AZ31B镁合金TIG焊接接头疲劳评定局部法研究
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摘要
基于轻质高强及矿产资源丰富的优势,镁合金已逐渐成为“陆海空天”交通运载装备中的重要结构材料。这些运载装备都是焊接结构,大部分都承受疲劳载荷的作用,潜在的危害大多是由于焊接接头疲劳失效引起的。而目前国内有关镁合金疲劳评定方法的研究还处于空白阶段,因此,研究镁合金焊接接头疲劳性能及其评定方法对镁合金结构的广泛应用与安全性具有重要的理论意义和实用价值。
本文主要采用有限元模拟手段研究AZ31B镁合金TIG焊接接头疲劳评定局部法中的临界距离法和临界面法,并将评定结果与疲劳试验结果进行对比分析。通过疲劳试验分析对接接头、横向十字接头和纵向十字接头三种接头型式的疲劳性能,观察不同接头型式的疲劳断裂位置,并通过扫描电镜分析其断裂类型；采用临界距离(CDM)法对对接接头、横向十字接头和纵向十字接头三种接头型式进行疲劳评定分析；采用考虑焊接残余应力的临界面(CPM)法对横向十字接头进行焊接温度场和残余应力场分析,并最终确定局部参量进行疲劳评定研究分析。
论文首先通过疲劳试验测试了AZ31B镁合金对接接头、横向十字接头和纵向十字接头三种典型焊接接头型式的疲劳性能,根据疲劳试验数据进行了S-N曲线拟合。按照国际焊接学会(IIW)推荐的数据处理方式,分析得出对接接头、横向十字接头和纵向十字接头型式的疲劳试验中值(存活率50%)S-N曲线的表达式分别为：lgN=13.45-4.491gΔσ、lgN=11.05-3.391gΔσ和lgN=11.38-3.371gΔσ。得出在2×106循环次数下,三种接头型式的疲劳强度分别为：39.17MPa、25.11MPa和32.15MPa。采用国际焊接学会推荐的S-N曲线斜率m=3.0时,相应的S-N曲线表达式分别为：lgN=11.01-3.001gΔσ、lgN=10.45-3.00lgΔσ和lgN=10.78-3.00lgΔσ。得出在2×106循环次数下,三种接头型式的疲劳强度分别为：37.04PMpa、24.19MPa和31.20MPa。对比分析发现,采用国际焊接学会推荐的m=3.0时得出的疲劳强度值与疲劳试验相比都较低,说明国际焊接学会推荐的方法对AZ31B镁合金来说是偏于安全的。
观察不同接头型式的疲劳断裂的宏观裂纹形貌,可以看出失效件的断裂位置均位于焊趾处。通过扫描电镜分析其断裂类型,分析发现AZ31B镁合金焊接接头疲劳断口中存在大量疏松的低熔共晶物,具有河滩状条纹、二次裂纹、舌状条纹、台阶状条纹等明显的解理断裂特征,可以认为AZ31B镁合金焊接接头的疲劳断裂属于典型的解理断裂。
采用临界距离(CDM)法中的点法(PM)和线法(LM)对AZ31B镁合金对接接头、横向十字接头和纵向十字接头进行疲劳评定。通过有限元软件模拟分析,观察不同接头型式的应力云图分布,可以看出三种接头型式的最大应力均位于焊趾处,即焊趾处为试件最有可能发生疲劳断裂的危险点。观察不同接头型式不同试件与最大主应力垂直方向(沿裂纹扩展方向)路径r上的应力分布状况,即△σ1-r曲线,计算出PM法和LM法的局部应力参量△σloc-p和△loc-L,拟合出相应的S-N曲线,并得出三种接头型式采用PM法时S-N曲线的表达式分别为：lgN=15.14-4.651gΔσ、lgN=12.03-3.391gΔσ和lgN=12.48-3.391gΔσ;采用LM时,分别为：lgN=14.94-4.551gΔσ、lgN=11.90-3.401gΔσ和lgN=11.99-3.371gΔσ。得出在2×106循环次数下,对接接头、横向十字接头和纵向十字接头采用PM法的疲劳强度分别为79.55MPa、49.10MPa和66.49Mpa;采用LM法的疲劳强度分别为79.01MPa、44.38MPa和48.93MPa。将评定结果与疲劳试验结果进行对比分析,结果表明,采用CDM法可以对AZ31B镁合金三种接头型式进行合理的疲劳评定,其评定结果与疲劳试验结果相符合,依据应力集中大小可以有效预测疲劳断裂位置。
采用考虑焊接残余应力的CPM法对AZ31B镁合金典型的角接头型式——横向十字接头进行了疲劳评定研究。通过采用有限元模拟软件对横向十字接头型式的实际焊接顺序模拟,进行焊接温度场和残余应力场耦合分析,观察四道焊缝残余应力场应力云图分布,发现四道焊缝均为焊趾处应力最大,并计算得出每道焊缝沿x、y、z三个方向的残余应力值。四道焊缝沿三个方向的残余应力大小也并不相同,相比而言,按照实际焊接顺序,第三道焊缝焊趾处的残余应力最大。根据有限元分析计算出试件局部最大主应力及应力比R=0,最终计算出各试件的局部参量βexp。在Dang Van疲劳评定准则基础上,将参数修正为α*=0.5,β*=55MPa,并拟合出Dang Van图形,即zmax—Pmax+1/3 tr(σres)曲线。从Dang Van图上可以看出处于修正后Dang Van线右上侧的试样发生疲劳断裂,处于Dang Van线左下侧的试样则不会发生断裂。说明修正的临界面法可以对AZ31B镁合金横向十字接头进行合理的疲劳评定分析,并可以根据应力集中大小预测试件可能的疲劳断裂位置。
Based on the advantages of light-weight, high-strength, and rich mineral resources, magnesium alloy have become a kind of important structural materials of traffic carrying equipments among "the land, the sea, the air and the aerospace field". Those traffic carrying equipments, which are mostly under fatigue load, are welded structures and their potential risks are usually caused by fatigue failure of welded joints, but at home, it is still blank on the research of fatigue assessment of magnesium alloy now. Therefore, the study of fatigue properties and fatigue assessment methods have great theory significance and practical value on extensive application and security of the magnesium alloy structure.
By element simulation, this paper maily studies fatigue assessment of TIG welding joints of AZ31B magnesium alloy by the critical distance method and the critical plane method, which along to the local approach, and compare the fatigue test results with simulation results. Fatigue properties of butt joint, transversr cross joint and longitudinal cross joint of AZ31B magnesium alloy by TIG welding were tested, and observe their failure position, moreover, the types of fracture were also obtained by SEM analysis. The three different types of welded joints were assessed using critical distance method (CDM); The fatigue assessment of trasverse cross joint was processed by analysis of temperature field and residual stress field based on critical plane method (CPM), which considered the effect of residual stress, and finally, the local parameter was defined.
The Fatigue properties of butt joint, transversr cross joint and longitudinal cross joint of AZ31B magnesium alloy by TIG welding were tested and the S-N curves were fitted. In the light of ways of how to manipulate data the international institute of welding (ⅡW) recommended, the expressions of the experiment mean S-N curves of welded joints:butt joint, transverse cross joint, and longitudinal cross joint, are brought out, they respectively are lgN=13.45-4.491gΔσ, lgN=11.05-3.391gΔσand lgN=11.38-3.371gΔσ. Under 2×106 cycles, the fatigue strength of butt joint, transverse cross joint and longitudinal cross joint are respectively 29.17MPa,25.11MPa and 32.15MPa. When the slope m of S-N curves the international institute of welding (ⅡW) recommended is 3.0, the expressions of S-N curves for three different types of welded joints are respectively lgN=11.01-3.001gΔσ, lgN=10.45-3.001gΔσand lgN=10.78-3.001gΔσ, then, under 2×106 cycles, the fatigue strength of butt joint, transverse cross joint and longitudinal cross joint are respectively 37.04MPa, 24.19MPa and 31.20MPa. By comparing the result, for each types of welded joins, the fatigue strength dealed with m=3.0 is lower than the result gained by fatigue test directly. It demonstrates that the method recommended by the international institute of welding is slanted to more safe for AZ31B magnusium alloy.
Observing the failure position of different types of welded joints, it shows that they all failure at welding toe. The fracture type were anlysed by scanning electron microscope, there were lots of low melting eutectic materials, and obvious cleaves fracture characteristics like the beach shape stripe, secondary cracks, tongue shape stripe, bench-shape stripes. So it indicates that the fatigue fracture type of welded joints of AZ31B magnesium alloy should belong to cleavage farture.
The fatigue assessment was investigated on butt joints, transverse cross joints and longitudinal cross joints of AZ31B magnesium alloy by using the point method (PM) and the line method (LM), which belong to the critical distance method (CDM). From the stress contour of different welded joints, the maximum stress all locates at the weld toe, where is the most possible fracture place of welded joints. For different types of welded joints, for each sample, the stress state of path r,Δσ1—r curves are absversed by simulates of finite element software, the path r is vertical to the direction of the maximum principal stress, and it also is the direction of fatigue crack propagation. Based on the local stress parametersΔσloc-p andΔσloc-L of the three types of welded joints, the corresponding S-N curves were fitted. For butt joint, transverse cross joint and longitudinal cross joint, the expressions of the S-N curves are respectively lgN=15.14-4.651gΔσ, lgN=12.03-3.391gΔσand lgN=12.48-3.391gΔσfor PM; and lgN=14.94-4.551gΔσ, lgN=11.90-3.401gΔσand lgN=11.99-3.371gΔσfor LM. Under 2×106 cycles, the fatigue strength of butt joints transverse cross joints and longitudinal cross joints are respectively 79.55MPa 49.10MPa and 66.48MPa for the PM,79.01MPa 44.38MPa and 48.93MPa for LM. Comparing the result of finite element analysis with the result of fatigue test, it shows that the CDM approach could be used in fatigue assessment on butt joints, transverse cross joints and longitudinal cross joints of AZ31B magnesium alloy. The assessing results of PM and LM are consistent with the result of fatigue test. Further more, according to the magnitude of stress concentration, the CDM approach could prognosticate the location of fatigue fracture.
The fatigue assessment of trasverse cross joint of magnesium alloy was studied using critical plane method (CPM) considering the effect of residual stress. By simulating the welding order of transverse cross joint and analysing temperature field and residual stress field, the residual stress contour of the four weld seam were gained, and it makes out that the maximum stress all mainly found in the weld toe, and the residual stress value of x, y, z directions were computed. Compare the residual stress value of three different directions of the four weld seam, it figures out that the residual stress value of the third weld seam is the maximum. According to the maximum principal stress of simulating, and as all know the stress ratio R=0, and finally, the local parameterβexp was calculated. Based on the Dang Van fatigue assessment criteria, the parameters corrected are a*=0.5, (3*=55MPa. Further more, the Dang Van figure was figured, it also isτmax—Pmax+1/3 tr(σres) curve. From the Dang Van figure, it illustrates that the result right upon the Dang Van line can failure; on the contrary, the result left under the Dang Van line can not faliure. The result demonstrates that the critical plane method coulde assess the transverse cross joint of AZ31B magnesium alloy reasonably.
本文主要采用有限元模拟手段研究AZ31B镁合金TIG焊接接头疲劳评定局部法中的临界距离法和临界面法,并将评定结果与疲劳试验结果进行对比分析。通过疲劳试验分析对接接头、横向十字接头和纵向十字接头三种接头型式的疲劳性能,观察不同接头型式的疲劳断裂位置,并通过扫描电镜分析其断裂类型；采用临界距离(CDM)法对对接接头、横向十字接头和纵向十字接头三种接头型式进行疲劳评定分析；采用考虑焊接残余应力的临界面(CPM)法对横向十字接头进行焊接温度场和残余应力场分析,并最终确定局部参量进行疲劳评定研究分析。
论文首先通过疲劳试验测试了AZ31B镁合金对接接头、横向十字接头和纵向十字接头三种典型焊接接头型式的疲劳性能,根据疲劳试验数据进行了S-N曲线拟合。按照国际焊接学会(IIW)推荐的数据处理方式,分析得出对接接头、横向十字接头和纵向十字接头型式的疲劳试验中值(存活率50%)S-N曲线的表达式分别为：lgN=13.45-4.491gΔσ、lgN=11.05-3.391gΔσ和lgN=11.38-3.371gΔσ。得出在2×106循环次数下,三种接头型式的疲劳强度分别为：39.17MPa、25.11MPa和32.15MPa。采用国际焊接学会推荐的S-N曲线斜率m=3.0时,相应的S-N曲线表达式分别为：lgN=11.01-3.001gΔσ、lgN=10.45-3.00lgΔσ和lgN=10.78-3.00lgΔσ。得出在2×106循环次数下,三种接头型式的疲劳强度分别为：37.04PMpa、24.19MPa和31.20MPa。对比分析发现,采用国际焊接学会推荐的m=3.0时得出的疲劳强度值与疲劳试验相比都较低,说明国际焊接学会推荐的方法对AZ31B镁合金来说是偏于安全的。
观察不同接头型式的疲劳断裂的宏观裂纹形貌,可以看出失效件的断裂位置均位于焊趾处。通过扫描电镜分析其断裂类型,分析发现AZ31B镁合金焊接接头疲劳断口中存在大量疏松的低熔共晶物,具有河滩状条纹、二次裂纹、舌状条纹、台阶状条纹等明显的解理断裂特征,可以认为AZ31B镁合金焊接接头的疲劳断裂属于典型的解理断裂。
采用临界距离(CDM)法中的点法(PM)和线法(LM)对AZ31B镁合金对接接头、横向十字接头和纵向十字接头进行疲劳评定。通过有限元软件模拟分析,观察不同接头型式的应力云图分布,可以看出三种接头型式的最大应力均位于焊趾处,即焊趾处为试件最有可能发生疲劳断裂的危险点。观察不同接头型式不同试件与最大主应力垂直方向(沿裂纹扩展方向)路径r上的应力分布状况,即△σ1-r曲线,计算出PM法和LM法的局部应力参量△σloc-p和△loc-L,拟合出相应的S-N曲线,并得出三种接头型式采用PM法时S-N曲线的表达式分别为：lgN=15.14-4.651gΔσ、lgN=12.03-3.391gΔσ和lgN=12.48-3.391gΔσ;采用LM时,分别为：lgN=14.94-4.551gΔσ、lgN=11.90-3.401gΔσ和lgN=11.99-3.371gΔσ。得出在2×106循环次数下,对接接头、横向十字接头和纵向十字接头采用PM法的疲劳强度分别为79.55MPa、49.10MPa和66.49Mpa;采用LM法的疲劳强度分别为79.01MPa、44.38MPa和48.93MPa。将评定结果与疲劳试验结果进行对比分析,结果表明,采用CDM法可以对AZ31B镁合金三种接头型式进行合理的疲劳评定,其评定结果与疲劳试验结果相符合,依据应力集中大小可以有效预测疲劳断裂位置。
采用考虑焊接残余应力的CPM法对AZ31B镁合金典型的角接头型式——横向十字接头进行了疲劳评定研究。通过采用有限元模拟软件对横向十字接头型式的实际焊接顺序模拟,进行焊接温度场和残余应力场耦合分析,观察四道焊缝残余应力场应力云图分布,发现四道焊缝均为焊趾处应力最大,并计算得出每道焊缝沿x、y、z三个方向的残余应力值。四道焊缝沿三个方向的残余应力大小也并不相同,相比而言,按照实际焊接顺序,第三道焊缝焊趾处的残余应力最大。根据有限元分析计算出试件局部最大主应力及应力比R=0,最终计算出各试件的局部参量βexp。在Dang Van疲劳评定准则基础上,将参数修正为α*=0.5,β*=55MPa,并拟合出Dang Van图形,即zmax—Pmax+1/3 tr(σres)曲线。从Dang Van图上可以看出处于修正后Dang Van线右上侧的试样发生疲劳断裂,处于Dang Van线左下侧的试样则不会发生断裂。说明修正的临界面法可以对AZ31B镁合金横向十字接头进行合理的疲劳评定分析,并可以根据应力集中大小预测试件可能的疲劳断裂位置。
Based on the advantages of light-weight, high-strength, and rich mineral resources, magnesium alloy have become a kind of important structural materials of traffic carrying equipments among "the land, the sea, the air and the aerospace field". Those traffic carrying equipments, which are mostly under fatigue load, are welded structures and their potential risks are usually caused by fatigue failure of welded joints, but at home, it is still blank on the research of fatigue assessment of magnesium alloy now. Therefore, the study of fatigue properties and fatigue assessment methods have great theory significance and practical value on extensive application and security of the magnesium alloy structure.
By element simulation, this paper maily studies fatigue assessment of TIG welding joints of AZ31B magnesium alloy by the critical distance method and the critical plane method, which along to the local approach, and compare the fatigue test results with simulation results. Fatigue properties of butt joint, transversr cross joint and longitudinal cross joint of AZ31B magnesium alloy by TIG welding were tested, and observe their failure position, moreover, the types of fracture were also obtained by SEM analysis. The three different types of welded joints were assessed using critical distance method (CDM); The fatigue assessment of trasverse cross joint was processed by analysis of temperature field and residual stress field based on critical plane method (CPM), which considered the effect of residual stress, and finally, the local parameter was defined.
The Fatigue properties of butt joint, transversr cross joint and longitudinal cross joint of AZ31B magnesium alloy by TIG welding were tested and the S-N curves were fitted. In the light of ways of how to manipulate data the international institute of welding (ⅡW) recommended, the expressions of the experiment mean S-N curves of welded joints:butt joint, transverse cross joint, and longitudinal cross joint, are brought out, they respectively are lgN=13.45-4.491gΔσ, lgN=11.05-3.391gΔσand lgN=11.38-3.371gΔσ. Under 2×106 cycles, the fatigue strength of butt joint, transverse cross joint and longitudinal cross joint are respectively 29.17MPa,25.11MPa and 32.15MPa. When the slope m of S-N curves the international institute of welding (ⅡW) recommended is 3.0, the expressions of S-N curves for three different types of welded joints are respectively lgN=11.01-3.001gΔσ, lgN=10.45-3.001gΔσand lgN=10.78-3.001gΔσ, then, under 2×106 cycles, the fatigue strength of butt joint, transverse cross joint and longitudinal cross joint are respectively 37.04MPa, 24.19MPa and 31.20MPa. By comparing the result, for each types of welded joins, the fatigue strength dealed with m=3.0 is lower than the result gained by fatigue test directly. It demonstrates that the method recommended by the international institute of welding is slanted to more safe for AZ31B magnusium alloy.
Observing the failure position of different types of welded joints, it shows that they all failure at welding toe. The fracture type were anlysed by scanning electron microscope, there were lots of low melting eutectic materials, and obvious cleaves fracture characteristics like the beach shape stripe, secondary cracks, tongue shape stripe, bench-shape stripes. So it indicates that the fatigue fracture type of welded joints of AZ31B magnesium alloy should belong to cleavage farture.
The fatigue assessment was investigated on butt joints, transverse cross joints and longitudinal cross joints of AZ31B magnesium alloy by using the point method (PM) and the line method (LM), which belong to the critical distance method (CDM). From the stress contour of different welded joints, the maximum stress all locates at the weld toe, where is the most possible fracture place of welded joints. For different types of welded joints, for each sample, the stress state of path r,Δσ1—r curves are absversed by simulates of finite element software, the path r is vertical to the direction of the maximum principal stress, and it also is the direction of fatigue crack propagation. Based on the local stress parametersΔσloc-p andΔσloc-L of the three types of welded joints, the corresponding S-N curves were fitted. For butt joint, transverse cross joint and longitudinal cross joint, the expressions of the S-N curves are respectively lgN=15.14-4.651gΔσ, lgN=12.03-3.391gΔσand lgN=12.48-3.391gΔσfor PM; and lgN=14.94-4.551gΔσ, lgN=11.90-3.401gΔσand lgN=11.99-3.371gΔσfor LM. Under 2×106 cycles, the fatigue strength of butt joints transverse cross joints and longitudinal cross joints are respectively 79.55MPa 49.10MPa and 66.48MPa for the PM,79.01MPa 44.38MPa and 48.93MPa for LM. Comparing the result of finite element analysis with the result of fatigue test, it shows that the CDM approach could be used in fatigue assessment on butt joints, transverse cross joints and longitudinal cross joints of AZ31B magnesium alloy. The assessing results of PM and LM are consistent with the result of fatigue test. Further more, according to the magnitude of stress concentration, the CDM approach could prognosticate the location of fatigue fracture.
The fatigue assessment of trasverse cross joint of magnesium alloy was studied using critical plane method (CPM) considering the effect of residual stress. By simulating the welding order of transverse cross joint and analysing temperature field and residual stress field, the residual stress contour of the four weld seam were gained, and it makes out that the maximum stress all mainly found in the weld toe, and the residual stress value of x, y, z directions were computed. Compare the residual stress value of three different directions of the four weld seam, it figures out that the residual stress value of the third weld seam is the maximum. According to the maximum principal stress of simulating, and as all know the stress ratio R=0, and finally, the local parameterβexp was calculated. Based on the Dang Van fatigue assessment criteria, the parameters corrected are a*=0.5, (3*=55MPa. Further more, the Dang Van figure was figured, it also isτmax—Pmax+1/3 tr(σres) curve. From the Dang Van figure, it illustrates that the result right upon the Dang Van line can failure; on the contrary, the result left under the Dang Van line can not faliure. The result demonstrates that the critical plane method coulde assess the transverse cross joint of AZ31B magnesium alloy reasonably.
引文
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[44]Radaj D. Design and analysis of fatigue resistants welded structures. Abington, Cambridge:Abington Publishing,1990.
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[47]Paris P., Erdogan F.. Analysis of Crack Growth Laws[J]. Journal of Basic Engineering, Transactioin of the ASME,1963,85:528-534.
[48]Madox S J. Calculation the fatigue strength of a welded joint using fracture mechanics. Met Con.1970.17(8):327-331.
[49]Madox S J. Assessing the significance of flaws in welds subject to fatigue. Weld J.1974. 53(9):401-409.
[50]Madox S J. Analysis of fatigue cracks in welds subjects to fatigue. Weld J.1975.11(2): 221-243.
[51]Madox S J. The effect of mean stress on fatigue crack propagation-a literature review. Int J Fract.1975.11(3):389-408.
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[53]Hobbacher A. The service fatigue strength of welded joints considered on a fracture-mechanical basis(German). Arch Eisenhutt.1977.48(2):109-114.
[54]Haibach E. Questions concerning the fatigue strength of welded joints considered from a conventional and a fracture-mechanical point of view(German). Schw Schn 1977.29(4):140-142.
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[61]Lazzarin P, Livieri P, Notch stress intensity factors and fatigue stress of Aluminium and Steel welded joints. Int. J. Fatigue.2001,23:225-232.
[62]Atzori B, Meneghetti P G, Sumel L. Estimation of the fatigue stress of light alloy welds by an equivalent notch stress analysis. Int. J. Fatigue.2002,24:591-599.
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[64]Taylor D, Bologna P, Bel Knani K. Prediction of fatigue location on a component using a critical distance method. Int. J. Fatigue.2000,22:735-742.
[65]Taylor D, Geometrical effects in fatigue:a unifying theoretical model. Int. J. Fatigue. 1999,21:413-420.
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[68]Sonsino C M, Radaj D, Brandt U, et al. Fatigue assessment of welded joints in AlMg4.5Mn Aluminium alloy (AA508) by local approaches. Int. J. Fatigue.1999, 21:985-999.
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