Damage Identification by Using a Self-Synchronizing Multipoint Laser Doppler Vibrometer

Abstract

The vibration-based damage identification method extracts the damage location and severity information from the change of modal properties, such as natural frequency and mode shape. Its performance and accuracy depends on the measurement precision. Laser Doppler vibrometer (LDV) provides a noncontact vibration measurement of high quality, but usually it can only do sampling on a single point. Scanning LDV is normally used to obtain the mode shape with a longer scanning time. In this paper, a damage detection technique is proposed using a self-synchronizing multipoint LDV. Multiple laser beams with various frequency shifts are projected on different points of the object, reflected and interfered with a common reference beam. The interference signal containing synchronized temporal vibration information of multiple spatial points is captured by a single photodetector and can be retrieved in a very short period. Experiments are conducted to measure the natural frequencies and mode shapes of pre- and postcrack cantilever beams. Mode shape curvature is calculated by numerical interpolation and windowed Fourier analysis. The results show that the artificial crack can be identified precisely from the change of natural frequencies and the difference of mode shape curvature squares.

Performance of Damage Identification Based on Directional Wavelet Transforms and Entopic Weights Using Experimental Shearographic Testing Results

“…This class of techniques involves all testing approaches based on modal analysis, including traditional testing with vibration sensors, like accelerometers or microelectromechanical system (MEMS) sensors [ 2 , 3 , 4 , 5 ]. A significant enhancement of sensitivity to structural damage in such techniques was achieved by an application of non-contact measurement systems, like laser Doppler vibrometers or interferometers used in electronic speckle pattern interferometry or shearography, which allowed the precision of measured quantities to be increased by up to several orders with respect to traditional contact sensors [ 6 , 7 , 8 , 9 , 10 ]. The details on the main principles of operation of shearography as well as the areas of potential applications can be found in review papers [ 11 , 12 , 13 ].…”