1. School of Mechatronics Engineering, Shenyang Aerospace University, Shenyang 110136, China;
2. Key Laboratory of Fundamental Science for National Defense of Aeronautical Digital Manufacturing Process, Shenyang Aerospace University, Shenyang 110136, China
Abstract:The growth behavior of naturally-initiated small cracks in single edge notched tensile (SENT) specimen of TB6 titanium alloy was studied. Fatigue experiments were conducted under constant amplitude loading with the stress ratios R of 0.1 and 0.5 at room temperature. Small cracks were allowed to be monitored by replica method during fatigue testing. Results show that at the same stress ratio, the initiation life of crack increases from 60% to 80% of the total fatigue life with the decrease of stress level. However, the stress level has no significant effect on the crack growth rate of TB6 titanium alloy. The crack growth rate at the early stage is greatly affected by the microstructure. Once the crack length reaches 200μm, the crack growth rate will increase rapidly regardless of grain boundary or grain orientation. Small cracks of TB6 titanium alloy are originated from the sample notch root in the form of corner crack, and the major part of total fatigue life is consumed in small fatigue crack initiation phase.
[1] MILLER K J. The behaviour of short fatigue cracks and their initiation Part II-a general summary[J]. Fatigue and Fracture of Engineering Materials and Structures, 1987, 10:93-113.
[2] NEWMAN J J, PHILIPS E P, SWAIN M H. Fatigue-life prediction methodology using small-crack theory[J]. International Journal of Fatigue, 1999, 21:109-119.
[3] LI W F, ZHANG X P. Investigation of initiation and growth behavior of short fatigue cracks emanating from a single edge notch specimen using in-situ SEM[J]. Materials Science and Engineering:A, 2001, 318:129-136.
[4] 陈勃,高玉魁,吴学仁,等. 喷丸强化7475-T7351铝合金的小裂纹行为和寿命预测[J]. 航空学报,2010,31(3):519-525. CHEN B, GAO Y K, WU X R, et al. Small crack behavior and fatigue life prediction for shot peening aluminum alloy 7475-T7351[J]. Acta Aeronautica et Astronautica Sinica, 2010, 31(3):519-525.
[5] 陈勃,吴学仁,刘建中. 基于小裂纹扩展的耐久性分析和经济寿命预测方法[J]. 机械强度,2004(增刊1):246-249. CHEN B, WU X R, LIU J Z. Durability analysis and economic life prediction using small crack theory[J]. Journal of Mechanical Strength, 2004(Suppl 1):246-249.
[6] 吴学仁,刘建中. 基于小裂纹理论的航空材料疲劳全寿命预测[J]. 航空学报,2006,27(2):219-226. WU X R, LIU J Z. Total fatigue life prediction for aeronautical materials by using small crack theory[J]. Acta Aeronautica et Astronautica Sinica, 2006,27(2):219-226.
[7] 童第华,吴学仁,刘建中,等. 基于小裂纹理论的铸造钛合金ZTC4疲劳寿命预测[J]. 材料工程,2015,43(6):60-65. TONG D H, WU X R, LIU J Z,et al. Fatigue life prediction of cast titanium alloy ZTC4 based on the small crack theory[J].Journal of Materials Engineering,2015,43(6):60-65.
[8] KUJAWSKI D. A new driving force parameter for crack growth in aluminum alloys[J]. International Journal of Fatigue, 2001, 23(8):733-740.
[9] KUJAWSKI D.A fatigue crack driving force parameter with load ratio effects[J]. International Journal of Fatigue,2001,23(1):239-246.
[10] DINDA S, KUJAWSKI D. Correlation and prediction of fatigue crack growth for different R-ratios using and parameters[J]. Engineering Fracture Mechanics, 2004, 71(12):1779-1790.
[11] 丁传富,刘建中,吴学仁. TC4钛合金和7475铝合金的长裂纹和小裂纹扩展特性的研究[J]. 航空材料学报,2005,25(6):11-17. DING C F, LIU J Z, WU X R. An investigation of small crack and long crack propagation behavior in titanium alloy TC4 and aluminum alloy 7475[J]. Journal of Aeronautical Materials,2005,25(6):11-17.
[12] CONNOLLEY T, REED P, STARINK M J. Short crack initiation and growth at 600℃ in notched specimens of Inconel 718[J]. Materials Science and Engineering:A, 2003,340(1/2):139-154.
[13] 张丽,吴学仁,黄新跃. GH4169合金自然萌生小裂纹扩展行为的试验研究[J]. 航空学报,2015, 36(3):840-847. ZHANG L, WU X R, HUANG X Y. Experimental investigation on the growth behavior of naturally initiated small cracks in superalloy GH4169[J]. Acta Aeronautica et Astronautica Sinica, 2015, 36(3):840-847.
[14] NISITANI H, GOTO M, KAWAGOISHI N. A small-crack growth law and its related phenomena[J]. Engineering Fracture Mechanics, 1992, 41(4):499-513.
[15] BLANKENSHIP C P, STARKE E A. The fatigue crack growth behavior of the Al-Cu-Li alloy weldalite 049[J]. Fatigue and Fracture of Engineering Materials and Structures,1991,14(1):103-114.
[16] DEMULSANT X, MENDES J. Microstructure effects on small fatigue crack initiation and growth in Ti6Al4V alloys[J]. Fatigue and Fracture of Engineering Materials and Structures,1995,18(12):1483-1497.