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For ships operating in combat situations, estimating the response of the bottom and side shells subjected to non-contact underwater blast load is of paramount importance. The damage severity in underwater explosion (UNDEX) depends not only on the shock factor but also on the type of fluid behind the structure (air or water). The response of a free-standing air-backed (AB) and water-backed (WB) plate has already been studied analytically by Liu and Young [1]. However, analysis for AB or WB conditions for fixed flexible plate structures has not been given due importance. In the present study, the equation of motion for the generalized single degree of freedom (SDOF) model for AB or WB plate, which accounts for its flexibility, is formulated from the principle of virtual work. However, since this model is constructed to have a global overview of the present problem without taking account of its inherent complexities, a detailed numerical investigation using MSC.DYTRAN solver is carried out for bare and stiffened plates. The results obtained from both the analytical model and numerical simulations clearly show significant reduction in displacement in case of WB condition compared to AB condition for equal shock factors. This study emphasizes the fact that WB condition can be used to our advantage in order to reduce damage associated with UNDEX in case of doublebottom or double-hull naval vessels.

While several studies have focused on the detection and localization of delamination in composite plates, few comprehensive studies have been performed for the identification of debond in stiffened metallic plates using vibration-based approaches. Therefore, this study is motivated by the need to evaluate the qualitative performance of existing damage detection algorithms, namely modal curvature, the gapped smoothing method (GSM), the generalized fractal dimension (GFD) and the wavelet transform coefficient (WTC), in detecting debond in stiffened metallic plates. Extensive experimental investigation is performed using laser Doppler vibrometer as a noncontact sensing device and LDS Permanent Magnetic Shaker as an actuator. The obtained results show high susceptibility to noise and lesser accuracy in locating the debond zone, except the WTC and GFD. However, the WTC fails to provide good results for higher debond lengths, and the GFD shows prominent false alarms at the free edges of the plates. To circumvent these difficulties, two different modifications of the fractal dimension algorithm, namely the modified GFD (MGFD) and the GFD with GSM (GFD-GSM), have been proposed. Extensive numerical simulations are further carried out using commercially available finite element package ANSYS 14.0 in order to examine the experimental findings. In contrast to most previous work, the signal-to-noise ratio (SNR) in the experimental data has been appropriately quantified and noise of the same SNR level has been synthetically generated and applied on the modal data obtained from numerical simulations. Comprehensive studies for different debond locations and lengths suggests a similar trend as that obtained from the experimental investigations. Finally, a study on damage severity has been performed using the WTC and proposed modifications of the GFD. It is found that the proposed modifications of the fractal dimension perform outstandingly well in all circumstances, and can be used as an excellent tool for debond localization and quantification.

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