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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1. Introduction Sandwich panel has been extensively developed in engineering application fields such as aerospace (Dayana et al., 2015) and ship structure (Sujiatanti et al., 2018; Ardhyananta et al., 2019). The sandwich panel is generally applied in ship structure to increase high bending stiffness to weight ratio. Consequently, large sandwich panels having relatively thin faceplate, but the thick core is more applied in modern construction. Specifically, marine sandwich panels are prone to debonding because of several reasons such as different adhesion properties of the interfaces layers, difficulties in controlling the proper bonding during the manufacturing process (Chen et al., 2017; Fatt and Sirivolu, 2017), significant distinctions between thickness and elastic moduli of the constitutive layers (Gaiotti and Rizzo, 2013; Tuswan et al., 2020). The presence of debonding can severely degrade the load-carrying capacity (Bragagnolo et al., 2020) and the vibration properties of the sandwich (Savic-Barcan et al., 2018). To maintain the integrity and safety of the structure, it is necessary to predict and detect the damage at the early stages. For large and complex structures such as ship structures that are difficult to assess, the use of local damage identification assessment can be costly and inefficient. The vibration-based damage assessment can classify as the non-destructive technique and can be cheap for large structures. This underlying principle of this method is to evaluate the modal parameters of the damaged structures such as the natural frequencies (Yang et al., 2016; Zhao et al., 2016), mode shapes (Kaveh and Zolghadr, 2015), and damping ratios (Cao et al., 2017). A review of the literature indicates that although extensive research has been conducted on the topic of free vibration, only a few results are reported on predictions of the dynamic properties in terms of evaluating the effect of debonding shapes in a complex sandwich structure, especially in the stern/ ramp door as well as other ship’s structural parameters. Debonding, which is one of the most critical failure criteria, has been investigated experimentally, analytically, and numerically (Saeid et al., 2016; Burlayenko and Sadowski, 2018; Wang et al., 2019). The effect of the debonding size, debonding location, and debonding type on the vibration characteristic is investigated in Burlayenko and Sadowski (2010). The influence of debonding is also investigated in ship deck plates made of the composite sandwich. Investigations have been performed for a clamped rectangular sandwich plate with an elliptical debonded area (Savic-Barcan et al., 2018). In another research, Tuswan et al. (2020) investigate the effect of local damage (debonding problem) on the modal parameters of the car deck model, which installed a sandwich plate system (SPS). It shows that higher natural frequencies are found to be more sensitive to the presence of debonding defects. All the mentioned results of dynamic analysis illustrate that they can be useful for the non-destructive damage detection of debonded sandwich plates. In terms of debonding modeling, most research is used both linear and nonlinear approaches. One of the approaches is based on the split-beam theory, where the decoupled layers are assumed to be able to penetrate each other or be constrained to move together (Pardoen, 1989; Jian and Hwu, 1995). To prevent overlapping between layers, the decoupled layers are linked by additional linear virtual spring elements and contact behavior between nodal surface is used (Burlayenko and Sadowski, 2010; Burlayenko and Sadowski, 2011a). In this paper, a free vibration analysis will be carried out to investigate the effect of small defects on the modal parameters in the complex structure. The stern/ramp door installed sandwich panels consist of steel faceplate and resin/clamshell core. The free vibration is investigated to compare the healthy model and debonded model by using the ABAQUS finite element software (ABAQUS, 2014). Four different debonding shapes with a 5% debonding ratio, such as circular, square, through-the-length, and through-the-width, are investigated. To model the debonded problem, the contact modeling “with” and “without” a piecewise linear spring between the debonded region are entirely compared. 2. Finite element modeling 2.1. Stern ramp/door model and material The model used in this analysis is a stern ramp/door of Ferry Ro-Ro 300 GT. Many ships with Ro-Ro capability, incorporate access by the bow as well as by the stern. It consists of a watertight axial ramp/door with entering and bridging flaps. The stern ramp is used for loading and discharging of cargo. It facilitates an efficient cargo flow and