2003 IRISH SCIENTIST YEAR BOOK

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Institute of Technology Tallaght
Kenneth Hughes
Finite Element Analysis and Modal Analysis of engineering products during dynamic impact loading

Droptest Rig with aluminium sample after impact test

The sudden application of loads on objects is an area of engineering where there is much uncertainty - e.g. dropping of a mobile phone or the impact of a bicycle helmet on a road after an accident. Traditionally the aim has been to estimate the maximum impact and base calculations of stresses and deformation around these figures. Recently it has been found that these estimations of loads are often less than that actually experienced by the object.

Our research evaluates an impact by modelling the whole event, the motion, the contact and the non-linear behaviour of the material. Modal analysis is being used to predict the permanent deformation of components when subjected to impact loads. The first stage of the investigation involves the modal testing of standard components - in this case aluminium plates - resulting in the extraction of the natural frequencies and mode shapes of the samples tested. From this, a mathematical summation is performed to extract the operating deflection shapes at specified frequencies. When the operating deflections for specified frequencies are known, it is then possible to input a zero frequency and mathematically extract a permanent deflection due to a known static input force at a known location.

Impact testing is conducted to obtain specimen deformations. Finite element analysis (FEA) is also used to predict the natural frequencies and mode shapes of the components. The impact loads are modelled using ANSYS with an explicit solver (Ls-Dyna). The material properties for these models are obtained from a stress strain test. Data extracted from testing and FEA models include displacement, acceleration, velocity, and reaction forces. Test, FEA and a mathematical model are finally compared and a conclusion made.

Contact: Kenneth Hughes, Department of Mechanical Engineering, IT Tallaght, Old Blessington Road, Tallaght, Dublin 24:
E-mail: [email protected]