Al-Zn-Mg合金焊接接头疲劳裂纹萌生特性研究
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摘要
A7N01铝合金是一种典型的Al-Zn-Mg合金,广泛用于高速列车结构,疲劳失效是该合金失效的主要形式。疲劳失效包括了疲劳裂纹萌生和扩展两个阶段,通过断裂力学可以较好的预测疲劳裂纹扩展寿命,但却无法预测裂纹萌生阶段的寿命,而这一阶段甚至能够占据疲劳总寿命的50%以上。
本文针对A7N01铝合金焊接接头进行了缺口疲劳试验,考察了焊缝、热影响区和母材的疲劳裂纹萌生特性。实验结果表明,在较高应力幅下,母材和焊缝区的疲劳裂纹萌生寿命差异较小,随着应力幅的降低,母材和焊缝区的裂纹萌生寿命的差别逐渐显现,与母材和焊缝相比,热影响区的疲劳裂纹萌生寿命值波动较大。母材的疲劳总寿命高于热影响区和焊缝的疲劳总寿命,焊缝的疲劳总寿命最低。三个区域疲劳裂纹萌生寿命占总寿命的比例与应力幅无关,该比例与微区性能关系密切,各区差异显著,其中焊缝最高,母材最低。
断口分析表明,疲劳裂纹在母材区萌生于富含Fe的未回溶大尺寸第二相粒子,在焊缝区萌生于晶界,晶界上存在元素偏析,热影响区疲劳裂纹萌生于边缘靠近焊趾处。通过显微组织分析发现,第二相粒子在加载前已经碎裂,晶界上有杂质元素Fe和合金元素Cr的富集,气孔主要位于熔合区。
利用连续损伤力学,基于微塑性假设构建了焊接接头的高周疲劳损伤模型。在该模型中,不考虑裂纹闭合效应,确定了损伤条件,提出了微塑性应变的对偶变量X,X为对微塑性变形起作用的短程内应力。通过裂纹萌生寿命曲线,对应于7N01铝合金得到损伤模型中的材料参数,确定了预测7N01铝合金焊接接头焊缝区、热影响区和母材的疲劳裂纹萌生寿命的具体表达式,预测了40MPa下焊接接头疲劳裂纹萌生寿命,与试验结果吻合较好。
Fatigue failure is the primary failure form of the A7N01 aluminum alloy belonged to Al-Zn-Mg alloy which is one of the main materials in the structure of high-speed vehicle. The fracture mechanics was applied to the fatigue crack propagation process and it has got an ideal solution. But thus far it couldn't predict the crack initiation life that could hold even more than 50 percent of the total fatigue life.
In this paper,the fatigue crack initiation properties in the weld metal, heat affected zone and base metal are detected by the notch fatigue test. Fatigue experiment results show that there is little difference in the fatigue crack initiation life between the base metal and the weld under the high stress amplitude. With the reducing of the stress amplitude, the difference appeared gradually. The fatigue crack initiation life of the heat affected zone doesn’t have obvious discipline in contrast with the base metal and weld. The total fatigue life of the base metal is higher than the heat affected zone and the weld that is the lowest. The proportion of the initiation life in the total fatigue life is closely related to micro-area properties while less affected with the changes of stress amplitude levels. In the weld, the initiation lifetime accounts more of the whole life than other zones.
The fractographs show that the fatigue crack initiates in different places, which are second phase particles rich in Fe in large size, the free surface along the grain boundaries and the pore, for base metal, weld and heat affected zone, respectively. By microstructure analysis it is found that second phase particles have crashed before loading, Fe and Cr are rich in the free surface. The pore in the heat affected zone locates at the fusion line.
Based on micro-plastic hypothesis, a continuum damage mechanics model of high cycle fatigue is presented. In this model, the crack closure isn't considered, the damage conditions is confirmed, the dual variable of the plastic strain X is put forward and X is the internal short-range stress that effects the micro-plastic. The parameters in the model are fitted from the crack initiation life curve of A7N01 aluminum alloy and the model can predict the fatigue crack initiation life in the three regions. It is found that the fatigue crack initiation life under 40MPa predicted by the model has a good accordance with the experiment results.
本文针对A7N01铝合金焊接接头进行了缺口疲劳试验,考察了焊缝、热影响区和母材的疲劳裂纹萌生特性。实验结果表明,在较高应力幅下,母材和焊缝区的疲劳裂纹萌生寿命差异较小,随着应力幅的降低,母材和焊缝区的裂纹萌生寿命的差别逐渐显现,与母材和焊缝相比,热影响区的疲劳裂纹萌生寿命值波动较大。母材的疲劳总寿命高于热影响区和焊缝的疲劳总寿命,焊缝的疲劳总寿命最低。三个区域疲劳裂纹萌生寿命占总寿命的比例与应力幅无关,该比例与微区性能关系密切,各区差异显著,其中焊缝最高,母材最低。
断口分析表明,疲劳裂纹在母材区萌生于富含Fe的未回溶大尺寸第二相粒子,在焊缝区萌生于晶界,晶界上存在元素偏析,热影响区疲劳裂纹萌生于边缘靠近焊趾处。通过显微组织分析发现,第二相粒子在加载前已经碎裂,晶界上有杂质元素Fe和合金元素Cr的富集,气孔主要位于熔合区。
利用连续损伤力学,基于微塑性假设构建了焊接接头的高周疲劳损伤模型。在该模型中,不考虑裂纹闭合效应,确定了损伤条件,提出了微塑性应变的对偶变量X,X为对微塑性变形起作用的短程内应力。通过裂纹萌生寿命曲线,对应于7N01铝合金得到损伤模型中的材料参数,确定了预测7N01铝合金焊接接头焊缝区、热影响区和母材的疲劳裂纹萌生寿命的具体表达式,预测了40MPa下焊接接头疲劳裂纹萌生寿命,与试验结果吻合较好。
Fatigue failure is the primary failure form of the A7N01 aluminum alloy belonged to Al-Zn-Mg alloy which is one of the main materials in the structure of high-speed vehicle. The fracture mechanics was applied to the fatigue crack propagation process and it has got an ideal solution. But thus far it couldn't predict the crack initiation life that could hold even more than 50 percent of the total fatigue life.
In this paper,the fatigue crack initiation properties in the weld metal, heat affected zone and base metal are detected by the notch fatigue test. Fatigue experiment results show that there is little difference in the fatigue crack initiation life between the base metal and the weld under the high stress amplitude. With the reducing of the stress amplitude, the difference appeared gradually. The fatigue crack initiation life of the heat affected zone doesn’t have obvious discipline in contrast with the base metal and weld. The total fatigue life of the base metal is higher than the heat affected zone and the weld that is the lowest. The proportion of the initiation life in the total fatigue life is closely related to micro-area properties while less affected with the changes of stress amplitude levels. In the weld, the initiation lifetime accounts more of the whole life than other zones.
The fractographs show that the fatigue crack initiates in different places, which are second phase particles rich in Fe in large size, the free surface along the grain boundaries and the pore, for base metal, weld and heat affected zone, respectively. By microstructure analysis it is found that second phase particles have crashed before loading, Fe and Cr are rich in the free surface. The pore in the heat affected zone locates at the fusion line.
Based on micro-plastic hypothesis, a continuum damage mechanics model of high cycle fatigue is presented. In this model, the crack closure isn't considered, the damage conditions is confirmed, the dual variable of the plastic strain X is put forward and X is the internal short-range stress that effects the micro-plastic. The parameters in the model are fitted from the crack initiation life curve of A7N01 aluminum alloy and the model can predict the fatigue crack initiation life in the three regions. It is found that the fatigue crack initiation life under 40MPa predicted by the model has a good accordance with the experiment results.
引文
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[3]陈小祝,匡永祥.铝及铝合金在交通运输业中的应用[J].铝加工,1993(2):1-4.
[4]薛华.高速列车用A6N01S和A7N01S铝合金焊接接头疲劳裂纹扩展速率研究[D].天津大学硕士论文,2007:1-7.
[5]钱友荣.疲劳裂纹萌生机制[J].兵器材料科学与工程,1986(11):17-25.
[6] J. Lemaitre. How To Use Damage Mechanics[J]. Nuclear Engineering and Design,1984,80(2):233-245.
[7] J.L. Chaboche, P.M. Lesne. A Non-linear Continuous Fatigue Damage Model[J]. Fatigue Fract. Engng Mater,Struct,1988, 11(1):1-17.
[8] Dusan Krajcinovic, Sreten Mastilovic. Some Fundamental Issues of Damage Mechanics[J]. Mechanics of Materials,1995,21(3):217-230.
[9] Hult. "Continuum Damage Mechanics—Capabilities Limitations and Promises" Mechanisms of Deformation and Fracture[J]. Pergamon Oxford,1979(5):233-347.
[10]余寿文,冯西桥.损伤力学[M].清华大学出版社,1997:1-6.
[11]李兆霞.损伤力学及其应用[M].科学出版社,2002:1-6.
[12]李灏.损伤力学基础[M].山东科学技术出版社,1992:209-215.
[13] J.L. Chaboche. Continuum Damage Mechanics:Part I—General Concepts[J]. Journal of Applied Mechanics.1988,55(1):59-64.
[14] June Wang. A Continuum Damage Mechanics Model for Low-Cycle Fatigue Failure of Metals[J]. Engineers Fracture Mechanics,1992,41(3):437-441.
[15] J. Lemaitre, J.P. Sermage and R. Desmorata. A two scale damage concept applied to fatigue[J]. International Journal of Fracture,1999,97(5): 67-81.
[16] Y.C. Xiao, S. Li and Z. Gao. A Continuum Damage Mechanics Model for High Cycle Fatigue[J]. International Journal of Fatigue,1998,20(7):503-508.
[17]肖迎春.疲劳损伤力学模型研究[D].北京航空航天大学博士论文,1998:58-64.
[18]余天庆,钱济成.损伤理论及其应用[M].国防工业出版社,1993:61-64.
[19] Guangxu Cheng, Jianzheng zuo, Zhiwen Lou, etal. Continuum Damage Modelof Low-Cycle Fatigue and Fatigue Damage Analysis of Welded Joint[J]. Engineering Fracture Mechanics,1996,55(1):155-161.
[20]邬华芝.钛合金焊接接头低周疲劳损伤模型研究[D].南京航空航天大学博士论文,2003:23-29.
[21]邬华芝,郭海丁,高德平等.焊接接头低周疲劳损伤分形演化模型[J].焊接学报,2003,24(1):88-90.
[22] N.A. Koneva, O.V. Sosnin, L.A. Teplyakona. Fatigue Evolution of Dislocation Substructures[J]. Novokuznetsk,2001(10):25-28.
[23]贾维平,李守新,王中光等.铜三晶体的循环形变行为及位错组态[J].材料研究学报,2000,14(6):619-623.
[24]李勇,李守新,李广义等.铜单晶驻留滑移带演化的实验观察与形成机制[J].自然科学进展,2001,11(6):625-630.
[25] W.P. Jia, J.V. Fernandes. Mechanical behaviour and the evolution of the dislocation structureof copper polycrystal deformed under fatigue-tension and tension-fatigue sequential strain paths[J]. Materials Science and Engineering,2003,348(5):133-144.
[26] H.L. Huang. A study of dislocation evolution in polycrystalline copper during low cycle fatigue at low strain amplitudes[J]. Materials Science and Engineering,2003,348(6):38-43.
[27] A.V. Gromova, Yu.F. Ivanov, S.V. Vorobyov. Ways of the dislocation substructure evolution in austenite steel under low and multicycle fatigue[J]. Procedia Engineering,2010(2):83-90.
[28] S.V. Konovalov, A.A. Atroshkina, Yu.F. Ivanov. Evolution of dislocation substructures in fatigue loaded and failed stainless steel with the intermediate electropulsing treatment[J]. Materials Science and Engineering,2010,527(7):3040-3043.
[29] Xiaoshan Liu, Guoqiu He, Xiangqun Ding. Fatigue behavior and dislocation substructures for 6063 aluminum alloy under nonproportional loadings[J]. International Journal of Fatigue,2009,31(8):1190-1195.
[30]范宋杰,何国球,刘晓山等. A356铝合金高周疲劳循环形变分析[J].上海金属,2007,29(6):23-26.
[31] Toshiyuki Fujii, Chihiro Watanabe, Yoshimichi Nomura. Microstructural evolution during low cycle fatigue of a 3003 aluminum alloy[J]. Materials Science and Engineering,2001,319(7):592-596.
[32] Shigenobu Kainuma, Takeshi Mori. A study on fatigue crack initiation point of load-carrying fillet welded cruciform joints[J]. International Journal ofFatigue,2008,30(9):1669-1677.
[33] Kwai S. Chan. Roles of microstructure in fatigue crack initiation[J]. International Journal of Fatigue,2010,32(6):1428-1447.
[34]张哲峰,段启强,王中光等.铜晶体的疲劳损伤微观机制[J].金属学报,2005(11):1143-1148.
[35]夏青,盐田俊雄.低碳钢疲劳裂纹的萌生及扩展的观察[J].洛阳工学院学报,1992,13(9): 57-61.
[36] H.L. Huang, N.J. Ho. The study of fatigue in polycrystalline copper under various strain amplitude at stage I: crack initiation and propagation[J]. Materials Science and Engineering,2000,293(5):7–14.
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