船体结构极限海况下逐次崩溃行为研究

研究船体结构在极限海况下所遭受的载荷以及相应的结构逐次崩溃行为对于确保在航船舶的安全或是设计出合理安全可靠的船舶结构,具有重要的意义。在该文的研究中,首先用基于势流理论的三维奇点分布法进行作用于船体的载荷和船体运动的计算;将得到的随时间变化的载荷施加到整船结构模型上;然后运用MSC.Marc软件,考虑几何非线性和材料非线性,进行结构逐次崩溃分析,可得到结构的极限承载能力。在明确作用于船体的外力和结构的耐力以后,即可进行船体结构安全性评估;可通过加大崩溃区域的板厚或加强筋尺寸的方式来提高结构的承载能力,增加船体结构的安全度。     

Impact response of rectangular and square stiffened plates supported on two opposite edges

Experimental drop weight impact tests have been performed to examine the dynamic response of small-scale stiffened plates struck laterally by a mass with a spherical indenter. The laboratory results are compared with numerical simulations. The plates stiffened with a flat bar or L profile are supported at two opposite edges and impacted at different velocities and locations along the span. The impact scenarios could represent incidents in marine structures, such as load actions due to dropped objects on decks. The experiments are conducted using a fully instrumented impact testing machine. The obtained force–displacement responses show a good agreement with the simulations performed by the LS-DYNA finite element solver. The finite element model includes defining the experimental boundary conditions so as to simulate small axial displacements of the specimen at the supports. This representation can be used to analyze the structural crashworthiness of similar marine structures under collision scenarios. The strain hardening of the material is defined using experimental data of quasi-static tension tests and the strain rate sensitivity is evaluated using standard coefficients of the Cowper–Symonds constitutive model. The results show that the plastic response of the specimens is highly sensitive to the amount of restraint provided at the supports. Furthermore, it is found that in most of the specimens the contribution of the stiffeners to the impact response is insignificant, since the ends of the stiffener are free at the unsupported edges and the specimens experience small axial displacements at the supports.     

Internally circumferentially cracked cylinders with functionally graded material properties

In this paper, internal circumferentially cracked hollow cylinders, which are assumed to be made of functionally graded materials, are studied. The conventional finite element method (FEM) is improved by introducing isoparametric transformation for simulating the gradient variations of material properties in finite elements. By using this improved FEM, the mode I stress intensity factors are calculated for cylinders subjected to uniform tension. Various types of functionally graded materials and different gradient compositions for each type are investigated. The results show that the material property distribution has a quite considerable influence on the stress intensity factors.     

The uncertainty of laminar burning velocity of premixed H2-air flame induced by the non-uniform initial temperature field inside the constant-volume combustion vessel

The initial temperature distribution of the combustible mixture has a significant effect on the measurement accuracy of the laminar burning velocity using the outwardly propagating spherical flame method. In the present study, the initial temperature fields inside the constant-volume combustion vessel were obtained by different arrangement methods of heater. Further, the effects of the non-uniformity of initial temperature field on the propagation processes of two-dimensional premixed H₂-air laminar flames were numerically studied. The results show that when the initial temperature field inside the vessel heated by heating tapes reaches a stable state, the temperature of H₂-air mixture tends to descend first and then rise along the gravity direction, which indicates that the non-uniformity of the temperature field increases with the actual delivered power. Compared with the uniform initial temperature field, the maximum relative deviation of laminar burning velocity of H₂-air mixture obtained in the non-uniform initial temperature field is 7% when the vessel is heated by heating tapes under the power of 669 W. However, the non-uniformity of the initial temperature field of the H₂-air mixture in the vessel obviously decreases and the maximum relative deviation of laminar burning velocity is only 2% when a simulated evenly arranged heater is employed to heat the vessel. Consequently, it is quite necessary to evaluate the non-uniformity of the initial temperature field inside the constant-volume combustion vessel before using the outwardly propagating spherical flame method to determine the laminar burning velocity.     

Numerical and experimental fatigue crack growth analysis in mode-I for repaired aluminum panels using composite material

Crack-front shape is an important parameter influencing the stress intensity factor and crack propagation rate in asymmetric repaired panels. In this study, the numerical and experimental fatigue crack growth behaviour of centrally cracked aluminum panels in mode-I condition repaired with single-side composite patches are investigated. It is shown that the crack growths non-uniformly from its initial location through the thickness of a single-side repaired panel. There is a good agreement between the propagated crack-front shapes obtained from finite element analysis with those obtained from the experiments for various repaired panels with different patch thicknesses. Furthermore, effects of plate and patch thickness on the crack growth life of the repaired panels are investigated. The experimental results show that the crack growth life of thin panels may increase up to 236% using a 16 layers patch. However, for thick panels, the life may extend about 21–35% using a 4 layers patch. Implementing of 8 and 16 layers patches has not a significant effect on the life extension of thick panels with respect to the 4 layers patch life.     

Residual ultimate strength assessment of stiffened panels with locked cracks

The residual ultimate strength of stiffened panels with locked cracks under axial compressive loading is analyzed. The influences of various geometrical characteristics of cracks and panels, such as the length and the orientation angle of cracks, are investigated by the nonlinear finite element analysis. The cracks are prevented from further propagation by holes with a diameter of 2 mm drilled at their tips. The finite element model has two bays in the longitudinal direction, employing periodical symmetrical boundary conditions at the ends of the loading edge. Beam tension test results are used to define the true stress–strain relationship of the material, which is used in the FE analyses to account for the nonlinear material property of steel.     

Localized indentation of sandwich panels with metallic foam core: Analytical models for two types of indenters

Sandwich structures with metallic foam core are sensitive to local indentation because of the low strength of the core and low bending stiffness of the thin face sheets. In this paper, local indentation response of sandwich panels with metallic foam core under a flat/spherical indenter was analyzed. The composite sandwich is modeled as an infinite, isotropic, plastic membrane on a rigid-plastic foundation. For simplicity, a quadratic polynomial displacement field was employed to describe the deformation of the upper face sheet. By using the principle of minimum work, explicit solutions for the indentation force and the sizes of the deformation regions were derived. The analytical results were verified by those from simulation by using the ABAQUS code, and they are in close agreement. Distribution of radial tensile strain of the upper face sheet and the ratio of energy dissipation of foam core to that of the upper face sheet were analyzed.     

甲烷/乙烷与甲烷/丙烷混合燃料着火特性

利用激波管与CHEMKIN软件研究了不同初始条件下乙烷和丙烷的掺混对甲烷着火延迟时间的影响规律,并从化学动力学角度分析了掺混乙烷和丙烷对甲烷着火延迟时间造成影响的原因。实验与模拟研究表明乙烷和丙烷的掺混会造成甲烷着火延迟时间的大幅度缩短,但随着温度的升高,其对甲烷着火延迟时间的影响逐渐变小。通过敏感性分析发现无论是甲烷/乙烷混合燃料还是甲烷/丙烷混合燃料,对着火促进最大的基元反应都是H+O₂=O+OH（R1）,在甲烷/乙烷和甲烷/丙烷混合燃料的着火反应中对着火抑制最大的两个基元反应是CH₄+H=CH₃+H₂（R128）和CH₄+OH=CH₃+H₂O（R129）。通过路径分析发现在甲烷/乙烷与甲烷/丙烷混合燃料中,随着混合燃料中乙烷与丙烷比例的增加,甲烷的主要反应路径基本不发生变化,主要影响了CH₃的消耗速率。