The irradiation of metals by energetic particles causes significant degradation of the mechanical properties, most notably an increased yield stress and decreased ductility, often accompanied by plastic flow localization. Such effects limit the lifetime of pressure vessels in nuclear power plants, and constrain the choice of materials for fusion-based alternative energy sources. Although these phenomena have been known for many years, the underlying fundamental mechanisms and their relation to the irradiation field have not been clearly demonstrated. Here we use three-dimensional multiscale simulations of irradiated metals to reveal the mechanisms underlying plastic flow localization in defect-free channels. We observe dislocation pinning by irradiation-induced clusters of defects, subsequent unpinning as defects are absorbed by the dislocations, and cross-slip of the latter as the stress is increased. The width of the plastic flow channels is limited by the interaction among opposing dislocation dipole segments and the remaining defect clusters.

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When irradiated, metals undergo significant internal damage accumulation and degradation of mechanical properties. Damage takes the form of a high number density of nanosize defect clusters (stackingfault tetrahedrons (SFTs) or interstitial loops). The alteration of mechanical properties is manifested in a hardening behavior and localized plastic deformation in defect-free channels. This work uses discrete dislocation dynamics (DD) to capture these effects. It sets the framework for the elastic interaction between gliding dislocations and defect clusters and details a scheme for loop unfaulting and absorption into dislocations. Here, it is shown that SFTs represents weaker pinning points for dislocation motion than parent dislocation loops. It is also shown that appreciable yield drop can be attributed to high density of defects decorating the dislocations. Strong obstacles cause dislocations in Cu to continually double cross slip causing the formation of defect-free channels. Finally, the correlation between yield stress increase and defect number density is in excellent agreement with the experiment.

A finite element model of the human dentate mandible has been developed to provide a comparison of fixation systems used currently for fracture reduction. Volume domains for cortical bone, cancellous bone, and teeth were created and meshed in ANSYS 8.0 based on IGES curves created from computerized tomography data. A unilateral molar clench was loaded on the model with a fracture gap simulated along the symphysis. Results based on Von Mises stress in cortical and cancellous bone surrounding the screws, and on fracture surface spatial fixation, show some relative differences between different screw-plate systems, yet all were judged to be appropriate in their reduction potential.

Recent advances in 3-D dislocation dynamics include the proper treatment of free surfaces in the simulations. Dislocation interaction and slip is treated as a boundary-value problem for which a zero-traction condition is enforced at the external surfaces of the simulation box. Here, a new rigorous method is presented to handle such a treatment. The method is semi-analytical/numerical in nature in which we enforce a zero traction condition at select collocation points on a surface. The accuracy can be improved by increasing the number of collocation points. In this method, the image stress-field of a subsurface dislocation segment near a free surface is obtained by an image segment and by a distribution of prismatic rectangular dislocation loops padding the surface. The loop centers are chosen to be the collocation points of the problem. The image segment, with proper selection of its Burgers vector components, annuls the undesired shear stresses on the surface. The distributed loops annul the undesired normal stress component at the collocation points, and in the process create no undesirable shear stresses. The method derives from crack theory and falls under “generalized image stress analysis” whereby a distribution of dislocation geometries or entities (in this case closed rectangular loops), and not just simple mirror images, are used to satisfy the problem’s boundary conditions (BCs). Such BCs can, in a very general treatment, concern either stress traction or displacements.

A self-consistent set of equations describing strain partitioning in planar bilayers is developed using a typical definition of strain, the assumption of a coherent interface and a mechanical equilibrium criterion. This approach eliminates the need for the concepts of lattice mismatch and compatibility of deformation, leading to a general solution for the strains and in-plane lattice parameter in bilayer structures. Using the strain equations, the strain energies in the system are calculated as a function of the epilayer to substrate thickness ratio. It was found that for a given substrate thickness, the epilayer strain energy contains a maximum at a layer thickness ratio of ∼1. The peak epilayer strain energy is only ∼25% of the maximum possible in the system. A criterion based on energy considerations is proposed for determining whether to use the epilayer or substrate dislocation formation energy when calculating the epilayer critical thickness. This criterion is applied to the GexSi1−x/Si(100) material system and is manifested by a kink in the critical thickness versus substrate thickness curves. The kink is interpreted as a boundary identifying whether a threading dislocation will most favorably be injected into the substrate or epilayer when the critical thickness is exceeded.

This work reports on a recent advancement in three-dimensional dislocation dynamics modelling. A method is presented for the treatment of dislocation image stresses resulting from the presence of nearby traction-free surfaces. The image stress-field of a dislocation segment below a finite-sized free surface is obtained by the distribution of N generally prismatic rectangular or square dislocation loops padding the area (the external bounding surfaces of the simulation box). The unphysical tractions created at surface collocation points by sub-surface crystal dislocations are annulled by proper determination of the loops' Burgers vectors. The image stresses on a dislocation segment are simply those stresses resulting from the surface loops. The accuracy can be improved by increasing the number of collocation points (i.e. surface loop density).

In this paper, we describe the strain-dependent behavior of an electric-LC ͑ELC͒ resonator unit cell, commonly used in metamaterial designs. We leverage analytic expression to understand the way strain manifests itself in a change in electromagnetic ͑EM͒ response. We verify the simplified physical models using full-wave simulations and generalize the trends to accommodate the strain profile for any arbitrary plane-stress loading scenario. © 2010 American Institute of Physics. ͓doi:10.1063/1.3507892͔ Metamaterials can greatly expand man's ability to control interactions with electromagnetic radiation and enable such phenomena as cloaking, 1,2 beyond diffraction-limited imaging, 3 gradient negative-index lenses, 4 and perfect absorbers. 5 They are a powerful concept by allowing designers to utilize geometry, and not just material properties, to engineer a structure's electromagnetic response; often providing properties not found in nature.However, transitioning metamaterials into real, operational systems requires knowledge of their behavior in relevant environments. Of significance is the role mechanical loading/strain plays in the electromagnetic response of a metamaterial. Mechanical strain is by definition, a deformation of the geometry of a structure. Since metamaterials rely so heavily on geometry for the desired response, it implies a direct causal relationship between applied strain and electromagnetic performance.Previous efforts investigated the strain 6 and temperature 7 dependent response of magnetic resonant elements; Melik et al. 6 even proposes using metamaterials as wireless strain gauges. Our efforts focused on a critical missing piece, the electric-LC resonator, depicted in Fig. 1. This structure operates at x-band, utilizing two parallel capacitors for enhanced resonant response. Figure 1 depicts the S-parameter curves for the cell.The mechanical model assumes the metamaterial unit cell is part of a large ͑Ͼ10 ͒, load-bearing structure. Mechanical loading on the cell is homogeneous, and the copper contributes insignificantly to the overall stiffness of the composite; therefore, the in-plane strain profile is approximately uniform across the unit cell ͑no local stiffening effects from the adhered copper͒. Due to the uniformity of the strain profile and the resultant absence of higher order differential terms, the linear system of Eq. ͑1͒ can be utilized to describe the deformed geometry of the unit cell as follows: 8The superscript 0 and 1 refer to the undeformed and deformed geometries, respectively, and the 3 ϫ 3 matrix is the mechanical strain tensor. The model accommodates different values of E ZZ in the substrate and copper, due to the differing mechanical properties in the materials. Additionally, the analysis was restricted to plane stress because 1,2 demonstrated that changes in EM performance due to curvature in the unit cell can be neglected. Therefore, terms E XZ and E YZ are equal to zero.The unit cell was modeled in ANSYS-HFSS, 9 utilizing equation surfaces that integrate the strain ...