International Journal of Innovation and Applied Studies ISSN 2028-9324 Vol. 2 No. 1 Jan. 2013, pp. 34-42 © 2013 Innovative Space of Scientific Research Journals http://www.issr-journals.org/ijias/ Corresponding Author: Israr Ullah (israrullah@gmail.com) 34 A Novel Method for Delamination Detection in Composites Israr Ullah and Jyoti K. Sinha School of MACE, The University of Manchester, Sackville Street, Manchester M13 9PL, UK Copyright © 2013 ISSR Journals. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT: Delamination detection in the composite structure based on the vibration responses when excited at the lower modes has been suggested here. It has been observed that the nonlinear interaction between the delaminated layers produce higher harmonics of the exciting frequency. These higher harmonics have observed to be useful for the detection and location of the delamination without comparing the vibration data from the healthy state of the composite structure. The paper presents the observation based on the finite element analysis of a composite plate with and without delamination and then on the experiments on the composite plates with and without delamination. KEYWORDS: Composite structures, Finite Element Modeling, Delamination, Nonlinear interaction, Vibration response. 1 INTRODUCTION Delamination detection in the composite structures is one of the active research areas since decades. Number of research has been carried out to meet this objective. The most commonly used method in the literature is based on the vibration analysis [1-23]. These vibration based studies include (a) change in modal parameters (natural frequencies and mode shapes) [1-4] or the vibration responses by comparing the data with the healthy state condition [1, 2, 6], (b) mapping the deflection shape of the surface of the composite structure either through the embedded sensors [19-20] or scanning the complete surface by the laser vibrometer [17-18]. The changes in the modal parameters may not be significant if the delamination size is small and the data from the healthy state may not be available for many existing composite structures. The array of the embedded sensors may also not be readily available in many cases. Moreover, the deflection mapping of the surface needs the structure to be excited at very high natural frequency in order of more than 10 kHz[15] which is also not a practical approach using the normal piezo-electrical shakers generally used in the vibration testing. Considering these limitations, a simple method which may be relatively easy to implement in practice and not requiring data from the healthy state condition is needed to meet the objective. Hence it is assumed that the excitation at a few lower modes by the conventional shaker is always possible and then scanning the complete composite surface through the laser vibrometer may not impose any practical limitations. With this assumption, the excitation of a composite plate with and without a delamination at a few lower modes and then the velocity responses measurements at number of points has been considered in the present study. Initially, the finite element (FE) model of a composite plate with and without delamination has been developed to simulate this experiment. The plate was then excited at a few lower modes so that the amplified steady state responses can be observed. The nonlinear interaction between the delaminated layers for the plate with delamination was also considered during the vibration response estimation. The breathing (opening and closing) of a crack in a structure during vibration always produce the higher harmonics of the exciting frequency [24], hence the similar responses are expected for the delaminated composite structure. In fact, it has been observed that the nonlinear interaction between the delaminated layers produce higher harmonics of the exciting frequency. These higher harmonics are then observed to be useful for the detection and location of the delamination without comparing the vibration data from the healthy state of the composite structure. The paper presents the observation based on the finite element analysis of a composite plate with and without delamination and then experimental validation on the composite plates with and without delamination.