Elimination or Significant Reduction of the Effects of Stress Concentrators by N
McCarter, Joan, Contributing Editor has reference to this Academic Journal, PHwiki organized this Journal Elimination or Significant Reduction of the Effects of Stress Concentrators by Nanosizing Collaborators: P. Deymier, MSE, Univ. of Arizona E. Enikov, AME, Univ. of Arizona C. Haynie, CEEM, Univ. of Arizona Central Theme: When spatial dimensions are below a material specific one, which most likely is in the nanometer range, stress concentrators become insignificant. Molecular Dynamics results in addition to design of experiments 10,000 layers of alternating metal/polymer. Each layer is 20-30nm thick. Courtesy: Sigma Tech. Intl., Inc. Components with Nanodimensional Structure Technologies presently exist, in addition to are becoming more efficient, in producing components of nanodimenions (nanolaminates, nanoflakes, nanofibers, comb, etc.) Nano-composite as long as energetic pigment applications. Courtesy: Sigma Tech. Intl., Inc. One Potential Important Future application: Hydrogen storage by adsorption, where intentional surfaces, pores (STRESS CONCENTRATORS) increase the surface area. Future Technologies The mechanical integrity is paramount, in addition to insensitivity to defects is a reliability design dream come true stress concentrators are ubiquitous
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Nature Knows Courtesy: Gao et al, 2003, Proc. Nat. Acad. Sci. The basic building components in many biological materials remarkable as long as their properties are at the nanoscale (mineral/organic). Why the Nanoscale Stress Concentrators Nano stress concentrators (NSCs) here indicate defects such as impurities, inclusions, cracks, pores; defects other than NSCs, e.g. dislocations, are also addressed yet they are considered part of the bulk material Examples: from roughness of substrate, impurities, pores, inclusions In biological materials NSCs are trapped proteins within mineral crystals during biomineralization. Consistency in these materials is remarkable. Size Effects Since Galileo Galilei in addition to Leonardo da Vinci Considering as long as a brittle material: g=1J/m2, E=100GPa, sth=E/30] in addition to a=p1/2 we obtain hcr = 30nm Note on Hall-Petch effects
Yield strength of electroplated Cu thin films as a function of film thickness t. In the plot of these nanoindentation experimental results, sizing to ~200nm, the yield stress was assumed as the 1/3 of the hardness. Courtesy of Volinsky in addition to Gerberich, 2003, Microel. Engr. Journal. Size Effects: Well Studied Insensitivity to NSCs has not been speculated, Plus difficulties in studying (experimental in addition to simulation) Koehler, 1970: a structure comprised of alternating layers of two suitable metals exhibits a resistance to plastic de as long as mation that would be greater than that expected from a homogenous alloy of the two. Below Certain Nanoscales: More than Size Effects Expected Behavior Insensitivity to Defects For NSC diameter = ½ thickness hcr MD Simulations (EAM) Cu Crystal with a NSC Pulled in (001) Over 2,000,000 atoms Many slip planes High strain rate Non periodic BC
MD Simulations (EAM) Cu Crystal w/o NSC Pulled in (001) Over 2,000,000 atoms Two slip planes High strain rate Non Periodic BC (111) slip planes as long as m Insensitivity to the NSCs Large system: 28.88×28.88×28.88 nm3 (over 2,000,000 atoms) 3.0×3.0x0.4 nm3 NSC Small system: 18.05×18.05×18.05 nm3 (~ 500,000 atoms) 3.0×3.0x0.4 nm3 NSC Why the Insensitivity Total number of atomistic defects, N, versus strain as long as the small Cu(001) system with in addition to without NSC. Plot was obtained from atom positions at five strain levels during de as long as mation. Role of Surfaces: ratio surface/volume ~ 1/a important as long as small a If, on average, the energy required as long as as long as ming each atomistic defect is constant, this explains insensitivity of the material to NSCs
With NSC Without NSC External surfaces start dominating as atomistic defect initiation sites Surfaces large system, high strain rate, non periodic BC With NSC Without NSC External surfaces start dominating as atomistic defect initiation sites Surfaces, small system, high strain rate, non periodic BC With NSC, side views Loaded surfaces start dominating as atomistic defect initiation sites as long as two large (infinite) lateral dimensions (periodic BC). NSCs stimulate the clustering of atomistic defects. Surfaces, small system, high strain rate, periodic BC Without NSC, side views
Slower Strain Rates Non Periodic BC Large System Small System Number of Atomistic Defects Versus Strain, (one realization, even though process is statistical) Strain Strain N N With NSC Without NSC Without NSC With NSC Slow strain rate, large system, no NSC in addition to 2 NSC sizes (3 curves) Slow strain rate, small system, no NSC in addition to 1 large NSC Slower Strain Rates Non Periodic BC Strain Strain Stress (Pa) Stress (Pa) Surprise: Slower Strain Rates Periodic BC Large System Small System Number of Atomistic Defects Versus Strain. NSCs stimulate the clustering of atomistic defects. Strain Strain N N Without NSC With NSC With NSC Without NSC
Slow strain rate, large system, no NSC in addition to 2 NSC sizes (3 curves) Slow strain rate, small system, no NSC in addition to 1 NSC Slower Strain Rates Periodic BC Strain Strain Stress (Pa) Stress (Pa) Without NSC Without NSC With NSC With NSC 1 With NSC 2 Slower strain rate, non periodic BC Large system with NSC Large system, no NSC Slower strain rate, non periodic BC side views Large system with NSC Large system, no NSC
Slower strain rate, periodic BC Large system with NSC Large system, no NSC Atomistic defects cluster at the loading surfaces Slower strain rate, periodic BC side views Large system with NSC Large system, no NSC Atomistic Defects cluster at the loading surfaces Experiments Nanoindentation: not appropriate as long as this work The very local indenter, which introduces a NSC, interacts strongly with pre-existing NSCs; two samples (films of different thickness) are unlikely to have the same NSCs positioned near the indenter in a similar fashion.
Experiments The SPM probe is pushed on the metal tubes lying on a flat wafer. Height image using contact mode (low resolution) after load is imposed by Force Volume SPM. (a) 1x1m, (b) 500x500nm; the marked area (red ellipse) was damaged during the Force Volume SPM. (c) Typical as long as ce displacement curve. Limitations . as long as ce volume SPM. Experiments Membrane tests Smooth Probe
Simulation of Experiments Simulation Issues The MD-FE interface (not resolved dispersion issues) use a wavelet-based absorbing interphase Propagation of atomistic defects in the FE domain use kMC (kinetic Monte Carlo) as intermediate technique to avoid artificial dislocation pileup Has been tested (Frantziskonis & Deymier, 2000) Conclusions For Cu subjected to tensile strain, the critical dimensions as long as the effects of NSCs are larger than the examined (up to) 28.8nm. Multiscale simulations are necessary to identify critical dimensions in addition to also examine slower strain rates. The spatial pattern of atomistic defects that develops during straining is different as long as a system with NSCs than one without NSCs. Yet, the number of atomistic defects (number of atoms with modified coordination number) seems to be independent of the NSCs. Samples larger than critical tend to cluster atomistic defects. Surfaces are instrumental in initiating atomistic defects. Surface Effects, also instrumental at macro-scales, are beneficial at nano-scales, i.e. they eliminate the effects of stress concentrators. Strain rate (1 order of magnitude difference) does not alter the conclusions Computer power in addition to experimental difficulties of the past did not allow one to even speculate that such a (materials processing in addition to reliability) dream may be true!
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