H Soleimani - MSc Student
A. S Nobari - Professor
H Shahverdi - Assitant Professor
J. E Jam - Professor Vibration and Dynamic Systems Research Laboratory, Department of Aerospace Engineering, Amirkabir University of Technology, Hafez Avenue, ۱۹۸۷۵۴۴۱۳, Tehran, Iran,

Applications of composite cylinders have been widely expanded in recent years. However, delamination which is a common defect in composite materials often propagates because of loading condition especially in vibrational environments or under impact loads. These kinds of damages should be taken under consideration due to avoid life design reduction or catastrophic failures. This paper presents an experimental low frequency investigation on the modal behavior of a delaminated filament wounded Glass/Epoxy cylinder. In order to show the delamination effect on modal parameters, the modal parameters of defected cylinder is compared with the reference state, i.e. healthy cylinder. The surface delamination is created in a healthy cylinder by low velocity impact loads and extended in four stages. Experimental modal analysis is implemented for each stage in order to indicate presence of the delamination. An experimental setup with free-free boundary condition is established with which composite cylinder is excited by a modal hammer while response is measured by a piezoelectric accelerometer. The comparison of natural frequencies, modal assurance criterion (MAC) and modal damping coefficients is illustrated between healthy and defected state of the cylinder for first eight modes. It is observed that damping ratios are more sensitive with respect to either natural frequencies or mode shapes. Moreover, it is revealed that the percentage of modal damping variation is higher in the first modes for small damage and lower in the first modes for bigger damages.

Experimental modal analysis, composite cylinder, delamination, natural frequency, mode shape, modal damping