Effect of low cycle fatigue pre-damage on very high cycle fatigue

Carbon-manganese steel is often applied in components of pipes in nuclear plant. Ultrasonic fatigue tests following low cycle fatigue （LCF） cycles damaged are used to study the strength of very high cycle fatigure （VHCF）. The comparison of test results of simple VHCF and cumulative fatigue （LCF plus VHCF） shows that LCF load influences the following VHCF strength. Continuum damage mechanics model is extended to VHCF region.

Low cycle fatigue behavior of high strength gun steels

The low cycle fatigue (LCF) behavior of two high strength steels, withnominal chemical compositions (mass fraction, %) of 0.40C-1.5Cr-3Ni-0.4Mo-0.2V (PCrNi3MoV) and0.25C-3Cr-3Mo-0.8Ni-0.1Nb (25Cr3Mo3NiNb), was investigated by using the smooth bar specimenssubjected to strained-controlled push-pull loading. It is found that both steels show cyclicsoftening, but 25Cr3Mo3NiNb steel has a lower tendency to cyclic softening. 25Cr3Mo3NiNb steel hashigher fatigue ductility, and its transition fatigue life is almost three times that of PCrNi3MoV.25Cr3Mo3NiNb steel also shows higher LCF life either at a given total strain amplitude above 0.5% orat any given plastic strain amplitude, despite its lower monotonic tensile strength than that ofPCrNi3MoV. It also means that 25Cr3Mo3NiNb steel can endure higher total strain amplitude andplastic strain amplitude at a given number of reversals to failure within 10~4. 25Cr3Mo3NiNb steelis expected to be a good gun steel with high LCF properties because only several thousand firingsare required for gun barrel in most cases.

Low cycle fatigue estimation of low yield point steel

With the development of technology for earthquake resistant,the research of the low yield point(LYP) steel which used for the fabrication of energy dissipation damper were paid more and more attention.The common studies of the low yield point steel is mainly about the performance with constant amplitude and constant frequency.The low cycle fatigue properties of low yield piont steel were studied by series of test with continuous varying amplitude and varying frequency with the materials testing system by us.The test results showed that low yield point steel of Baosteel have excellent low cycle fatigue properties,which meet the requirement for steel used for the fabrication of energy dissipation damper completely.The low cycle fatigue performance of low yield point steel of Baosteel mainly depended on the amplitude in test.And the effect of varying frequency for the low yield point steel was more less than varying amplitude.

Effect of laser shock peening on combined low- and high-cycle fatigue life of casting and forging turbine blades

Laser shock peening （LSP） is a novel effective surface treatment method to improve the fatigue performance of turbine blades. To study the effect of LSP on combined low- and high-cycle fatigue （CCF） life of turbine blades, the CCF tests were conducted at elevated temperatures on two types of full-scale turbine blades, which were made of K403 by casting and GH4133B by forging. Probabilistic analysis was conducted to find out the effect of LSP on fatigue life of those two kinds of blades. The results indicated that LSP extended the CCF life of both casting blades and forging blades obviously, and the effect of LSP on casting blades was more evident; besides, a threshold vibration stress existed for both casting blades and forging blades, and the CCF life tended to be extended by LSP only when the vibration stress was below the threshold vibra- tion stress. Further study of fractography was also conducted, indicating that due to the presence of compressive residual stress and refined grains induced by LSP, the crack initiation sources in LSP blades were obviously less, and the life of LSP blades was also longer; since the compressive residual stress was released by plastic deformation, LSP had no effect or adverse effect on CCF life of blade when the vibration stress of blade was above the threshold vibration stress.

Predicting the Fatigue Life in Steel and Glass Fiber Reinforced Plastics Using Damage Models

Three cumulative damage models are examined for the case of cyclic loading of AISI 6150 steel, S2 glass fibre/epoxy and E glass fibre/epoxy composites. The Palmgren-Miner, Broutman-Sahu and Hashin-Rotem models are compared to determine which of the three gives the most accurate estimation of the fatigue life of the materials tested. In addition, comparison of the fatigue life of the materials shows the superiority of AISI 6150 steel and S2 glass fibre/epoxy at lower mean stresses, and that of steel to the composites at higher mean stresses.

Masing Behavior and Microstructural Change of Quenched and Tempered High-Strength Steel Under Low Cycle Fatigue

Low cycle fatigue behavior of a quenched and tempered high-strength steel(Q960 E) was studied in the strain amplitude ranging from ± 0.5% to ± 1.2% at room temperature. As a result of fatigue loading, the dislocation structural evolution and fracture mechanism were examined and studied by transmission electron microscopy and scanning electron microscopy(SEM). The results showed that this Q960 E steel showed cyclic softening at different strain amplitudes, and the softening tendency was more apparent at strain amplitude of ±(0.6–1.2)% than that at ± 0.5%. The reduction in dislocation density with increasing strain amplitude is responsible for the softening tendency of cyclic stress with the strain amplitude. The material illustrates near-Masing behavior at strain amplitude ranging from ± 0.6% to ± 1.2%. The near-Masing behavior of Q960 E high-strength steel can be the result of stability of martensite lath at different strain amplitudes. Partial transformation from martensite laths to dislocation cells is responsible for the derivation from ideal Masing behavior. In the SEM examination of fracture surfaces, transgranular cracks initiate on the sample surface. Striations can be found during the crack propagation stage.

Effects of Strain Rate on Low Cycle Fatigue Behaviors of High-Strength Structural Steel

The low cycle fatigue （LCF） behavior of a high-strength structural steel was investigated in the strain rate range of 4×10^-6 -0.12 s^-1 （0. 001-3 Hz） under constant total strain （±1%） control. The cyclic stress response at all strain rates exhibited behavior of rapid softening in the early stage of fatigue life and subsequent saturation up to failure. It was found that the stress amplitude, the plastic strain amplitude, the plastic strain energy density and the fatigue life depend mainly on the strain rate. The strain rate of 0. 012 s-1 was found as a transition point where the LCF of the steel showed different behavior from low strain rate to high strain rate. The relationship between the time to failure and strain rate was expressed well by a power law relation. The fracture surfaces of the fatigue sam-ples were characterized by using a scanning electron microscope （SEM） and the fracture mechanisms were discussed in terms of time-dependent deformation of the steel.

Energy-Based Prediction of Low Cycle Fatigue Life of High-Strength Structural Steel

Energy-based models for predicting the low-cycle fatigue life of high-strength structural steels are presented. The models are based on energy dissipation during average of cycles, cycles to crack propagation and total cycles to failure. Plastic strain energy per cycle was determined and found as an important characteristic for initiation and propagation of fatigue cracks for high-strength structural steels. Fatigue strain-life curves were generated using plastic energy dissipation per cycle (loop area) and compared with the Coffin-Manson relation. Low cycle fatigue life was found similar from both methods. The material showed Masing-type behavior. The cyclic hysterisis energy per cycle was calculated from cyclic stress-strain parameters. The fracture surfaces of the fatigue samples were characterized by scanning electron microscope and the fracture mechanisms were discussed.

A damage-coupled multi-axial time-dependent low cycle fatigue failure model for SS304 stainless steel at high temperature

Based on the time-dependent strain cyclic characteristics and fatigue behaviors of SS304 stainless steel under multi-axial cyclic loading at 700 ？ C, and in the frame of unified visoco-plastic cyclic constitutive model and continuum damage mechanics theory, the damage-coupled multi-axial time-dependent constitutive model and fatigue failure model were proposed. In the model, the evolution equation of damage was introduced in and the time-dependent effects, e.g. holding time, loading rate, were taken into account. The model was applied to the simulation of whole-life cyclic deformation behaviors and prediction of LCF life for SS304 stainless steel in multiaxial time-dependent low cycle fatigue tests. It is shown that the simulated results agree well with experimental ones.

Improved Approach to Determine the Material Parameters for a Combined Hardening Model

For the simulation of isothermal mechanically loaded components, it is indispensable to have a material model, which describes the material behavior very accurately. In this case, a combined hardening model was chosen in order to reflect the prevalent deformation behavior. The combined hardening model enables simulation independent of the number of load cycles and the chosen strain amplitude. The main point is the declaration of the parameters from the chosen material model. This work deals with the estimation of the parameters. For validation and as input data of the here defined approach low cycle fatigue (LCF) tests were performed on cast aluminum and at 250°C. The comparison of the test results and the simulations indicated that σmax from the simulated hysteresis lies inside a range of ±5% referred to the test results.

EH36钢焊接接头焊趾处应力集中对高低周复合疲劳的影响

为研究船舶结构用钢EH36钢焊接接头焊趾处应力集中对高低周复合疲劳的影响,对不同焊缝形貌20 mm厚EH36钢双面全熔透对接接头进行高低周复合疲劳试验.研究发现,高低周复合疲劳对接头寿命的降低幅度随着高周应力比的增大而增大;结合数字图像相关法测量结果分析,发现焊趾处应力集中系数越大,接头的高低周复合疲劳寿命越低,其降低幅度随着高周应力比的增大而增大.基于试验数据对模型进行验证,结果表明,应力集中系数高的接头疲劳寿命低且寿命预测结果分散性增大;Palmgren-Miner模型和Zhu模型由于未充分考虑高低周耦合损伤作用,导致寿命预测结果相对危险.修正后Zhu模型在充分考虑低周疲劳对高低周复合疲劳寿命的影响后,预测寿命大于实际寿命的预测点较少,预测结果相对安全,且分散性较小,误差也较小.

热冲击高强钢焊接接头的低周疲劳及断裂机理研究

为了研究热冲击下高强钢焊接接头的低周疲劳性能及断裂机理,用电弧焊焊接Q345结构钢,分别在-20~400℃、-40~650℃、-60~900℃下对焊接接头进行循环冲击,用疲劳试验机加载低周疲劳载荷,分析热冲击温度对低周疲劳性能及疲劳强度的影响,并用扫描电子显微镜等分析焊接接头的断裂机理。结果表明:相同疲劳加载次数下,焊接接头的总应变幅值随热冲击循环次数的增加匀速下降;相同热冲击循环次数下,疲劳加载次数越多,焊接接头的应变幅值越小;相同疲劳加载次数和相同热冲击循环次数下,随冲击温度范围的扩大,焊接接头的总应变幅值逐渐减小,但总应变幅值整体变化与冲击温度范围正相关。

高压涡轮盘螺栓孔疲劳模拟件设计及试验

航空发动机关键件采用模拟件开展试验可在一定程度上替代真实结构试验,从而大幅节省试验周期和成本。为满足某型发动机高压涡轮盘定寿需求,减少真实轮盘低循环疲劳的试验成本,本文采用考虑应力梯度影响的结构特征模拟件设计方法,设计了该发动机高压涡轮盘螺栓孔部位的结构特征模拟件,开展了特征模拟件及轮盘真实结构的低循环疲劳试验;对于模拟件试验结果,应用统计学方法得到了具有一定置信度和可靠度的模拟件寿命和分散度,采用该分散度计算得到的模拟件安全循环寿命和轮盘实际试验结果相比,模拟件安全循环寿命小于轮盘实际低循环疲劳寿命,误差为14.69%,该螺栓孔模拟件设计及低循环疲劳试验研究可用于指导实际轮盘的定寿工作,具有一定的工程参考价值。

基于流场换热的阀壳瞬态温度场分析及强度评估

文章基于浸入边界法,对汽轮机阀壳进行了流场分析,得到了阀壳在不同开度下的换热系数场和温度场,根据机组启停曲线分析了阀壳瞬态温度场,并在循环热载荷和压力载荷下的得到了阀壳在启停过程的低周疲劳和长时运行时的高温蠕变,为研制更加安全、高效的汽轮机阀壳提供了重要理论依据。

Inconel718榫接结构高低周微动疲劳试验研究

航空发动机涡轮叶片在实际服役环境中,受到循环往复的高/低周载荷,这是一种典型的高低周微动疲劳工况。涡轮叶片由于微动疲劳频繁引发疲劳断裂问题,严重影响着航空发动机的安全运行。为了研究热端部件材料的高低周微动疲劳问题,设计一种Inconel718榫连接模拟件,对其开展高低周复合微动疲劳试验。共设计3种不同载荷/频率的试验工况,对比不同工况下模拟件的寿命分布情况。结果表明:高周/低周载荷对榫连接结构的微动疲劳寿命有显著影响,随着高周/低周载荷或者频率的增大,涡轮叶片榫接部位微动疲劳寿命均随之降低;试验结果证明榫接结构高低周微动疲劳试验较好地还原了涡轮叶片实际服役情况;通过显示动力学有限元分析,所预测的裂纹萌生位置与试验结果中裂纹萌生位置分散性较小。

航空液压管路高低周复合疲劳寿命预测研究

为有效预测航空液压管路的疲劳寿命,在分析现有液压管路寿命预测方法不足的基础上,针对飞机液压管路受力特点,分析了液压管路的载荷类型,并对其进行了载荷分解,进而提出了一种基于高低周复合载荷的疲劳寿命预测方法。为验证所提方法的有效性,以某型航空液压管路为例,利用Ansys Workbench软件对其进行仿真分析,在此基础上,对所分析液压管路分别进行了考虑高低周载荷作用和不考虑高低周载荷作用的疲劳寿命预测。对比寿命预测结果发现,高低周复合疲劳载荷对液压管路的寿命影响较大,在预测液压管路的疲劳寿命时,需同时考虑高周疲劳载荷和低周疲劳载荷的作用。

高温氧化对粉末高温合金微观结构和疲劳性能影响研究

为了探究涡轮盘用粉末高温合金表面氧化对其低周疲劳性能的影响,分别针对第三代镍基粉末高温合金的粗晶(Coarse grain,CG)和细晶(Fine grain,FG)材料开展氧化时间对其疲劳性能影响机理的研究。通过在700℃空气环境下开展不同时长的高温预氧化实验和低周疲劳(Low-cycle fatigue,LCF)实验,使用SEM和EDS表征LCF断口、表面氧化层结构成分及其强化相形貌变化,揭示LCF裂纹萌生机理。实验结果表明,氧化层厚度随氧化时间而增加,氧元素以氧化侵入的形式进入基体;相同氧化时间下,CG抗氧化性能优于FG;疲劳裂纹萌生于氧化侵入和亚表面夹杂物等应力集中部位,在实验温度下LCF寿命受氧化作用和夹杂物共同影响;高温氧化作用下氧化层呈现分层结构,外层为NiO,中间层为含有Cr_(2)O_(3),TiO_(2)等复杂氧化物及尖晶石相(NiCr_(2)O_(4))的混合层,内层为Al_(2)O_(3);CG二次γ’相氧化后平均尺寸增加,FG二次γ’相平均尺寸没有明显的变化,但两种组织氧化后在晶界附近均观察到长条状的二次γ’相,表明高温氧化作用下晶界优先遭到破坏。

无纺结构麻纤维复合材料疲劳性能及失效机理

近年来,以可持续性为特征的麻纤维因其在环境可接受性和商业可行性方面的优势而备受关注。以麻纤维代表之一的洋麻毡为增强材料,不饱和聚酯(UP)为基体,采用手工叠层和模压成型的方法制备了洋麻/UP复合材料。设计低周疲劳(低速低循环疲劳)和高周疲劳(高速多循环疲劳测试)来探讨其疲劳性能和疲劳极限,重点观察不同疲劳测试条件下麻纤维的断裂形态和失效机理。

M24高强度螺栓低周疲劳试验与理论研究

为探究M24高强度螺栓低周疲劳破坏机理,对18个20MnTiB材质的10.9级M24高强度螺栓进行了低周疲劳试验研究,并针对典型螺栓试件进行宏观和微观断口分析。另外,基于拟合Coffin-Manson公式和幂指函数公式,探讨了疲劳试验加载频率与螺栓低周疲劳寿命之间的关系,并采用ABAQUS和Fe-Safe软件建立有限元模型,进一步探讨了螺栓低周疲劳断裂机理及其影响因素。结果表明:10.9级M24高强度螺栓低周疲劳破坏模式主要分为脆性断裂和非脆性破坏,且疲劳断裂方式与加载频率无明显相关性;疲劳断口扩展区面积占比较少,瞬断区面积较大且更粗糙;疲劳源形核期短,裂纹形核容易,且断口边缘分布多个疲劳源,造成裂纹扩展过程中产生大量二次裂纹,降低了疲劳裂纹扩展寿命;螺栓内部含碳杂质会促进裂纹的萌生与扩展,降低了螺栓的裂纹萌生寿命;当加载频率为0.2 Hz时,螺栓低周疲劳寿命关系更契合幂指函数公式;当加载频率为0.5 Hz时,Coffin-Manson公式拟合程度更好;数值模拟所得螺栓应力集中位置与低周疲劳试验中试件断裂位置基本吻合;Fe-Safe软件模拟高强度螺栓低周疲劳寿命具有一定可行性。

激光冲击强化对Ti-6Al-4V榫齿结构高低周复合疲劳性能的影响

为了探究激光冲击强化对Ti-6Al-4V合金榫齿结构高低周复合疲劳行为的影响,本文对经过激光冲击强化处理的Ti-6Al-4V榫齿结构的表面完整性特征和高低周复合疲劳性能进行测试。结果表明:相比于未经过激光冲击强化处理的原始榫齿结构,激光冲击榫齿结构的高低周复合疲劳寿命提高729%。激光冲击改变了榫齿结构接触区域的微动磨损特征。原始榫齿结构表现为大尺寸的脱层磨损坑,坑底部诱发微裂纹并向基体内扩展,而激光冲击榫齿结构表现为小尺寸脱层薄片。此外,激光冲击还降低榫齿结构的疲劳裂纹扩展速率。激光冲击强化抗高低周复合疲劳作用显著,这主要归因于激光冲击强化层改变疲劳裂纹萌生方式,并抑制裂纹早期扩展。