摘要
在550 ℃高温环境下,对Ti60合金试件开展疲劳-蠕变交互试验,在疲劳应力σmax=450 MPa、应力比R=0.1的试验条件下,研究不同蠕变应力对钛合金疲劳蠕变行为的影响。根据试验数据,基于Norton模型提出一种能在样本有限的情况下,仅通过短时间的试验来预测较长时间的蠕变应力门槛值的预测模型。将该模型预测结果与最大轴向应力法计算结果进行对比,并将两者预测结果均与升降法所得试验结果进行了对比,相对误差均在2%以内,表明该预测模型能较为准确地预测疲劳蠕变试验条件下蠕变应力门槛值。
近年来随着航空航天技术的不断发展,航空飞行器的速度取得了重大突破,更高飞行速度的服役环境对飞机结构材料的要求也随之增加。超音速飞机在高速飞行情况下会产生大量的热,并且在短时间内无法及时有效地散热,同时伴随着机械载荷,在此种工况下,传统的铝合金已经无法满足高温环境下强度需要,而高温钛合金因其耐腐蚀、耐高温等特
对疲劳蠕变交互作用的研究,目前主要基于损伤力学和断裂力学理论,通过损伤参量和损伤累积研究材料的失效机理以及裂纹萌生寿
疲劳蠕变试验采用

图1 疲劳蠕变试件示意图
Fig. 1 Fatigue creep specimen

图2 疲劳蠕变试验波形示意图
Fig. 2 Waveform of fatigue creep test
以550 ℃下疲劳最大应力水平σmax=450 MPa、蠕变应力σcreep=450 MPa为例,试验结果如

图3 550 ℃,σmax=450 MPa,σcreep=450 MPa时的应变-时间曲线
Fig. 3 Strain-time curve at 550 ℃,σmax=450 MPa and σcreep=450 MPa

图4 疲劳蠕变应变时间曲线(试验时长50 h,疲劳载荷=450 MPa)
Fig. 4 Strain-time curve in fatigue-creep test(test duration 50 h and fatigue load =450 MPa)

图5 疲劳蠕变应变时间曲线(100 h)
Fig. 5 Strain-time curve in fatigue-creep test(100 h)
从
从
2009年陈学东
, | (1) |
式中:Nc-f为疲劳蠕变寿命;C和m为材料常数;σNmax为最大轴向应力。
在等温(给定的相同温度)及相同保载时间的试验条件下,利用
, | (2) |
式中:σcreep为蠕变应力;tc-f为试验时长,单位为h;C1和m1为材料参数。

图6 基于最大轴向应力计算蠕变应力流程图
Fig. 6 Flow chart of creep stress calculation based on maximum axial stress model
按照上述流程,计算达到不同应变量下,进行50 h试验的蠕变应力门槛值的各材料参数(见
应变量/% | C1 | m1 |
---|---|---|
0.3 | 580.764 | -0.155 |
0.4 | 616.595 | -0.156 |
0.5 | 639.735 | -0.155 |
0.6 | 662.217 | -0.153 |
0.7 | 682.339 | -0.156 |
0.8 | 695.024 | -0.154 |
1.0 | 721.107 | -0.155 |
应变量/% | 蠕变应力门槛值/MPa |
---|---|
0.3 | 316.71 |
0.4 | 334.00 |
0.5 | 349.50 |
0.6 | 363.96 |
0.7 | 370.65 |
0.8 | 380.50 |
1.0 | 393.24 |
3.1节所提到的预测模型步骤较复杂,计算不同应变量下的门槛值时均需要重新通过试验数据确定相关参数。下面以平均应变速率为依据,提出步骤更为简洁的蠕变应力门槛值预测模型。
试验中观察到无论试验时长为50 h或100 h,蠕变曲线第2阶段的线性应变增加速率随蠕变应力σcreep的变化而改变,且随着蠕变应力σcreep的增加,蠕变应变速率也随之增加,二者具有明显正相关性,在实际应用中,蠕变第1阶段与第3阶段时间占比小,应变速率变化快且不稳定,而蠕变第2阶段在整个蠕变过程中时间占比较大,速率稳定,且不同的σcreep对应的应变速率有明显差异,根据疲劳蠕变过程中不同蠕变应力σcreep对应的平均应变变化率提出疲劳蠕变试验中蠕变应力门槛值的预测模型。
根据目前试验所得数据,不同蠕变应力下试验的应变速率不同,且随着蠕变应力的增大,蠕变速率也随之增加,故基于Norton模
, | (3) |
式中:t为疲劳蠕变试验时长; σcreep为蠕变应力;b1和c1为拟合参数,在双对数坐标系下进行线性拟合确定。
(4) |
在
, | (5) |
, | (6) |
, | (7) |
式中,σd为蠕变应力门槛值。以

图7 基于平均应变速率计算蠕变应力流程图
Fig. 7 Flow chart of creep stress calculation based on average strain rate model

图8 蠕变应力应变速率曲线
Fig. 8 Creep stress-strain rate curve
在双对数坐标下,根据
。 | (8) |
根据
应变量/% | 蠕变应力门槛值/MPa |
---|---|
0.3 | 332.40 |
0.4 | 347.06 |
0.5 | 358.86 |
0.6 | 368.81 |
0.7 | 377.43 |
0.8 | 385.07 |
1.0 | 398.17 |
为验证3.1和3.2节中模型的有效性,用升降

图9 50 h疲劳蠕变试验升降图
Fig. 9 Up-down method of fatigue-creep test in 50 h

图10 100 h疲劳蠕变试验升降图
Fig. 10 Up-down method of fatigue-creep test in 100 h
图中,“×”表示经过试验时长后试件应变达到1%的数据,“○”表示经过试验时长,试件应变未达到1%的数据,根据2组升降图可确定试验时长50 h和100 h,疲劳最大应力水平450 MPa,应变达到1%的蠕变应力σcreep为
, | (9) |
。 | (10) |
运用3.1节方法计算所得50 h应变达到1%的蠕变应力为
。 | (11) |
运用3.2节方法计算所得50 h与100 h应变达到1%的蠕变应力为
(12) |
。 | (13) |
试验结果与模型预测结果对比及误差如
模型 | 试验时长/h | 达到应变量/% | 预测蠕变应力/MPa | 试验蠕变应力/MPa | 误差/% |
---|---|---|---|---|---|
基于最大轴向应力模型 | 50 | 1.00 | 393.24 | 392.50 | 0.19 |
基于平均应变速率模型 | 50 | 1.00 | 398.17 | 392.50 | 1.45 |
100 | 1.00 | 358.86 | 352.50 | 1.82 |
由
按照3.1和3.2节的2种预测蠕变应力门槛值的模型,计算不同应变量下试验时长50 h的蠕变应力门槛值,结果对比如
应变量ε/% | 基于最大轴向应力计算蠕变应力门槛值/MPa | 基于平均应变速率计算蠕变应力门槛值/MPa |
---|---|---|
0.3 | 316.71 | 332.40 |
0.4 | 334.00 | 347.06 |
0.5 | 349.50 | 358.86 |
0.6 | 363.96 | 368.81 |
0.7 | 370.65 | 377.43 |
0.8 | 380.50 | 385.07 |
1.0 | 393.24 | 398.17 |
从

图11 2种模型蠕变应力门槛值计算结果对比
Fig. 11 Comparison of calculation results of creep stress threshold values of two models
1)Ti60合金在550 ℃下进行疲劳蠕变试验结果显示,在试验过程中,试件有明显的蠕变损伤过程,即第1阶段应变速率由快变慢,第2阶段应变速率保持不变,以及应变速率持续增加的第3阶段;以
2)当固定试验温度、疲劳蠕变试验中的疲劳载荷、保载时间等因素时,材料的应变速率随着蠕变应力的不同而不同,且蠕变应力越大,应变速率越大。根据此现象提出的疲劳蠕变预测模型的操作简单,精度较高。
3)所提出蠕变应力门槛值的预测模型仅仅考虑蠕变应力单个因素的影响,但金属在高温环境下的疲劳蠕变交互作用影响因素十分复杂,从试验结果看,疲劳和蠕变2种失效形式存在明显的交互作用,未来的研究可以不仅仅局限于蠕变应力的影响,也将疲劳失效考虑其中,使得疲劳蠕变预测模型体系更加完善、准确。
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