Nonlinear Modal Testing and Analysis of Multiple Degree of Freedom Engineering Structures using a Frequency Domain method

Lead Research Organisation: University of Bristol
Department Name: Aerospace Engineering


Despite the combined improvements in computing power and modelling capabilities one major hindrance to optimal design is the present incapability to capture experimentally (identify) nonlinear dynamic behaviour which characterise many engineering structures. It can be argued that a step-change in the accuracy of structural dynamics prediction capability (i.e. the accuracy of the models used for simulations) can only occur if the nonlinear characteristics are opportunely captured and modelled. Currently, it is standard engineering practice to apply techniques that can provide an answer to whether or not the system being tested is nonlinear but do not help to identify the type and magnitude of nonlinearity. There is thus scope for implementing, within a standard dynamic experiment, a method for 'detection and quantification' of the nonlinear behaviour. Experimental Modal Analysis, or Modal Testing, is practiced on a daily basis in dynamics labs of universities and industries alike. The final goal is to transform a set of measured data into a set of modal parameters which eventually will allow the (re)construction (or validation) of a spatial model (most notably a Finite Element model). However, the theory developed to perform modal analysis so far ceases to be valid if the vibrating structure exhibits nonlinear behaviour. The work proposed in this project intends to initiate a more structured effort for the continuation and the expansion of theory and practice of modal analysis in nonlinear structural dynamics. This proposal responds to the need of introducing a novel modal analysis technique for nonlinear structures which aims, at least to a first degree of approximation, to extract type and magnitude of structural nonlinearity from measured frequency response functions. This is necessary in order to progress beyond the current boundaries of the well established linear techniques set decades ago and which no longer suffice to respond the expanding capabilities of simulation methods such as the finite elements.

Planned Impact

The main aim of this First Grant research project is to introduce a new, state-of-the-art, modal testing and analysis techniques in order to identify and characterise structural nonlinearities in complex engineering structures by using standard testing methods. As extensively discussed in the Case for Support , the research activity proposed here is fundamental for defining a novel procedure which enables advances in dynamics and vibration testing and analysis. The outcomes of this research are beneficial for both academic and industrial communities. The research's results will form the basis from which more studies can stem in the field of modelling nonlinearities (which are now limited by the identification techniques available). An outcome of this proposed project is also the publication on the web of a library of numerical FRFs of common nonlinearities. This tool, which is missing at the present, is the first step during any study on this topic and will enable the scientific community to use existing data with its consequent advantages. Besides, the research's product will benefit most UK engineering companies, as vibration test is practised in all engineering disciplines and, presently, a capability for identifying and characterise (at least to a first degree of approximation) the nonlinear dynamics is missing. The linear modal test and analysis procedures and techniques are fully mature. However, as computer power increases and model becomes more refined, there is need of complementing this expansion by providing experimental procedure and techniques which allow the nonlinear behaviour to be identified as this is the only way for a refined model to reproduce the reality. There exists a strong demand from industry to research in this field and large R&D investments have been made by major companies in order to pursue this goal; this research will provide a strong contribution to the engineering and scientific community by providing the state-of-the-art methodology for nonlinear dynamics identification and lay the basis for further research and development. The ultimate goal is to capture the nonlinear dynamic behaviour so that this can be modelled and implemented in, for example, FE models which will thus allow unprecedented predictive capability. The industrial relevance of the proposed research is such that it has attracted great interest from three major UK based companies: AgustaWestland (a cutting-edge company for the production and design of helicopters), Rolls-Royce Plc (one of the world largest aero-engine manufacturing companies) and Airbus UK (part of a world leading aircraft manufacturer). Each of these three companies have expressed intention of following closely the development of the project by nominating a technical monitor. In order to provide an efficient communication/dissemination mechanism with the industry, a series of bi-annual workshops is envisaged with a kick-off presentation to be held at the start of the research activity. The main purpose of these technical meetings is (a) to guarantee that the research has a practical aspect and (b) to ensure a much needed technological transfer between academia and industry (so that the project outcomes will be successfully exploited by leading UK industries in a short term scenario). The intellectual property of the research results will belong to the UoB.


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Description This project investigated the problem of identifying parameters for nonlinear structural dynamic systems with multiple degrees of freedom. The key findings are that for systems with amplitude dependent stiffness and damping some identification can be carried out based on simplifying assumptions that each resonance peak is governed by a single nonlinear oscillator equation. In fact there is coupling between the resonances in many cases, so further work is required to investigate this more complex situation.
Exploitation Route Modal testing techniques are used extensively in the aerospace industry, and for civil engineering structures. The results from this project can be used in this context. The algorithms developed in this project have been investigated for commercial exploitation by the aerospace industry and related software developers.
Sectors Construction,Energy