Department of Mechanical and Nuclear Engineering, Penn State University, University Park, Pennsylvania 16802, United States.
Nano Lett. 2011 Jun 8;11(6):2510-6. doi: 10.1021/nl201083t. Epub 2011 May 18.
Classical fracture mechanics as well as modern strain gradient plasticity theories assert the existence of stress concentration (or strain gradient) ahead of a notch tip, albeit somewhat relaxed in ductile materials. In this study, we present experimental evidence of extreme stress homogenization in nanocrystalline metals that result in immeasurable amount of stress concentration at a notch tip. We performed in situ uniaxial tension tests of 80 nm thick (50 nm average grain size) freestanding, single edge notched aluminum specimens inside a transmission electron microscope. The theoretical stress concentration for the given notch geometry was as high as 8, yet electron diffraction patterns unambiguously showed absence of any measurable stress concentration at the notch tip. To identify possible mechanisms behind such an anomaly, we performed molecular dynamics simulations on scaled down samples. Extensive grain rotation driven by grain boundary diffusion, exemplified by an Ashby-Verrall type of grain switching process, was observed at the notch tip to relieve stress concentration. We conclude that in the absence of dislocations, grain realignment or rotation may have played a critical role in accommodating externally applied strain and neutralizes any stress concentration during the process.
经典断裂力学以及现代应变梯度塑性理论都断言在缺口尖端前方存在应力集中（或应变梯度），尽管在韧性材料中这种现象有所缓解。在本研究中，我们提供了纳米晶体金属中极端应力均匀化的实验证据，这导致在缺口尖端处产生无法测量的大量应力集中。我们在透射电子显微镜内对 80nm 厚（平均晶粒尺寸为 50nm）的独立式单边缺口铝制样品进行了原位单向拉伸测试。对于给定的缺口几何形状，理论上的应力集中高达 8，但电子衍射图谱明确显示在缺口尖端处没有任何可测量的应力集中。为了确定这种异常现象背后的可能机制，我们对缩小的样品进行了分子动力学模拟。在缺口尖端观察到由晶界扩散驱动的大量晶粒旋转，例如 Ashby-Verrall 型晶粒切换过程，以缓解应力集中。我们的结论是，在没有位错的情况下，晶粒重新排列或旋转可能在适应外部施加的应变方面发挥了关键作用，并在该过程中消除了任何应力集中。
