侯方
, 张军辉
HOU Fang, ZHANG Junhui
文献标识码: 分类号 O346.2 文章编号 1005-3093(2016)07-0481-08
通讯作者:
收稿日期: 2015-10-14
网络出版日期: 2016-07-25
版权声明: 2016 《材料研究学报》编辑部 《材料研究学报》编辑部
基金资助:
作者简介:
本文联系人: 王清远，教授
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摘要
使用自主搭建的高温超声疲劳实验系统在常温和600℃进行CrMoW转子钢1×1010周次超高周疲劳实验, 研究了CrMoW转子钢在工作温度下的超高周疲劳性能。结果表明: 在600℃和常温下转子钢的S-N曲线均呈连续下降型, 试件在109周次后仍发生疲劳断裂。在常温实验条件下疲劳寿命小于1×107周次试件的裂纹多萌生在表面, 而高于1×107周次的多以内部萌生为主。在高温实验环境下裂纹也在内部夹杂处萌生, 且裂纹萌生方式对疲劳寿命没有决定性作用。鱼眼定量分析结果表明, CrMoW转子钢在600℃和常温下裂纹扩展的应力强度因子门槛值分别为3.4 MPam1/2和1.0 MPam1/2。
关键词：
Abstract
The fatigue properties of CrMoW rotor steel were investigated by a high temperature ultrasonic fatigue system. Fatigue tests of CrMoW rotor steel up to 1×1010 cycles had been conducted at room temperature and 600℃ respectively. Results reveal that the type of S-N curves present continuously descending at room temperature and 600℃. Fatigue fracture occured over 109 cycles. Fractograph of specimens show that Crack initiate mainly from the surface for the specimens that fatigue life is less than 1×107 cycles, but crack initiate mostly from inclusions for those that fatigue life is greater than 1×107 cycles at room temperature. It is found that crack can also initiate at inclusions, but the modes of crack initiation do not play a decisive role for the fatigue life at high temperature. Threshold values of fatigue crack growth obtained by measuring the size of fisheye are 3.4 MPam1/2 and 1.0 MPam1/2 at room temperature and 600℃ respectively.
Keywords：
为了提高发电效率、节约煤炭资源, 火力发电汽轮机转子的工作温度和压力不断提高。因此, 要求用于制造汽轮机转子部件的材料具有更高的力学性能。目前, 用于制造超超临界汽轮机组高压转子的CrMoW转子钢是在9%Cr1Mo钢的基础上添加W发展出来的新型耐热钢, 其最高使用温度为610℃, 具有良好的淬透性、断裂韧性、抗疲劳破坏性能及在长期高温状况下良好的组织稳定性。已有学者对这种材料进行了微观组织和疲劳断裂力学方面的研究。吴海利等[1]研究了其室温低周疲劳特性, 赵鹏等[2]研究了该材料在蠕变疲劳载荷下的棘轮效应。但是, 国内外对转子钢疲劳研究集中在低周疲劳方面[3-6], 高周疲劳[7]的研究报道很少。研究发现, 汽轮机转子在服役多年后高温低应力区的疲劳损伤远大于低温高应力区, 长期高温循环交变载荷对转子钢的疲劳性能有较大的影响[8]。新型超超临界汽轮机高压转子的设计服役期为30年, 其间不仅承受由热应力引起的低周荷载, 还有高达4×1010周次高温低应力疲劳载荷。目前还没有关于CrMoW转子钢高温超高周疲劳性能的研究。要实现1010周次以上的超高周疲劳循环加载, 如采用传统的50 Hz疲劳实验机大约需要6年的时间, 而超声疲劳实验系统的加载频率为20 kHz, 只需要140 h。用超声疲劳实验系统研究了不同种类的材料[9-14], 结果都验证了超声疲劳加速实验方法的准确性和和可靠性。已有的高温超声疲劳实验[15-19]均在商用超声疲劳实验机的基础上增加感应加热设备, 并已成功地对高温单晶镍基合金[15, 17]、铝合金[18]、耐高温钢[16]和钛合金[19]进行了研究。
基于超超临界汽轮机高压转子的实际工作温度为600℃, 本文使用自主搭建的高温超声疲劳实验系统进行CrMoW转子钢在室温和600℃下的超高周疲劳实验, 研究高温及夹杂物对转子钢疲劳性能的影响。
实验用材料X12CrMoWVNbN10-1-1的化学成分与基本力学性能, 分别列于表1和表2。图1表明, 转子钢的微观组织为高温回火马氏体, 马氏体板条宽度介于0.2-2 μm, 可以看到原奥氏体晶界, 晶粒尺寸约为80 μm。在原奥氏体晶界和马氏体板条界上存在着大量颗粒状析出物, 该弥散析出相沉淀于奥氏体晶界与马氏体板条界, 起到钉扎位错、稳定板条界面及亚晶界的作用, 可有效提高高温蠕变强度。
图1 CrMoW转子钢的微观组织结构
Fig.1 Microstructures of CrMoW rotor steel (a) optical, (b) SEM
表1 材料化学成分(质量分数, %)
Table 1 Chemical composition of the materials(%, mass fraction)
| C | Cr | Mo | W | Ni | Fe |
|---|---|---|---|---|---|
| 0.13 | 10.37 | 1.08 | 1.01 | 0.80 | 86 |
表2 材料力学性能参数
Table 2 Mechanical properties of the materials
| Temperature/℃ | E/GPa | Rm/MPa | Rp0.2/MPa | Hardness(HV) |
|---|---|---|---|---|
| 20℃ | 210 | 900 | 780 | 296.8 |
| 600℃ | 138 | 550 | 490 | - |
实验用试件的形状及尺寸如图2所示。试件表面经过机械抛光, 用原子力显微镜(AFM)对未实验件的中部进行了光洁度检测, 测试区域的表面形貌如图2c所示。使用软件分析出其表面粗糙度值Ra为0.16。常温实验使用经典的沙漏型试件, 以得到更大的实验应力区来提高准确性, 高温实验采用狗骨型试件。使用有限元软件计算验证试件尺寸, 设计加载频率为20 kHz。
图2 超声实验试件及其表面形貌
Fig.2 Specimen of ultrasonic testing (a) room temperature; (b) high temperature (unit: mm); (c) surface topography
常温疲劳实验在岛津USF-2000超声疲劳实验机上完成。共振频率为20 kHz, 当试件因裂纹萌生而致其共振频率超出20±0.5 kHz范围或到达实验预设循环周次时, 实验终止。实验中采用间歇振动加载, 并用压缩冷干空气对试件进行冷却。
高温实验在自主搭建的超声高温实验系统上完成。该系统由超声疲劳实验机、试件加热装置、测温装置三部分组成, 其示意图在图3中给出, 实验加热装置为定制高频感应加热器。测温装置为NEC红外热像仪(精度±0.1℃), 实验开始前用热电偶标定红外测温仪以保证测温的准确性, 。
为了使试件中部实验段在高温实验中温度恒定, 采用间歇振动加载, 控温软件根据由红外测温仪测得的试件中部实时温度值, 用PID算法调控感应加热器的输出功率, 使实验温度稳定。图4给出了在高温实验过程中24 h内试件实验应力区的温度变化曲线, 可见实验区温度波动为±3℃。
图5和图6分别给出了转子钢在常温和600℃的疲劳S-N曲线, 图中实心圆点表示裂纹萌生于内部夹杂, 空心圆点表示裂纹萌生于表面, 箭头所标注数据点表示应力循环至1×1010周次仍未发生断裂。整体上看, 常温和高温下试件疲劳寿命均随应力的增大而降低, 试件在109后仍会发生断裂, 不存在传统的疲劳极限。常温下S-N曲线的离散性大于高温。
在常温下S-N曲线在107周次附近有一个拐点, 107周次后疲劳强度下降的趋势小于107周次前。常温试件裂纹萌生方式没有明确的疲劳寿命分界, 从105到1010周次内既有内部萌生亦有表面萌生。进一步统计发现, 寿命小于107周次的试件以表面萌生为主, 占据寿命小于107周次试件总数的85%; 寿命大于107周次试件以内部萌生为主。此外, 寿命小于107周次的内部萌生试件集中在460 MPa至470 MPa低应力区, 导致此应力区疲劳寿命有较大的离散性。结合常温超高周实验常见S-N曲线类型与机理, S-N曲线在107周次附近出现拐点是裂纹萌生方式从表面萌生向内部萌生转变所致。
600℃的S-N曲线的拐点位于1×108周次附近, 寿命在1×108周次后疲劳强度的下降趋势大于1×108周次前。根据高温超声疲劳文献[15-18, 20], 裂纹多萌生于试件表面, 没有发现常温中典型的内部夹杂萌生。本文实验发现, 在高温下裂纹也会从内部夹杂处萌生, 且内部萌生方式均匀分布于各个循环周次。值得注意的是, 内部萌生点与表面萌生点所构成的S-N曲线变化趋势相似, 表明裂纹萌生方式不对试件疲劳寿命起决定性作用。
图7和图8分别给出了常温与600℃下疲劳强度和疲劳强度与其对应温度下拉伸强度比值随循环周次的变化。在常温和高温下疲劳强度均随着循环周次的增加而减小, 两者间的差值随着周次的增加而增大。在常温下疲劳强度在108周次后的下降趋势逐渐减小, 疲劳强度趋近450 MPa。在600℃疲劳强度的下降趋势相近, 疲劳强度随着周次的增加连续下降。在常温下疲劳强度与拉伸强度的比值在108周次为0.52, 1010周次为0.5, 结果与拉伸强度小于1200 MPa的低合金钢一致[21]。在600℃该比值随周次增加而下降, 106周次为0.42, 1010周次下降至0.20。这表明, 高温会极大降低材料的疲劳性能, 且使材料疲劳性能的降低程度随着循环周次的增加不断增大。由此可推断, 600℃的S-N曲线在108出现拐点与试件在高温环境下的实验时间有关。在高温超声疲劳系统中107周次仅需0.6 h, 高温对材料表面的损伤较小, 随着实验时间的增加高温损伤的累积效应导致材料抗拉性能急剧下降, 表层氧化膜更易破裂[19], 从而使S-N曲线在107周次至108周次间出现拐点。
图8 疲劳强度与拉伸强度比值随着疲劳寿命的变化
Fig.8 Ratio of fatigue strength to tensile strength varies with fatigue life
在常温实验条件下, 断口呈现表面与内部夹杂两种萌生机制。图9给出了在常温实验条件下疲劳裂纹从表面及内部夹杂物萌生的典型电镜图。从图9a可清晰观察到裂纹的萌生区、慢速扩展区、快速扩展区。从图9b可见裂纹萌生于试件表面, 并以穿晶方式扩展。从图9c可清晰观察到内部夹杂物形成的鱼眼特征。图9d为夹杂物放大图, 表格所列为夹杂物EDS分析结果, 表明夹杂物主要成分为Al的氧化物。夹杂物周围分布着白色细颗粒, 该粗糙区域被Sakai[22]称为FGA(fine granular area)。
图9 常温试件的典型断口形貌
Fig.9 Typical fractographs of room temperature specimens (a) σ=510 MPa, Nf=1.3806×106; (b) σ=510 MPa, Nf=1.3806×106; (c) σ=490 MPa, Nf=1.5442×107; (d) σ=490 MPa, Nf=1.5442×107
图10给出了S-N曲线中低应力(460 MPa-470 MPa)短寿命区试件断口, 可观察到裂纹萌生于次表面夹杂物, 且没有鱼眼特征形成。试件内部的夹杂物与基体因弹性模量的不同, 在振动中因形变不协调而导致局部的应力集中, 最终形成裂纹源。近表面夹杂物导致的应力集中程度大于内部夹杂。此外, 微裂纹经过较短的循环即扩展至试件表面形成表面裂纹而进一步缩短疲劳寿命, 导致材料疲劳可靠性的降低。
图10 短寿命试件的裂纹萌生区形貌
Fig.10 Fractographs of short-life specimens (a) σ=470 MPa, Nf=8.5764 ×105; (b) σ=465 MPa, Nf=3.0132 ×105
图11a和图11c分别给出了在600℃实验条件疲劳裂纹从表面和内部夹杂物萌生的典型电镜图。在高温环境下试件表面与空气中的氧气结合形成的氧化层, 降低表层的强度。在高频振动情况下裂纹易从该处萌生以至试件表面在整个实验应力幅值下均可能成为裂纹的萌生处, 如图11b中所注。图11c为高温环境下内部萌生试件断口图, 可观察到鱼眼特征。图11d为鱼眼区放大图, 裂纹萌生源为内部夹杂物。鱼眼宏观形态与常温的相似, 在夹杂物周围也存在粗糙区。对比常温与高温下宏观断口图, 可发现在高温下断口的粗糙度比常温的高, 表明在高温下裂纹的扩展速度比常温的高。
图11 高温试件的典型断口形貌
Fig.11 Typical fractographs of high temperature specimens (a) σ=125 MPa, Nf=6.6955×108; (b) σ=125 MPa, Nf=6.6955×108; (c) σ=185 MPa, Nf=7.2652 ×106; (d) σ=185 MPa, Nf =7.2652 ×106
鱼眼内FGA区面积与裂纹扩展门槛值相关, 是已被证实的结论[23]。文献[24]的结果表明, 在常温下用Murakami提出的公式(1)计算出的FGA区应力强度因子范围△KFGA与实验测得的裂纹扩展门槛值△Kth基本一致。基于高温下鱼眼特征与常温相似, 尝试将公式(1)也用于在600℃夹杂周围FGA区的计算。
测量常温和600℃下断口FGA区面积, 结合公式(1)计算出△KFGA值, 结果如图11所示。常温下△KFGA平均值为3.4 MPam1/2, 子样标准差为0.24。600℃下△KFGA平均值为1.0 MPam1/2, 子样标准差为0.17。文献[24]通过裂纹扩展实验测得一种马氏体钢裂纹扩展门槛值△Kth为3.7 MPam1/2, 与本次实验通过FGA区面积计算所得△KFGA相近。600℃下断口鱼眼特征及定量计算所得△KFGA的规律与常温下的实验结果具有很好的一致性。因此可以推断, 600℃下裂纹扩展门槛值约为1.0 MPam1/2。根据公式(1), 600℃下裂纹扩展门槛值比常温下的小, 与实验结果吻合得很好。高温降低了裂纹扩展的门槛值, 导致裂纹在600℃下承受比常温更小的应力便能开始慢速扩展。
图12给出了常温和600℃下内部萌生试件夹杂物尺寸与疲劳寿命间的关系, 其中夹杂物尺寸用其直径大小表示。可以看出, 常温下内部萌生试件夹杂物尺寸随着疲劳寿命的增加整体呈下降趋势, 600℃的规律则恰好相反, 且呈现很高的线性。在常温和600℃间所呈现的规律不同缘于其裂纹萌生机理的差异。在常温环境下, 随着应力的降低裂纹从内部夹杂处萌生, 且寿命随着夹杂物尺寸的减小增加。在600℃内部夹杂与表面氧化层一直呈竞争趋势, 裂纹从表面氧化层萌生的概率随着实验周次的增加而增大, 因此内部夹杂物萌生的临界尺寸随着周次的增加而增大。
图12 夹杂物的直径与疲劳寿命的关系
Fig.12 Relationship between the diameter of inclusion and fatigue life
在两种实验环境下材料内部夹杂物尺寸的变化范围应相近, 但由图13可见, 在常温环境下夹杂物的最小尺寸明显大于600℃下的最小值。文献[25] 的结果表明, 当材料中夹杂物尺寸小于一定值时裂纹不会从内部夹杂处萌生。由此可以断定, 在常温环境下当转子钢试件中夹杂物直径小于22 μm时, 裂纹在1010周次内不会从内部夹杂处萌生。在高温下夹杂尺寸与寿命间的关系表明, 在600℃也存在夹杂尺寸的临界值, 但是其值是一个与疲劳寿命相关的变量。因此, 控制转子钢中夹杂物尺寸是提高其超高周疲劳性能和可靠性的有效手段。基于技术与经济的考虑, 夹杂物的尺寸并非越小越好, 须根据设计疲劳寿命和服役环境给出合理的上限值。
1. 在常温和高温环境下CrMoW转子钢在109后仍会发生疲劳断裂, 不存在传统意义下的疲劳极限。
2. 在常温下内部夹杂物使材料的疲劳性能降低, 次表面夹杂物使材料的疲劳可靠度极大降低。
3. 在高温环境下疲劳裂纹也会从内部夹杂处萌生, 并形成鱼眼特征; 裂纹的萌生方式对材料的疲劳寿命没有决定性的作用。
4. 根据FGA区特征计算出常温和600℃裂纹扩展的门槛值分别为3.2 MPam1/2和1.0 MPam1/2。
The authors have declared that no competing interests exist.
| [1] |
Low cycle fatigue behaviors of X12CrMoWVNbN10-1-1 steel for rotors at room temperature, X12CrMoWVNbN10-1-1转子钢室温低周疲劳特性 .
采用应变控制对超超临界汽轮机 转子钢X12CrMoWVNbN10-1-1进行室温低周疲劳试验,并对试验结果进行了讨论.拟合了循环应力-应变曲线和应变-寿命曲线,得到该材料的室 温低周疲劳特性参数,包括Ramberg-Osgood参数和Manson-Coffin参数,推测出该材料的转变寿命.对低周疲劳试验开始和结束两个阶 段的拉压应力峰值、拉压卸载弹性模量以及迟滞回线面积进行了对比分析,并对不同应变幅控制的低周疲劳对应的应力变化进行了讨论.
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| [2] |
Experimental study on ratchet effect of steel X12CrMoWVNbB10-1-1 for USC steam turbine rotors, 超超临界汽轮机转子X12CrMoWVNbB10-1-1钢棘轮效应的试验研究,
以超超临界汽轮机转子材料X12Cr Mo WVNbB10-1-1钢为对象,研究了在蠕变疲劳载荷下该材料的棘轮效应,探讨了在高温环境下不同峰值保持时间对棘轮效应的影响以及蠕变和棘轮之间的交 互作用,并分析了棘轮效应的存在对寿命评估的影响.结果表明：保持时间会影响纯交变载荷下产生的非弹性应变循环累积εR1,在较低保持时间时,εR1不会 出现安定现象,从而抑制了保持时间段内产生的蠕变应变循环累积εR2的演化;εR2对材料寿命的影响可以归类为与时间有关的蠕变损伤;提出采用εR1和循 环塑性应变之和来度量棘轮应变εR1的损伤;针对不同的损伤机制,利用分区分析的方法可以进行精确的寿命评估.
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| [3] |
Effect of loading rate on low-cycle fatigue properties of 30CrMo1V rotor steel, 加载速率对30Cr1Mo1V汽轮机转子钢低周疲劳特性的影响,
以30Cr1Mo1V汽轮机转子钢为研究材料,选取0.1%/s、0.3%/s和0.5%/s的加载速率,采用控制总应变的方法,在RDL 05电子蠕变疲劳试验机上研究了加载速率对材料低周疲劳特性的影响.同时,还提出在538℃下加载速率对30Cr1Mo1V汽轮机转子钢应力和低周疲劳寿命影响的关系式以及低周疲劳寿命与总应变幅值的关系式.结果表明:在538℃时,随着加载速率的提高,转子钢循环应力增大,低周疲劳寿命延长;在同一应变下,加载速率越大,所对应的应力幅值越大;随着应变幅值的增大,应力增大,低周疲劳寿命缩短.
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| [4] |
Study on softening property of 30Cr1Mo1V steel under high temperature low-cycle fatigue,
30Cr1Mo1V钢高温低周疲劳中的软化特性,
软化是高温长期运行部件的主要 特征,软化意味着材料性能的降低。为了探讨软化对材料低周疲劳特性的影响,对30Cr1Mo1V转子钢进行了应变控制中断低周疲劳试验。试验温度为 540℃和565℃,应变幅为0.2%～1.0%,采用拉压对称三角波。基于试验结果,建立了疲劳损伤与低周疲劳寿命之间的关系。通过对试验试样的维氏硬 度进行检测,得到了带有硬度修正的低周疲劳应变-寿命修正公式,并对修正公式的有效性进行了检验。结果表明,硬度修正公式在循环寿命小于0.5倍寿命分数 时有效,而在循环寿命大于0.5倍时,则存在高估剩余寿命的危险。在同一硬度下,对不同温度和应变,损伤是不同的,应变越小,损伤越大;温度越高,损伤越 小
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| [5] |
Failure analysis of a steam turbine rotor,
The fracture of the shaft was located at the first stage [2] and the fatigue fracture extended over roughly 75% of the initial cross section. Primary failure causes were identified by the analyses performed, and the observed fracture mechanism was traced back to high cycle fatigue damage. The origin of the fatigue phenomenon can be traced to the stress field generated on the rotor surface by both the frequent startup cycles and the blade fixing method.
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| [6] |
Comparison of various 9-12%Cr steels under fatigue and creep-fatigue loadings at high temperature, |
| [7] |
Microstructure characteristics and temperature-dependent high cycle fatigue behavior of advanced 9% Cr/CrMoV dissimilarly welded joint, |
| [8] |
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| [9] |
Gigacycle fatigue of ferrous alloys,
The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1聽脳聽10cycles at an ultrasonic frequency of 20 kHz. A good agreement is found with the results from conventional tests at a frequency of 25 Hz by Renault between 10and 10cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 10cycles, and the fatigue endurance stress continues to decrease with increasing number of cycles to failure between 10and 10cycles. The evolution of the temperature of the specimen caused by the absorption of ultrasonic energy is studied. The temperature increases rapidly with increasing stress amplitudes. There is a maximum temperature between 10and 10cycles which may be related to the crack nucleation phase. Observations of fracture surfaces were also made by scanning electron microscopy (SEM). Subsurface cracking has been established as the initiation mechanism in ultra-high-cycle fatigue (>10cycles). A surface subsurface transition in crack initiation location is described for the four low-alloy high-strength steels and a SG cast iron.
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| [10] |
Piezoelectric fatigue testing machines and devices,
During the 1990s several methods have been developed around the word in order to test specimens at very high fatigue life. In our laboratory an ultrasonic fatigue testing system was designed and built 15 years ago to determine crack growth threshold and–curve of metals. Those first results were published in ASTM STP 1231 (1994), ASTM STP1411 (2002) and in proceedings of Fatigue 2002. This paper sums up the progress of such fatigue testing machines and devices from this date.
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| [11] |
Device for carrying out environmental very high cycle fatigue tests with ultrasonic excitation in asymmetric push-pull mode,
The increasing lifetime of many engineering components leads to a growing demand for accelerated testing methods. Fatigue failure of components submitted to cyclic loading at stress levels below the endurance limit occurs even beyond 10(7) cycles which has been the traditional limit for fatigue testing in most laboratories. Test programs covering this range of cycles on servo-hydraulic or resonance machines are very time consuming. Therefore methods for very high cycle fatigue (VHCF) testing at ultrasonic frequencies have been developed and are now used routinely. These methods rely on the formation of a longitudinal standing ultrasonic wave inside a test specimen. The wave exerts an alternating tensile and compressive stress on the specimen. Because of their origin in a standing wave, the tensile and compressive stresses usually have the same magnitude, i.e. the test is carried out under fully reversed conditions. Several test rigs have been proposed and built to overcome this drawback by coupling an ultrasonic loading device with a classical uniaxial test bench and superposing the ultrasonic stress to a constant or slowly varying stress. We present a different approach for overcoming that limitation where the constant stress is generated by a pressure difference. This approach is especially useful for testing in hazardous environments since all movable parts like pull rods passing through the walls of the test chamber are avoided.<br/>We describe the design and the performance of such a VHCF device and present first test results demonstrating the deterioration of the lifetime of Inconel 718 specimens in high pressure gaseous hydrogen compared to argon. (C) 2013 Elsevier Ltd. All rights reserved.
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| [12] |
The present situation and future problems in ultrasonic fatigue testing-mainly reviewed on environmental effects and materials’screening.
Finally, a couple of future problems to be solved in ultrasonic fatigue testing are touched on briefly.
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| [13] |
On the mechanism of very high cycle fatigue in Ti-6Al-4V,
ABSTRACT abstract: The laughter of Alexander, the unvanquished hero, is a wordless message, a sign of his mastery and intelligent force, a sign of his art in deciphering signs. Alexander’s laughter signifies that true force scorns force, just as true eloquence scorns eloquence. Intelligence turns force into a dynamic and laughing force and laughter thus becomes a fundamental capacity of force in order to better conceive the phenomenon of life and survival. It is true ritual laughter, “theurgic” laughter. The laughter of Alexander in India was perhaps the laughter of a holy man as defined by the sage Yang Xiong.
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| [14] |
Very high cycle fatigue measuring techniques,
Ever since high-strength steels were found to fail below the traditional fatigue limit when loaded with more than 10 8 cycles, the investigation of metals’ and alloys’ very high cycle fatigue properties has received increased attention. A lot of research was invested in developing methods and machinery to reduce testing times. This overview outlines the principles and testing procedures of very high cycle fatigue tests and reports findings in the areas of crack formation, non-propagating small cracks, long crack propagation and thresholds. Furthermore, superimposed and variable amplitude loading as well as frequency effects are reported.
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| [15] |
High-temperature ultrasonic fatigue testing of single-crystal superalloys,
A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of single-crystal superalloys used in aircraft engine turbine blades. In this development, a commercial ultrasonic fatigue testing machine was considerably modified to achieve high-temperature fatigue testing. The testing system was first applied to a heat-resistant steel at 650 degrees C to check its accuracy, and next to SC superalloy samples at 1000 degrees C. As a result, the ultrasonic fatigue testing showed good agreement with the conventional fatigue testing, demonstrating the high accuracy of the developed system. In these results. the SC superalloys showed no fatigue limit, indicating gigacycle fatigue tests to be necessary. (C) 2011 Elsevier B.V. All rights reserved.
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| [16] |
Ultrasonic fatigue tests at high temperature on an austenitic steel,
The objective of this paper is to study the gigacycle fatigue behavior of an austenitic steel at temperatures of 60002°C and 70002°C under fully reverse loading ( R =611). Numerical simulation by finite element method (FEM) was used to design the specimens and to analyze the effects of the variation in the dynamic Young modulus with temperature from measurements of the ultrasonic resonance frequency. Finally, new stress-life curves for this material are presented for a lifetime range from 10 5 to 10 9 cycles at room temperature, 60002°C and 70002°C.
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| [17] |
Ultrasonic fatigue of a single crystal Ni-base superalloy at 1000℃,
An ultrasonic fatigue testing system capable of operating at temperatures up to 1000°C has been developed and utilized to study the fatigue behavior of a single crystal superalloy (PWA 1484) at a temperature of 1000°C and loading frequency of approximately 20kHz. The stress-life data generated from the ultrasonic testing system were comparable to those from conventional servo-hydraulic fatigue tests for similar single crystal alloys. Interior Ta-rich carbides were the major microstructural feature responsible for crack initiation in the alloy. Crack growth under ultrasonic loading frequency at 1000°C for PWA 1484 occurred in a crystallographic manner on {111} octahedral slip planes, in contrast to the normal Mode-I growth mode typically observed for single crystal superalloys at high temperature (>850°C) with conventional servo-hydraulic loading frequencies (<100Hz).
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| [18] |
Effects of microstructure and temperature on fatigue behavior of E319-T7 cast aluminum alloy in very long life cycles,
The effects of microstructure and temperature on the fatigue behavior of a commercial Al–Si–Cu alloy used in automotive engine components were investigated for lifetimes as long as 10 9 cycles using ultrasonic fatigue instrumentation operating at 20kHz. The primary finding of this study is that the influence of microstructure on the cyclic properties is greater than the influence of the testing temperature. Fractographic studies indicated that most fatigue cracks initiate from microshrinkage pores located at or very near to the specimen surface, while a much smaller number of cracks initiate from twin boundaries. Increasing test temperature resulted in a modest decrease in endurance limit by about 12% from 20 to 150°C, while a significant decrease in endurance limit by about 23% was observed from 150 to 250°C at high number of cycles. Using fatigue data developed in this study, a statistical model, the random fatigue-limit model (RFL), was evaluated for its utility in estimating fatigue behavior in the gigacycle regime.
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| [19] |
High-temperature ultra-high cycle fatigue testing of TC17 titanium alloy, Journal of Aerospace Power, 29(7), TC17钛合金高温超高周疲劳实验 .
自主研发了高温超声疲劳（20kHz）实验系统,并完成了TC17钛合金在室温、200℃和 350℃条件下的疲劳性能实验研究.结果表明：TC17钛合金的动态弹性模量随温度升高呈线性减小.S-N曲线在室温下呈直线下降趋势,但在200℃和 350℃条件下,S-N曲线在疲劳寿命为107周次处出现明显的拐点.断口分析表明疲劳裂纹萌生于试件表面或次表面,没有发现裂纹萌生于内部的情 形,TC17钛合金的裂纹萌生可不依赖于试件内部的夹杂物或缺陷.高温不仅促进了裂纹的萌生还促进了裂纹的扩展.
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| [20] |
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| [21] |
Gigacycle fatigue properties of 1800MPa class spring steels.
ABSTRACT Fatigue tests up to 10cycles were carried out for two spring steels (Heats A and D1) and one valve spring steel (Heat F) with tensile strength, , of 1720, 1725 and 1764 MPa, respectively. The size and composition of inclusions in Heats Dl and F were controlled. The surface-type fracture occurred at shorter lives below 10cycles, while the fish-eye-type fracture occurred at longer lives. The fatigue limit, , at 10cycles was 640 MPa for Heats A and D1 and 700 MPa for Heat F. AlOinclusions for Heat A and both TiN inclusions and matrix cracks, i.e. internal facets, for Heat F were observed at the fish-eye-type fracture sites, while only matrix cracks were observed for Heat Dl. ODA, i.e. optically dark area, which is considered to be related to hydrogen effects, were formed around AlOand TiN inclusions. Fatigue tests were also conducted after specimens were heated up to 573 K in high vacuum of 2 脳 10Pa. The heat treatment eliminated matrix cracks for Heat D1 and the fatigue limit at 10cycles recovered to the estimated value of 920 MPa from the equation = 0.53 for the surface fracture. These results suggest that inclusions control and hydrogen influence the gigacycle fatigue properties for these high strength steels. In addition, it is expected that the creation of a martensite structure with a high resistance to hydrogen effects in the inclusion-controlled steel could achieve the higher fatigue limit estimated for the surface-type fracture.
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| [22] |
Characteristic S-N properties of high-carbon-chromium-bearing steel under axial loading in long-life fatigue,
ABSTRACT Owing to difficult economical conditions, machines and structures often have to be used beyond the design lifetime. In this study, fatigue properties of a bearing steel in the long-life region were experimentally examined under cyclic axial loading. The complicated S–N behaviour was well explained as a combination of S–N curves for surface-induced fracture and interior inclusion-induced fracture. Fish-eye marks were always observed on the fracture surfaces of specimens, which failed in the latter fracture mode, and an inclusion was found at the center of the fish-eye. Finally, it was found that the fatigue fracture of this steel in the long-life region occurred through the following three processes: (i) formation of the characteristic area as a fine granular area (FGA), (ii) crack propagation to form the fish-eye and (iii) rapid crack propagation to cause the catastrophic fracture.
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| [23] |
Essential characteristics and influential factors for very-high -cycle fatigue behavior of metallic materials, Acta Metallurgica Sinica, 45(7), 770(2009)(洪友士, 赵爱国, 钱桂安, 合金材料超高周疲劳行为的基本特征和影响因素, |
| [24] |
Very high cycle fatigue behaviour of 2000-MPa ultra-high-strength spring steel with bainite-martensite duplex microstructure,
ABSTRACT The very high cycle fatigue and fatigue crack growth (FCG) behaviours of 2000-MPa ultra-high-strength spring steel with different bainite–martensite duplex microstructures (designated as B-M1 and B-M2) obtained through isothermal quenching and fully martensite (designated as M) for comparison were studied in this paper by using ultrasonic fatigue testing and compact-tension specimens. It was found that for the B-M1 sample with well-controlled thin and uniformly distributed bainite, the fatigue crack threshold Δis higher and FCG rate /at an early stage is lower than those of the M sample. Therefore, the former has rather longer fatigue life at high stress amplitude, though both have almost identical fatigue strength. However, the fatigue properties of bainite–martensite duplex microstructure are significantly deteriorated with the formation of large bainite. Furthermore, like that of the M sample, the S–N curves of the B-M1 and B-M2 samples also display continuous declining type and fish-eye marks were always observed on the fracture surface in the case of internal fractures, which were mainly induced by inclusion. A granular bright facet (GBF) was observed in the vicinity around the inclusion. For each of the three samples, the stress intensity factor range at the boundary of inclusion (Δ) decreases with increasing the number of cycles to failure (), while the stress intensity factor range at the front of GBF(Δ) is almost constant with and equals to its Δ. This indicates that Δmight be the threshold value governing the beginning of stable crack propagation.
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| [25] |
Estimation of the critical size of inclusion in high strength steel under high cycle fatigue condition, Acta Metallurgica S inica, 41(11), 高周疲劳条件下高强钢临界夹杂物尺寸估算,
依据Murakami的"夹杂物等效投影面积模型"估算了在高周 疲劳条件下一定硬度(或强度)高强钢的"临界夹杂物尺寸".估算结果表明,随着钢硬度(或强度)的增加,"临界夹杂物尺寸"逐渐减小;"临界夹杂物尺寸" 也受构件表面机加工粗糙度的影响,表面越光洁,这个尺寸也越小.从本文几种钢的实验数据以及其它已发表的数据都可以间接证明,估算的临界尺寸是合理的.
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