Benito deCelis

Bio: Benito deCelis is an academic researcher. The author has contributed to research in topics: Crack growth resistance curve & Brittleness. The author has an hindex of 1, co-authored 1 publications receiving 171 citations.

Molecular dynamics simulation of crack tip processes in alpha‐iron and copper

Abstract: The intrinsic crack tip processes of either propagation by cleavage or blunting by the nucleation of dislocations from the nonlinearly stressed region at the crack tip have been simulated by a molecular dynamics approach in alpha‐iron and in copper, utilizing the Johnson and Morse potentials, respectively, and a new fixed stress boundary condition at the border between the inner discrete region and the outer anisotropic linear continuum. The simulations showed that alpha‐iron is inherently brittle, and fails by cleavage along a cube plane when the stress intensity factor reaches the critical Griffith value. No dislocations are nucleated in iron and even the development of restricted crack tip twinning in special orientations does not alter this intrinsic brittleness. In copper crack tip blunting at a level somewhat less than the Griffith stress intensity factor always prevented brittle crack growth by cleavage. Thus, copper is inherently ductile. Because it permitted the unhindered development of substantial nonlinear crack tip displacements, and did not prevent dislocations from penetrating through the border between the inner nonlinear material and the outer linear continuum, the new stress boundary condition was found to be far superior to the fixed or flexible boundary conditions used at this border by previous investigators. This is reflected in the observed critical stress intensity factors for brittle cleavage that were found to be nearly equal to the expected Griffith value for the stress boundary condition while the displacement boundary conditions gave results nearly three times higher.

Role of material microstructure in plate stiffness with relevance to microcantilever sensors

Abstract: This work examines the effect of microstructure upon microcantilever bending stiffness. An existing beam theory model, based upon an isotropic Hooke's law constitutive relationship, is compared to a model based upon a micropolar elasticity constitutive model. The micropolar approach introduces a bending stiffness relation which is a function of any two independent elastic constants of the Hooke's law model (e.g., the elastic modulus and the Poisson's ratio), and an additional material constant (called γ). A consequence of the additional material constant is the prediction of an increased bending stiffness as the cantilever thickness decreases—a stiffening due to the material microstructure which becomes measurable at micron-order thicknesses. Polypropylene microcantilevers, which have a non-homogeneous microstructure due to their semi-crystalline nature, were fabricated via injection molding. A nanoindenter was used to measure their stiffness. The nanoindenter-determined stiffness values, which include the effect of the additional micropolar material constant, are compared to stiffness values obtained from beam theory. The nanoindenter stiffness values are seen to be at least four times larger than the beam theory stiffness predictions. This stiffening effect has relevance in future MEMS applications which employ materials with non-homogeneous microstructures instead of the conventional MEMS materials (e.g., silicon, silicon nitride), which have a very uniform microstructure.

Embrittlement of interfaces by solute segregation

Abstract: We discuss theoretical models of interfacial embrittlement by solute segregation. Of properties susceptible to alteration by segregation, the ideal work of interfacial separation, 2 γ int , is predicted to have an important but probably not exclusive role in controlling embrittlement. A thermodynamic framework for estimating 2 γ int from data available through free surface and grain boundary adsorption studies is outlined, and relevant segregation energies are given for carbon, phosphorus, tin, antimony and sulphur segregation in iron. Data from intergranular fracture experiments involving these same segregants is also summarized in an attempt to test the idea that segregation-induced embrittlement (or ductilization) can be understood in terms of the segregant's effect on 2 γ int . Uncertainties in present data do not allow a convincing test, but it is not implausible that the deleterious effects of phosphorus, tin, and sulphur in iron can be understood in this way. The effect of carbon does not seem to be similarly understandable, although that may be due to the inappropriateness of the only available surface segregation data in that case, which are for a (001) surface rather than a general polycrystalline surface created by intergranular fracture.

Crack propagation in b.c.c. crystals studied with a combined finite-element and atomistic model

Abstract: A new method for combined finite-element and atomistic analysis of crystal defects has been developed. The coupling between the atomistic core and the surrounding continuum is described in terms of non-local elasticity theory. Static and dynamic tests demonstrate the satisfactory performance of this method. The model is applied to crack propagation on cleavage and non-cleavage planes in b.c.c. crystals, using potentials for iron and tungsten as examples. On the cleavage planes, pronounced directions of ‘easy’ crack propagation, besides less favourable directions, are found. On the preferred cleavage plane {100}, easy propagation is possible in any macroscopic direction by microscopic facetting of the crack front into easy directions, while on the secondary cleavage plane {110}, there is only one macroscopic direction in which cracks will propagate easily. On all other planes studied here, plastic processes at the crack tip (twinning and/or dislocation emission) intervene before brittle crack prop...

Multiparadigm modeling of dynamical crack propagation in silicon using a reactive force field

Abstract: We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for several thousand atoms near the crack tip, while more than 100 000 atoms are described with a nonreactive force field. ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including changes in crack dynamics for different crack orientations.