• Israr Ullah

Student thesis: Phd


Composite materials are in use in several applications, for example, aircraft structural components, because of their light weight and high strength. However the delamination which is one of the serious defects often develops and propagates due to vibration during the service of the structure. The presence of this defect warrants the design life of the structure and the safety. Hence the presence of such defect has to be detected in time to plan the remedial action well in advance. There are a number of methods in the literature for damage detection. They are either "baseline free/reference free method" or using the data from the healthy structure for damage detection. However very limited vibration-based methods are available in the literature for delamination detection in composite structures. Many of these methods are just simulated studies without experimental validation. Grossly 2 kinds of the approaches have been suggested in the literature, one related to low frequency methods and other high frequency methods. In low frequency approaches, the change in the modal parameters, curvatures, etc. is compared with the healthy structure as the reference, however in the high frequency approaches, excitation of structures at higher modes of the order of few kHz or more needed with distributed sensors to map the deflection for identification of delamination. Use of high frequency methods imposes the limitations on the use of the conventional electromagnetic shaker and vibration sensors, whereas the low frequency methods may not be feasible for practical purpose because it often requires data from the healthy state which may not be available for old structures. Hence the objective of this research is to develop a novel reference-free method which can just use the vibration responses at a few lower modes using a conventional shaker and vibration sensors (accelerometers/laser vibrometers). It is believed that the delaminated layers will interact nonlinearly when excited externally. Hence this mechanism has been utilised in the numerical simulations and the experiments on the healthy and delaminated composite plates. Two methods have been developed here - first method can quickly identify the presence of the delamination when excited at just few lower modes and other method identify the location once the presence of the delamination is confirmed. In the first approach an averaged normalised RMS has been suggested and experimentally validated for this purpose. Latter the vibration data have then been analysed further to identify the location of delamination and its size. Initially, the measured acceleration responses from the composite plates have been differentiated twice to amplify the nonlinear interaction clearly in case of delaminated plate and then kurtosis was calculated at each measured location to identify the delamination location. The method has further been simplified by just using the harmonics in the measured responses to identify the location. The thesis presents the process of the development of the novel methods, details of analysis, observations and results.
Date of Award31 Dec 2011
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorJyoti Sinha (Supervisor)


  • Delamination, Structural Health Monitoring, Composite, Vibration, Plate

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