Free-edge sensor placement for identifying vibration modes of structures subjected to impact loadings using fiber Bragg gratings

Summary For modal testing, active vibration control, or structural health monitoring, determination of sensing locations becomes a crucial issue once the interesting mode has nodal lines on which the sensors are bonded. Theoretically, only nodal points but not nodal lines would exist along free edges of the structures. Thus, measurements of vibrations along free edges of structures would maximize the number of detecting (i.e., for monitoring application) or observing (i.e., for active control application) modes. However, conventional sensors such as strain gauges or accelerometers can only be bonded on flat or smooth surfaces. In this work, fiber Bragg gratings, which possess linear geometric shape, are employed to investigate the feasibility of detecting more vibration modes along free edges of structures. A traction-free aluminum solid with symmetric geometry subjected to various impact loadings is considered. To verify the capability of the free-edge bonded FBG sensors for detecting vibration modes from impact-induced responses, relationships between sensing locations, impact locations, and intensities of modes, are discussed with the aid of mode shapes obtained by the finite element me\thod. Within frequencies up to 30 kHz, 14 out-of-plane dominated and 25 in-plane dominated modes in frequency domain are examined. Multiple modes, that is, repeated roots, two or more modes with identical frequencies due to symmetric geometry, are also analyzed. Our experimental results show that placing the FBGs along free edges of structures to acquire and analyze more vibration modes is feasible. Copyright © 2016 John Wiley & Sons, Ltd.