Laboratory for Dynamics of Machines and Structures 
The mass normalisation of the displacement and strain mode shapes in a strain experimental modal analysis using the mass-change strategy
 T. Kranjc, J. Slavič and M. Boltežar
Journal of Sound and Vibration, Volume 332, Issue 26, 23 December 2013, Pages 6968-6981

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Abstract
The classic experimental modal analysis (EMA) is a well-known procedure for determining the modal parameters. The less frequently used strain EMA is based on a response measurement using strain sensors. The results of a strain EMA are the modal parameters, where in addition to the displacement mode shapes the strain mode shapes are also identified. The strain EMA can be used for an experimental investigation of a stress-strain distribution without the need to build a dynamical model. It can also be used to determine the modal parameters when, during modal testing, a motion sensor cannot be used and so a strain sensor is used instead. The displacement and strain mode shapes that are determined with the strain EMA are not mass normalised (scaled with respect to the orthogonality properties of the mass-normalised modal matrix), and therefore some dynamical properties of the system cannot be obtained. The mass normalisation can be made with the classic EMA, which requires the use of a motion sensor. In this research a new approach to the mass normalisation in the strain EMA, without using a motion sensor, is presented. It is based on the recently introduced mass-change structural modification method, which is used for the mass normalisation in an operational modal analysis. This method was modified in such a way that it can be used for the mass normalisation in the strain EMA. The mass-normalised displacement and strain mode shapes were obtained using a combination of the proposed approach and the strain EMA. The proposed approach was validated on real structures (beam and plate).
Authors

R&D

Tadej Kranjc, PhD

  Ydria Motors d.o.o.
  tadej.kranjc@si.ebmpapst.com
  +386 1 4771 228
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Associate Professor

Janko Slavič, PhD

  Ladisk, Faculty of Mechanical Engineering, University of Ljubljana
  janko.slavic@fs.uni-lj.si
  +386 1 4771 226
jankoslavic     jankoslavic    
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Professor

Miha Boltežar, PhD

  Ladisk, Faculty of Mechanical Engineering, University of Ljubljana
  miha.boltezar@fs.uni-lj.si
  +386 1 4771 608
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