Applying deformable structures in underwater environment is a hot topic in ocean development. In this study, an underwater arch-shaped deformable structure filling the swollen hydrogel beads in its flexible outer frame was proposed. Based on discrete element method, after the calibrations of the mesoscopic numerical parameters of the flexible outer frame and hydrogel beads, numerical simulations of bending tests on different arch-shaped deformable structures under a central load of 10N, were established to study deformations and force chains in these structures. The results showed the swollen beads filled in structure could help to achieve the desired structural shape and strength, and the filling degree of the beads offered more influence on the composite structure's bending resistance than the bead's radius.
With the rapid development of human social life, terrestrial resources are over-exploited. Countries all over the world have paid their attention to developing underwater resources in the ocean. In the process of deep-sea development in full swing, underwater unmanned robots are widely used in the construction of deep-sea marine development platforms and underwater structures due to their flexibility, such as the Bluefin Spray Glider produced by General Dynamics of USA.
However, the traditional underwater unmanned robot and underwater structure are large and heavy before entering the water, and their process of entering the water is time-consuming, labor-intensive and financial-intensive. To improve work efficiency, deployable deformable structures can be applied in this field, which have already been conducted in many other fields like aerospace (Meguro, Harada and Watanable, 2004; Perrygo, 1999; Guo, Shi, Li, Deng and Liu, 2018) and civil engineering (Fish, 1974; Gong, Li and Song, 2010; Moritz, Bernd, Doug and Hanspeter, 2012; Li, Guo, Gong, Qing and Li, 2016). Additionally, deployable deformable structures offer advantages including low stow volume, light weight, low cost, easy extensibility, and ability to mitigate other limitations of underwater unmanned robot and underwater structure. Li et al. (2017, 2020) proposed to apply deployable deformable structures whose main frame was an inflatable tubular structure made of synthetic fabrics in deep-sea exploration. Comer and Levy (2012) have proven that the inflatable beam can be considered a standard Euler-Bernoulli Beam. Apedo et al. (2010, 2014) proposed a 3D Timoshenko beam with a homogeneous orthotropic woven fabric through three-dimensional nonlinear finite element analysis of inflatable beams, which were made of modern textile materials and could be used as a load-bearing beams or arches when inflated. However, the inflatable deployable method has limitation to the need of storing hydraulic pressure in advance to complete the gas reserve, so as to provide inflation for the inflatable deployable structure, in the underwater environment. Some researchers and scholars have proposed replacing gas with hydrogel beads to expand the deployable deformable structure in underwater environment. The hydrogel beads swell and retain a significant fraction of the water they absorb (Sachiko, Shuichi and John, 2008; Ahmed, 2015), which have already been applied in many fields, like biomedical science (Stamatialis, Papenburg, Gironés, Saiful, Bettahalli, Schmitmeier and Wessling, 2008), coal dehydration (Sun, Zhang, Shi, Tang, and Wu, 2002), sensor (Liu, Tang, Tham, Yuk, Lin, Lu and Zhao, 2017) and concrete (Kong and Zhang, 2014). The swollen hydrogel beads have toughness to obtain a certain strength (Ahmed, 2015). Xie et al. (2019) conducted numerical simulations about the bending tests of beam-shaped deformable structures composed of various hydrogel beads and inflatable outer frames through discrete element method, and the results showed that the filling degree and radii of the hydrogel beads had a direct effect on the deformation resistance of the composite deformable structures. Li et al. (2019) proposed an underwater inflatable tubular structure of nylon sleeves filled with hydrogel beads, and carried out bending tests of the inflatable tubular structures under different lengths, slenderness ratios and bead densities; their results provided the experimental Young's modulus and deformation of the inflatable tubular structures under different conditions. In addition, researches on the application of hydrogel beads to control deformable structures in civil engineering are rarely reported.
