Effect of Damage on the Modal Parameters of Composite Plates

Abstract

In the present work, the effect of delaminations on the natural frequencies and mode shapes of composite panels has been analyzed, both experimentally and by finite element technique. Bi-woven glass cloth with epoxy resin as matrix is made use of to prepare the laminate. Vacuum-bagging method was employed in the fabrication of specimens. Specimens were prepared with 3 mm thickness and tested for three different boundary conditions, viz., clamped-free-free-free (C-F-F-F), clamped-free-clamped-free (C-F-C-F) and clamped-clamped-clamped-clamped (C-C-C-C). The traditional “strike method” has been used to measure the vibration properties. The modal characteristics of the specimens have been obtained by studying their impulse response. Each specimen has been subjected to impulses through a hard-tipped hammer which is provided with a force transducer and the response measured through the accelerometer. The impulse and the response are processed through a computer aided fast Fourier transform (FFT) analyzer. Vibration parameters such as natural frequencies, modal damping and the corresponding mode shapes were extracted and results are presented. In addition, modal assurance criterion (MAC) has been calculated as a quality indicator for mode shapes evaluated through experiments.

Keywords: Composite plates, natural frequency, mode shape and modal assurance criterion