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.

Effect of Rare Earth Elements on Low Frequency Fatigue of Hot Rolled Steel 60CrMnMo at High Temperature

The effect of rare earth(RE) on low frequency high temperature fatigue strength of hot rolled steel 60CrMnMo was investigated. The results show that by adding a certain amount of RE in steel 60CrMnMo, its serviceable life during low frequency fatigue test at high temperature can be improved.The number of cycles for fracture was increased by 18.5%～28.8%. The number of large inclusion particleswas reduced and the composition of inclusions and their shape were changed as well. The strip distances on fatigue fracture surface were decreased. It is concluded that the optimum amount of RE addtion is about 0.05 wt%～0. 10 wt%.

A PROBABILISTIC ASSESSMENT OF TEMPERATURE EFFECTSON THE LOW CYCLE FATIGUE BEHAVIOUR OF 1Cr_(18)Ni_9Ti STEEL WELD METAL

Based on the test results obtained from the single-step test and the incremental-step test at room temperature and 240℃, a probabilistic assessment of temperature effects on the cyclic stress-strum response and the fatigue life of 1Cr18Ni9Ti steel weld metal is performed. In orber to assess the temperature effect on cyclic stress amplitude where there is a scatter of the material cyclic constitution, a probabilistic assessment approach on the basis of probabilistic modified Ramberg-Osgood relations is introduced.The investigation shows that the cyclic stress amplitude and the scatter of cyclic stress amplitude data are decreased at 240℃. Similarly, from the consideration of the fatigue life scatter a probabilistic assessment of temperature effect on the fatigue life is suggested on the basis of probabilistic Langer S-N relations. The investigation shows that the crack initiation life is increased and the scatter of crack initiation life data is decreased at 240℃.

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.

EFFECT OF MICROSTRUCTURE OF ALUMINIDE COATING ON HIGH-CYCLE FATIGUE PERFORMANCE OF COATING-SUPERALLOY SYSTEM

The high-cycle fatigue performance of different microstructures of aluminide coating- superalloy system has been studied at 900℃.The single phase coating of coarse equiaxial grain NiAI(β)has unfavorable eJfect onJatigue life of the coating-superalloy.The fatigue life may shorten if the coating of NiAl(β)was an enrichment of coarse refractory metal grains. While an improvement can be made by dispersing numerous secondary phase particles such as extreme.fine γ′,quasi-σ-phase and others.

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.

Investigation on LCF Behavior of Welded Joint at Di erent Temperatures for Bainite Steel

The current research of low cycle fatigue(LCF) is mainly focused on the components with uniform microstructure.Compared with these typical components, LCF behavior of welded components are more complex due to their great gradient microstructure, especially for di erent temperature. In this paper, LCF properties were conducted on the welded joint at di erent temperatures for bainite steel, and the failure mechanism was systematically discussed.Fatigue parameters derived from fitting curves indicated that welded joint had worse plastic deformation resistance and experienced more significantly strain hardening e ect at 300 °C. The joint failed in the weld metal at room temperature, which attributed to the softening in weld metal combined with cyclic strain hardening e ect in heata ected zone, which meant the joint was more sensitive with the hardness at this condition. When it came to 300 °C,more cracks appeared near to HAZ and the heterogeneous distributed surface inclusion was responsible for the fracture transition to HAZ adjacent to bainite steel rather than the softest zone in HAZ, reflecting the joint was more sensitive with the surface inclusion at 300 °C. This research could support the design on loading of welded component at di erent temperature, and further ensure the safe operation.

Fatigue Property of Basalt Fiber-Modified Asphalt Mixture under Complicated Environment

The fatigue property of asphalt mixtures under complicated environment （low-temperature bending performance, chloride penetration, freezing-thawing cycle and their coupling effect） and the improvement effect for relevant property of basalt fiber-reinforcing asphalt mixture under complicated environment are studied. Two grading types of asphalt mixtures, AC-16I and AC-13I, are chosen, whose optimum asphalt-aggregate ratio and optimum dosage of basalt fiber are determined by the Marshall test. The standard specimens are made firstly, and then the low temperature bending tests of asphalt mixture and basalt fiber-reinforced asphalt mixture under the coupling effect of the chloride erosion and freezing-thawing cycle have been carried out. Finally, the fatigue property tests of asphalt mixture and basalt fiber-reinforced asphalt mixture under complex environment are performed on MTS material testing system. The results indicate that the tensile strength, the maximum curving tensile stress, the curving stiffness modulus, and fatigue properties of asphalt mixture are influenced by the coupling effect of the chloride erosion and freezing-thawing cycle. The low-temperature bending performance and fatigue property of asphalt mixtures under complicated environment can be greatly improved by adding moderate basalt fiber. The dense gradation asphalt mixture possesses stronger ability to resist adverse environmental effects under the same condition.

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.

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.

温度和应变幅对亚稳奥氏体不锈钢S321低周疲劳行为的影响

在室温和650℃的环境下对S321、S321H不锈钢进行了0.5%、0.7%、0.9%应变幅下的低周疲劳试验,并在室温下对S321和S321H开展了多个寿命点的疲劳加载历史试验,结合X射线衍射定量分析测算形变诱发马氏体的含量。结果表明:形变诱发马氏体的生成会导致奥氏体钢持续循环硬化,室温下合金在初次硬化后继续二次硬化,而在650℃下没有形变诱发马氏体生成,合金在初次硬化后进入循环稳定阶段。循环过程中,碳含量越低,形变诱发马氏体含量越多,合金的循环硬化程度更高。两种温度下,两种材料的疲劳寿命在低应变幅下都相差不大,应变幅越大,两种材料的疲劳寿命相差越大,因此,可以考虑服役条件为低应变幅时使用S321不锈钢代替S321H不锈钢。

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。

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.

High-temperature failure mechanism and defect sensitivity of TC17 titanium alloy in high cycle fatigue

Crack initiation is an essential stage of fatigue process due to its direct effect on fatigue failure.However,for titanium alloys in high-temperature high cycle fatigue(HCF),the crack initiation mechanisms remain unclear and the understanding for the defect sensitivity is also lacking.In this study,a series of fatigue tests and multi-scale microstructure characterizations were conducted to explore the high-temperature failure mechanism,and the coupled effect of temperature and defect on TC17 titanium alloy in HCF.It was found that an oxygen-rich layer(ORL)was produced at specimen surface at elevated temperatures,and brittle fracture of ORL at surface played a critical role for surface crack initiation in HCF.Besides,internal crack initiation with nanograins at high temperatures was a novel finding for the titanium alloy.Based on energy dispersive spectroscopy,electron backscatter diffraction and transmission electron microscope characterizations,the competition between surface and internal crack initiations at high temperatures was related to ORL at surface and dislocation resistance in inner microstructure.The fatigue strengths of smooth specimens decreased at elevated temperatures due to the lower dislocation resistance.While the fatigue strengths of the specimens with defect were not very sensitive to the temperatures.Finally,a fatigue strength model considering the coupled effect of temperature and defect was proposed for TC17titanium alloy.

Corporate Effects of Temperature and Strain Range on the Low Cycle Fatigue Life of a Single-Crystal Superalloy DD10

Low cycle fatigue behavior of a nickel-based single-crystal superalloy DD10 was investigated at 760 and 980 ℃ under different strain ranges. Results show that the fatigue life （Nf） of DD10 alloy exhibits different temperature dependence under various strain ranges. Under low strain range, the alloy exhibits a longer Nf at 760 ℃ than that at 980 ℃. However, under high strain range, a reverse result is obtained. This difference can be attributed to the change of dominant damage modes under various test conditions, which is manifested in different modes of crack initiation （crack nucleation and its early propagation）. At 760 ℃, the crack initiates at pores in subsurface due to local stress concentration. This process is mainly controlled by plastic amplitude and plastic property, but not affected by oxygen-induced damage before the crack propagates to the surface. At 980 ℃, the crack initiates at surface instead of pores due to the more homogeneous plastic deformation and the disharmony between the external oxidation layer and the bulk material when the strain amplitude is high. At that temperature, the process is mainly controlled by oxidation damage and strain amplitude simultaneously. Therefore, under high strain range, the crack initiation is much easier at 760 ℃ due to plastic deformation and the poor plasticity, while under low strain range obvious oxidation damage at 980℃ may accelerate the crack initiation.

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.

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.

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.

低熔点合金高低温循环浸渍杨木的性能及机理研究

利用气-液相变自驱动原理,选用Sn-Bi-Pb低熔点合金(LMPA),通过木材含水率的调控及采用高低温交替循环浸渍法,在不破坏木材本身结构的条件下制备了金属化杨木,探讨了低熔点合金在木材中的渗透机理,并分析了金属化杨木的增重率、吸水率、顺纹抗压强度及导热性能。结果表明:提高木材的含水率并在高低温循环条件下浸渍有助于LMPA渗透到木材导管中,当杨木的含水率为60%时,其增重率高达52.47%;浸泡144 h后的吸水率比对照样减小了78.4%,顺纹抗压强度提高了33.75%,导热系数是对照样的2.6倍,在地暖地板领域显示出较好的应用前景。

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。