Quantitative in Situ SEM High Cycle Fatigue: The Critical Role of Oxygen on Nanoscale-Void-Controlled Nucleation and Propagation of Small Cracks in Ni Microbeams

Abstract

This Letter presents a quantitative in situ scanning electron microscope (SEM) nanoscale high and very high cycle fatigue (HCF/VHCF) investigation of Ni microbeams under bending, using a MEMS microresonator as an integrated testing machine. The novel technique highlights ultraslow fatigue crack growth (average values down to ∼10 m/cycle) that has heretofore not been reported and that indicates a discontinuous process; it also reveals strong environmental effects on fatigue lives that are 3 orders of magnitude longer in a vacuum than in air. This ultraslow fatigue regime does not follow the well documented fatigue mechanisms that rely on the common crack tip stress intensification, mediated by dislocation emission and associated with much larger crack growth rates. Instead, our study reveals fatigue nucleation and propagation mechanisms that mainly result from room temperature void formation based on vacancy condensation processes that are strongly affected by oxygen. This study therefore shows significant size effects governing the bending high/very high cycle fatigue behavior of metals at the micro- and nanoscales, whereby the stress concentration effect at the tip of a growing small fatigue crack is assumed to be greatly reduced by the effect of the bending-induced extreme stress gradients, which prevents any significant cyclic crack tip opening displacement. In this scenario, ultraslow processes relying on vacancy formation at the subsurface or in the vicinity of a crack tip and subsequent condensation into voids become the dominant fatigue mechanisms.

Development of fatigue testing system for in-situ observation of stainless steel 316 by HS-AFM & SEM

“…These slip bands are the result of local plasticity on active slip systems and result in local elevations and depressions of the surface; generally termed surface extrusions and intrusions [4]. While several theoretical and computational methods have been proposed and developed to study fatigue crack initiation and growth, such as [5], [6], [15], [7]- [14], physical evidence and validation data for fatigue nucleation, initiation and growth relies upon the development of experimental techniques and observations. Recent advances in modelling, such as crystal plasticity and dislocation dynamics approaches, have afforded the opportunity to model mesoscale plasticity in considerable detail.…”

“…This information is critical to underpin and validate developments of current models, especially those focussing on nucleation and micro (early) growth stages [16]. Most historic observations of fatigue crack nucleation and growth have been performed by optical microscopy [11], limited transmission electron microscopy (TEM) [12] and/or scanning electron microscopy (SEM) [13], [14], [15]. Although notable insight has been obtained by these techniques, the fidelity of information on local plasticity was compromised by limited resolution of these microscopes and/or their inability to measure true sample topography.…”

Computational and experimental study of crack initiation in statistical volume elements

“…The loading sequence consist of applying an equivalent displacement controlled simulation for a fully reversed bending rotation angle of 25 mrad to the nodes at the micro-beam free end with the support end fixed. A rotation angle of 25 mrad is equivalent to an experimental microbeam fatigue test ran at ̴ 470 MPa with a fatigue crack nucleation life, that lies between 1.42 x10 7 and 2.34 x10 7 cycles in a vacuum environment [22].…”

Cyclic Deformation of Microcantilevers Using In-Situ Micromanipulation

“…with R σ = -1. For MEMS materials, high cycle fatigue studies at microscale have been performed through the use of micro-resonators [26,27] and other special devices [28][29][30].…”