应力比对TC4钛合金超高周疲劳失效机理的影响
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摘要
采用频率为100 Hz的电磁谐振疲劳试验机进行疲劳拉伸试验,研究了两种应力比(R=0. 1和-1)对TC4钛合金的超高周疲劳失效机理的影响.结果表明,两种应力比下的S-N曲线都呈现"双线"型,但各自表示的意义及失效机理不同.当R=0. 1时,TC4钛合金的疲劳失效形式有两种,即由加工缺陷诱发的表面失效和内部鱼眼失效,这两种失效形式都伴随着颗粒平面(Facet)出现;而当R=-1时,仅存在表面失效,且无Facet的出现.基于断裂力学的讨论可知,在正应力比及真空环境下,对应小裂纹扩展的门槛值更低,更有利于裂纹扩展及Facet的形成. TC4钛合金的整个内部疲劳失效过程及机理可解释为:(1)滑移线或滑移带在部分α晶粒上的出现;(2)微裂纹的萌生和接合;(3)颗粒亮区(GBF)的形成;(4)鱼眼的形成;(5)鱼眼外的失稳裂纹扩展;(6)最终的瞬时断裂.
As technology has developed along with the increasing mechanical demands placed upon it,the need for fatigue exceeding 107 cycles or even longer for machines and components is necessary,not only for safety and reliability,but also for minimizing the economic and human costs brought about by failure. Titanium alloys have been one of the most widely used and most important materials in the aerospace domain owing to their superior properties of high strength-weight ratio and good temperature resistance. Studies have shown that S-N curves of TC4 alloy exhibit a linearly decreasing tendency and no fatigue limit around 107 cycles under very highcycle fatigue. Thus,fatigue strength design according to the traditional standard is adventurous to some extent. In this study,an electromagnetic resonant fatigue testing machine at a frequency of 100 Hz was employed to carry out fatigue tests and investigate the influence of two stress ratios( R = 0. 1 and-1) on TC4 titanium alloy under a very high-cycle fatigue regime. The results show that S-N curves under each of the two stress ratios present the so called"duplex characteristic"while their respective failure mechanisms are different. The fracture of specimens under R = 0. 1 corresponds to two modes,i. e.,surface failure induced by machining defects,and interior fisheye failure,accompanied by the appearance of facets. The horizontal part of the S-N curve at stress ratio of 0. 1 represents the transition stress between the surface failure and the interior failure,beyond which the surface failure can take place,while surface failure without facets only occurs under R =-1. Based on fracture mechanics,the threshold value of small crack growth is lower under a positive stress ratio and in a vacuum,which is more conducive to crack propagation and formation of facets. From these results,the interior fatigue failure process and mechanism of TC4 titanium can be explained as follows:( 1) the appearance of slip lines or bands inpartial α grain;( 2) initiation and coalescence of micro-crack;( 3) formation of granular bright facets( GBF);( 4) formation of fisheye;( 5) unstable crack propagation outside the fisheye;( 6) instantaneous fracture.
As technology has developed along with the increasing mechanical demands placed upon it,the need for fatigue exceeding 107 cycles or even longer for machines and components is necessary,not only for safety and reliability,but also for minimizing the economic and human costs brought about by failure. Titanium alloys have been one of the most widely used and most important materials in the aerospace domain owing to their superior properties of high strength-weight ratio and good temperature resistance. Studies have shown that S-N curves of TC4 alloy exhibit a linearly decreasing tendency and no fatigue limit around 107 cycles under very highcycle fatigue. Thus,fatigue strength design according to the traditional standard is adventurous to some extent. In this study,an electromagnetic resonant fatigue testing machine at a frequency of 100 Hz was employed to carry out fatigue tests and investigate the influence of two stress ratios( R = 0. 1 and-1) on TC4 titanium alloy under a very high-cycle fatigue regime. The results show that S-N curves under each of the two stress ratios present the so called"duplex characteristic"while their respective failure mechanisms are different. The fracture of specimens under R = 0. 1 corresponds to two modes,i. e.,surface failure induced by machining defects,and interior fisheye failure,accompanied by the appearance of facets. The horizontal part of the S-N curve at stress ratio of 0. 1 represents the transition stress between the surface failure and the interior failure,beyond which the surface failure can take place,while surface failure without facets only occurs under R =-1. Based on fracture mechanics,the threshold value of small crack growth is lower under a positive stress ratio and in a vacuum,which is more conducive to crack propagation and formation of facets. From these results,the interior fatigue failure process and mechanism of TC4 titanium can be explained as follows:( 1) the appearance of slip lines or bands inpartial α grain;( 2) initiation and coalescence of micro-crack;( 3) formation of granular bright facets( GBF);( 4) formation of fisheye;( 5) unstable crack propagation outside the fisheye;( 6) instantaneous fracture.
引文
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[10] Yu Y,Gu J L,Bai B Z,et al. Very high cycle fatigue mechanism of carbide-free bainite/martensite steel micro-alloyed with Nb. Mater Sci Eng A,2009,527(1-2):212
[11] Xu X X,Yu Y,Cui W L,et al. Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex mircrostructure. Int J Miner Metall Mater,2009,16(3):285
[12] Neal D F,Blenkinsop P A. Internal fatigue origins in two alphabeta titanium alloys. Acta Metall,1976,24(1):59
[13] Lütjering G. Influence of processing on microstructure and mechanical properties of(α+β)titanium alloys. Mater Sci Eng A,1998,243(1-2):32
[14] Bache M R. A review of dwell sensitive fatigue in titanium alloys:the role of microstructure,texture and operating conditions.Int J Fatigue,2003,25(9-11):1079
[15] Li W,Sakai T,Li Q,et al. Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime. Mater Sci Eng A,2011,528(15):5044
[16] Murakami Y. Metal Fatigue:Effects of Small Defects and Nonmetallic Inclusions. 1st ed. Oxford:Elsevier Science Ltd,2002
[17] Sakai T,Sato Y,Oguma N. Characteristic S--N properties of high-carbon-chromium-bearing steel under axial loading in longlife fatigue. Fatigue Fract Eng Mater Struct,2002,25(8-9):765
[2] Oguma H,Nakamura T. The effect of microstructure on very high cycle fatigue properties in Ti-6Al--4V. Scripta Mater,2010,63(1):32
[3] Stanzl-Tschegg S E,Mayer H. Fatigue and fatigue crack growth of aluminium alloys at very high numbers of cycles. Int J Fatigue,2001,23(Suppl 1):231
[4] Deng H L,Li W,Sun Z D,et al. A prediction model for the very high cycle fatigue life for inclusion-FGA(fine granular area)-fisheye induced fatigue failure. Chin J Eng,2017,39(4):567(邓海龙,李伟,孙振铎,等.基于夹杂-细晶粒区--鱼眼疲劳失效的超长寿命预测模型.工程科学学报,2017,39(4):567)
[5] Liu X L,Sun C Q,Zhou Y T,et al. Effects of microstructure and stress ratio on high-cycle and very high cycle fatigue behavior of Ti--6Al-4V alloy. Acta Metall Sin,2016,52(8):923(刘小龙,孙成奇,周砚田,等.微结构和应力比对Ti-6Al--4V高周和超高周疲劳行为的影响.金属学报,2016,52(8):923)
[6] Crupi V,Epasto G,Guglielmino E,et al. Influence of microstructure[alpha+beta and beta]on very high cycle fatigue behavior of Ti-6Al--4V alloy. Int J Fatigue,2017,95:64
[7] Sakai T,Takeda M,Shiozawa K,et al. Experimental reconfirmation of characteristic S-N property for high carbon chromium bearing steel in wide life region in rotating bending. J Soc Mater Sci Jpn,2000,49(7):779
[8] Shiozawa K,Lu L,Ishihara S. S-N curve characteristics and subsurface crack initiation behavior in ultra-long life fatigue of a high carbon-chromium bearing steel. Fatigue Fract Eng Mater Struct,2001,24(12):781
[9] Shiozawa K,Murai M,Shimatani Y,et al. Transition of fatigue failure mode of Ni-Cr-Mo low-alloy steel in very high cycle regime. Int J Fatigue,2010,32(3):541
[10] Yu Y,Gu J L,Bai B Z,et al. Very high cycle fatigue mechanism of carbide-free bainite/martensite steel micro-alloyed with Nb. Mater Sci Eng A,2009,527(1-2):212
[11] Xu X X,Yu Y,Cui W L,et al. Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex mircrostructure. Int J Miner Metall Mater,2009,16(3):285
[12] Neal D F,Blenkinsop P A. Internal fatigue origins in two alphabeta titanium alloys. Acta Metall,1976,24(1):59
[13] Lütjering G. Influence of processing on microstructure and mechanical properties of(α+β)titanium alloys. Mater Sci Eng A,1998,243(1-2):32
[14] Bache M R. A review of dwell sensitive fatigue in titanium alloys:the role of microstructure,texture and operating conditions.Int J Fatigue,2003,25(9-11):1079
[15] Li W,Sakai T,Li Q,et al. Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime. Mater Sci Eng A,2011,528(15):5044
[16] Murakami Y. Metal Fatigue:Effects of Small Defects and Nonmetallic Inclusions. 1st ed. Oxford:Elsevier Science Ltd,2002
[17] Sakai T,Sato Y,Oguma N. Characteristic S--N properties of high-carbon-chromium-bearing steel under axial loading in longlife fatigue. Fatigue Fract Eng Mater Struct,2002,25(8-9):765