The ultimate strength of doubler plate reinforced Y-joints under compression loading

It is common practice in the offshore industry to solve the punching shear problem due to compression by using doubler plate. The finite-element method is a useful tool for studying this problem. The aim of this paper is to study the static strength of doubler plate reinforced Y-joints subjected to compression loading. The finite-element method is adopted in numerical parametric studies. The individual influences of the geometric parameters βand τd （ doubler plate to chord wall thickness ratio） and ld/d1 （ dubler plate length to brace diameter ratio） on the ultimate strength are made clear. The results show the size of plate may have important effects on the strength of reinforced joints. It is found that the ultimate strength of Y-joints reinforced with appropriately proportioned doubler plates can be greatly improved nearly up tothree times to un-reinforced Y-joints.     

复杂应力状态下无筋板格极限强度研究

无筋板格是波浪中航行船舶的基本结构单元,精确评估无筋板格极限强度对保证船舶结构安全性具有重要意义.本文基于弹性大挠度理论分析和刚塑性分析,给出了复杂应力状态下无筋板格极限强度计算方法;定性的研究了各参数对无筋板格极限强度的影响;比较了剪应力与其它应力作用的合成分析方法和分离分析方法,验证了分离分析方法的可行性和高效性;开展了本文方法与经验公式及ABS规范公式的比较研究.     

Ultimate strength design of stiffened plates under axial compression and bending

The basis for design of stiffened plates under longitudinal compression is outlined and predictions using several codes are compared against numerical results from an inelastic beam-column formulation and test results. In order to explore the inherent differences in column behaviour separately from discrepancies arising due to plate panel behaviour, the code predictions are re-evaluated adopting a common plate panel effective width formulation. On this basis, a critical review of code methods is made and some modifications are proposed.

The effect of the magnitude and direction of applied uniform bending on the axial capacity of stiffened plates is investigated by comparing two alternative design approaches, namely an interaction equation and a method based on the Perry equation, against results from numerical analyses and from rigid plastic theory. The interaction equation is invariably more conservative than the Perry approach but its simplicity tends to be convenient for routine design applications. Finally, results of numerical analyses, together with experimental results from previous studies, on continuous stiffened plates under combined axial compression and lateral pressure are presented and available design guidance is discussed.     

Effect of pitting corrosion on the ultimate strength of steel plates subjected to in-plane compression and bending

Corrosion pits with a circular cone shape are typically observed on coated hold frames of aged bulk carriers which carry exclusively coal and iron ore. In order to ensure the safety of these types of bulk carrier, it is necessary to understand the effect of pitting corrosion on the local strength of hold frames. In order to investigate this effect, a series of nonlinear finite-element (FE) analyses has been performed with pitted plates subjected to in-plane compressive loads and bending moments. It has been shown that the ultimate compression load or bending moment of pitted plates is smaller than that of uniformly corroded plates in terms of average thickness loss, and that predictions of the ultimate strength using the average thickness loss at the minimum cross section would be conservative. In order to establish a method of evaluating strength reduction due to pitting corrosion, it is important to identify the failure mode that would be most detrimentally affected by pitting corrosion. It was found that the reduction of the ultimate compressive load or bending moment due to pitting corrosion is smaller than that of the tensile strength in terms of equivalent thickness.     

Strength and deformability of corroded steel plates under quasi-static tensile load

The objective of this study was to estimate the strength and deformability of corroded steel plates under quasi-static uniaxial tension. In order to accurately simulate this problem, we first estimated the true stress–strain relationship of a flat steel plate by introducing a vision sensor system to the deformation measurements in tensile tests. The measured true stress–stain relationship was then applied to a series of nonlinear implicit three-dimensional finite element analyses using commercial code LS-DYNA. The strength and deformability of steel plates with various pit sizes, degrees of pitting intensity, and general corrosion were estimated both experimentally and numerically. The failure strain in relation to the finite element mesh size used in the analyses was clarified. Two different steels having yield ratios of 0.657 and 0.841 were prepared to examine the material effects on corrosion damage. The strength and deformability did not show a clear dependence on the yield ratios of the present two materials, whereas a clear dependence was shown with respect to the surface configuration such as the minimum cross-sectional area of the specimens, the maximum depth of the pit cusp from the mean corrosion diminution level, and pitting patterns. Empirical formulae for the reduction of deformability and the reduction of energy absorption of pitted plates were proposed which may be useful in strength assessment when examining the structural integrity of aged corroded structures.     

Probabilistic modelling of the ultimate strength of ship plates with non-uniform corrosion

This paper presents the results of a parametric study of probabilistic modelling of the ultimate strength of ship plates with non-uniform corrosion represented by random fields. The load-shortening behaviour of the plates with non-uniform reduction of thickness due to corrosion under longitudinal compression is obtained using a general-purpose nonlinear finite element analysis program. A nonlinear time-dependent corrosion model is used to define the probabilistic characteristics of the random fields based on corrosion data measured in plate elements at different locations of bulk carriers. Based on the probabilistic models derived by Monte Carlo simulation, equations to predict the mean and the 5 % characteristic value of the ultimate strength of plates with non-uniform corrosion are developed. Finally a regression equation is proposed to take into account the effect of non-uniform corrosion patterns in the predictions of the ultimate strength of plates with uniform corrosion.     

Statistical characterization of surfaces of corroded steel plates

The statistical characteristics of corroded steel plate surfaces exposed to marine environments are of interest for assessing longer term structural safety and integrity using probabilistic methods. This requires information about the variability of corrosion loss and pitting over surfaces. The present paper reports on the observed statistical character of the surfaces of 10 large (1.2 m × 0.8 m × 3 mm thick) steel plates exposed in temperate climate marine immersion, tidal and splash zones for 2.5 years. For the analysis the plates were cut into smaller segments that were mechanically scanned to obtain digitised surface topographies. These were then analysed to estimate the correlation structure and the standard deviation of the surface topography. Considerable differences were found for these and for the mean corrosion loss between different exposure zones. For any one segment the surface topography was found to be highly statistically dependent, implying that smaller coupon sizes can provide adequate estimates of corrosion loss. From this it may be inferred that the deepest pits are not statistically independent as commonly assumed in extreme value statistical representations.     

联合载荷作用下撑杆结构极限强度研究

超大型浮体结构在复杂海洋环境下承受多轴载荷共同作用,使得刚度较弱的撑杆结构极易发生破坏,从而影响整个浮体的安全和可靠性。文章以超大型浮体撑杆结构为研究对象,基于模型相似理论,进行了压扭联合载荷作用下撑杆结构极限强度试验模型设计,并开展模型试验研究。通过数值仿真方法与试验结果的对比分析,验证了数值仿真方法的正确性,并据此开展实尺度撑杆结构在不同扭转载荷作用下的压缩极限强度数值仿真计算,给出了压扭联合载荷作用下撑杆结构极限承载能力计算简化公式,为撑杆结构设计提供支撑。     

循环载荷下箱型梁极限强度性能实验研究

通过系列箱型梁模型实验,研究了箱型梁在极值循环弯曲载荷下的极限承载性能。分别对四个加筋箱型梁模型进行了循环载荷下的四点纯弯实验,实验分别采取单向及双向循环载荷两种施加方式。在单向循环弯曲实验中,模型的后续循环的极限强度与前一循环的后极限强度阶段的卸载点接近,但塑性变形有明显增加,极限承载能力下降显著;双向循环弯曲中,反向弯矩虽然抵消了部分塑性变形,但箱型梁的极限承载能力仍有明显下降。实验表明,实验加载过程中,箱型梁在承受极值循环载荷初期,其构件崩溃速率较缓,而一旦进入后极限阶段,崩溃速率显著加快;箱型梁在极值循环弯曲载荷下的极限承载性能,即后极限强度性能,相比一次性极限强度值逐步下降。     

船体梁在循环载荷下的极限强度研究

在船体极限强度的研究中,对船体到达极限强度后的剩余承载能力的研究非常重要,其关系到船体结构的生命力设计,用于判断船舶破损情况下是否还能保持一定的自存能力,制定救援方案以及作为船舶在极端情况下安全性的判断依据。目前此类研究多是针对单次加载,而在实际海洋环境中,船舶会受到交变载荷的作用,如果超过弹性范围,会留下塑性应变,这些残留应变会影响船舶最终承载能力的大小。本文以逐步崩溃法为基础,用Fortran语言开发了计算程序,该程序可以得到船体梁在极限强度后的承载能力,同时通过递增塑性法来模拟循环加载带来的影响,并采用非线性有限元法进行对比验证。     

含初始变形船体加筋板有效宽度及剩余极限强度研究

为了评估舰船结构损伤后的剩余强度,对船体加筋板出现初始几何变形后,参与总纵强度的有效宽度和加筋板剩余极限强度进行研究。将加筋板受到垂直于平面压力后的变形,作为其初始几何变形,改变变形的方向和大小,利用有限元软件Ansys对加筋板结构进行线性和非线性分析。定义了板有效宽度计算方法,对不同变形方向和变形幅值时板的有效宽度和加筋板的极限强度进行对比分析,并拟合得到了计算板有效宽度和加筋板极限强度的经验公式。结果表明,初始几何变形会削弱加筋板结构的强度。在对损伤后船体结构强度进行分析和校核时,提出的经验公式可以直接用来计算板的有效宽度和加筋板的极限强度。     

点蚀损伤钢板在空爆冲击波载荷作用下的动态响应研究

为探讨固支点蚀损伤钢板结构在空爆冲击波载荷作用下的动态响应,利用有限元分析软件Ansys/LSDYNA,开展了空爆冲击波对固支点蚀钢板板的毁伤作用数值模拟计算,探讨了点蚀损伤深度、点蚀损伤分布密度及点蚀损伤分布位置对固支钢板结构在空爆冲击波载荷作用下动态响应的影响。结果表明,点蚀损伤深度、点蚀损伤分布密度均对板的动态承载能力有较显著影响,而点蚀损伤分布位置则影响非常小。     

复合材料夹层板架结构在组合载荷作用下的极限强度研究

随着复合材料船舶建造尺寸越来越大,结构极限强度评估具有重要意义。本文基于后屈曲理论,通过渐进失效分析方法对复合材料夹层板架结构在组合载荷作用下的极限强度展开研究。首先通过与相关复合材料层合板试验及数值仿真结果进行对比,验证了本文渐进失效分析方法的准确性。然后,以复合材料夹层板架结构作为船舶上层建筑并考虑其受力特性,对具有初始缺陷且在轴向和侧向压力同时作用下的复杂受力状态的夹层板架结构进行计算,得到夹层板架结构的首层失效强度以及最终承载能力,并对失效位置做出预报。     

水下爆炸载荷作用下舰船结构极限强度研究

在对水下爆炸载荷作用下典型舰船结构损伤研究的基础上,分析了塑性变形和各种破口形状尺寸等受损情况下Nishihara箱形梁的极限强度,得出结论:有破口的箱形梁未必比有塑性变形的极限强度小,若中剖面破口长度相等,则破口面积越大极限强度越小。利用NAPA软件建立典型舰船的模型得出设计载荷并导入MSC.Patran划分网格、定义属性并施加载荷与边界条件,运用MSC.Dytran模拟水下爆炸载荷高瞬态非线性分析,通过MSC.Nastran与工程软件MARS对该模型进行极限强度非线性分析对比,提出了一种对真实爆炸损伤状态下的舰船结构极限强度计算方法,证明其运用于结构设计校核极限强度的有效性和安全性。     

An overview of buckling and ultimate strength of spherical pressure hull under external pressure

The load-carrying capability of spherical shells under external pressure has been the subject of a long history and many theoretical and experimental studies have been carried out. However, from a comparative study on the design rules for the minimum thickness of the deep manned spherical shells from various classification societies, significant differences have been found. This indicates that these design rules need to be updated and unified like Common Structural Rules for tankers and bulk carriers. In order to lay a foundation for this target, a systematic study is carried out to develop a consistent calculation method for predicting the ultimate strength of spherical pressure hull under external pressure. This is the first paper of a series of three for reporting this study and in this paper, a critical review on the buckling and ultimate strength of spherical pressure hulls is carried out and further problems to be studied are identified. This could lay a solid foundation for the further study.     

基于J积分的含裂纹加筋板在单轴拉伸载荷下的极限强度计算

为研究含裂纹加筋板的极限拉伸强度,本文建立一系列不同长细比、不同裂纹长度、不同裂纹位置的含裂纹加筋板有限元模型,并基于J积分理论对其在单轴拉伸载荷下的极限强度进行了计算。结果发现含裂纹加筋板极限拉伸强度随加筋板长细比的增大略有减小,但减小的程度并不明显;含裂纹加筋板极限拉伸强度随裂纹长度的增大而减小,且减小的幅度逐渐增大;加强筋上的裂纹对含裂纹加筋板极限强度的影响小于底板上的裂纹,而裂纹同时出现在底板和加强筋上时对含裂纹加筋板极限拉伸强度的影响最大。表明含贯穿型裂纹的加筋板在单轴拉伸载荷下的剩余强度对加筋板长细比不敏感,而对裂纹长度较为敏感。     

An approximate method to generate average stress–strain curve with the effect of residual stresses for rectangular plates under uniaxial compression in ship structures

The average stress–strain curve for rectangular plates under uniaxial compression in ship hull structures is generated on the basis of the existing design formulae. The dual-term equation for ultimate strength of uniaxially compressed plates is employed to generate the average stress–strain curve for long plates. Two alternatives proposed by Faulkner and Frankland, respectively, are considered. An approximate method to take into account the effect of residual stresses is proposed. By investigating the variation of the stress distribution at different stages of compression, the average stress–strain relationship of the rectangular long plates with the effect of residual stresses is derived from equilibrium. The behaviors of the wide plates are also discussed. The assumption proposed by Valsgard for the idealized stress distribution of wide plates under uniaxial compression is adopted. The average stress–strain relationship for wide plates is derived from the existing design formulae. Two equations proposed by Hughes and recommended by API for critical stress of the central portion of the wide plate are considered. The effect of residual stresses is also taken into account for wide plates.