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.     

随机海况下的船体结构疲劳裂纹扩展

传统的疲劳问题一般都是采用S—N曲线及Miner累计损伤理论进行的,文章在基于断裂力学的基础上,通过权函数法来计算应力沿裂纹面的非线性效应力强度因子,鉴于船舶在海洋环境中受到载荷的随机性,充分考虑加载次序、过载峰和应力比等对裂纹扩展的影响,给出了船舶结构疲劳裂纹在随机载荷下的扩展寿命的计算过程。     

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.     

Numerical simulations of fatigue crack initiation and propagation based on re-tensile plastic zone generating load criterion for in-plane gusset welded joints

Many accidents are caused by fatigue in welded built-up steel structures, and so it is important to estimate the fatigue lives of such structures quantitatively for safety reasons. By assuming that fatigue cracks cannot grow without an accumulation of alternating tensile/compressional plastic strain, one of the authors identified an improved effective stress intensity factor range ΔK _RPG based on the re-tensile plastic zone generating (RPG) load, which represents the driving force for fatigue cracks, and suggested that ΔK _RPG should be used as the parameter to describe fatigue crack growth behavior. The “FLARP” numerical simulation code in which ΔK _RPG is implemented as the fatigue crack growth parameter, was developed in order to predict fatigue crack initiation and propagation behavior. In this paper, it is demonstrated that FLARP gives accurate estimates for fatigue life by comparing the estimated fatigue crack growth curves and S–N curves with the experimental results for in-plane gusset welded joints, which are used in many welded steel structures. Moreover, the effect of induced bending moment due to the linear misalignment in the out of plane direction on the fatigue strength of in-plane gusset welded joints is investigated through numerical simulations.     

复杂载荷作用下潜艇结构疲劳裂纹扩展预报方法

统一疲劳裂纹扩展模型是课题组在McEvily模型基础上提出来的,它将疲劳裂纹扩展的3个扩展区域统一起来,并能解释更多的疲劳试验现象.本文介绍了统一疲劳裂纹扩展模型的基本表达式.将此模型与焊缝焊趾表面裂纹应力强度因子的计算方法结合起来,探讨复杂载荷作用下潜艇结构疲劳裂纹扩展预报方法.将服从Weibull分布的随机载荷系列编排为升序、降序载荷谱及随机载荷谱,预报潜艇锥柱结合壳焊缝焊趾处表面裂纹在3种载荷谱下的疲劳裂纹扩展情况,并分析随机载荷谱下载荷次序效应及初始裂纹尺寸对疲劳裂纹扩展行为的影响.结果表明,载荷次序效应对潜艇结构疲劳寿命的影响很明显,且合理的确定初始裂纹尺寸对潜艇结构的疲劳寿命预报是非常重要的.     

不同数值积分方式对海洋结构物疲劳裂纹扩展模拟结果的影响

海洋结构物的疲劳寿命预报是当前的研究热点,文章根据第二作者率领的课题组提出的海洋结构物疲劳寿命预报统一方法比较了疲劳裂纹扩展模拟的三种数值积分方法：逐周数值积分法、△N积分法和△a积分法,并分析了每种方法的适用条件。随后,以中心表面裂纹承受单向交变拉伸载荷的平板为例,分别运用三种方法计算了裂纹扩展寿命和最终裂纹尺寸。同时,研究了△N和△a取值不同对结果的影响。最后,综合考虑计算耗时和计算结果精度,给出了△N积分法和△a积分法的建议值,即△N/N≤1.0%,△a≤0.1 mm。     

潜艇锥柱典型节点表面裂纹扩展数值模拟

锥柱结合处是潜艇结构疲劳破坏的热点区域。本文以潜艇锥柱结合壳结构典型节点为研究对象,以断裂力学为理论基础,使用APDL语言对ANSYS软件进行2次开发,分析潜艇耐压壳结构典型节点表面裂纹在交变载荷作用下的扩展过程,并与试验结果进行对比,结果表明本方法可较好地模拟表面裂纹的疲劳扩展。     

A reliability approach to fatigue crack propagation analysis of ship structures in polar regions

Fatigue is a common failure mode in ship structures. For structures with an initial crack, the fatigue crack propagation behavior needs to be considered. The purpose of this study is to establish a procedure for analysis of fatigue crack propagation of ship structures in combination with reliability methods. The stress intensity factor (SIF) and geometry correction factor are calculated by means of finite element analysis. Validation for the SIF calculation is achieved by comparing the computed results with those based on related solutions. Since fatigue damage usually occurs in weld areas, the effect of such components on the fatigue crack propagation behavior was also considered in this work. The Paris law in combination with the Monte Carlo technique are employed for the fatigue crack propagation analysis in this study. Reliability updating based on inspection for cracks is also carried out. A computer program was developed for the purpose of fatigue crack propagation analysis within the framework of reliability methods. An application example of fatigue crack propagation in relation to the hull of the icebreaker Xuelong 2 is presented. The sensitivity of the procedure to key analysis parameters (sample size, initial crack size) is also considered. Finally, the effect of low temperatures on the computed results is also analyzed.     

一种基于累积损伤理论和裂纹尖端弹塑性应力场的裂纹扩展预测方法

文章提出了一种预测疲劳寿命的应力循环叠加和损伤循环叠加的双重叠加方法。该方法依据累积损伤理论,通过分析变幅载荷下裂纹尖端的应力场,采用应力循环叠加的方法反映残余应力的影响;同时利用SWT应力应变和寿命的关系,循环叠加裂尖损伤。该方法能够很好地反映过载时裂纹扩展的迟滞效应,适合变幅载荷下疲劳寿命的评估分析。     

不同的裂纹尺寸对椭圆形裂纹应力强度因子影响的研究

针对平板椭圆形裂纹,提出了椭圆裂纹求解应力强度因子的解析解法,该方法是建立在弹性断裂理论及复变函数理论基础上的。文中针对椭圆裂纹,在具有不同的外荷载,以及不同的椭圆裂纹形状情况下,对应力强度因子进行较为全面的仿真分析。得到了应力强度因子和椭圆裂纹长短轴及所处位置,以及外荷载之间的关系变化的曲线。利用所建立的计算模型,可以非常迅速地求得不同几何参数情况下椭圆裂纹在复杂应力场中沿裂纹周边不同位置的应力强度因子。并对三种不同椭圆裂纹形状情况下的应力强度因子进行了分析和比较。     

Fatigue crack propagation and computational remaining life assessment of ship structures

The fitness for serviceability of structural members of marine structures in which fatigue cracks might be found during in-service inspection is investigated in order to prevent instantaneous failures of ships, as well as a loss of serviceability such as the oil- and/or watertightness of critical compartments. The essential features of fatigue crack propagation and the remaining life assessment are discussed in the first part of the paper, where the effects of weldment, complicated stress distributions including stress biaxialities at three-dimensional structural joints, structural redundancy, and crack curving are found to be of primary importance. The second part of the paper contains a discussion of an advanced numerical simulation method for the remaining life assessment, in which the above-mentioned effects of fatigue crack propagation are taken into account. The simulated crack paths and the fatigue crack propagation lives are found to be in fairly good agreement with the experimental results.     

基于裂纹扩展理论的船体结构疲劳评估

疲劳破坏是船舶结构的主要破坏形式之一。为了保证船舶结构有足够的疲劳强度,各国船级社、船厂等均建立了船舶结构疲劳强度校核规范作为船舶疲劳评估的指导性文件,尽管这些规范均是建立在S-N曲线方法基础上的,但由于S-N曲线方法存在自身无法克服的缺陷(如忽略材料的初始缺陷等),对同一节点进行计算得到的疲劳寿命大相径庭。该文作者在基于裂纹扩展理论的基础之上,给出了一套详细的船体结构疲劳评估方法,并应用此方法对大型船舶结构典型节点的疲劳寿命进行评估,以期能为完善船舶结构疲劳寿命的评估提供参考。     

一种复杂载荷作用下船舶结构疲劳裂纹扩展预报方法

风暴模型是Tomita等提出的用来评估船舶结构疲劳强度的一种随机波浪载荷简化模型,它能表达波浪载荷是与时间相关的随机过程。文中介绍了风暴模型及波浪诱导应力短期分布的基本特征。将风暴模型和裂纹扩展率单一曲线模型及焊趾表面裂纹应力强度因子的计算方法结合起来,探讨了复杂载荷作用下船舶结构疲劳裂纹扩展预报方法。并用权函数法计算了给定残余应力分布的表面裂纹应力强度因子。预报了对接焊接接头焊趾处表面裂纹在风暴波浪载荷作用下的疲劳裂纹扩展行为,结果表明风暴的大小、顺序,初始裂纹尺寸及残余应力对裂纹扩展行为影响明显。合理的风暴模型参数及初始裂纹尺寸的确定对船舶结构的疲劳寿命预报是非常重要的。     

考虑残余应力的加筋板结构疲劳裂纹应力强度因子计算方法研究

文章基于温度梯度法对焊接残余应力分布进行了模拟。通过对有限元网格划分规则的讨论,确定了数值模拟过程中应力强度因子计算的最佳方案,同时,采用Green函数法以及Faulkner模型给出了残余应力作用下应力强度因子的理论值。通过有限元模拟与理论计算的结果比较,验证了常规载荷以及残余应力作用下,含中心裂纹平板以及含初始裂纹加筋板结构应力强度因子计算结果的准确性,建立了考虑残余应力影响的结构裂纹扩展模拟流程。最后,通过拉伸载荷下加筋板裂纹扩展模型试验对数值计算方法进行了验证,为加筋板结构裂纹扩展规律研究以及裂纹扩展寿命预报奠定了基础。     

Numerical study on fatigue crack growth at a web-stiffener of ship structural details by an objected-oriented approach in conjunction with ABAQUS

It is necessary to manage the fatigue crack growth (FCG) once those cracks are detected during in-service inspections. This is particular critical as high strength steels are being used increasingly in ship and offshore structures. In this paper, a simulation program (FCG-System) is developed utilizing the commercial software ABAQUS with its object-oriented programming interface to simulate the fatigue crack path and to compute the corresponding fatigue life. In order to apply FCG-System in large-scale marine structures, the substructure modeling technique is integrated in the system under the consideration of structural details and load shedding during crack growth. Based on the nodal forces and nodal displacements obtained from finite element analysis, a formula for shell elements to compute stress intensity factors is proposed in the view of virtual crack closure technique. Neither special singular elements nor the collapsed element technique is used at the crack tip. The established FCG-System cannot only treat problems with a single crack, but also handle problems with multiple cracks in case of simultaneous but uneven growth. The accuracy and the robustness of FCG-System are demonstrated by two illustrative examples. No stability and convergence difficulties have been encountered in these cases and meanwhile, insensitivity to the mesh size is confirmed. Therefore, the FCG-System developed by authors could be an efficient tool to perform fatigue crack growth analysis on marine structures.     

双轴加载下船底纵骨疲劳寿命试验与数值模拟研究

以实尺度制作了疲劳试验的试件,对45000DWT江海直达散货船船底纵骨与水密及非水密实肋板连接处进行了双轴恒幅循环载荷下的疲劳试验,记录了热点处的疲劳裂纹扩展长度和对应循环次数,获得船底纵骨疲劳特性;根据不同节点处裂纹萌生和扩展情况,得出非水密节点处是疲劳危险处;用动态应变仪记录热点处的应变,根据新版《船体结构疲劳强度指南》中的热点应力法计算热点应力,用试验数据拟合了一条S-N曲线,与指南提供的D曲线进行了对比分析,验证了D曲线的合理性。同时用MSC.Patran软件建立有限元模型,选取2007版《船体结构疲劳强度指南》中名义应力法对应结构形式的F曲线,用MSC.fatigue模块计算了相应节点的总疲劳寿命,将有限元结果和试验结果进行对比,验证了热点应力法的准确性。     

复杂应力场中裂纹疲劳扩展寿命预报

船舶作为一种大型焊接结构,其疲劳热点部位的应力应变场分布很复杂,要预报这些部位的裂纹疲劳扩展寿命,必须解决复杂场中裂纹的应力强度因子计算及其裂纹扩展方向问题.该文对船舶肘板处两种不同原因产生的裂纹的扩展路径、扩展速率进行了研究.裂纹扩展方向用第一主应力准则确定,在裂纹扩展方向上给定不同的裂纹增量,得到不同长度裂纹的复合裂纹等效应力强度因子,并拟合这些计算结果得出船舶肘板的应力强度因子计算式.结合裂纹扩展率单一曲线模型对肘板裂纹扩展寿命进行了预报,预报结果与实验结果符合得较好,说明所采用的方法可行.对建立船舶典型节点的裂纹扩展寿命预报方法有参考价值.     

在随机载荷作用下用断裂力学法估算潜艇耐压壳体的疲劳寿命和裂纹尺寸

潜艇在上浮下潜的过程中承受着反复作用的由外部水压力所产生的随机载荷,耐压壳体面临着疲劳破坏。本文引入了潜艇的疲劳载荷谱,模拟了潜艇随机载荷历程,并对一次载荷历程估算了疲劳寿命;同时模拟了100次载荷历程,分别对疲劳寿命和裂纹尺寸进行了统计,结果表明裂纹的疲劳寿命符合威布尔分布,裂纹扩展尺寸服从对数正态分布。该结论与目前疲劳可靠性分析中使用的对数正态分布模型和威布尔分布模型相一致。     

不同侧支孔布置形式下的船舶系缆力数值模拟*

对于闸底长廊道侧支孔输水系统的水动力学研究,目前主要采用物理模型试验和现场原型观测方法,均需消耗大量的人力和物力;采用方便快捷且不受试验条件及尺度效应影响的数值计算方法的研究成果相对较少。通过闸室船舶系缆力数值模拟对不同侧支孔布置形式下输水系统水动力学特性进行系统性研究,总结分析其布置原则,可为船闸闸底长廊道侧支孔输水系统的设计和优化提供理论依据,并为类似工程提供借鉴,具有较重要的学术价值和实际意义。