Modern Practice in Stress and Vibration Analysis VI

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Authors: Angela Cerrini, P. Johannesson, Stefano Beretta
Abstract: To face the increasing demand for long lasting, versatile and performing machines, a detailed analysis of the load conditions is required especially when structural integrity assessment has to be achieved. Usually acquisitions of load histories are shorter than the machine working life and an extrapolation of the signal for the total service life is needed. Traditional methods for load spectra extrapolation are based on conservative choices in terms of worst case scenario. Methods based on extreme value statistics have been developed. The problem addressed in this paper concerns the extrapolation of load histories on a welded boom in which different manoeuvres are superimposed. Different ways of extrapolating the load measurement have been derived, both in time domain and in Markov domain, in order to account for the superposition of bigger and more damaging cycles and smaller cycles caused by two different service operations.
Authors: D. Hickey, Keith Worden, Jan R. Wright, J.E. Cooper
Abstract: Normal mode force appropriation is a method of physically exciting and measuring the undamped natural frequencies and normal mode shapes of a structure. Traditionally used in the aerospace industry for ground vibration testing, it is capable of accurate normal mode estimates. The method attempts to determine multi-point force vectors that will induce single mode behaviour, thus allowing each mode to be viewed in isolation. However it fails to tackle changing dynamic response with forcing level of excitation in nonlinear systems. The method of Force Appropriation for Nonlinear Systems or FANS, produces a special appropriated force vector resulting in nonlinear response. The structure responds dominantly in the target linear mode shape permitting the direct nonlinear characteristics of that mode to be identified in the absence of cross coupling effects.
Authors: M. Borowiec, Grzegorz Litak, Michael I. Friswell
Abstract: This paper examines the dynamics of a single degree of freedom nonlinear model, representing a quarter of an automobile with a semi-active, nonlinear suspension. Assuming that the kinematic excitation caused by the road surface profile is harmonic, the principal resonance and frequency entrainment are obtained for regions of the model parameters. Changing the excitation frequency and road profile amplitude we analyze possible chaotic vibrations and bifurcations of the system.
Authors: D. Hickey, M. Haroon, Douglas E. Adams, Keith Worden
Abstract: In real mechanical situations it is a certainty there will be non-linear behaviour present at a range of frequencies and amplitudes. It is not, however, always possible to have a priori knowledge of the input to a system. A method has been developed by Adams to allow the experimental engineer to overcome such problems. The technique is addressed in this paper and applied to both simulated and experimental data. The method makes use of time domain characterisation via work and characteristic diagrams and also the frequency domain approach toward non-linear identification from feedback of the outputs, (NIFO). This paper attempts to use these time and frequency domain techniques to locate, characterise and quantify non-linear behaviour using both simulated and experimental data. The approach to this work is to obtain simulated data from a quarter car model and real data taken from an experimental rig. The data will be taken at a variety of frequencies and amplitudes and the above time and frequency domain techniques will be applied.
Authors: Andrew J. Hull, David A. Hurdis
Authors: Vijay Sahadevan, Yoann Bonnefon, Tim Edwards
Abstract: This paper presents a two-stage meta-heuristic approach to producing weight-optimised solutions needed prior to the detailed finite element analysis of composite wing. Composite wing covers are assumed to take the form of a group of stiffened sub-panels with varying skin and stiffener geometries according to the wing layout and loads. A population of limited solutions satisfying various design constraints was created using layout (skin and stiffener geometry), selected lay-ups, rule based stacking sequence and various assumed loads. The closed form analytical solutions of flat stiffened orthotropic plates are used for calculating buckling reserve factors and strength margins. For each sub-panel, a meta-heuristic rule was imposed to search for a suitable combination of skin and stiffener geometry. The criterion used was minimum weight satisfying laminate continuity accounting for manufacturability. Later, the optimised solutions for each sub-panel are converted into a format supported by the conventional finite element tool (NASTRAN). The use of meta-heuristic approach and their automation in Visual Basic for Applications resulted in fast convergence and potential time-saving compared to genetic algorithms.
Authors: A. Israr, Matthew P. Cartmell, Marek Krawczuk, Wiesław M. Ostachowicz, Emil Manoach, Irina Trendafilova, E.V. Shishkina, Magdalena Palacz
Abstract: Recent NATO funded research on methods for detection and interpretation methodologies for damage detection in aircraft panel structures has motivated work on low-order nonlinear analytical modelling of vibrations in cracked isotropic plates, typically in the form of aluminium aircraft panels. The work applies fundamental aspects of fracture mechanics to define an elliptical crack, and the local stress field and loading conditions, arbitrarily located at some point in the plate, and then derives an analytical expression for this that can be incorporated into the PDE for an edge loaded plate with various possible boundary conditions. The plate PDE is converted into a nonlinear Duffing-type ODE in the time domain by means of a Galerkin procedure and then an arbitrarily small perturbation parameter is introduced into the equation in order to apply an appropriate solution method, in this case the method of multiple scales. This is used to solve the equation for the vibration in the cracked plate for the chosen boundary conditions, which, in turn, leads to an approximate analytical solution. The solution is discussed in terms of the perturbation approximations that have been applied and highlights the phenomenology inherent within the problem via the specific structures of the analytical solution.
Authors: Daniel G. Gorman, Irina Trendafilova, A.J. Mulholland, Jaromír Horáček
Abstract: When carrying out a vibration modal analysis of a structure it is usually assumed that the structure is in vacuo. However as structures become increasingly light and thin walled due to the development of high grade corrosion resistant alloys, this fundamental assumption is becoming increasingly strained. In this paper we will highlight the analysis and the implications of structural/fluid interaction on modal analysis for the purpose of prediction of dynamic response and for vibration health monitoring.
Authors: S.M.R. Alavi, Mohammad Mohammadi Aghdam, A. Eftekhari
Abstract: This article presents apparently the first application of Meshless local Petrov-Galerkin (MLPG) method for 3-D elasticity analysis of moderately thick rectangular laminated plates. As with other Meshless methods, the problem domain is represented by a set of spread nodes in all three dimensions of the plate without configuration of elements. The Moving Least-Squares (MLS) method is applied to construct the required shape functions. A local asymmetric weak formulation of the problem is developed and MLPG is applied to solve the governing equations. Direct interpolation method is employed to enforce essential boundary conditions. Details of formulation, numerical procedure, convergence and accuracy characteristics of the method are investigated. Results are compared, where possible, with other analytical and numerical methods and show good agreement.
Authors: N. Ahmed, A.V. Mitrofanov, Vladimir I. Babitsky, Vadim V. Silberschmidt
Abstract: Ultrasonically assisted turning (UAT) is a novel material-processing technology, where high frequency vibration (frequency f ≈ 20kHz, amplitude a ≈15μm) is superimposed on the movement of the cutting tool. Advantages of UAT have been demonstrated for a broad spectrum of applications. Compared to conventional turning (CT), this technique allows significant improvements in processing intractable materials, such as high-strength aerospace alloys, composites and ceramics. Superimposed ultrasonic vibration yields a noticeable decrease in cutting forces, as well as a superior surface finish. A vibro-impact interaction between the tool and workpiece in UAT in the process of continuous chip formation leads to a dynamically changing stress distribution in the process zone as compared to the quasistatic one in CT. The paper presents a three-dimensional, fully thermomechanically coupled computational model of UAT incorporating a non-linear elasto-plastic material model with strain-rate sensitivity and contact interaction with friction at the chip–tool interface. 3D stress distributions in the cutting region are analysed for a representative cycle of ultrasonic vibration. The dependence of various process parameters, such as shear stresses and cutting forces on vibration frequency and amplitude is also studied.

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