Bounds and estimates for the effect of strain gradients upon the effective plastic properties of an isotropic two-phase composite

Predictions are made for the size effect on strength of a random, isotropic two-phase composite. Each phase is treated as an isotropic, elastic-plastic solid, with a response described by a modified deformation theory version of the Fleck-Hutchinson strain gradient plasticity formulation (Fleck and Hutchinson, J. Mech. Phys. Solids 49 (2001) 2245). The essential feature of the new theory is that the plastic strain tensor is treated as a primary unknown on the same footing as the displacement. Minimum principles for the energy and for the complementary energy are stated for a composite, and these lead directly to elementary bounds analogous to those of Reuss and Voigt. For the case of a linear hardening solid, Hashin-Shtrikman bounds and self-consistent estimates are derived. A non-linear variational principle is constructed by generalising that of Ponte Castañeda (J. Mech. Phys. Solids 40 (1992) 1757). The minimum principle is used to derive an upper bound, a lower estimate and a self-consistent estimate for the overall plastic response of a statistically homogeneous and isotropic strain gradient composite. Sample numerical calculations are performed to explore the dependence of the macroscopic uniaxial response upon the size scale of the microstructure, and upon the relative volume fraction of the two phases.     

Behaviour of thick composite tubes considering of delamination

The present paper deals with plane finite element analysis of thick composite tubes. Thick composite tubes are commonly used in marine industry and in deep-water offshore applications. Two kinds of interlaminar delamination type defect in a thick walled cylinder subjected to external pressure were confronted; an annular or ring like delamination and a strip delamination. Two finite element models were developed to predict the strain energy release rate at the delamination fronts. In these models the effects of the processing history of the composite material in the form of a uniform thermal load were also included to simulate the state of the residual stress in the composite. The considered defects are studied by means of the effect of buckling, investigating the annular and the strip delamination buckling, and the subsequent loss of load carrying capacity of the delaminated region.     

Effects of microscopic boundary conditions on the deformation behavior of small-volume metallic glasses

Although large-volume metallic glasses (MGs) are susceptible to shear localization due to their intrinsically strain-softening response, recent experiments and molecular dynamics simulations have shown that small-volume MGs samples are able to exhibit work hardening response. Here, we seek to address two issues regarding the mechanical response of small-volume MGs at low homologous temperatures from a continuum-based modeling perspective: (1) are MGs capable of exhibiting a work hardening response, and (2) what is the physical mechanism which causes its work hardening response?Along with implementing a recently-developed finite-deformation, strain gradient plasticity-based constitutive model for MGs into a self-developed finite-element code, we study the tensile response of small-volume MG samples of various sizes through finite-element simulations. Our simulations show that small-volume MG samples are capable of exhibiting a work hardening response provided the following conditions are met: (a) the sample size is small enough, and (b) the appropriate microscopic boundary conditions for the free volume are imposed on the sample.     

A linked FEM-damage percolation model of aluminum alloy sheet forming

A so-called damage percolation model is linked with a finite element model of a sheet forming process to offer a comprehensive study of ductile damage evolution. In the current study, a damage percolation code is linked with LS-DYNA, an explicit dynamic FEM code used to introduce local strain gradients and compliance effects due to damage-induced softening. The linked model utilizes a Gurson-based yield surface to account for the softening effects of void damage, while the local damage development and void linkage events are modeled using the damage percolation code. The percolation code accepts detailed second phase particle fields from image analysis of a 2.0×1.6 mm optical micrograph of AA5182 aluminum alloy sheet. The model is applied to a stretch-flange stamping process which is known to be a damage-sensitive operation. The critical conditions for fracture are predicted for various initial stretch flange hole sizes.     

A homogenization theory of strain gradient single crystal plasticity and its finite element discretization

In this study, a homogenization theory based on the Gurtin strain gradient formulation and its finite element discretization are developed for investigating the size effects on macroscopic responses of periodic materials. To derive the homogenization equations consisting of the relation of macroscopic stress, the weak form of stress balance, and the weak form of microforce balance, the Y-periodicity is used as additional, as well as standard, boundary conditions at the boundary of a unit cell. Then, by applying a tangent modulus method, a set of finite element equations is obtained from the homogenization equations. The computational stability and efficiency of this finite element discretization are verified by analyzing a model composite. Furthermore, a model polycrystal is analyzed for investigating the grain size dependence of polycrystal plasticity. In this analysis, the micro-clamped, micro-free, and defect-free conditions are considered as the additional boundary conditions at grain boundaries, and their effects are discussed.     

Plated and Intact Femur Strains in Fracture Fixation Using Fiber Bragg Gratings and Strain Gauges

This paper presents an experimental methodology to determine plated and intact femur strains using fiber Bragg gratings and strain gauges. A plated and an intact synthetic femur were used and loaded under a simplistic static load of 600 N. A stainless steel (316L) plate was used to fixate a simulated 45° fracture on one femur. Strains were recoded at the same sites on both femurs. Strain shielding is shown to be more pronounced at the distal region of the plated femur. The experimental methodology based on fiber Bragg grating sensors is a novel approach to assess bone plate strains, which could also be used to obtain biologic tissue and implant surface strains in locations where conventional strain gauge use is not technically feasible.     

Cation-Promoted Strain-Release-Driven Access to Functionalized Azetidines from Azabicyclo[1.1.0]butanes

Access to 1,3-functionalized azetidines through a diversity-oriented approach is highly sought-after for finding new applications in drug-discovery. To this goal, strain-release-driven functionalization of azabicyclo[1.1.0]-butane (ABB) has generated significant interest. Through appropriate N-activation, C3-substituted ABBs are shown to render tandem N/C3-fucntionalization/rearrangement, furnishing azetidines; although, modalities of such N-activation vis-à-vis N-functionalization remain limited to selected electrophiles. This work showcases a versatile cation-driven activation strategy of ABBs. And capitalizes on the use of Csp³ precursors amenable to forming reactive (aza)oxyallyl cations in situ. Herein, N-activation leads to formation of a congested C−N bond, and effective C3 activation. The concept was extended to formal [3+2] annulations involving (aza)oxyallyl cations and ABBs, leading to bridged bicyclic azetidines. Besides the fundamental appeal of this new activation paradigm, operational simplicity and remarkable diversity should engender its prompt use in synthetic and medicinal chemistry.     

Misorientation/local plastic strain manifestations in chemical etching color

Cold plastic deformation produces misorientations inside the crystal grains, and the distribution of the misorientation is quite crucial to understand the deformation behavior of the metals or alloys. The misorientation manifestations in chemical etching contrast are investigated in this study in the case of cold-deformed iron. The chemical etching is performed by using nital, while the crystal orientation is determined by electron backscatter diffraction (EBSD). The correlation between the chemical etching contrast and crystal orientation have been studied in both cold-deformed and undeformed iron. The results clearly show that the chemical etching contrast strongly reflects the crystallographic orientation. The gradual change in chemical etching contrast inside the individual deformed grains gives information of both the misorientation and local plastic strain within the grains. This method can provide an easy and alternative way to qualitatively understand the misorientation and local plastic strain distributions in the microstructures.     

Spin currents in graphene under tension

Based on the Tight-Binding model, we have asymmetric massless Dirac fermions as the carriers in graphene under tension. Because of uniaxial strain, the velocities of Dirac fermions depend on their directions. This work studies the effect of the uniaxial strain on the spin transport through a single magnetic barrier of the strained graphene system. The result shows that graphene has a great potential for applications in nano-mechanical spintronic devices. This is because of strain in graphene can induce the spin-dependent pseudo-potentials at the barrier to control the spin currents of the junction.