张力腿系缆系统涡激振动疲劳可靠性分析

张力腿平台的系缆系统在波流的联合作用下非常容易发生疲劳破坏.文章通过多项Galerkin方法对张力腿系缆系统的涡激振动方程进行求解,获得了系统的非线性动力响应.并综合考虑疲劳评估中相关参数和程序的不确定性,运用断裂力学的方法对系缆系统进行疲劳可靠性评估.文中还分别运用FORM和蒙特卡罗仿真对具体算例进行了计算.     

Second-order perturbation added mass fluctuation on vertical vibration of tension leg platforms

Total motion in waves can be considered as a superposition of the motion of the body in still water and the forces on the restrained body. In this study the effect of added mass fluctuation on vertical vibration of tension leg platforms (TLP) in the case of vibration in still water for both free and forced vibration subjected to axial load at the top of the leg is presented. This effect is more important when the amplitude of vibration is large. Also this is important in fatigue life study of tethers. The structural model used here is very simple. Perturbation method is used to formulate and solve the problem. First- and second-order perturbations are used to solve the free and forced vibrations, respectively.     

张力腿平台拉索的疲劳损伤

目前在分析张力腿平台(TLP)疲劳损伤时,通常限定平台只发生小位移,未考虑不利工况下平台发生有限位移的情况.在分析模型中考虑有限大位移后,会引入多种非线性因素,也会使TLP的动力响应明显不同于小位移情况,因此研究TLP有限位移下非线性波浪载荷引起的疲劳损伤十分重要.首先求得随机海浪作用下TLP拉索应力的时间历程曲线,并采用雨流法对其进行计数得到疲劳载荷谱;然后依据Miner累积损伤模型,求得TLP拉索在不利工况下的疲劳损伤;最后对ISSCTLP平台拉索在某海况下进行了疲劳损伤短期计算,并说明TLP拉索疲劳可靠度的计算方法.     

Numerical simulation of fatigue crack propagation under biaxial tensile loadings with phase differences

Fatigue crack propagation under the biaxial tensile loading, which loading directions are normal and parallel to the initial crack position, is highlighted in this study. Most of in-service structures and vessels are subjected to many types of loading. Generally, these loadings have different axial components with different phases. However, the structural integrities of structures and vessels are evaluated according to design codes based on theoretical and experimental investigations under a uniaxial loading condition. Most of these codes are based on the S–N curves approach. An approach that does not use S–N curves has been favored by researchers, with the fracture mechanics approach preferred for evaluating the fatigue life of structures. An advanced fracture mechanics approach was developed based on the Re-tensile Plastic zone Generating (RPG) stress criterion for fatigue crack propagation. In this study, fatigue crack propagation tests under biaxial loading with six different phase and loading conditions are performed and the effect of the phase difference under biaxial loading is evaluated. A numerical simulation method of fatigue crack propagation based on the RPG stress criterion under different biaxial loading phase conditions is presented and compared to measured data.     

功率谱密度法的随机疲劳试验研究

本文讨论了随机疲劳载荷谱的设计和修正,随机疲劳试验的过程,实测随机数据的采集、计数和统计分析。介绍了功率谱密度法随机疲劳试验的实施。该方法包含的载荷成分比较全面,载荷的分布规律更接近于实际,其分析结果对预估疲劳寿命有更高的精度。     

Simulation-based fatigue crack management of ship structural details applied to longitudinal and transverse connections

It is of continuing importance for ship structural design to establish a system to compute the growth behavior of fatigue cracks propagating in structural details. In the present paper, a simulation program is developed for multiple fatigue cracks propagating in a three-dimensional stiffened panel structure, where it can predict fatigue crack lives and paths by taking into account the interaction of multiple cracks, load shedding during crack propagation and welding residual stress. Various fatigue crack propagations in longitudinal stiffeners of ship structures are investigated by both the present simulation method and experiments. From these results, it is found that the crack propagation may considerably change, depending on the loading conditions, structural details and residual stress distributions. This means that one could possibly manage to avoid fatal damage of the skin-plate by properly designing the structural details. Furthermore, these results may imply a possibility to realize a rational fatigue crack management if one can estimate the fatigue crack-propagation behavior during the ship lifecycle. The present simulation program may offer a useful numerical tool for this purpose.     

A state-of-the-art review on fatigue life prediction methods for metal structures

 Metals are the most widely used materials in engineering structures, and one of the most common failure modes of metal structures is fatigue failure. Although metal fatigue has been studied for more than 160 years, many problems still remain unsolved. In this article, a state-of-the-art review of metal fatigue is carried out, with particular emphasis on the latest developments in fatigue life prediction methods. All factors which affect the fatigue life of metal structures are grouped into four categories: material, structure, loading, and environment. The effects of these factors on fatigue behavior are also addressed. Finally, potential problems to be resolved in the near future are pointed out. Received: January 7, 2002 / Accepted: March 25, 2002     

近海结构疲劳计算中波浪与结构建模探讨

文章根据作者已提出的基于海况频度分布的统计分类及非线性动力学计算的疲劳评估方法,通过进一步改进波浪能的输入,提出了一种更为有效的计算方法。通过将这一方法运用到一典型导管架结构的疲劳计算,文中首先研究了波浪方向对疲劳破坏的影响,进而分析了波浪谱中若干重要参数及HHT-α时间积分中步长对疲劳计算结果的影响。通过对结构反应的统计研究,发现海况的恶劣程度并不直接影响结构响应非高斯趋势的显著程度。文中提出的方法在减少不确定性的同时又不降低安全标准,有效提高了疲劳计算的精确度,从而可以降低相关近海结构建设和维护成本。文中的计算方法已被成功地应用于北海及墨西哥湾数座近海结构在风和波浪荷载作用下的疲劳评估。     

A simplified life estimation method for the spherical hull of deep manned submersibles

Creep and fatigue are involved in the loading of deep manned submersible, which is a rather complex variable amplitude pattern. Dwell effects resulting in lower life than pure fatigue are observed in the prior experimental research while no proper prediction methods are available to explain the phenomenon. Recently, the authors proposed a modified crack growth rate model to explain the creep effect under the cyclic loading which is validated for the crack growth of some titanium alloys under cyclic creep loading, however, its application is restricted to the condition where lots of parameters have to be determined based on many experiments and it is not very convenient in the material selection and primary design stage of a component. In this paper, a simplified prediction model for the load pattern of constant amplitude cyclic loading is proposed aiming at one of the most applicable materials for the pressure hull of submersibles, Ti–6Al–4V ELI. The method can be easily used to estimate the life of the pressure hull based on two series of basic mechanical properties of the material and validated by the modified crack growth rate model with parameters determined by large amounts of experimental data.     

深海张力腿非线性动力学分析

深海复杂载荷易引发张力腿涡激振动。考虑涡激载荷、流体阻尼力、预张力和平台运动等耦合作用,根据Hamilton原理建立张力腿空间非线性动力学方程,然后采用有限差分法进行数值求解。考虑南海海况,讨论典型张力腿平台动力响应特点,通过研究可为深海平台的设计和安全运行提供技术参考。     

Multi-scale modeling of fatigue crack propagation applied to random sequence of clustered loading

Fatigue crack propagation in marine structures is obviously governed by mechanics of the considerably different four levels of multi-scale problems. Problems of structural response to environmental loads have length scale of several hundred meters, whereas possible detectable size of cracks from initial defects in a weld is of the order of millimeters. Once a fatigue crack initiates, crack tip plasticity is of the order of several grain sizes, while the resulting fatigue crack growth in each load cycle is of the order of nanometers. In our previous work, the first author and their associates have developed the so-called CP-System, which can treat the first two multi-level problems as an integrated system. Furthermore, we have incorporated the third level of mechanics by using the stress intensity range corresponding to the repeated tensile plastic deformation ahead of the crack tip. In the present paper, we shall discuss a more rational integral equation-based formulation in order to integrate the third and fourth levels of micro-mechanics to the first two levels of continuum mechanics.The method is then applied to fatigue crack propagation under the effects of random sequence of clustered loading. As an example of the random sequence of clustered load, we shall use the so-called “storm model”. In the crack propagation simulation, we have to take into account of the plastic wake on the crack surfaces, whose thicknesses are influenced by the material parameters involved in the crack growth model. These parameters are first identified by the fatigue tests under combined constant and random loading using a CT specimen. Then, fatigue crack growth is investigated by numerical simulation and fatigue tests for various random sequences of clustered loading. The experimental and numerical results agree quite well with each other, and fatigue crack propagation is found to be considerably retarded under random sequence loading, so that the conventional equivalent stress approach may provide rather conservative results to the real seaway loading.     

Experimental and numerical investigation of fatigue damage due to wave-induced vibrations in a containership in head seas

Fatigue cracks have been known to occur in welded ships for several decades. For large ocean-going ships wave-induced vibrations can, depending on trade and design, cause up to 50% of the fatigue damage. The vibrations may be due to springing and whipping effects. In this paper, we address the fatigue damage caused by wave-induced vibrations in a containership of newer design trading in the North Atlantic. The fatigue damage was obtained both experimentally and numerically. The experimental results were found from tests performed with a flexible model of the ship, while the numerical predictions were done using nonlinear hydroelastic strip theory. The measurements showed that the wave-induced vibrations contributed approximately 40% of the total fatigue damage. The numerical method predicted the wave frequency damage well, but was found to overestimate the total fatigue damage by 50%. This was mainly due to an overprediction of the wave-induced vibrations. The discrepancy is partly related to three-dimensional (3D) effects which are not included in the two-dimensional (2D) slamming calculation, and partly to an overprediction of the springing contribution. Moreover, the numerical method does not account for the steady wave due to forward speed. By using a simplified approach we show that high-frequency damage can be significantly reduced by including the steady wave for the relevant vessel, implying better agreement with the experimental results.     

The subjective measurement of seafarers’ fatigue levels and mental symptoms

Human error is the most important factor causing many ship accidents in maritime industry despite advanced technology and international regulations. Fatigue in seafarers is a well-known problem and a serious cause of ship accidents. There are many factors unique to the marine environment raising the potential for fatigue at sea. Due to the difficulties in measuring human fatigue and also in suggesting fatigue to be a root cause of accident, it is important to devise methods to detect and quantify the fatigue and mental symptoms. In this study, ‘Piper Fatigue Scale’ (PFS) has been used for measuring fatigue level and ‘Symptom Checklist 90- Revised’ (SCL-90-R) for detecting the severity of mental symptoms. Data analyses were performed using the SPSS (Statistical Package for the Social Sciences) software. According to the results of PFS analysis, a slight degree of fatigue is detected in all sub-dimensions of the scale. According to the results of SCL-90-R analysis, the distress of mental symptoms perceived by seafarers is not generally highly detected. In conclusion, the purpose of this study is to determine, by using subjective measurements, the fatigue level and mental symptoms among seafarers caused by working conditions on-board.     

大型散货船结构节点疲劳寿命的分析计算

根据应力幅长期分布的Ｗｅｉｂｕｌｌ分布假设及结构材料疲劳的Ｓ－Ｎ曲线和线性累积损伤Ｍｉｎｅｒ准则，使用数值分析与解析相结合的混合法对３４０００ＤＷＴ大潮型散货船结构进行了分析计算，结果令人满意，证实本文介绍的方法解决船体结构在疲劳载荷作用下疲劳寿命估算的实用方法。     

Fatigue tests on grouted connection segment specimens in offshore wind turbine structures considering water ingression

Grouted connections (GCs) are widely used to connect superstructures and driven piles in offshore wind turbine structures. They resist fatigue loading in marine splash zones and even submerged environments. In this paper, six GC segment specimens were designed and tested under fatigue loading in both the air and water ingression conditions. The results in the air condition showed that for the specimens with lower loading ranges, the strain distributions and residual displacements stabilized after 20 thousand load cycles. These conditions persisted until the end of the test with two million cycles, with only a few tiny cracks appearing on the grout material surface. Meanwhile, wide cracks and grout material exfoliation were found in specimens with higher loading ranges. The residual displacement accumulated gradually, which eventually caused the termination of the test when it reached 5 mm after 1.3 million cycles. The results in the water ingression condition showed that the water had entered into the micro-cracks of the grout material, which severely degraded the fatigue behavior of the GC specimens. Even in lower loading ranges, specimens W-1 and W-2 only endured 0.264 million and 64 thousand load cycles before the displacement of the top clamp reached −10 mm. Compared with two specimens tested in the air, with a total displacement of less than −0.7 mm after two million load cycles, the severe deteriorating effect of water ingression on the fatigue behavior of GCs was apparent.     

Fracture mechanics analysis for mooring chain links subjected to out-of-plane bending (OPB)

A fracture mechanics (FM) based investigation on the mechanism of out-of-plane bending (OPB) between mooring chain links and its effects on fatigue lives of mooring chain links are conducted. Four types of OPB problems that mooring chain links laying on the chain wheel, chain links passing over the bending shoe, chain links constraint provided by the chain hawse, and chain links constraint provided by the chain stopper are considered. Tension ranges of mooring lines are calculated based on the combined loading process induced by the motions of wave frequency (WF) and low frequency (LF). Initial cracks are assumed to propagate from surfaces of chain links and stress intensity factors are calculated in terms of stress ranges determined by a finite element (FE) analysis. The results show that fatigue lives of mooring chain links are decreased significantly due to OPB effects. In addition, the increase of the number of pockets of chain wheel mitigates OPB effects on fatigue lives of mooring chain links laying on the chain wheel, and the increase of the track diameter would reduce OPB effects on fatigue lives of mooring chain links passing over the bending shoe as well. However, for chain links constraint provided by the chain hawse, the diameter of mooring chain hawse has no significant effect on fatigue lives of the mooring chain links subject to OPB if without the abrupt change of the contact conditions between chain links and chain hawse. For mooring links constraint provided by chain stoppers considering the effect of proof loading test, fatigue lives of mooring chain links would drop significantly with the increase of interlink angles and friction coefficient.     

A regression and beam theory based approach for fatigue assessment of containership structures including bending and torsion contributions

Container shipping has been expanding dramatically during the last decade. Due to their special structural characteristics, such as the wide breadth and large hatch openings, horizontal bending and torsion play an important role to the fatigue safety of containerships. In this study the fatigue contributions from vertical bending, horizontal bending and torsion are investigated using full-scale measurements of strain records on two containerships. Further, these contributions are compared to results from direct calculations where a nonlinear 3D panel method is used to compute wave loads in time domain. It is concluded that both bending and torsion have significant impacts on the fatigue assessment of containerships. The stresses caused by these loads could be correctly computed by full-ship finite element analysis. However, this requires large computational effort, since for fatigue assessment purposes the FE analysis needs to be carried out for all encountered sea states and operational conditions with sufficient time steps for each condition. In this paper, a new procedure is proposed to run the structure finite element analysis under only one sea condition for only a few time steps. Then, these results are used to obtain a relationship between wave loads and structural stresses through a linear regression analysis. This relation can be further used to compute stresses for arbitrary sea states and operational conditions using the computed wave loads (bending and torsion moments) as input. Based on this proposed method for structure stress analysis, an efficient procedure is formulated and found to be in very good agreement with the full-ship finite element analysis. In addition it is several orders of magnitude more time efficient for fatigue assessment of containership structures.     

随机载荷作用下的疲劳裂纹扩展

船舶等许多工程结构在服役过程中的受载荷历程是一个随机过程.而变幅载荷下的载荷相互作用对疲劳裂纹扩展寿命将产生显著的影响.因此研究随机载荷作用下的裂纹扩展及定量计算对船舶结构的疲劳寿命预测的可靠性是十分重要的.该文提出了一个基于有效应力强度因子,以应力比和裂纹尖端塑性区尺寸为主要参数的随机载荷作用下疲劳寿命预测模型.该模型用于预测几种载荷谱作用下的裂纹扩展试验,结果表明预测结构和实验结果符合得很好.     

A computational approach for fatigue crack propagation in ship structures under random sequence of clustered loading

The authors have developed a simulation program, CP-System, for multiple cracks propagating in a three-dimensional stiffened panel structure, where through-the-thickness crack propagation is formulated as a two-dimensional in-plane problem, and the crack propagation behavior is simulated by step-by-step finite element analyses. In order to evaluate the fatigue lives of marine structures accurately, it is necessary to take into account the load histories induced by sea waves, which may be composed of a random sequence of certain clustered loads with variable stress range. In the proposed crack growth model, the crack opening and closure behavior is simulated by using the modified strip yielding model, and the effective tensile plastic stress intensity range, ΔK _RP, is calculated by considering the contact of plastic wake along the crack surfaces. The adequacy of the proposed crack growth model is examined by comparison with fatigue tests under non-constant-amplitude loading. The usefulness of the developed method is demonstrated for a ship structural detail under certain simulated load sequences. It is shown that the fatigue crack growth of a ship structure is significantly retarded due to the load interaction effects, so that the conventional method for fatigue life assessment may predict a relatively conservative fatigue life of a structure.     

基于改进子模型技术的TLP平台疲劳分析

研究适用于TLP平台的疲劳分析方法。首先建立TLP平台的整体结构模型,运用基于累积损伤理论的疲劳谱分析法,根据整体分析初步确定TLP平台疲劳关键节点位置,即疲劳敏感部位。然后建立疲劳敏感部位的中间模型与精细子模型,运用改进的子模型技术分析计算得到关键节点的热点应力,并根据疲劳谱分析法计算关键节点的疲劳寿命,并将之与传统子模型技术的计算结果比较。分析表明,改进的子模型技术比传统方法的应力计算结果更加精确,有利于得到更加真实的疲劳寿命计算结果。