Study of the vibration of bulkhead-stiffened cylindrical shells by laser-based methods

The first part of this dissertation work deals with an experimental study of the vibration behavior of bulkhead stiffened cylindrical shells by using laser-based vibration measurement methods. Holographic interferometry and laser speckle photography are first demonstrated on revealing the dynamic behavior of a 22 ft long cylindrical shell. These methods are thereafter further explored to study the vibration characteristic of cylindrical shells with different stiffeners such as a full bulkhead or a partial bulkhead. Many experimentally obtained holograms and specklegrams reveal interesting features of the vibration of bulkhead stiffened cylindrical shells. The experimentally obtained results are compared with those obtained from a finite element model developed by General Dynamic Electric Boat Division, and the finite element model is generally validated. Mode localization theory is used to explain some interesting findings in experiments and the reason of some discrepancies between the finite element analysis and experiment results. The presence of irregularities in a weakly coupled structure such as a bulkhead-stiffened cylindrical shell is shown to be able to localize the modes of vibration and inhibit the propagation of vibration within the shell. A numerical simulation based on the finite element modal analysis indicates the validation of this explanation of the experimental findings. Thereafter, the eigensolutions of disordered, plate-stiffened cylindrical shell stiffened are derived by the use of receptance method. Numerical calculations are thereafter performed based upon this model and indeed reveal the exist of localized vibration in this kind of structure. This analytical study provides physical insights into the mode localization phenomenon in stiffened cylindrical shell type of structures from a more systematic manner. The conditions and the effect of mode localization on natural frequencies and mode shapes of cylindrical shell structure are also studied. The relationship between natural frequency loci veering, mode shape localization of steady state vibration and pass- stop-band behavior of wave propagation are established in stiffened cylindrical shell structures. It is hoped that this finding can be used to emphasis the importance of mode localization when the numerical models are applied to the study of dynamic behavior of real engineering structures. Finally, a novel laser-based vibration measurement system is developed. Rather than using a photographic camera as in conventional speckle photography, this system utilizes a CCD camera and an interfaced computer to perform the digital speckle image processing. Several algorithms are developed or found to extract the mode shapes related to the vibration of objects from captured digital speckle images. The algorithms can be generally categorized into the following aspects as spatial domain filtering (e.g., image edge detection operation), spectrum domain filtering (e.g., Fourier transform operation), spatial domain correlation and image blurring model. The computer generated or processed images can provide a Chladni-like pattern showing the mode shapes of vibrating objects. This new laser-based vibration measurement technique not only inherits the common merits shared by conventional optical schemes such as whole-field, non-contact and non-destruction, but also possesses its unique features of noise-resistance, simplicity, flexibility and, most of all, practicability. The feasibility of this new technique had been proven by successful field tests at the shipyard of General Dynamic Electric Boat Division.