Notch effects on high-cycle fatigue properties of Ti-6Al-4V ELI alloy at cryogenic temperatures

Notch effects on the high-cycle fatigue properties of the forged Ti-6Al-4V ELI alloy at cryogenic temperatures were investigated. Also, the high-cycle fatigue data were compared with the rolled Ti-5Al-2.5Sn ELI alloy. The one million cycles fatigue strength (FS) of the smooth specimen for the forged Ti-6Al-4V ELI alloy increased with a decrease of test temperature. However, the FS of each notched specimen at 4 K were lower than those at 77 K. On the other hand, the FS of the smooth and the notched specimens for the forged Ti-6Al-4V ELI alloy at 4 K were lower than those for the rolled Ti-5Al-2.5Sn ELI alloy. This is considered to be the early initiation of the fatigue crack in the forged Ti-6Al-4V ELI alloy compares with the forged Ti-5Al-2.5Sn ELI.     

The cyclic strain resistance, fatigue life and final fracture behavior of magnesium alloys

This paper summarizes the results of a comprehensive study on the cyclic strain resistance, low-cycle fatigue life and fracture behavior of three rapidly solidification processed magnesium alloys. Test specimens of the magnesium alloy were cyclically deformed under fully-reversed total strain amplitude control straining, over a range of strain amplitudes, giving less than 10⁴ cycles to failure. The cyclic stress response characteristics, strain resistance and low-cycle fatigue life of the alloys are discussed in light of alloy composition. All three alloys follow the Basquin and Coffin-Manson strain relationships, and exhibit a single slope for the variation of cyclic elastic and cyclic plastic strain amplitude with reversals-to-fatigue failure. The cyclic stress response characteristics, fatigue life and final fracture behavior of the alloy are discussed in light of competing and synergistic influences of cyclic total strain amplitude, response stress, intrinsic microstructural effects and dislocation-microstructural feature interactions during fully-reserved strain cycling.     

MICROSTRUCTURAL EFFECT ON LOW CYCLE FATIGUE BEHAVIOUR IN Ti-ALLOYS UNDER BIAXIAL LOADING

Abstract— Low cycle fatigue tests under axial, torsional and combined axial-torsional loading were conducted using thin-wall tubular specimens of Ti-6A1–4V titanium alloys. Two kinds of alloys with different microstructures, the (α+β) and β alloys, were investigated in fatigue tests at room temperature. When the failure life was correlated with the equivalent plastic strain, the life in axial loading shifted toward the lower life region compared with those in other loading modes in both alloys. Dominant surface cracks propagated in mode I under axial and combined loading in the two alloys. Although growth by the mode II type was predominant under torsional loading, the growth direction of the main crack coincided with the specimen axis in the (α+β) alloy, but the circumferential direction in the β alloy. The cracking morphology depended on the microstructure, especially under the torsional mode of loading, and was simulated successfully by using the proposed model for crack initiation.     

Al-5.4Zn-2.6Mg-1.4Cu合金板材的低周疲劳行为

进行Al-5.4Zn-2.6Mg-1.4Cu合金板材的室温低周疲劳实验,对比研究了轴向平行于轧制方向(RD方向)和垂直于轧制方向(TD方向)试样的低周疲劳行为。结果表明：对于0.4%~0.8%的外加总应变幅,RD和TD方向合金试样的循环应力响应行为均呈现出循环稳定;对于相同的外加总应变幅,TD方向合金的循环应力幅值高于RD方向,而RD方向合金的疲劳寿命高于TD方向。对于RD和TD方向,Al-5.4Zn-2.6Mg-1.4Cu合金的塑性应变幅、弹性应变幅与载荷反向周次均呈线性关系。在低周疲劳加载条件下,裂纹在疲劳试样的自由表面以穿晶方式萌生和扩展。     

Ti-6Al-4V钛合金的疲劳裂纹扩展规律

针对熔模铸造Ti-6Al-4V钛合金的等幅疲劳裂纹扩展速率和疲劳裂纹扩展门槛值进行了研究。结果表明：该钛合金CT试样的疲劳裂纹扩展门槛值高于CCT试样的疲劳裂纹扩展门槛值,同一类试样的疲劳裂纹扩展门槛值随着应力比的增加呈下降趋势;疲劳裂纹扩展速率随着平均应力的增加以及应力水平的增加而增大;根据疲劳裂纹扩展试验数据拟合了Ti-6Al-4V钛合金Paris方程和Walker方程中的相关材料参数,以为材料的使用寿命评估及损伤容限设计提供参考。     

Strain Accumulation in Ti-6Al-4V during Fatigue at High Mean Stress

The effect of mean stress and frequency on the high cycle fatigue behavior of Ti-6Al-4V has been investigated. It has been shown that a transition in the fatigue behavior occurs at a stress ratio of approximately 0.7. Above this value, the material exhibits measurable strain accumulation and necking. Since Ti-6Al-4V is susceptible to room temperature creep, an empirical model was developed using static creep data in an attempt to predict the cyclic behavior of the material. The model was unable to account for the large amounts of strain seen experimentally. In addition, closer examination of the data revealed that the deformation was more closely related to the number of cycles than to time.     

Ti60钛合金室温保载疲劳性能及断裂行为

研究高峰值应力条件下Ti60钛合金双态组织和片层组织的低周疲劳与保载疲劳性能,利用金相显微镜(OM)、扫描电子显微镜(SEM)和电子背散射衍射(EBSD)等观察和分析Ti60钛合金的显微组织与疲劳断裂行为。结果表明:显微组织对低周疲劳性能影响不大,但显著影响保载疲劳性能,双态组织保载疲劳敏感性大于片层组织;保载情况下,疲劳寿命显著下降;随峰值应力的提高,疲劳寿命下降,保载疲劳敏感性增加;相同循环周次内,保载疲劳塑性应变累积大于低周疲劳,双态组织的塑性应变累积大于片层组织;低周疲劳裂纹萌生于试样表面,为单裂纹源,而保载疲劳裂纹为内部多源萌生;断口表面均存在准解理小平面,双态组织断口准解理小平面密度大于片层组织。     

Cyclic deformation behavior and fatigue lives of ultrafine-grained Ti-6AL-4V ELI alloy for medical use

It was shown that introducing an ultrafine-grained (UFG) microstructure in pure metals as well as some alloys leads to strongly enhanced fatigue properties. The cyclic deformation behavior of UFG Ti-6Al-4V ELI (extra low interstitials) alloy is studied by both strain and stress controlled fatigue tests using plastic strain amplitudes between 3 × 10^?4 and 5 × 10^?3 and stress amplitudes ranging from 550 to 670 MPa. The UFG microstructures were obtained by equal channel angular pressing (ECAP) with different number of passes followed by a subsequent thermomechanical treatment (TMT). When compared to the conventional grain (CG) size counterpart, the UFG alloy exhibited a pronounced enhancement in the fatigue life in the S–N (Wöhler) diagram. It was also shown that additional UFG processing prior to TMT did not result in any further improvement of the fatigue resistance. Furthermore, microstructural investigations revealed a high cyclic stability of the UFG microstructure.     

Effect of heat treatment on strain–controlled fatigue behavior of cast Mg–Nd–Zn–Zr alloy

The influence of heat treatment on the strain-controlled fatigue behavior of cast NZ30?K alloy was investigated. Compared with the as-cast and solutionized (T4) alloys, the peak-aged (T6) and over-aged (T7) counterparts have a higher cyclic stress and a lower plastic strain value due to the precipitation strengthening. The as-cast and T4-treated alloys have a higher fatigue strength/yield strength ratio than the aged alloys, which is mainly attributed to their higher cyclic hardening. Under stress-controlled loading, the aged alloys show lower hysteresis energies than the as-cast and T4-treated counterparts, leading to longer fatigue lifetimes. For the T4-treated alloy, the cyclic hardening and fatigue failure are controlled by the dislocations-slip and twinning, while for both the as-cast and T6-treated counterparts, they are controlled by the dislocation-slip. For the T7-treated alloy, cyclic deformation and failure behavior are mainly dependent on dislocations-slip and grain boundary sliding.     

一种新的多轴高周疲劳模型

将疲劳强度以上加载等效为塑性应变,建立了塑性应变与加载应力呈线性关系的表达式,由此得到循环加载的塑性应变能。该塑性应变能使材料微观组织结构发生不可逆变化而引起等效宏观应力。假定该应力符合一种特定的分布函数,导出其最大应力与外加应力叠加达到材料本征断裂应力时的裂纹成核寿命,从而并由微裂纹引起上述两部分应力变化,得到继续加载直至宏观裂纹出现的疲劳寿命。所建立的多轴疲劳寿命公式由3个材料参数表达,并通过单轴疲劳试验数据确定。初步研究表明：该模型对所引用的多轴疲劳试验数据有很好的预测能力。     

Ti-6Al-4V合金的室温蠕变行为

研究了Ti-6Al-4V钛合金板材的室温蠕变行为及其对合金后续使用性能的影响.结果 表明:合金的宏观织构、应力水平以及预塑性应变都显著影响其室温蠕变行为.在加载方向上合金的<0001>峰值极密度越高,则其加工硬化指数越大、蠕变指数越小、室温蠕变性能越好.足够大的应力,是合金发生室温蠕变的必要条件.只有在蠕变应力不小于0...     

考虑应变路径的多轴低周疲劳寿命预测模型

通过分析材料在多轴非比例加载下产生附加强化的机理,该文以拉扭薄壁管试件为研究对象,分析了临界平面上的应变状态,并在此基础上以塑性应变能为控制参数定义表征多轴低周疲劳寿命对应变路径依赖性的非比例度。基于多轴疲劳临界损伤面原理,应用von-Mises 准则和本文定义的应变路径非比例度参数建立起能反映应变路径对非比例附加强化影响的多轴低周疲劳寿命预测模型。利用该模型预测08X18H10T 不锈钢、Ti-6Al-4V合金、S460N 钢和2.25Cr-1Mo 钢这4 种材料的多轴疲劳寿命,并与试验值进行比较。结果表明:该模型的预测结果与试验结果吻合良好,能同时适用于比例与非比例加载,预测精度较高,便于工程应用。     

Three-dimensional mixed-mode fatigue crack growth in a functionally graded titanium alloy

The implementation of unitized structure in the aerospace industry has resulted in complex geometries and load paths. Hence, structural failure due to three-dimensional mixed-mode fatigue crack growth is a mounting concern. In addition, the development of functionally graded materials has further complicated structural integrity issues by intentionally introducing material variability to create desirable mechanical behavior. Ti-6Al-4V β-STOA (solution treated over-aged) titanium is a functionally graded metallic alloy that has been tailored for superior fatigue crack growth and fracture response compared with traditional titanium alloys. Specifically, the near-surface material of Ti β-STOA is resistant to fatigue crack incubation and the interior is more resistant to fatigue crack growth and fracture. Therefore, Ti β-STOA is well suited for applications where surface cracking is a known failure mode. Advances in experimental testing have shown that complex loading conditions and multi-faceted materials can be tested reliably. In this paper, the authors will experimentally generate three-dimensional mixed-mode surface crack data in functionally graded Ti-6Al-4V β-STOA and comment on the effect of the material tailoring.     

4.5Ni钢表面裂纹的低周疲劳扩展行为研究

采用悬臂弯曲加载方式，以总应变范围作为受检和控制参数，分析了高强度４．５Ｎｉ钢表面裂纹的低周疲劳扩展行为，给出了裂纹扩展速率ｄ（２ａ）／ｄＮ与总应变范围ΔεＴ的关系式及关系曲线。同时对弯曲加载条件下低周疲劳损伤断口微观形貌进行了观察分析。指出４．５Ｎｉ钢的低周疲劳裂纹扩展方式主要是穿晶，疲劳辉纹为晶体学延性辉纹，疲劳裂纹扩展属于塑性钝化模型机制。     

Microstructural influences on the growth of small cracks in Ti-6Al-4 V

ABSTRACT The effects of microstructure on the growth of small cracks in Ti-6Al-4 V under fatigue loading are presented. The small crack growth is compared with large crack growth. Two large crack tests were performed at a stress ratio of 0.4 and a frequency of 15 Hz. For small crack growth tests, double edge notch specimens were loaded under constant amplitude at four maximum stresses with a stress ratio of 0.4 and a frequency of 15 Hz. A plastic replication technique was used to monitor the small fatigue crack growth rate. The microstructure consists of bands of α and β phases. The present study indicates that the crack growth direction and shape are dependent upon the grain size and grain orientations, and that the crack growth rate seems to be affected by the spacing of α-rich and β-rich bands. Small cracks are propagated at stress intensity factors well below the large crack threshold stress intensity factor.     

Cyclic stress–strain response and crystal plasticity finite element analysis of AISI 9310 steel in biaxial fatigue loading

There are still many gaps in the research on the multiaxial fatigue failure mechanism of the gear shaft. In this paper, cyclic stress–strain response and biaxial fatigue damage characteristics of gear steel AISI 9310 were investigated. The specimens showed obvious cyclic softening characteristics at all phase angles, and the softening rate was directly associated with the initiation and propagation of cracks. The fractographies at different phase angles revealed that the specimens under out-of-phase loading suffered fatigue failure caused by a single crack source on the surface, while the fatigue crack under in-phase loading was gathered together by the propagation of different crack sources. Finally, the established crystal plastic finite element model showed a good prediction of the plastic strain energy density at different phase angles, and the maximum error was 13.03%. Furthermore, a biaxial fatigue life prediction method was proposed, with a maximum error of 39.5%.     

Microstructural features of Ti-6Al-4V manufactured via high power laser directed energy deposition under low-cycle fatigue

Laser additive manufacturing(LAM)technique has unique advantages in producing geometrically com-plex metallic components.However,the poor low-cycle fatigue property(LCF)of LAM parts restricts its widely used.Here,the microstructural features of a Ti-6Al-4 V alloy manufactured via high power laser directed energy deposition subjected to low-cycle fatigue loading were studied.Before fatigue loading,the microstructure of the as-deposited parts was found to exhibit a non-homogeneous distribution of columnar prior-β grains(200-4000 μm)at various scanning velocities(300-1500 mm/min)and rela-tively coarse α-laths(1.0-4.5 μm).Under cyclic loading,fatigue microcracks typically initiated within the aligned α phases in the preferred orientation(～45° to the loading direction)at the surface of the fatigue specimens.Fatigued Ti-6Al-4V exhibited a single straight dislocation character at low strain amplitudes(＜0.65％)and dislocation dipoles or even tangled dislocations at high strain amplitudes(＞1.1％).In addition,dislocation substructure features,such as dislocation walls,stacking faults,and disloca-tion networks,were also observed.These findings may provide opportunities to understand the fatigue failure mechanism of additive manufactured titanium parts.     

Texture effects under tension and torsion loading conditions in titanium alloys

The paper derives from a major research programme on texture evolution and characterisation in the titanium alloys Ti-6Al-4V and Ti550. The present publication focuses on the mechanical characterisation of the texture in rolled plate. It focuses specifically on monotonic strengths and strain control fatigue under tension and torsion loading. Dependencies of fatigue performance on specimen orientation are interpreted through EBSD evaluation of basal and prism plane intensities within the present materials. The cyclic deformation and mechanisms of failure are related to the relative magnitude of tensile and shear stress components in the tension and torsion loading modes. Stress relaxation is shown to play an important part in these processes. Its relationship to available slip systems is discussed     

Cyclic stress response and deformation behaviour of precipitation-hardened aluminium-lithium alloys

The cyclic stress response of two lithium-containing aluminium alloys aged to contain ordered precipitates was studied in different environments over a range of plastic strains. The specimens were cycled using tension-compression loading under total strain control. The peak-aged Al---Li---Mn alloy cyclically hardened to failure, whereas the peak-aged Al---Li---Cu alloy displayed softening for most of the fatigue life. The presence of shearable softening for most of the fatigue life. The presence of shearable precipitates in the two alloys results in a local decrease in resistance to dislocation movement, leading to a progressive loss of ordering contributions to hardening and slip concentration. This, coupled with the presence of precipitate free zones, promotes strain localization in intense slip bands and results in early crack nucleation. Transmission electron microscopy observations revealed homogeneous deformation in specimens cycled at high plastic strain amplitudes. However, at lower plastic strain amplitudes, deformation was inhomogeneous in the two alloy systems with the formation of intense planar slip bands. Results of this study reveal that the initial hardening observed is due to dislocation-dislocation and dislocation-precipitate interaction and that the softening observed in the Al---Li---Cu alloy is a mechanical and not an environmental effect.     

Fatigue of engineering structures under combined nonproportional loads: An overview

Nonproportionality in fatigue is a matter of loading sequences acting on structures. In such loading cases, a life assessment‐based search of the initiation site is required. This means that for a variety of potential sites, their stress‐strain sequences must be determined and the corresponding fatigue lives have to be calculated. For the determination of local stress‐strain sequences, a superposition of the action of loads at all times for all potential sites must be performed. The superposition has not only to be performed at a particular time of a load reversal but also for a sufficiently large number of intermediate points in time. For the assessment of crack initiation under local nonproportional stress‐strain sequences, a multiaxial fatigue damage hypothesis must be specified. Due to the superiority with respect to accuracy, critical plane approaches are the favourite ones when dealing with nonproportional fatigue of ductile metals. For describing fatigue crack growth, the time sequences for the stress intensity factors corresponding to the 3 opening modes must be obtained by superposition. A mixed‐mode hypothesis must be applied to predict both the direction and the increment of crack growth at each crack front point. A first prediction trial should be based on an appropriate extension of the maximum tangential stress hypothesis. An application of a variant of the maximum shear stress hypothesis might be an amendment. Especially in the case of crack turning, the consideration of crack face contact is mandatory.