Chapter 8 Scanning LDV Measurement Technology for Vibration Fatigue Testing Fabrizio Magi, Dario Di Maio, and Ibrahim Sever Abstract Vibration fatigue testing is a verification method for structural components. It is rapid and cost efficient, and it is often performed for aero-engine components because it replicates closely stresses under normal operation conditions. This testing methodology is well known for metallic components but it can also be applied to composites for studying failure caused by high cycle fatigue. Resonant frequency decay is usually the main parameter used to assess fatigue behaviour of metallic components at a given excitation level. However for composites this alone is not very useful and several measurement parameters need to be monitored in order to understand the behaviour and develop a set of failure criteria. Scanning LDV is an excellent measurement system which enables a variety of options thanks to its non-contact nature. This manuscript will show how this system can be used for monitoring several parameters during vibration fatigue testing. A custom made control panel called MONTEVERDI and the independent use of the scanning mirrors allow the SLDV to perform several tasks: (1) Phase Lock Loop (PLL) to excite the component always at resonance, (2) measuring of Operational Deflection Shape (ODS) using either step or continuous scanning method, (3) custom calibrated strain measurement and (4) phase portrait for nonlinear vibration analysis. This paper will present the great potential of using SLDV for performing time consuming vibration fatigue testing via an automated control panel. Keywords Scanning laser Doppler velocimeter • Automated testing • Vibration fatigue • Operative deflection shape 8.1 Introduction Vibration fatigue is a testing technique that allows a rapid and cost-efficient evaluation of the fatigue life of a component. It consists of exciting a structure by an excitation force at any resonance frequency, causing the structure to respond according to its dynamic characteristics. This method closely replicates the operational conditions of components undergoing vibratory stresses, allowing high excitation levels with relatively low input power. Vibration fatigue testing is well standardized for metals. It was developed during the 1950s [1–3] by Lazan et al. and it consisted in exciting a specimen around its resonant frequency by rotating an eccentric mass. The vibration amplitude was fed back to the controller thanks to an accelerometer mounted on the top of the specimen grip. In spite the reliability of accelerometers, sometimes there is a need of non-contact sensor for measuring vibration amplitude without interfering with the system response. Non-contact control systems rely on either video extensometer sensors or laser triangulation sensors [4], with the drawback of being dedicated to measure the vibration of a single point. This manuscript shows how to carry out an automated fatigue test to characterise the fatigue behaviour of composite components using a Scanning Laser Doppler Velocimeter (SLDV) system. The SLDV system is used (1) for performing both an amplitude and a phase lock loop control, (2) for calibrating the displacement with the relative strain and (3) for measuring important dynamic parameters such as nonlinearities in the vibration and ODS. F.Magi Advanced Composite Centre for Innovation and Science (ACCIS), University of Bristol, University Walk, BS8 1TR Bristol, UK e-mail: fabrizio.magi@bristol.ac.uk D. DiMaio ( ) Department of Mechanical Engineering, University of Bristol, University Walk, BS8 1TR Bristol, UK e-mail: dario.dimaio@bristol.ac.uk I. Sever Rolls-Royce plc, Derby DE24 8BJ, UK © The Society for Experimental Mechanics, Inc. 2016 J. De Clerck, D.S. Epp (eds.), Rotating Machinery, Hybrid Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-30084-9_8 87
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