Pokorný Pavel, Vojtek Tomáš, Jambor Michal, Náhlík Luboš, Hutař Pavel
Institute of Physics of Materials, Czech Academy of Sciences, Žižkova 22, 616 00 Brno, Czech Republic.
Central European Institute of Technology (CEITEC), Brno University of Technology, Purkyňova 123, 612 00 Brno, Czech Republic.
Materials (Basel). 2021 May 13;14(10):2530. doi: 10.3390/ma14102530.
Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.
载荷幅值小于疲劳裂纹扩展阈值的欠载循环通常被认为是无意义的循环，对工程结构部件的疲劳寿命没有任何影响。然而，本文表明，在某些情况下，这些欠载循环会显著延缓随后的裂纹扩展。这种现象是在低于阈值的循环加载过程中氧化物诱导裂纹闭合发展的结果。分别使用能谱技术和局部聚焦离子束切割测量氧化物碎片层的宽度和厚度，实验描述的疲劳裂纹扩展迟缓效应得到了支持。对于施加的欠载循环次数越多、幅值越大，迟缓效应和氧化物碎片量都越大。裂纹闭合测量显示在欠载循环期间闭合水平逐渐增加。在低空气湿度下测试的试样，以及裂纹开放时间与施加欠载循环所需时间相同的试样，均未显示出迟缓效应。这些结果可以增进我们对环境对疲劳裂纹扩展影响的理解，以及理解实验室测试结果与服役部件疲劳寿命之间的差异。
