高锰奥氏体TWIP钢的循环变形及疲劳裂纹扩展行为研究
发布时间：2018-09-01 15:00
【摘要】：随着汽车工业的发展,减轻车重、降低能耗、减少环境污染已成为现代汽车的发展趋势。提高汽车用钢板的强度是降低钢板厚度、减轻车重从而降低能耗的有效途径。新型高锰奥氏体TWIP钢兼具高强度、高塑性,被认为是最具发展潜力的汽车结构用钢。作为结构材料,TWIP钢在实际服役过程中必然会受到交变载荷的作用,导致其疲劳失效。因此,研究高锰奥氏体TWIP钢的疲劳行为不仅具有实用价值,也可以为结构件的抗疲劳设计和安全使用提供可靠的理论依据。本文以高锰奥氏体TWIP钢为研究对象,在室温下研究了它的低周疲劳行为,分析了应变范围和应变速率对高锰奥氏体TWIP钢低周疲劳行为的影响;研究了它的疲劳裂纹扩展行为,分析了晶粒尺寸、应力比和合金元素Al添加对TWIP钢疲劳裂纹扩展行为的影响规律。得到以下主要结果:低周疲劳实验结果表明,高锰奥氏体0Al钢在循环变形过程中出现了动态应变时效,且此动态应变时效与应变速率有关。在固定应变范围为1.4%时,在低应变速率(8×10-4 s~(-1)和2×10-3 s~(-1))下,应力-应变滞后回线上无锯齿波动现象。而在中等应变速率(8×10-3 s~(-1))下,出现了锯齿波动现象,但随着循环周次的增加,锯齿波动幅度减弱,并逐渐消失。在较高应变速率(2×10-2 s~(-1))下,锯齿波动幅度增大,且同样随着循环周次的增加而减弱,但并没有消失,它伴随整个疲劳循环过程。高锰奥氏体0Al钢在低周疲劳循环变形过程中出现循环硬化/软化现象,此循环硬化/软化行为的程度与外加应变范围和应变速率密切相关。在应变速率为8×10-3 s~(-1)、且在较高应变范围(1.0%~1.4%)下,高锰奥氏体0Al钢表现为初始阶段的循环硬化和循环饱和之后、循环软化直至失效。而在低应变范围(0.8%)下,高锰奥氏体0Al钢在经过循环硬化达到饱和后,只表现出轻微的循环软化,而且在循环失效前还经历了一个循环稳定阶段。此外,在恒定应变范围为1.4%、不同应变速率下,高锰奥氏体0Al钢的循环应力响应均表现为初始循环硬化、循环饱和、循环软化直至失效。在恒定应变速率(8×10-3 s~(-1))下,高锰奥氏体0Al钢的低周疲劳寿命随着应变范围的增大而降低,且遵循Coffin-Manson关系。因此,利用文中获得的疲劳性能参数,可以较为准确地预测高锰奥氏体0Al钢的低周疲劳寿命。在恒定应变范围(1.4%)时,低周疲劳寿命随着应变速率的增大而降低。这是因为,在高应变速率下,低周疲劳循环过程中每一个循环所累积的塑性损伤高于低应变速率。此外,在较高应变速率时出现的动态应变时效会使材料在循环变形过程中产生较多的不均匀变形和驻留滑移带。这些局部不均匀变形和驻留滑移带有利于裂纹形核、促进疲劳裂纹扩展,从而降低其疲劳寿命。无论是粗晶粒还是细晶粒高锰奥氏体0Al钢,其疲劳裂纹扩展抗力随着循环应力比的降低而增大。在高、低应力比(0.6和0.1)下,粗晶粒高锰奥氏体0Al钢的疲劳裂纹扩展门槛值ΔKth均高于细晶粒钢的裂纹扩展门槛值。在Paris区,随着应力强度因子范围ΔK的逐渐增大,晶粒尺寸对疲劳裂纹扩展速率的影响减弱。晶粒尺寸对疲劳裂纹扩展门槛值的影响,主要是由裂纹闭合和平面滑移性的不同引起的。此外,在粗晶粒高锰奥氏体0Al钢的疲劳裂纹扩展过程中产生的裂纹分叉也会消耗很多的能量,从而降低了裂纹尖端的有效驱动力,有利于疲劳裂纹扩展门槛值的升高。基于传统二维观察方法和三维同步辐射CT技术研究了高锰奥氏体0Al和3Al钢在应力比为0.1时的疲劳裂纹扩展行为。结果表明,高锰奥氏体0Al钢的疲劳门槛值高于3Al钢。这是因为在近门槛区,疲劳裂纹闭合对高锰奥氏体0Al钢的影响高于3Al钢。而且在3Al钢中,Al的加入增加了其层错能,从而降低了材料的平面滑移性,导致3Al钢的循环滑移可逆性降低。此外,裂纹在扩展过程中,两两裂纹片之间的相交也会降低裂纹尖端的有效驱动力,从而降低裂纹扩展速率。
[Abstract]:With the development of automotive industry, reducing vehicle weight, energy consumption and environmental pollution has become the development trend of modern automobiles. Increasing the strength of automotive steel sheet is an effective way to reduce the thickness of steel sheet, reduce vehicle weight and reduce energy consumption. As a structural material, TWIP steel will inevitably be subjected to alternating load in the actual service process, resulting in fatigue failure. Therefore, the study of fatigue behavior of high manganese austenitic TWIP steel is not only of practical value, but also provides a reliable theoretical basis for fatigue design and safe use of structural parts. The low cycle fatigue behavior of austenitic TWIP steel was studied at room temperature, and the effects of strain range and strain rate on the low cycle fatigue behavior of high manganese austenitic TWIP steel were analyzed. The main results are as follows: The results of low cycle fatigue test show that dynamic strain aging occurs during cyclic deformation of high manganese austenitic 0Al steel, and this dynamic strain aging is related to strain rate. There is no sawtooth fluctuation on the line, but the sawtooth fluctuation appears at moderate strain rate (8 x 10-3 s-1). However, with the increase of cycle number, the sawtooth fluctuation amplitude decreases and gradually disappears. At higher strain rate (2 x 10-2 s-1), the sawtooth fluctuation amplitude increases and decreases with the increase of cycle number, but it does not disappear. The cyclic hardening/softening behavior of high manganese austenitic 0Al steel during low cycle fatigue cyclic deformation is closely related to the applied strain range and strain rate. In addition, in the low strain range (0.8%), the high manganese austenitic 0Al steel shows only slight cyclic softening after cyclic hardening and saturation, and it also undergoes a cyclic stable stage before cyclic failure. The cyclic stress response of high manganese austenitic 0Al steel is shown as initial cyclic hardening, cyclic saturation, cyclic softening until failure at different strain rates. The low cycle fatigue life of high manganese austenitic 0Al steel decreases with the increase of strain range at constant strain rate (8 *10-3 s~(-1)), and follows the Coffin-Manson relationship. Therefore, the low cycle fatigue life of high manganese austenitic 0Al steel can be predicted more accurately by using the fatigue performance parameters obtained in this paper. The low cycle fatigue life decreases with the increase of strain rate in the constant strain range (1.4%). This is because the plastic damage accumulated in each cycle during the low cycle fatigue cycle at high strain rate. The damage is higher than the low strain rate. In addition, the dynamic strain aging at higher strain rate will result in more inhomogeneous deformation and resident slip bands during cyclic deformation. These local inhomogeneous deformation and resident slip bands are beneficial to crack nucleation, promote fatigue crack propagation and reduce fatigue life. The fatigue crack growth resistance of coarse grain or fine grain high manganese austenitic 0Al steel increases with the decrease of cyclic stress ratio. At high and low stress ratios (0.6 and 0.1), the fatigue crack growth threshold value Kth of coarse grain high manganese austenitic 0Al steel is higher than that of fine grain steel. The influence of grain size on the threshold value of fatigue crack growth is mainly caused by the difference of crack closure and plane slip. In addition, the bifurcation of cracks produced in the process of fatigue crack growth of coarse grain high manganese austenitic 0Al steel will consume a lot. Based on the traditional two-dimensional observation method and three-dimensional synchrotron radiation CT technique, the fatigue crack propagation behavior of high manganese austenitic 0Al and 3Al steels at the stress ratio of 0.1 was studied. This is because the effect of fatigue crack closure on high manganese austenitic 0Al steel is higher than that of 3Al steel in the near threshold region. Moreover, the addition of Al increases the stacking fault energy of 3Al steel, which decreases the planar slip of the material and results in a decrease in the reversibility of cyclic slip of 3Al steel. Intersection will also reduce the effective driving force at the crack tip, thus reducing the crack growth rate.
【学位授予单位】：燕山大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.1
本文编号：2217552
[Abstract]:With the development of automotive industry, reducing vehicle weight, energy consumption and environmental pollution has become the development trend of modern automobiles. Increasing the strength of automotive steel sheet is an effective way to reduce the thickness of steel sheet, reduce vehicle weight and reduce energy consumption. As a structural material, TWIP steel will inevitably be subjected to alternating load in the actual service process, resulting in fatigue failure. Therefore, the study of fatigue behavior of high manganese austenitic TWIP steel is not only of practical value, but also provides a reliable theoretical basis for fatigue design and safe use of structural parts. The low cycle fatigue behavior of austenitic TWIP steel was studied at room temperature, and the effects of strain range and strain rate on the low cycle fatigue behavior of high manganese austenitic TWIP steel were analyzed. The main results are as follows: The results of low cycle fatigue test show that dynamic strain aging occurs during cyclic deformation of high manganese austenitic 0Al steel, and this dynamic strain aging is related to strain rate. There is no sawtooth fluctuation on the line, but the sawtooth fluctuation appears at moderate strain rate (8 x 10-3 s-1). However, with the increase of cycle number, the sawtooth fluctuation amplitude decreases and gradually disappears. At higher strain rate (2 x 10-2 s-1), the sawtooth fluctuation amplitude increases and decreases with the increase of cycle number, but it does not disappear. The cyclic hardening/softening behavior of high manganese austenitic 0Al steel during low cycle fatigue cyclic deformation is closely related to the applied strain range and strain rate. In addition, in the low strain range (0.8%), the high manganese austenitic 0Al steel shows only slight cyclic softening after cyclic hardening and saturation, and it also undergoes a cyclic stable stage before cyclic failure. The cyclic stress response of high manganese austenitic 0Al steel is shown as initial cyclic hardening, cyclic saturation, cyclic softening until failure at different strain rates. The low cycle fatigue life of high manganese austenitic 0Al steel decreases with the increase of strain range at constant strain rate (8 *10-3 s~(-1)), and follows the Coffin-Manson relationship. Therefore, the low cycle fatigue life of high manganese austenitic 0Al steel can be predicted more accurately by using the fatigue performance parameters obtained in this paper. The low cycle fatigue life decreases with the increase of strain rate in the constant strain range (1.4%). This is because the plastic damage accumulated in each cycle during the low cycle fatigue cycle at high strain rate. The damage is higher than the low strain rate. In addition, the dynamic strain aging at higher strain rate will result in more inhomogeneous deformation and resident slip bands during cyclic deformation. These local inhomogeneous deformation and resident slip bands are beneficial to crack nucleation, promote fatigue crack propagation and reduce fatigue life. The fatigue crack growth resistance of coarse grain or fine grain high manganese austenitic 0Al steel increases with the decrease of cyclic stress ratio. At high and low stress ratios (0.6 and 0.1), the fatigue crack growth threshold value Kth of coarse grain high manganese austenitic 0Al steel is higher than that of fine grain steel. The influence of grain size on the threshold value of fatigue crack growth is mainly caused by the difference of crack closure and plane slip. In addition, the bifurcation of cracks produced in the process of fatigue crack growth of coarse grain high manganese austenitic 0Al steel will consume a lot. Based on the traditional two-dimensional observation method and three-dimensional synchrotron radiation CT technique, the fatigue crack propagation behavior of high manganese austenitic 0Al and 3Al steels at the stress ratio of 0.1 was studied. This is because the effect of fatigue crack closure on high manganese austenitic 0Al steel is higher than that of 3Al steel in the near threshold region. Moreover, the addition of Al increases the stacking fault energy of 3Al steel, which decreases the planar slip of the material and results in a decrease in the reversibility of cyclic slip of 3Al steel. Intersection will also reduce the effective driving force at the crack tip, thus reducing the crack growth rate.
【学位授予单位】：燕山大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.1
【参考文献】
相关博士学位论文 前1条
1 黄宝旭;氮、铌合金化孪生诱发塑性（TWIP）钢的研究[D];上海交通大学;2007年
相关硕士学位论文 前1条
1 刘春月;汽车用TWIP钢压缩变形行为研究[D];太原理工大学;2010年
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