Numerical simulation of fatigue crack propagation in heterogeneous geomaterials under varied loads using displacement discontinuity method

Heterogeneous brittle geomaterials are highly susceptible to cyclic loads.They contain inherent flaws and cracks that grow under fatigue loads and lead to failure.This study presents a numerical model for analyzing fatigue in these materials based on the two-dimensional(2D)boundary element method and linear elastic fracture mechanics.The process is formulated by coupling the displacement discontinuity method with the incorporation technique of dissimilar regions and the governing equations of fatigue.The heterogeneous media are assumed to consist of materials with different properties,and the interfaces are assumed to be completely bonded.In addition,the domains include multiple cracks exposed to constant and variable amplitude cyclic loads.The stress intensity factor is a crucial parameter in fatigue analysis,which is determined using the displacement field around crack tips.An incremental crack growth scheme is applied to calculating the fatigue life.The growth rate values are employed to estimate the length of crack extension when there are multiple cracks.The interaction between cracks is considered,which also includes the coalescence phenomenon.Finally,various structures under different cyclic loads are examined to evaluate the accuracy of this method.The results demonstrate the efficiency of the proposed approach in modeling fatigue crack growth and life estimation.The behavior of life curves for the heterogeneous domain was as expected.These curves illustrate the breakpoints caused by utilizing discrete incremental life equations.At these points,the trend of the curves changed with the material properties and fatigue characteristics of the new material around the crack tips.

The Effects of Fatigue Cracks on Fastener Loads during Cyclic Loading and on the Stresses Used for Crack Growth Analysis in Classical Linear Elastic Fracture Mechanics Approaches

High strength threaded fasteners are widely used in the aircraft industry, and service experience shows that for structures where shear loading of the joints is significant, like skin splices, fuselage joints or spar caps-web attachments, more cracks are initiated and grow from the edges of the fastener holes than from features like fillets radii and corners or from large access holes. The main causes of this cracking are the stress concentrations introduced by the fastener holes and by the threaded fasteners themselves, with the most common damage site being at the edge of the fastener holes. Intuitively, it is easy to visualize that after the crack initiation, during the growth stages, some of the load transferred initially by the fastener at the cracked hole will decrease, and it will be shed to the adjacent fasteners that will carry higher loads than in uncracked condition. Using currently available computer software, the method presented in this paper provides a relatively quick and quantitatively defined solution to account for the effects of crack length on the fastener loads transfer, and on the far field and bypass loads at each fastener adjacent to the crack. At each location, these variations are determined from the 3-dimensional distribution of stresses in the joint, and accounting for secondary bending effects and fastener tilt. Two cases of a typical skins lap splice with eight fasteners in a two rows configuration loaded in tension are presented and discussed, one representative for wing or fuselage skins configurations, and the second case representative for cost effective laboratory testing. Each case presents five cracking scenarios, with the cracks growing from approx. 0.03 inch to either the free edge, next hole or both simultaneously.

FATIGUE RELIABILITY DESIGN AND ESTIMATION METHOD FOR VARIABLE LOADING

A method is presented for estimating fatigue reliability under variable loading, which isbased on load cycles-fatigue life interference theory as well as cumulative fatigue damageanalysis. The basic opinion is that for variable loading the increment of failure probability pro-duced by each load cycle is determined by the stress level as well as the damage state at whichthis load cycle applies Contrast to 'conditional reliability-equivalent life methodology'. this meth-od calculates the equivalent cycle numbers between different stress levels according to cumulativefatigue damage rule but not equivalent failure probability.

Effect of fatigue load on the bending tribo-corrosion-fatigue behaviors between the main cable wires

The main cable bent around the saddle of the suspension bridge is subjected to the wind,the vehicle,the bridge’s own weight and the corrosive media.The coupling of the three loads and the environments causes the friction,the corrosion,and the fatigue(tribo-corrosion-fatigue)among the wires inside the main cable.In this paper,a wire bending tribo-corrosion-fatigue test rig was designed and developed.The effect of fatigue load on the bending friction behaviors between the cable wires in ultrapure water and 3.5%NaCl solution was explored.The tribological properties and electrochemical corrosion behaviors under different fatigue loading ranges were investigated.The tribo-corrosion-fatigue interaction between the cable wires was quantitatively characterized,and the mechanism of the interaction was analyzed.The results demonstrate that the increasing fatigue load exacerbates the coupling damage of the cable wires attributed to the enhanced interaction.The findings carry theoretical importance when assessing the main cable’s deterioration and the load-bearing safety of a suspension bridge.

FATIGUE LIFE PREDICTION OF RC BEAMS STRENGTHENED WITH PRESTRESSED CFRP UNDER CYCLIC BENDING LOADS

The application of prestressed carbon reinforced polymer （prestressed CFRP） in reinforced concrete （RC） members can improve the mechanical properties of strengthened structures and strengthening efficiency. This paper proposed a semi-empirical prediction fornmla of fatigue lives of the RC beams strengthened with prestressed CFRP under bending loads. The formula is established based on the fatigue life prediction method of RC beams and fatigue experimental data of non-prestressed CFRP reinforced beams done before. Fatigue effect coefficient of the formula was confirmed by the fatigue experiments of the RC beams strengthened with prestressed carbon fiber laminate （prestressed CFL） under cyclic bending loads. Fatigue lives of the strengthened beams predicted using the formula agreed well with the experimental data.

A general model of fatigue crack growth under variable amplitude loading

A general model of fatigue crack growth(FCG) in ductile alloys under variable amplitude loading is proposed based on the passivation-lancet theory and the crack closure concept.The model can capture the interactions of single cycle overloading and underloading,sequential loading and spectrum loading effectively.Moreover,the retardation effect due to overloads and the acceleration effect due to underloading can be described quantitatively by a transition function of crack opening stress.The fatigue test data in 2024-T351,2024-T3 and 7075-T6 aluminum alloys under different types of variable amplitude loading and spectrum loading are used to validate the general model and the predictions by the general model are in good agreement with the test data.Furthermore,the predictions are also compared with the existing models,including FASTRAN,AFGROW and the state-space model,and the comparison results show that the general model predicts the FCG process more accurately.

Modification of Wirsching’s Model for Fatigue Reliability Analysis

First, Wirsching's model, which is widely employed in fatigue reliability anlysis of marine and offshore structures, is analysed systematically. It is found that the very important random variable A in Wirsching's model can not be directly determined from fatigue experiment because of the irreversibility of fatigue test, and in fact, what Wirsching studied from testing results is not A but a of the statistical Miner's rule. Second, by use of the statistical Miner's rule, a modified Wirsching's model is proposed. Thirdly and more importantly, based on the two-dimensional probabilistic Miner's rule, a new model is established for fatigue reliability analysis of structural components subjected to specified cyclic loading of variable amplitude or stochastic time history. In the end, an example is presented, from which it will be seen that this new model is very convenient to use and feasible to engineering practice.

PROBABILISTIC MINER’S RULE IN FATIGUE RELIABILITY

Firstly, constant amplitude P-S_a-S_m-N_c surface family is established.Secondly, four basic assumptions, i.e., monotonically increasing, non-coupling, separability andnonintersecting of fatigue damage accumulation are proposed from the viewpoint of both damagemechanics and fracture mechanics. Then the individual isodamage D-S_a-S_m-N surface under constantamplitude loading is constructed and the two-dimensional individual Miner's rule is derived.Consequently, the two-dimensional probabilistic Miner's rule (TPMiner) is established and proved fora population subjected to variable amplitude loading. Finally, with successfully experimentverification, TPMiner proves be to very useful and feasible in fatigue reliability theory.

基于等效裂纹法的铝合金T形焊接接头疲劳寿命评估

针对焊接缺陷的多样性、复杂性及随机分散性等特征,提出一种以一组高应力疲劳试验结果作为参考点的等效裂纹法开展焊接结构的疲劳寿命研究。以疲劳试验结果作为参考,假设一个完整板中的初始裂纹等效焊接结构中的缺陷及应力集中效应,基于线弹性断裂力学和Paris公式计算等效裂纹扩展寿命作为焊接结构的疲劳失效寿命。提出基于等效裂纹法的结构疲劳寿命计算流程;开展铝合金T形焊接接头的疲劳试验以及拟合P-S-N曲线;基于T形焊接接头的焊根失效模式采用等效裂纹方法预测T形接头的计算P-S-N曲线,并与试验拟合曲线进行对比,相对误差为10%,验证了该方法的可行性与有效性;最后,通过一组变幅载荷谱疲劳试验与预测P-S-N曲线对比,相对误差均小于15%,同时也验证了该方法对于变幅载荷的适用性,实现了基于一组疲劳试验结果预测全范围T形焊接接头疲劳寿命的评估。以较少的测试和成本实现确定的疲劳寿命,简化了概率疲劳寿命的评估过程,该方法可以推广于其他焊接接头寿命评估中。

岛礁环境对吊耳疲劳性能影响研究

目的 研究吊耳在岛礁环境下的腐蚀特征及其对疲劳性能的影响。方法 通过开展吊耳试验件的岛礁自然环境试验,测试其不同在试验周期的疲劳性能,分析断口形貌、元素含量变化,研究腐蚀对吊耳抗疲劳性能的影响。结果 在1a的岛礁自然环境试验中,吊耳的腐蚀部位主要为表面镀层,并且呈非均匀腐蚀特征。随着试验时间的延长,吊耳平均疲劳断裂循环次数呈下降趋势,疲劳断口的韧性断裂特征减弱,并且氢含量有所增加。吊耳内部存在MnS夹杂物,位于韧窝底部。结论 吊耳表面点蚀坑、内部MnS夹杂物对疲劳载荷作用下裂纹的萌生和生长起到促进作用,降低了其抗疲劳性能,应加强腐蚀防护和质量控制。

柔塔型风力发电机组的转矩控制器设计

风力发电机组(以下简称风电机组)的转矩控制,需要兼顾发电效率与机组载荷,常采用恒转速-变转速-恒转速策略。针对柔塔型风电机组塔架频率低、在变转速区存在共振点,引起塔架疲劳载荷急剧增加的问题,提出一种分段式变转速策略:将传统的变转速区分解成2个变转速区和1个快速穿越区,在边界区通过及时下拉或上拉电磁转矩,使风轮转速快速上升或下降,实现共振区的快速穿越;动态调整PI转矩输出极限值,实现变转速区与恒转速区之间的平滑切换。建立用于转矩控制器设计的动力学模型,根据模型完成控制器参数设计,并对控制器的稳定性做出校验。通过GH Bladed仿真分析和风电现场测试验证,表明设计出的转矩控制器能在变转速区使机组快速穿越塔架共振区,有效避免塔架共振的发生,且在恒转速区能够较好地稳定转速,确保在不增加风电机组疲劳载荷的同时还有足够的稳定裕度,具有较高的工程应用价值。

FGH96高温变幅疲劳载荷试验及损伤累积模型研究

针对涡轮盘用镍基高温合金FGH96,开展两级疲劳载荷试验以及谱载荷试验。试验载荷设计参考了涡轮盘关键点载荷及实测飞行任务谱特征。采用常见损伤累积模型对试验结果进行分析。结果显示,FGH96在低-高载荷次序下循环分数和大于1,在高-低载荷次序下循环分数和小于1,符合常见非线性疲劳损伤累积模型的预期;对于谱载荷的疲劳寿命,由Chaboche非线性疲劳损伤模型与时间分数模型组成的损伤累积模型的预测则偏保守,且具有较高的预测精度,预测结果均在2倍偏差带内。

循环荷载下四筒导管架基础抗弯承载特性试验研究

海上风电基础在25 a服役期内会面临千万次的周期性荷载,为了研究循环荷载对四筒导管架基础抗弯极限承载力的影响,在砂土地基中开展了物理模型试验,介绍了四筒吸力式导管架筒型基础,并对砂土地基进行制备测试,系统分析了循环荷载幅值比、3种不同的变幅加载方式对循环后单向极限承载力的影响,并对循环累积转角进行分析。同时将抗弯承载力与上述变量变化的关系进行了函数拟合。结果表明:循环荷载幅值比和抗弯承载力近似呈正比增长,筒周塑性变形区域的增大会导致循环累积转角呈现非线性增加。无论是增序还是减序方式的加载变幅值循环加载试验,对于超过8万次的长期循环作用来说,海上风力机的抗弯极限承载力的变化并不明显。

Fatigue damage rule of LY12CZ aluminium alloy under sequential biaxial loading

A series of biaxial two-level variable amplitude loading tests are conducted on smooth tubular specimens of LY12CZ alumin- ium alloy. The loading paths of 90° out-of-phase, 45° out-of-phase and 45° in-phase are utilized. The fatigue damage cumulative rules under two-level step loading of three loading paths are analyzed. By introducing a parameter a which is a function of the phase lag angle between the axial and the torsional loading, a new multiaxial nonlinear fatigue damage cumulative model is proposed. The proposed model is evaluated by the experimental aluminium alloy, and multi-level loading of 45 steel. Fatigue lives data for two-level loading, multi-level loading of LY12CZ predicted are within a factor of 2 scatter band.

非对称循环载荷下TA2工业纯钛疲劳损伤行为的预测

对TA2工业纯钛进行应力控制的非对称载荷疲劳试验,研究了不同最大应力(245,253,270,300 MPa)下纯钛的疲劳寿命和断口形貌,分析了非对称循环载荷下的应变演变以及应力-应变响应;基于连续损伤力学理论,建立了基于棘轮应变和总应变能密度的疲劳损伤演变预测模型,对纯钛的疲劳损伤过程进行预测,并与试验结果进行对比。结果表明:随着最大应力的降低,TA2工业纯钛的疲劳寿命延长。当最大应力为245,253 MPa时,塑性韧断区面积较小,断口中出现疲劳裂纹扩展区,失效机制为疲劳开裂及裂纹扩展失效,疲劳应变演变由加载初始阶段、稳定阶段和失效阶段组成,随最大应力的增加,塑性应变能密度以及相同疲劳寿命对应的平均应变均增大。当最大应力为270,300 MPa时,断口主要由纤维状的塑性韧断区组成,失效机制为循环塑性大变形失效,此时疲劳应变演化的稳定阶段不明显,随最大应力的增加,塑性应变能密度增大幅度较大,且当循环次数达到70%疲劳寿命时,270 MPa最大应力下的平均应变较大。基于棘轮应变的疲劳损伤演变预测模型对于疲劳中后期(循环次数不低于40%疲劳寿命)损伤值的预测效果较好,最大相对误差不超过12.5%;基于总应变能密度的疲劳损伤演变预测模型预测的损伤值相对误差基本在23%以内,整体预测稳定性较差。

交变双轴加载下钛合金微动磨损的仿真及试验研究

微动磨损是导致机械连接结构失效的主要形式之一。目前大多数研究都在恒定法向接触载荷工况下开展。然而实际工程中,广泛存在着可变法向载荷和轴向载荷的多轴加载工况,导致目前恒定法向载荷下微动磨损的研究结果不能很好地解释多轴交变加载工况下的微动磨损行为。因此,以钛合金的微动磨损为例,进行了交变双轴加载下微动磨损的仿真分析和试验测试研究。首先,建立了受循环交变双轴加载作用下的微动磨损模型;然后,对不同法向载荷幅值、位移载荷幅值、双轴加载相位差下的微动磨损行为进行了仿真,并根据Q-P曲线分析方法对这些复杂微动情况下的微动磨损机制进行了讨论;最后,在双轴加载的微动磨损试验机上进行了双轴微动磨损试验,验证了该有限元模型磨损形貌的合理性。研究结果表明:交变法向载荷的磨损形貌不同于恒定法向载荷的磨损形貌,交变下法向载荷均值与恒定法向载荷保持不变,交变法向载荷下的磨损深度及宽度均大于恒定法向载荷下的磨损深度及宽度。位移幅值和相位差一定,随着法向载荷幅值的增大,磨损宽度和深度均增加,磨损形貌由“W”型转变为“W+V”型;法向载荷幅值和相位差一定,磨损宽度和深度均随位移幅值的增大而增大;法向载荷幅值和位移幅值一定,改变双轴加载相位差,0°相位差最大磨损深度出现在接触后缘,180°相位差最大磨损深度出现在接触前缘,90°和270°相位差的前、后缘磨损深度极大值接近。

基于弯曲疲劳试验的发动机曲轴疲劳试验设计探讨

为了提高发动机曲轴部件工作的稳定性与可靠性,对某汽车发动机曲轴为试验对象,将其分成多个单拐试样,分别使用电磁谐振加载法与电液伺服加载法进行弯曲疲劳试验。试验结果表明,电液伺服加载法对弯矩的控制更为合理、精准;电磁谐振加载法可以有效缩短试验时间,减小试验成本;不同试验方法下获得的疲劳极限值并不相同,要结合曲轴试验目的选取适宜的试验方法;两种试验方式下曲轴失效形式均是连杆颈辊压槽处出现断裂。由此可以得出:两种加载方式得到的疲劳极限数值偏差幅度为4.4%;发动机曲轴部件中最为薄弱的部分在连杆颈辊压槽处。

抽水蓄能电站顶盖螺栓应力分析

抽水蓄能机组工况变化复杂,顶盖在运行过程中受到复杂的水推力作用,会使顶盖螺栓的应力发生变化,特别是在机组甩负荷过程中,螺栓的受力特性直接关系到机组的运行安全,因此,有必要全面了解顶盖螺栓在典型工况下的应力水平。本文利用电阻应变测量法,在抽水蓄能电站顶盖螺栓的中间截面布置应力测点,测试螺栓在预紧状态、甩50%负荷、甩100%负荷工况下的应力。结果表明,顶盖圆周方向上的螺栓受力不均匀,螺栓在安装后预紧存在损失的现象;机组甩负荷过程中,螺栓受到一定的弯曲载荷,在顶盖合缝面处的螺栓的应力幅值最高。

复杂交变载荷下的微动运行机制影响因素分析

微动问题是导致机械连接结构失效的主要因素之一,目前大多数研究都在恒定法向接触载荷工况下开展。而许多实际工程问题中,存在着可变法向载荷和轴向载荷的多轴加载工况,致使微动运行机制的研究结果无法很好地解释多轴加载工况下的微动行为。为此,对复杂交变载荷下的微动运行机制影响因素进行了分析,以期为双轴加载工况下的微动失效行为提供有力的分析方法。首先,在传统微动疲劳模型的基础上,分析了受循环交变法向载荷和轴向载荷共同作用下的微动摩擦学行为和动力学特征,提出了一种Q-P曲线分析方法;然后,推导了双轴交变载荷参数与微动运行状态之间的函数表达式;最后,讨论了不同的加载参数(法向载荷的均值P m、法向载荷的幅值P a、远端载荷的均值F B,m、远端载荷的幅值F B,a、双轴加载相位差φ和刚度比R sti)对Q-P曲线形状和位置的影响。研究结果表明:比例加载情况下,Q-P曲线为直线函数;非比例加载情况下,Q-P曲线为椭圆函数;加载参数会影响Q-P曲线的形状和位置。Q-P曲线分析方法为复杂交变载荷作用下的微动失效分析提供了有效的表征方法。

Fatigue crack growth behavior of a 170 mm diameter stainless steel straight pipe subjected to combined torsion and bending load

In a nuclear powerplant,the rotary equipment,such as a pump directly fitted with hanger in the piping system,experiences torsional and bending loads.Higher crack growth rate occurs because of this torsional load in addition to the bending load.Hence,it is necessary to study the fatigue behavior of piping components under the influence of combined torsional and bending load.In this study,experimental fatigue life evaluation was conducted on a notched stainless steel SA312 Type 304LN straight pipe having an outer diameter of 170 mm.The experimental crack depth was measured using alternating current potential drop technique.The fatigue life of the stainless steel straight pipe was predicted using experiments,Delale and Erdogan method,and area-averaged root mean square-stress intensity factor approach at the deepest and surface points of the notch.Afterward,the fatigue crack growth and crack pattern were discussed.As a result,fatigue crack growth predicted using analytical methods are in good agreement with experimental results.