequency range (2, 11 and 17 Hz) and indicated that no resonances were displayed up to a frequency of about 550 Hz. No significant vibration attenuation was shown at frequencies lower than 150 Hz. The two materials cut from the gloves that were labelled as anti-vibration gloves (AV) indicated resonance at frequencies of 150 and 160 Hz. However, the non-glove material that did not meet the requirements for AV gloves showed resonance at 250 Hz. The attenuation for the three materials was found at frequencies of 315 Hz and 400 Hz. The level and position of the true resonance frequencies were found to vary between samples and individual subjects