PSI - Issue 81

Anandito Adam Pratama et al. / Procedia Structural Integrity 81 (2026) 58–65

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al., 2025). In addition to shock wave effects, subsequent gas bubble pulsations can further intensify structural damage, underscoring the importance of understanding both explosion mechanisms and dynamic structural response during the design and analysis of marine structures, as discussed by Fathallah et al. (2015). The severity of explosion loading is commonly characterized using the Shock Factor (SF), as discussed by Guo et al. (2017), Liang et al. (2025), and Yao et al. (2025). The SF formulation incorporates key parameters such as explosive charge mass ( ), stand-off distance ( ), and incident angle ( ) , as originally proposed by Gupta et al. (2010), and provides a practical metric for comparing UNDEX scenarios and evaluating loading intensity on marine structures. However, many previous investigations have relied primarily on the SF value alone, raising questions regarding its adequacy in fully representing the severity of UNDEX events under varying structural and geometric conditions. Given that naval vessels may encounter explosions with diverse charge weights, stand-off distances, and attack geometries, the relationship between shock velocity, loading severity, and geometric configuration must be clearly defined, as emphasized by Keil (1961). For high-explosive threats such as naval mines, the severity of loading is often associated with the energy density of the shock wave impinging on the vessel hull. To mitigate the adverse effects of such extreme loading conditions, recent developments in materials and structural engineering have introduced advanced solutions to improve shock resistance. Among these, sandwich panels with two face sheets bonded to a lightweight core have attracted considerable attention due to their high structural efficiency, superior energy absorption, low weight, and cost-effectiveness, as reported by Qu et al. (2025). Their widespread adoption in the automotive, aerospace, and marine sectors is further supported by advantages in manufacturability, impact resistance, and overall structural performance, as demonstrated in studies by Fhandy et al. (2025), Liu et al. (2022), Nurcholis et al. (2025), Sarwoko et al. (2024), and. In marine applications, sandwich panels have been shown to effectively absorb shock energy, redistribute stresses, and reduce blast-induced deformation, thereby enhancing the structural integrity of ship hulls and underwater structures, as documented by Manalo et al. (2010). This study aims to evaluate the limitations of using Shock Factor (SF) as a standalone metric for real-world UNDEX scenarios under different structural conditions. Accordingly, this study also presents a numerical analysis of the mechanical response of sandwich panels with four cellular core configurations (S-core, U-core, X-core, and Y-core) to UNDEX shock loads. Simulations were conducted using an explosion scheme with a constant SF reference to evaluate each core configuration equally. The results of this study are expected to contribute to the development of naval vessel hull designs and maritime infrastructure that are more resistant to underwater explosion threats. 2. UNDEX Loading Underwater explosions (UNDEX) exhibit more complex characteristics than air explosions, as they generate significantly higher pressures accompanied by very short shock wave durations, as reported by Jen (2009). This complexity is further amplified by the higher density and lower compressibility of water, which allows shock waves to propagate more efficiently with minimal attenuation. As a result, UNDEX can be particularly destructive, even at considerable distances from the source. The effects on marine structures are generally classified into local damage, such as tearing or denting of plating, and global responses, including overall hull whipping and vibration. According to Arons and Yennie (1948) and Wang et al. (2014), an underwater explosion releases energy rapidly, with its dissipation occurring in three primary forms. The most considerable portion, about 57%, is carried by the shock wave. Around 37% is associated with the secondary-wave pulse generated by the explosion bubble, while the remaining ~6% is dissipated as heat. The shock wave occurs immediately after the explosion, with very high pressure, but lasts only a few milliseconds. Next, a bubble pulse is formed, which produces a lower load but lasts longer, even up to several seconds, and can be repeated, with the pressure decreasing as the bubble collapses. Fig. 1 shows this phenomenon, which is further explained by Snay (1957).

Fig. 1. Shock wave and bubble pulsation phenomena in UNDEX by Snay (1957).