Introduction to Laser Doppler Vibrometer

The composites are gaining importance as structural material in many critical and advanced applications. Their extensive use is still lagging due to the lack of efficient and reliable damage detection methods which could ensure sustained reliability and well being of the structure over its planned service life. There is a growing need for continuous monitoring of structures made of advanced composites to avert catastrophic failures and to provide confidence for the rapid introduction of these high performance and heterogeneous materials into service. The dynamic responses of structure offer unique information on defects inside the structure. Changes in the physical properties of the structures due to damage alter the dynamic responses such as natural frequencies, modal damping and mode shapes. These changes in physical parameters can be extracted to estimate damages in the structure by experimental modal analysis. In this module, dynamic responses of glass-epoxy composite laminates with different ply orientations are studied. Laser Doppler Vibrometer has been used to capture the vibration characteristics of dynamically excited healthy and delaminated composite laminates. Delaminations have been introduced in the laminates at different locations to simulate damage situations. The effect of delamination on modal characteristics (natural frequencies, modal damping and mode shapes) of the composite laminates has been analyzed for the purpose of damage detection, its location and severity. The first four natural frequencies recorded during experimental modal analysis for healthy laminates are compared with a standard FE software ABAQUS. Perceptible changes in natural frequencies, damping and mode shapes are observed in all delaminated specimen during experimental modal analysis. Laminates with longitudinal and transverse direction fiber arrangements are more prone to these changes while cross ply laminates are only marginally affected. A change in modal damping in a locality is a good indicator of potential delaminating site.