Laser vibration measurements and data processing for structural diagnosticon composite material

In recent years, the identification and characterization of defects in mech anical and civil structures have been widely investigated by processing of vibration measurement results. In this field laser Doppler vibrometry (LDV) offers large potentials in terms of spatial resolution and reduced intrusi vity. In this work LDV technique has been applied to measure delamination e xtension, depth, and location in composite materials. A lumped parameter mo del has been developed to describe the dynamic behavior of a delaminated co mposite panel with the aim of determining which kind of measurement data mu st be extracted for damage monitoring and to design efficient postprocessin g algorithms for experimental LDV data. It is shown that the root-mean-squa re (rms) values computed in different frequency bands are indicators of the delamination depth. In fact, the vibration exhibits a higher rms value in higher frequency bands, if the defect is deeper. Following the model result s, an experimental investigation by LDV has been performed on panels with k nown detachments. Accuracy of results has been checked by comparison with t hermal tomography, which at present is one of the most used measurement tec hniques for monitoring the state of composite materials. Experimental resul ts show the effectiveness of the model and the real applicability of the pr oposed technique. The presented methodology has proved to be efficient to d etermine the delamination depth. Also issues connected with structural exci tation and measurement uncertainty are addressed. (C) 2000 American Institu te of Physics. [S0034-6748(00)05301-6].