Is Stress Concentration Relevant for Nanocrystalline Metals?

Abstract

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.

Fracture of graphene: a review

“…The low toughness was attributed to the large number of defects along grain boundaries and triple junctions. It was further shown that the strength of a nanocyrstalline graphene strip becomes insensitive to pre-existing flaws when its width falls below 17.16 nm, i.e., the strip will always fail at the theoretical strength regardless of the flaw size, a state commonly referred to as flaw tolerance (Gao et al 2003;Gao and Chen 2005;Kumar et al 2009;Keten et al 2010;Kumar et al 2011;Qin and Buehler 2011;Kumar et al 2013;Gu et al 2013); see Fig. 11a-b.…”

In-Situ Measurements of Free-Standing, Ultra-Thin Film Cracking in Bending

“…The minimum available FIB current of 10pA is capable of producing a notch approximately 200 nm in width due to resolution limits for a typical FIB. Consequently, for a 100 nm thick beam with an 800 nm span, the notch as fabricated by the FIB would have been about one fourth of the length of the beam, as also observed by Kumar et al [30,31]. To improve this for the 100 nm thick beams, a much sharper notch was produced by a novel method utilizing the fully converged TEM beam to locally sputter material, resulting in a notch radius of approximately 5 nm.…”

In Situ Microstructural Control and Mechanical Testing Inside the Transmission Electron Microscope at Elevated Temperatures

“…Such multiple location crack nucleation indicates flaw insensitivity, which has been observed in the literature for nano-crystalline films. 29 We also attempted to repeat this experiment at 600°C; however, with very little change in fracture toughness. This is because even though the high temperature and stress increased the grain size and introduced some annealing twins, we did not observe significant differences in dislocation-based toughening.…”