PSI - Issue 27

Tuswan Tuswan et al. / Procedia Structural Integrity 27 (2020) 22–29 Tuswan et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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the damaged regions, especially in higher modes. When the debonding between the faceplates and the core of the sandwich increases, the shear stress transfer area between the faceplates and the core will decrease. The decrease in the transfer area causes a decrease in the stiffness and strength of the sandwich. Generally, the numerical results demonstrated that the local interfacial damage in stern ramp/door models leads to the significant shifts of the natural frequencies with respect to intact models, as visualized in Fig. 6a. The frequency shifts are the difference between the natural frequency of the initial model and the natural frequency of the debonded model. In most cases, the frequencies shift of debonded models increases due to a loss in stiffness caused by initial debonding, and the mode shapes contain along with a global shape and a local deformation in the discontinuity region. Besides, the frequencies change more rapidly as a mode number increases, although the trend of frequencies shift is irregular. The debonding in the lower modes is violated due to the local thickening phenomenon caused by debonding, which for lower modes made the frequencies of the damaged plates even higher than intact ones (Paolozzi and Peroni, 1990; Hou and Jeronimidis, 1999). The local thickening phenomenon can be illustrated in Fig. 4a. Moreover, the presence of debonding defect has a significant effect on the natural frequencies. The identification to determine the debonding presence in the model should be performed in high modes. Fig. 5 illustrates the comparison of natural frequencies of intact and debonding models without spring contact modeling with different debonding shapes analyzed in the first ten-mode. Debonding causes a decrease in natural frequency even in the lower mode, especially in the square and circular debonding shape. However, it seems there is an insignificant natural frequency decrease in the through-the-length, and through-the-width debonding. Compared with debonded modeling with spring element contact, the frequency shift without spring contact modeling is more significant even in the lower modes. As observed of the result in Fig. 5, the neglecting of contact leads to incorrect results, which significantly overestimate the value of natural frequency. Thereby, using the spring contact model, the calculated responses of the model with debonding are dampened compared to the model without contact. It is because contact elements are used to prevent the detached models from overlapping each other. Therefore, the model of the local contact-impact behavior is necessary to accurately represent the dynamic response of the models (Burlayenko and Sadowski, 2011b). 4. Conclusions The paper has investigated the effect of small defects on the modal parameters of the stern/ramp door installed sandwich panels. The free vibration is investigated to compare the healthy model and debonded model analyzed in the first ten-mode. Four different debonding shapes with a 5% debonding ratio, such as circular, square, through-the length, and through-the-width, were studied. In terms of conta ct modeling, the comparison of “with” and “without” a piecewise linear spring contact are analyzed. Summing up the results obtained in this study, the result has been shown that the debonding defect reduced the natural frequencies, especially in the square and circular debonding shapes. There is no significant frequency shift in the through-the-length, and through-the-width debonding modeling. It proves that the localized debonding shape is more sensitive to the natural frequency changes. The higher natural frequencies are found to be more sensitive to the presence of debonding problems. Using such shifts directly resulting from the comparison of the natural frequency of both intact and debonded stern ramp/ door, the diagnostic can be performed. Acknowledgments The research leading to these results has received funding from Directorate of Research and Community Services, Ministry of Research, Technology and Higher Education, The Republic of Indonesia under The Master’s Degree Program Leading to Doctoral Degree for Excellent Bachelor Graduates (PMDSU) research scheme. References ABAQUS: Abaqus Analysis User Guide, Simulia, 2016. Ardhyananta, H., Sari, E.N., Wicaksono, S.T., Ismail, H., Tuswan, Ismail, A., 2019. Characterization of Vinyl Ester Bio-resin for Core Material Sandwich Panel Construction of Ship Structure Application: Effect of Palm Oil and Sesame Oil. AIP Conference Proceedings 2202, 1-5.