Hull impact response is a focus of attention in issues such as impact environment, vibration and noise, and structural damage. To a certain extent, the hull structure can be simplified into a beam model. Under impact loads, the beam model usually exhibits dynamic responses that are a superposition of overall low frequencies and local medium-high frequencies. Currently, scholars usually conduct research independently on the responses of each frequency band, while the mechanism of how structural response evolves from the high frequency to the low frequency is still unknown. In this paper, the entropy-theoretic characterization method is proposed. The effects of load amplitude, load type, statistic element size and element type on the entropy increase process and relaxation time are further investigated by the full free beam model. Results show that the evolution process of impact response of beam structures includes three stages, and the definition of structural response entropy can effectively reflect the evolution process of structural response, which can be used as a quantitative index to effectively describe the characteristics of structural response evolution and will provide new basic theory support for engineering applications.

Dynamic stress response is an important parameter to describe the degree of impact damage of a structure under underwater explosion and able to act as a significant reference for structural safety assessment. However, the dynamic response of a structure presents a strongly nonlinear process, and for large underwater engineering structures, it is difficult to accurately obtain analytic solutions in the whole frequency range simply through theoretical analysis methods. From a statistical perspective, this paper systematically describes and derives the impact input and dissipation process in the form of energy. Within the linear elasticity, the connection between structural strain and energy is derived from the energy density relationship during the process of structural vibration, achieving the prediction of structural stress. The critical input of the system under the critical elasticity is inversely derived based on the structure’s yield strength, which provides a technical reference for the safety assessment of large underwater structures under impact. After verifying by underwater explosion experiments, the relative error obtained through the prediction method and experiments is within 17%, with an average error of 7.52%.

Pressure hulls of underwater vehicles, typically configured as spherical or cylindrical structures, are vulnerable to localized damage when subjected to shock loads from underwater explosions. This study establishes theoretical models for deformation patterns and deformation center deflection of such hulls under underwater shock by integrating fluid-structure interaction theory, localized deformation mechanics, and structural dynamics. Systematic investigations are conducted to evaluate the influence of hull thickness h, diameter D, length L, shock factor I, and hydrostatic pressure p infinity. The deformation center deflection w0 is found to root scale as w0 proportional to h-2 with thickness, w0 proportional to D with diameter, w0 proportional to L-0.5 with length, w0/D proportional to I2 with shock factor, and w0/D proportional to p infinity with hydrostatic pressure. Results show that spherical hulls of equal diameter exhibit superior shock resistance, and hull thickness plays a particularly significant role in mitigating shock effects. The proposed deformation patterns and deflection metrics provide a practical basis for evaluating shock resistance and assessing pressure-bearing capacity and implosion risks in damaged underwater hulls.

As a core parameter characterizing the ability of materials and structures to dissipate vibrational energy, the loss factor directly affects a structure’s vibration and noise reduction performance and dynamic stability. Analyzing this parameter is of guiding significance to structural design in precision machinery, aerospace, shipbuilding, and civil engineering. An Energy Dissipation Boundary Spectrum is defined, aiming to reveal their frequency‑domain attributes and intrinsic coupling with dimensionless structural features. enabling synchronous analysis of the frequency‑domain distribution of the structural loss factor together with structural characteristics, and the distribution rules in the spectrum is visualized. Research indicates that the structural loss factor exhibits a general trend of “higher values at low frequencies and lower values at high frequencies” in the frequency domain. Structural characteristics manifest as the loss factor increasing with the aspect ratio, with an approximate increase of 1

With growing demands for vibration mitigation and blast resistance of marine engineering structures, fiber-metal laminate (FML) structures are increasingly applied in key marine engineering fields. However, research on the damage characteristics and numerical prediction methods of large-scale FML structures under underwater contact explosions remains insufficient. To address this gap, underwater contact explosion tests with a 150 g TNT-equivalent charge are conducted on FML cylindrical shells at 8 m water depth to obtain realistic structural damage characteristics. Meanwhile, a user-defined shell-element constitutive model accounting for anisotropic strain-rate effects is established, and numerical simulations are performed using the coupled Eulerian-Lagrangian (CEL) method. The results indicate that, near the charge, the dominant damage modes in the carbon fiber layers are tensile-driven fiber fracture and fiber pull-out; in the variable cross-section regions at the axial ends, shear damage and delamination prevail; and at the tips of circumferential cracks, compression-induced fiber buckling is observed. Pronounced interfacial debonding between the carbon fiber laminate and steel layer is associated with stress-wave reflections at dissimilar-material interfaces. The primary damage to the carbon fiber laminate exhibits a dominant cross-shaped pattern, while distinct circumferentially propagating cracks are observed in the inner steel layer. Compared with test results, the simulation yields relative errors of approximately 5.6

The vibration and noise reduction characteristics of submersibles have been extensively investigated. Composite materials have various applications in automotive, aerospace, and other fields because of their excellent damping, corrosion resistance, specific strength, and other properties. However, compared with steel, composites are still deficient in terms of stability, stiffness, and economy. Composites also present structural dynamic parameter properties that differ from those of steel structures, which limit their application on submarines. This study aims to improve the vibration and noise reduction performance of submarines and further enhance the application of composite materials in submarine vehicles. For this purpose, the structural design of a laminated reinforced cylindrical shell made of steel and composite materials is conducted, and a traditional form of steel comparative structure with equal mass and shape is designed to contrast with the laminated model. The modal characteristics reflect the inherent frequency features of a structure. By altering the natural frequency, the model can be shifted away from the excitation frequency, which avoids intense resonance and effectively suppresses sound radiation caused by resonance. Therefore, the characteristics of the two structures in terms of modal and damping are compared and explored through experiments and simulations. The results show that the relative error of the modal between experiment and computation does not exceed 6%. After replacing 40% of the mass of steel with carbon fiber, the first three orders of the intrinsic frequency of the structure are increased by more than 13%, the amplitude of the transfer function is reduced by 9.6%, and the damping is improved by more than 75%. Therefore, the vibration and noise reduction characteristics of submersibles have been improved.

The transient statistical energy analysis (TSEA) method is suitable for solving high-frequency structural dynamic response problems, and has been widely used in aerospace and marine fields. However, for complex structures such as hull shells and multi-compartment submersibles, there are problems such as a cumbersome computational process and huge computational volume. This paper proposes the Iterative TSEA (ITSEA) method, which utilizes an iterative approach to solve the energy balance equation combined with initial conditions. Validation shows that compared to TSEA, the ITSEA algorithm saves 40% of computational parameters and processes, while the relative error of the energy decay curve after 20 iterations is no more than 2.1%. ITSEA offers significant advantages for large-scale shock response calculations for complex structures, and for structures with more than 30 subsystems, the improvement in computational efficiency becomes significant.

Damage to the ship can threaten its overall strength, which in turn affects the structural safety of the hull and its subsequent use. Recognizing ongoing damage immediately and effectively is crucial. This paper examines a method based on curvature modes to identify ship damage. The effectiveness of this method is investigated when the ship experiences simultaneous and successive damage. The ship is simplified as a hull girder with free boundary conditions. Six different damage cases are analyzed through numerical simulations and experiments. Results show that the curvature mode index can effectively identify two separate damages when the ship is damaged in two locations simultaneously. Additionally, when the ship is subjected to continuous attacks, the curvature mode index can accurately detect new damage without being affected by existing damage. The proposed method offers a novel approach to double damage identification in hull girders, providing valuable insights for real-time ship safety assessment.

Deep-sea pressure shells are at risk of implosion, making the study of implosion load characteristics crucial. This study utilizes the CEL numerical method to develop an implosion model for a standard titanium alloy spherical pressure shell and conducts simulation calculations for deep-sea implosion under different conditions. It reveals varying implosion characteristics with changes in the implosion strength parameter Pλ∗. Notably, there are significant differences in the peaks of implosion load in different directions for small Pλ∗. As Pλ∗ increases, the influence of the shell on the implosion load weakens, leading to a process closer to ideal bubble collapse. Additionally, the energy proportion of the implosion load is proportional to Pλ∗0.5. By introducing the linear buckling mode of the shell as the initial geometric imperfection, the influence of imperfection shape and amplitude on the implosion process and load is analyzed in detail. It is found that the amplitude has a greater impact on the implosion. Increasing the imperfection amplitude leads to a higher peak and energy proportion of implosion load, and advances the time to reach the peak.

Large ships and submarines are the main equipment and mobile platforms supporting three-dimensional naval space operations. However, with the increasing damage power and strike accuracy of warheads, plus the diversity of destroying elements, their living environment is facing increasingly severe threats. It is thus necessary to work on the damage and protection of naval vessels subjected to blast loading. The blast-induced damage and protection of naval vessels is an interdisciplinary issue that involves the integration of explosion mechanics, fluid mechanics, structural mechanics, materials science and other disciplines in which many challenges are yet to be addressed. In view of some of the key issues involved, this paper systematically summarizes the research progress on blast load characteristics, damage mechanisms and protection technologies and equipment at home and abroad in recent years. The shortcomings of the current research are pointed out and the development trends of ship explosion damage and protection are predicted. This study can provide valuable references for future research on the efficient damage and protection of naval vessels.

In order to facilitate the development of the impact test and restore the original impact load effect, this paper adopts the combined sine wave method and the composite fundamental wave method to reconstruct the impact load of the marine multi-diaphragm isolator. The advantages and disadvantages of the two methods are analyzed, and the consistency of the response effect of the components under the two shock loads is studied. The results show that the diaphragm isolator demonstrates excellent shock resistance and displacement limitation. The response effect of the isolator under the combined sine wave is different from the original response, and the reduction degree of medium and high frequency load spectrum is poor, which has certain limitations. Compared to the structural response under the original impact load, the acceleration response error under the composite fundamental wave load is 9.37%, and the top vertical deformation error is 12.9%. The composite fundamental wave method effectively recovers the shock load by transforming the characteristic parameters of the base waveform.

In order to facilitate the development of the impact test and restore the original impact load effect, this paper adopts the combined sine wave method and the composite fundamental wave method to reconstruct the impact load of the marine multi-diaphragm isolator. The advantages and disadvantages of the two methods are analyzed, and the consistency of the response effect of the components under the two shock loads is studied. The results show that the diaphragm isolator demonstrates excellent shock resistance and displacement limitation. The response effect of the isolator under the combined sine wave is different from the original response, and the reduction degree of medium and high frequency load spectrum is poor, which has certain limitations. Compared to the structural response under the original impact load, the acceleration response error under the composite fundamental wave load is 9.37%, and the top vertical deformation error is 12.9%. The composite fundamental wave method effectively recovers the shock load by transforming the characteristic parameters of the base waveform.

Ship damage identification is a new field of ship research, which is of great significance to damage control, life and property safety and strategic decision-making, but there is no effective method for ship damage identification. The structural system changes caused by ship damage are usually represented by the change of system mass, stiffness and energy dissipation characteristics, and then the change of structural dynamic characteristic parameters, which provides a theoretical basis for ship damage identification. In this paper, the ship is simplified as a beam model with fully free boundary conditions. Based on the dynamic characteristic parameter of the modal shape, the damage identification of the ship under six damage conditions is carried out through numerical simulation and experimental verification by using COMAC and the derived indexes of curvature mode and modal flexibility. The results show that the first four orders of curvature mode difference mean index and the curvature index of modal flexibility change rate can accurately locate the damage of hull beam structure; The damage degree of hull beam can be identified by the change of curvature mode difference index, but there are some errors in this method; The curvature index of modal flexibility change rate is positively correlated with the damage degree, which can reflect the damage degree of the structure. The research results of this paper can provide new methods and ideas for hull beam damage identification.

Pressure hulls is an essential part of the deep-sea submersible and may implode to emit shock pressure wave. This paper establishes a deep-sea implosion model based on the Eulerian finite element method, and two implosion conditions, ellipsoidal and egg-shaped, are studied. It is found that the pressure load at the gauging point in the long semi-axis direction is always greater than that in the short semi-axis direction when the two models implosion. Introducing the radius ratio β, it is found that the pressure load after the implosion of the two models increases first and then decreases with the increase of β. Then the dimensionless number Ma is introduced to simulate the change of pressure load under implosion in different water depths. It is found that pressure peak Pm∗ decreases by a factor of Ma−0.5 as Ma increases, and the time Tm∗ to reach the pressure peak increases linearly with the rise of Ma. Therefore, the pressure load caused by implosion can be reduced by reasonably selecting the radius ratio of the pressure hull. In addition, the implosion load when a plurality of pressure hulls are arranged together can be reduced by organizing them along the short semi-axis direction.

It is essential for decision makers to obtain real-time information on damage to a ship after it has been damaged, but there is no effective method currently. A method for identifying ship damage based on frequency is proposed, which is a data-driven approach using neural networks. The frequency database under different damage conditions is obtained by migration matrix method, optimized by normalization method, and trained by Probabilistic Neural Network (PNN) to form the agent model. Considering the limitations in the deployment of measurement points in practical applications, an optimization method based on Modal Assurance Criterion (MAC) that takes into account the offset of the measurement points is introduced and studied. Methods presented are experimentally validated by a simplified beam model which simulates free boundary conditions of ship. Results show that the agent model constructed can accurately identify the damage location. The damage quantification error decreases with the increase of the preset damage extent, which indicates that the accuracy is higher for larger damage. The test scheme optimization method can effectively obtain the best measurement point placement scheme for efficient measurement of modal parameters. The results of this paper can provide technical and methodological support for the real-time identification of ship damage.

针对目前在船舶爆炸损伤试验中不能有效获得船舶损伤数据,将爆炸过程中船舶船体梁总振动固有频率与爆炸损伤前完好船体总振动固有频率进行对比,建立完好船体和损伤船体多种假想损伤的固有频率数据库,采用神经网络智能方法得到频率改变特征参数与损伤参数的映射关系,对船舶结构损伤状态进行判断.以一艘1500吨级的船舶为研究对象,通过仿真计算船舶结构损伤前后各阶固有频率,基于频率改变特征参数与损伤参数的神经网络智能方法识别船舶结构损伤位置和损伤程度,损伤位置全部定位正确,损伤程度平均精度达97.91％.该方法适用于爆炸损伤条件下的船舶结构损伤识别,新颖有效.

Ship damage identification is very important for damage control, personal and property safety, and strategic decision. In this paper, a novel method for hull girder damage identification is proposed based on mode cur-vature shapes(MCS). A 4000-ton ship, which is simplified as a hull girder, is taken as the research object to validate this method through six damage scenarios represented by different stiffness loss. The natural frequencies and mode shapes are obtained by the finite element method, and then the MCS was derived by a central dif-ference approximation. An improved damage indicator(DI) is defined to locate the damage. The effects of noise and FE modeling uncertainty on DI are further discussed. The damage extent under different damage scenarios is derived by the perturbation theory and MCS differences are used for damage quantitation. Results show that DI has significant damage localization ability in single or multiple damage cases. However, this MCS-based damage quantitation method derived by perturbation theory has a large error when quantifying tiny damage. The research results of this paper can provide a new method and idea for hull girder damage identification and help check the safety of the ship.

针对水下爆炸载荷作用下桁架箱体浮筏的冲击环境特性问题,采用数值仿真和声固耦合的方法,利用Abaqus软件进行水下非接触爆炸模拟,对具有双层平台的浮筏的垂向冲击环境分布特性以及减振器的刚度和数量对其垂向冲击环境的影响开展了研究.结果表明:上下平台冲击环境沿纵向分布变化趋势基本一致,沿纵向上冲击环境在位于支柱之间的板架处出现峰值,在两舷侧减振器的缓冲作用下,浮筏迎背爆面冲击环境接近,最大差异仅为11％;浮筏减振器的刚度和数量都会对上下平台的冲击环境造成影响.本文分析结果可为安装在浮筏上的设备抗冲击提供参考.

舰船冲击环境是舰船设备抗冲击评估的基础输入数据,其最可靠的来源是实船抗冲击试验,其中低频冲击环境的测试工程上常采用低频振子、簧片仪等专用仪器,这些传感器存在过于笨重、仪器安装相对困难的问题,直接采用加速度计测试结果由于"零飘"导致误差大,其结果不可靠.为此,通过对低频冲击环境形成机理的分析,指出大型舰船低频冲击环境主要来源于舰船低频总振动,并提出一种基于实船抗冲击试验舰船总振动应变的间接测试方法.通过千吨级舰船实船的试验数据证明了该方法的有效性,加速度计实测数据与低频振子测得的数据相比,平均偏差为42.9％,新方法与低频振子测得的数据相比,平均偏差为12.6％.该技术与传统方法相比具有传感器安装简便、测点数目需求低和有效覆盖船长范围大的优势,对实船抗冲击试验的低频冲击环境测试具有重要的参考价值.