Assessment of the stiffened panel performance in the OTEC seawater tank design:Parametric study and sensitivity analysis

Ocean thermal energy conversion(OTEC)is a process of generating electricity by exploiting the temperature difference between warm surface seawater and cold deep seawater.Due to the high static and dynamic pressures that are caused by seawater circulation,the stiffened panel that constitutes a seawater tank may undergo a reduction in ultimate strength.The current paper investigates the design of stiffening systems for OTEC seawater tanks by examining the effects of stiffening parameters such as stiffener sizes and span-over-bay ratio for the applied combined loadings of lateral and transverse pressure by fluid motion and axial compression due to global bending moment.The ultimate strength calculation was conducted by using the non-linear finite element method via the commercial software known as ABAQUS.The stress and deformation distribution due to pressure loads was computed in the first step and then brought to the second step,in which the axial compression was applied.The effects of pressure on the ultimate strength of the stiffener were investigated for representative stiffened panels,and the significance of the stiffener parameters was assessed by using the sensitivity analysis method.As a result,the ultimate strength was reduced by approximately 1.5%for the span-over-bay ratio of 3 and by 7%for the span-over-bay ratio of 6.

An Efficient Method for Local Buckling Analysis of Stiffened Panels

The local buckling of stiffened panels is one of possible failure modes and concerned by engineers in the preliminary design of lightweight structures. In practice,a simplified model,i.e.,a rectangular plate with elastically restrained along its unloaded edges,is established and the Ritz method is usually employed for solutions. To use the Ritz method,however,the loaded edges of the plate are usually assumed to be simply supported. An empirical correction factor has to be used to account for clamped loaded edges. Here,a simple and efficient method,called the quadrature element method(QEM),is presented for obtaining accurate buckling behavior of rectangular plates with any combinations of boundary conditions, including the elastically restrained conditions. Different from the conventional high order finite element method(FEM),non-uniformly distributed nodes are used,and thus the method can achieve an exponential rate of convergence. Formulations are worked out in detail. A computer program is developed. Improvement of solution accuracy can be easily achieved by changing the number of element nodes in the computer program. Several numerical examples are given. Results are compared with either existing solutions or finite element data for verifications. It is shown that high solution accuracy is achieved. In addition,the proposed method and developed computer program can allow quick analysis of local buckling of stiffened panels and thus is suitable for optimization routines in the preliminary design stage.

Formulation of Reduction Rate for Ultimate Compressive Strength of Stiffened Panel Induced by Opening

The main objective of this study is to numerically investigate the characteristics of ultimate compressive strength of stiffened panels with opening and also to fit the design-oriented formulae. For this purpose, three series of well executed experimental data on longitudinally stiffened steel plates with and without opening subjected to the uniform axial in-pane load which is carried out to study the buckling and post-buckling up to the final failure are chosen. Also, a nonlinear finite element method capable of efficiently analyzing the large elasto-plastic deflection behavior of stiffened panels is developed and used for simulation. The feasibility of the present simulation process is confirmed by a good agreement with the experimental results. More case studies are developed employing the simulation process to analyze the influence of various design variables on the reduction rate of ultimate strength of stiffened panel induced by opening. Based on the computed results, two design formulae are fitted and the accuracy of design formulae is studied. Furthermore, the viability of the design formulae for practical engineering is proved.

Reliability of the Ultimate Strength of Ship Stiffened Panel Subjected to Random Corrosion Degradation

Attentions have been increasingly paid to the influence of the corrosion on the ultimate strength of ship structures. In consideration of the random characteristics of the corrosion of ship structures, the method for the ultimate strength analysis of the ship stiffened panel structure subjected to random corrosion degradation is presented. According to the measured corrosion data of the bulk carriers, the distribution characteristics of the corrosion data for the stiffened panel on the midship deck are analyzed, and a random corrosion model is established. The ultimate strength of the corroded stiffened panel is calculated by lhe nonlinear finite element analysis. The statistical descriptions of the ultimate strength of the corroded stiffened panel are defined through the Monte Carlo simulations. A formula is proposed on the ultimate strength reduction of the stiffened panel as a function of the corrosion volume. The reliability analysis of the ultimate strength of the corroded deck stiffened panel is performed. It shows that both the corrosion data of the deck stiffened panel and the ultimate strength of the random corroded deck stiffened panel follow the log-normal distribution. The ultimate stress ratio of the stiffened panel is inversely proportional to the corrosion volume ratio.

Numerical Analysis of Static Performance Comparison of Friction Stir Welded versus Riveted 2024-T3 Aluminum Alloy Stiffened Panels

Most researches on the static performance of stiffened panel joined by friction stir welding（FSW） mainly focus on the compression stability rather than shear stability. To evaluate the potential of FSW as a replacement for traditional rivet fastening for stiffened panel assembly in aviation application, finite element method（FEM） is applied to compare compression and shear stability performances of FSW stiffened panels with stability performances of riveted stiffened panels. FEMs of 2024-T3 aluminum alloy FSW and riveted stiffened panels are developed and nonlinear static analysis method is applied to obtain buckling pattern, buckling load and load carrying capability of each panel model. The accuracy of each FEM of FSW stiffened panel is evaluated by stability experiment of FSW stiffened panel specimens with identical geometry and boundary condition and the accuracy of each FEM of riveted stiffened panel is evaluated by semi-empirical calculation formulas. It is found that FEMs without considering weld-induced initial imperfections notably overestimate the static strengths of FSW stiffened panels. FEM results show that, buckling patterns of both FSW and riveted compression stiffened panels represent local buckling of plate between stiffeners. The initial buckling waves of FSW stiffened panel emerge uniformly in each plate between stiffeners while those of riveted panel mainly emerge in the mid-plate. Buckling patterns of both FSW and riveted shear stiffened panels represent local buckling of plate close to the loading corner. FEM results indicate that, shear buckling of FSW stiffened panel is less sensitive to the initial imperfections than compression buckling. Load carrying capability of FSW stiffened panel is less sensitive to the initial imperfections than initial buckling. It can be concluded that buckling loads of FSW panels are a bit lower than those of riveted panels whereas carrying capabilities of FSW panels are almost equivalent to those of riveted panels with identical geometries. Finite element method for simulating static performances of FSW and riveted stiffened panels is proposed and evaluated and some beneficial conclusions are obtained, which offer useful references for analysis and application of FSW to replace rivet fastening in aviation stiffened panel assembly.

Ultimate Compressive Strength Prediction for Stiffened Panels by Counterpropagation Neural Networks(CPN)

Stiffened Panels are important strength members in ship and offshore structures. A new method based on counterpropagation neural networks (CPN) is proposed in this paper to predict the ultimate compressive strength of stiffened panels. Compared with two-parametric polynomial, this method can take more parameters into account and make more use of experimental data. Numerical study is carried out to verify the validation of this method. The new method may find wide application in practical design.

Structural-acoustic optimization of stiffened panels based on a genetic algorithm

For the structural-acoustic radiation optimization problem under external loading,acoustic radiation power was considered to be an objective function in the optimization method. The finite element method(FEM) and boundary element method(BEM) were adopted in numerical calculations,and structural response and the acoustic response were assumed to be de-coupled in the analysis. A genetic algorithm was used as the strategy in optimization. In order to build the relational expression of the pressure objective function and the power objective function,the enveloping surface model was used to evaluate pressure in the acoustic domain. By taking the stiffened panel structural-acoustic optimization problem as an example,the acoustic power and field pressure after optimized was compared. Optimization results prove that this method is reasonable and effective.

Numerical and Experimental Study on Stiffened Composite Panel Repaired by Bolted Joints under Compressive Load

Numerical and experimental study was conducted to investigate the failure mode and strength performance of stiffened composite panel repaired by bolted joints under compressive load, and the results were then compared with those from virgin stiffened composite panel without any damage. A finite element analysis model was established for repaired and virgin stiffened composite panels under compressive load, the 3D Hashin criteria was applied to identify the composite structure failure, and the secondary stress criteria was adopted to identify the adhesive failure between the base laminate and the stiffener. The failure modes of repaired stiffened composite panels were stiffened composite panels breaking off along the bolt joints. The experimental results were consistent with the finite element analysis results, indicating the reliability of the finite element analysis model.

Manufacturing cost optimization and estimation combined computer of composite-stiffened panels

The primary goal of this study is to fully grasp the production flow of the new processing technologies for manufacturing composite stiffened panels incorporating cost as one of the design variables early in the design process. An approach is presented to determine the optimum process for cost as objective function. A cost estimation model is established based on the integrally molding process. In the model,the cost drivers which are related to the manufacture processes in terms of material,labor,tool and equipment costs are taken into account. At the same time,estimation software combined computer is developed to aid optimization design. A case of manufacturing composite stiffened panels with T-shaped stiffeners is examined. Excellent agreement shows the optimum process for cost is obtained for the composite stiffened panel with cocuring. It is also revealed that the estimation software combined computer is efficient. The estimation methodology is valid to guide design of the manufacturing process for the composite-stiffened panel.

典型钛合金壁板高温环境下振动疲劳分析与试验研究

振动和高温一直是影响飞机结构安全的重要因素,受到振动载荷和高温环境联合作用的飞机结构更容易出现疲劳破坏。本研究中的典型钛合金壁板结构形式包括加筋壁板和蜂窝壁板,通过改变壁板厚度、加筋间距以及结构连接方式,使用分析和试验的手段对壁板的振动特性、振动疲劳寿命及破坏部位开展规律性研究。研究发现,加筋壁板破坏部位出现在加筋或连接角片上,蜂窝壁板在8 h试验时间内未出现破坏,壁板厚度、加筋间距以及结构连接形式对壁板振动疲劳寿命影响较小。

中心开孔加筋壁板压缩性能研究

为了研究飞机结构中不同开孔尺寸加筋壁板在压缩载荷下的屈曲行为和后屈曲行为,本文设计了相应的试验方法和试验夹具,完成了壁板压缩试验。得到了不同开孔尺寸加筋壁板的屈曲/破坏载荷、失稳过程及破坏模式。结果表明:加筋壁板的屈曲模式是筋条间蒙皮和筋条外蒙皮发生相反的变形,且屈曲模态随载荷的增加发生多次跳变;加筋壁板的压缩破坏模式是在屈曲变形的基础上筋条伴有明显的变形。针对不同开孔尺寸的加筋壁板进行压缩加载有限元仿真,得到的屈曲与破坏模式和试验的吻合,屈曲/破坏载荷与试验结果的误差在4%以内,验证了有限元模型的有效性。随着开孔尺寸的增加,加筋壁板的屈服载荷先缓慢减小后快速增大;壁板的破坏载荷逐渐减小,最终结构失去后屈曲承载能力。

平纹编织复合材料加筋壁板低速冲击及冲击后压缩性能多尺度分析

该文通过构建平纹编织复合材料(plain woven composites,PWC)的微/细/宏观多尺度模型,对复合材料T型和工型加筋壁板的低速冲击(low-velocity impact,LVI)和冲击后压缩(compression after impact,CAI)性能进行分析。根据PWC内部结构特征,构建微观和细观代表性体积单元(representative volume element,RVE)模型,通过施加不同的载荷条件,预测其等效力学参数。同时,基于局部均匀化方法,将细观RVE模型转化为包含0°和90°子胞的等效交叉层合板(equivalent cross-ply laminate,ECPL)胞元。通过ECPL胞元建立PWC加筋壁板的宏观分析模型,并进行LVI和CAI性能分析。首先进行了能量为8 J、10 J和12 J的LVI试验和多尺度模拟,研究了PWC加筋壁板在LVI载荷下的力学行为。随后,对不同能量冲击后的加筋壁板进行CAI试验和数值模拟,研究其冲击后压缩剩余力学性能和损伤行为。最后,通过对比分析不同能量冲击下的LVI和CAI试验和预测结果,发现多尺度模型对加筋壁板力学性能的预测误差均小于6%,验证了该多尺度模型的有效性。试验和预测结果表明,在CAI载荷作用下,PWC加筋壁板的蒙皮部分沿着冲击损伤的位置发生完全断裂,其中,纤维断裂和层间分层成为PWC加筋壁板的主要损伤形式。此外,工型加筋壁板表现出更好的抵抗压缩强度衰减能力。

带筋壁板筋条双侧激光喷丸成形研究

带筋壁板能显著提高飞行器的结构效率与气密性,是现代航空航天飞行器广泛采用的结构形式,而激光喷丸成形作为新型无模柔性成形工艺,是带筋壁板成形的有效工艺途径。带筋壁板的可喷丸区域包括筋条双侧与内外侧蒙皮,考虑筋条双侧喷丸可精确调控带筋壁板的几何形状,首先对带筋壁板筋条双侧激光喷丸的变形特征进行分析,并基于激光喷丸固有应变理论推导筋条双侧喷丸等效弯矩解析公式,进而建立筋条双侧喷丸等效弯矩数值模型。研究表明,建立的等效弯矩模型可为筋条双侧喷丸的带筋壁板激光喷丸成形的变形预测与工艺规划提供有效途径。

裂纹加筋和缺口加筋对钢加筋板极限强度的影响

This paper numerically evaluates the effect of the crack position on the ultimate strength of stiffened panels.Imperfections such as notches and cracks in aged marine stiffened panels can reduce their ultimate strength.To investigate the effect of crack length and position,a series of nonlinear finite element analyses were carried out and two cases were considered,i.e.,case 1 with thin stiffeners and case 2 with thick stiffeners.In both cases,the stiffeners have the same cross-section area.To have a basis for comparison,the intact panels were modeled as well.The cracks and notches were in the longitudinal and transverse direction and were assumed to be in the middle part of the panel.The cracks and notches were assumed to be through the thickness and there is neither crack propagation nor contact between crack faces.Based on the numerical results,longitudinal cracks affect the behavior of the stiffened panels in the postbuckling region.When the stiffeners are thinner,they buckle first and provide no reserved strength after plate buckling.Thus,cracks in the stiffeners do not affect the ultimate strength in the case of the thinner stiffeners.Generally,when stiffeners are thicker,they affect the postbuckling behavior more.In that case,cracks in the stiffeners affect the buckling and failure modes of the stiffened panels.The effect of notch was also studied.In contrast to the longitudinal crack in stiffeners,a notch in the stiffeners reduces the ultimate strength of the stiffened panel for both slender and thick stiffeners.

考虑Ⅲ型断裂的复合材料加筋壁板界面损伤扩展

复合材料加筋壁板筋条-蒙皮界面损伤研究中Ⅲ型断裂能的作用往往被忽略,本文结合物理试验和数值方法对复合材料开剖面工形加筋壁板和闭剖面Ω形加筋壁板开展了后屈曲筋条-蒙皮界面损伤扩展研究。利用考虑法向压缩抑制界面剪切失效机制的新形界面损伤起始准则和考虑Ⅲ型断裂的Reeder界面损伤扩展准则对两种构形加筋壁板后屈曲历程进行数值模拟分析,对比两种构形壁板的屈曲模式、破坏形式以及筋条-蒙皮界面损伤扩展时的各形能量释放率的分布情况。得出闭剖面Ω形加筋壁板筋条-蒙皮界面主要在面外剥离力的作用下损伤扩展,I型断裂能在界面的损伤起始和扩展中起主要作用;开剖面工形加筋壁板界面主要在剪力的作用下损伤扩展,Ⅲ型断裂能在损伤横向扩展中起主导作用。上述研究阐述了各形断裂能在复合材料加筋壁板界面损伤扩展过程中的作用机理,为复合材料加筋壁板后屈曲数值模拟分析方法研究奠定基础。

弹性边界约束加筋板结构振动特性与功率流分析

从能量传输角度开展弹性边界约束条件下加筋薄板结构振动特性研究。加筋薄板结构振动系统横向振动位移采用边界光滑傅里叶级数展开,在加筋薄板端部引入平动约束弹簧与旋转约束弹簧,通过设置约束弹簧的刚度数值模拟任意边界条件。基于能量原理描述弹性边界约束加筋薄板结构振动系统的总势能、总动能与外激励做功项,结合瑞利-里兹方程得到系统的标准特征方程,弹性边界约束加筋薄板结构的振动响应特性可以通过求解标准特征方程得到。通过数值算例,验证本方法在求解弹性边界约束下加筋薄板结构振动特性的准确性,探究加强筋位置、高宽比以及边界约束弹簧刚度对系统振动特性、能量流响应的影响规律。本方法具有快速收敛、高效、高精度的特点,可为研究弹性边界约束加筋薄板结构动力学行为提供有效分析手段。

复合材料加筋板高能量低速冲击损伤特性数值研究

高能量大面积钝物冲击对复合材料飞机运营安全带来较大的威胁。采用Hashin准则和双线性渐进损伤模型模拟层内损伤,采用Cohesive界面元模拟层间损伤,采用Ogden超弹性材料模型作为橡胶材料本构模型,并进行了模型有效性验证,针对复合材料加筋板开展了高能量软体、硬体低速冲击下的损伤特性数值研究。分别分析了加筋板不同位置在刚性冲头和橡胶冲头高能量低速冲击下的冲击响应和损伤过程,对比研究了在冲击过程中加筋板变形和损伤模式。结果表明,刚性冲击下接触压力较大,局部变形和损伤更严重。橡胶冲击下橡胶变形起到了一定缓冲和分散压力的作用,但接触力峰值与刚性冲击相比高出约23%。冲击位置位于筋条时在刚性冲击下的损伤最严重,损伤主要位于蒙皮、筋条腹板和缘条,在橡胶冲击下损伤主要位于蒙皮背部和筋条腹板。与刚性冲击相比,不同位置橡胶冲击下的损伤较小且几乎外部目视不可见。

复合材料帽形加筋壁板加捻区质量问题研究

帽形加筋壁板结构的复合材料制件具有比强度高、比刚度高、可设计性良好等优点,在航空航天等工程领域得到了广泛的应用,而加筋结构的三角空隙必须使用捻条进行填充。采用“湿长桁+湿蒙皮”共固化工艺制造帽形加筋壁板复合材料制件时,发现捻条的外形会显著影响制件的成型质量。为了提高帽形加筋壁板制件的成型质量,研究了不同材质的长桁芯模、捻条制备方法、捻条填充量对加捻区长桁及蒙皮质量的影响。结果表明:由专用的加捻模具制备并在(60±5)℃的温度下预压实后,捻条预成型体的外形更精确,与其他结构配合更好;当捻条的填充量修正系数为1.1时,可有效避免蒙皮和长桁出现纤维褶皱;所选用的硅橡胶芯模固化成型后加捻区质量控制更好。

复合材料帽型加筋壁板典型件共固化成型技术研究

文章对复合材料帽型加筋壁板典型件进行了共固化成型技术研究,通过研究该典型件结构发现,该复合材料典型件结构复杂,需要实现加筋与壁板的一体化组合加工。因此,文章采用了共固化成型技术,该技术可以通过一次压力将多个构件进行联合固化,从而实现构件的一体化,且该技术具有成本低、生产效率高的优势,可以推广应用于类似结构的复合材料制造。