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Increasingly, the essential, robust character of many nanoscale devices requires knowledge of their fracture toughness. For most brittle materials the technique of choice has been indentation mechanics but little insight into the fracture mechanism(s) has resulted since these have generally been treated as brittle fracture dominated by the true surface energy. Linear elastic fracture mechanics approaches have been invoked to describe indentation fracture but do not address why the surface energy from fracture toughness is most often slightly or even substantially greater than the true surface energy. In the present study we invoke a crack extension force correlation that demonstrates why this is the case at least in fracture measurements based on indentation mechanics. The proposed correlation is different from previous ones in that it focuses on observations of indentation-induced dislocation activity prior to fracture. Allowing the resistance side of the crack extension force analysis to incorporate small amounts of plasticity gives a relationship that is consistent with 22 relatively brittle intermetallics, semiconductors and ceramics. This explains why measured strain energy release rates can be 2 to 5 times as large as surface energies measured in vacuum or calculated by pseudopotentials using the local density approximation. [ABSTRACT FROM AUTHOR]

Gao et al. (PNAS, 100, 5597–5600 (2003)) have argued that load-bearing mineralized hard tissues, including bones, shells, and teeth, are nanocomposites, in which the mineral phase has nanoscale dimensions that ensure optimum strength and flaw tolerance. In particular, it has been claimed that the thickness of these brittle building blocks, being smaller than a critical size, h *, of the order of tens of nanometers, renders them insensitive to the presence of crack-like flaws and enables them to achieve near-theoretical strength, which is why Nature employs nanoscale features in mineralized biological composites. We find this point of view, which Gao et al. and others have quoted in subsequent publications and presentations, unpersuasive and present several counterexamples which show that biological structures, as a result of being comprised of relatively fragile constituents that fracture at stress levels several orders of magnitude smaller than the theoretical strength, adopt various strategies to develop mechanical responses that enable them to mitigate catastrophic failure. Nanoscale structural features are not a result of an innate resistance to very high stresses. [ABSTRACT FROM AUTHOR]

An efficient Monte Carlo procedure is presented for characterizing the propagation of a crack in a material whose fracture toughness is a random field. The simulations rely on accurate approximate solutions of the integral equations that govern the dislocation densities, stress intensity factors, and energy release rates of curvilinear cracks. For a plate containing an edge crack that propagates towards a subsurface crack representing a traction-free boundary, results for the distributions of crack trajectories, critical applied far-field stresses, and nominal fracture toughness are presented for various parameters that quantify the randomness of the material's critical energy release rate. A demonstrative probabilistic model for crack trajectories is built and size effects are discussed. [ABSTRACT FROM AUTHOR]

LPCVD polysilicon films exhibit tensile or compressive residual stresses and stress gradients, depending on deposition temperature. The stresses, which are a result of an “intrinsic” growth eigenstrain, ℇg, produce undesired curvatures in released structures. A combined experimental/computational design procedure is presented for controlling the curvature of thin films comprised of tensile layers, deposited at 570°C, alternated with compressive layers deposited at 615°C. Experimental measurements are first used to calculate the through-the-thickness variation of ℇg for both temperatures. This information is in turn incorporated into a mechanical model that predicts, for prescribed parameters that define the geometry, elastic moduli, and eigenstrain distribution, the stress distribution before release, and the curvature upon release, of multilayered films. Comparisons of predicted and measured average stress before release of a number of films, and of the curvature upon release of a circular micromirror device, provide a preliminary assessment of the semi-empirical model, which when combined with optimization algorithms can be used to develop recipes (thicknesses of the individual layers of a multiplayer device) that will achieve prescribed curvature according to given constraints. [ABSTRACT FROM AUTHOR]

Shows that the resistance of the shell of the conch Strombus gigas to catastrophic fracture can be understood quantitatively by invoking two-energy dissipating mechanisms, multiple micro-cracking in the outer layers and crack bridging in the middle layers. Methods; Results; Conclusion.

Decreasing scales effectively increase nearly all important mechanical properties of at least some "brittle" materials below 100 nm. With an emphasis on silicon nanopillars, nanowires, and nanospheres, it is shown that strength, ductility, and toughness all increase roughly with the inverse radius of the appropriate dimension. This is shown experimentally as well as on a mechanistic basis using a proposed dislocation shielding model. Theoretically, this collects a reasonable array of semiconductors and ceramics onto the same field using fundamental physical parameters. This gives proportionality between fracture toughness and the other mechanical properties. Additionally, this leads to a fundamental concept of work per unit fracture area, which predicts the critical event for brittle fracture. In semibrittle materials such as silicon, this can occur at room temperature when the scale is sufficiently small. When the local stress associated with dislocation nucleation increases to that sufficient to break bonds, an instability occurs resulting in fracture. [ABSTRACT FROM AUTHOR]

This paper presents the design, fabrication, and mechanical testing of a bioinspired composite structure with characteristic dimensions on the order of tens of microns. The microarchitecture, designed and fabricated using microelectromechanical systems (MEMS) technology, involves two distinct length scales and represents the first attempt at mimicking the crossed-lamellar microstructure of molluscan shells such as the giant Queen conch, Strombus gigas, which contains features with dimensions spanning five distinct length scales. The displacement control capabilities of a nanoindenter enabled the observation of the graceful failure of the micro-composite under three point bending and, in turn, the measurement of its post-peak load-displacement response and work of fracture. [ABSTRACT FROM AUTHOR]