The enhancement of diffusion by strain of InAs quantum dots in a GaAs matrix

We investigate the annealing behavior of Photoluminescence (PL) from self-assembled InAs quantum dots (QDs) with different thicknesses GaAs cap layers. The diffusion introduced by annealing treatment results in a blue-shift of the QD PL peak, and a decrease in the integrated intensity. The strain present in QDs enhances the diffusion, and the QDs with the cap layers of different thicknesses will experience a strain of different strength. This can lend to a, better understanding of the larger blue-shift of the PL peak of the deeper buried QDs, and the different variance of the full width at half maximum of the luminescence from QDs with the cap layers of different thicknesses.

Spin splitting modulated by uniaxial stress in InAs nanowires

We theoretically study the electronic structure, spin splitting, effective mass, and spin orientation of InAs nanowires with cylindrical symmetry in the presence of an external electric field and uniaxial stress. Using an eight-band k center dot p theoretical model, we deduce a formula for the spin splitting in the system, indicating that the spin splitting under uniaxial stress is a nonlinear function of the momentum and the electric field. The spin splitting can be described by a linear Rashba model when the wavevector and the electric field are sufficiently small. Our numeric results show that the uniaxial stress can modulate the spin splitting. With the increase of wavevector, the uniaxial tensile stress first restrains and then amplifies the spin splitting of the lowest electron state compared to the no strain case. The reverse is true under a compression. Moreover, strong spin splitting can be induced by compression when the top of the valence band is close to the bottom of the conductance band, and the spin orientations of the electron stay almost unchanged before the overlap of the two bands.

Strain gradient theory based on a new framework of non-local model

A new framework of non-local model for the strain energy density is proposed in this paper. The global strain energy density of the representative volume element is treated as a non-local variable and can be obtained through a special integral of the local strain energy density. The local strain energy density is assumed to be dependent on both the strain and the rotation-gradient. As a result of the non-local model, a new strain gradient theory is derived directly, in which the first and second strain gradients, as well as the triadic and tetradic stress, are introduced in the context of work conjugate. For power law hardening materials, size effects in thin metallic wire torsion and ultra-thin cantilever beam bend are investigated. It is found that the result predicted by the theoretical model is well consistent with the experimental data for the thin wire torsion. On the other hand, the calculation result for the micro-cantilever beam bend clearly shows the size effect.

Catastrophic rupture of heterogeneous brittle materials under impact loading

The catastrophic failure of heterogeneous brittle materials under impact loading is not fully understood. To describe the catastrophic failure behavior of heterogeneous brittle materials under impact loading, an elasto-statistical-brittle (ESB) model is proposed in this paper. The ESB model characterizes the disordered inhomogeneity of material at mesoscopic scale with the statistical description of the shear strength of mesoscopic units. If the applied shear stress reaches the strength, the mesoscopic unit fails, which causes degradation in the shear modulus of the material. With a simplified ESB model, the failure wave in brittle material under uni-axial compression is analyzed. It is shown that the failure wave is a wave of strain or particle velocity resulted from the catastrophic fracture in an elastically stressed brittle media when the impact velocity reaches a critical value. In addition, the failure wave causes an increase in the rear surface velocity, which agrees well with experimental observations. The critical condition to generate failure wave and the speed of failure wave are also obtained.

Stress analysis of CR superferric magnet

Finite Element Method is used in this article to analyze the stress of CR superferric magnet. Magnetic force and the stress caused by this force are calculated. The thermal stress and strain of the coil caused by cooling down is also analyzed. The result will be taken as a check for the design of the coil and coilcase, and also as a reference for the optimization of further design and quench protection.

Stress Analysis of the Superconducting Magnet of LPT

The superconducting magnet of the LPT (Lanzhou Penning trap) consists of nine coaxial coils. The maximum magnetic field is 7 T and thus results in a large magnetic force. In order to assure the mechanical stability, it is necessary to do the stress analysis of the magnet system. The 3D Finite Element Analysis of thermal and mechanical behavior was presented in this paper. For the numerical simulation and analysis of the phenomena inside the structure, the ADINA and TOSCA code were chosen right from start. The ADINA code is commonly used for numerical simulations of the structure analysis [1] and the TOSCA code is professional software to calculate the magnetic field and Lorentz Forces. The results of the analysis were evaluated in terms of the stress and deformation.

Uniaxial deformation of overstretched polyethylene: In-situ synchrotron small angle X-ray scattering study

Synchrotron small angle X-ray scattering was used to study the deformation mechanism of high-density polyethylene that was stretched beyond the natural draw ratio. New insight into the cooperative deformational behavior being mediated via slippage of micro-fibrils was gained. The scattering data confirm on the one hand the model proposed by Peterlin on the static structure of oriented polyethylene being composed of oriented fibrils, which are built by bundles of micro-fibrils. On the other hand it was found that deformation is mediated by the slippage of the micro-fibrils and not the slippage of the fibrils. In the micro-fibrils, the polymer chains are highly oriented both in the crystalline and in the amorphous regions. When stretching beyond the natural draw ratio mainly slippage of micro-fibrils past each other takes place. The thickness of the interlamellar amorphous layers increases only slightly. The coupling force between micro-fibrils increases during stretching due to inter-microfibrillar polymer segments being stretched taut thus increasingly impeding further sliding of the micro-fibrils leading finally to slippage of the fibrils.

Structural evolution of tensile-deformed high-density polyethylene during annealing: Scanning synchrotron small-angle X-ray scattering study

The structural evolution of high-density polyethylene subjected to uniaxial tensile deformation was investigated as a function of strain and after annealing at different temperatures using a scanning synchrotron small-angle X-ray scattering (SAXS) technique. The results confirm that in the course of tensile deformation intralamellar block slips were activated at small deformations followed by a stress-induced fragmentation and recrystallization process yielding thinner lamellae with their normal parallel to the stretching direction. The original sheared lamellae underwent severe internal deformation so that they were even less stable than the newly developed thinner lamellae. Accordingly, annealing results in a melting of the original crystallites even at moderate strains where the stress-induced fragmentation and recrystallization just sets in and generates a distinctly different form of lamellar stacks aligned along the drawing direction. It was found that the lamellae newly formed during stretching at moderate strains remain stable at lower temperature. Only at a very high annealing temperature of 120 degrees C can they be melted, leading to an isotropic distribution of the lamellar structure.

Preparation of fully cross-linked CNBR/PP-g-GMA and CNBR/PP/PP-g-GMA thermoplastic elastomers and their morphology, structure and properties

Binary CNBR/PP-g-GMA and ternary CNBR/PP/PP-g-GMA thermoplastic elastomers were prepared by reactive blending carboxy nitrile rubber (CNBR) powder with nanometer dimension and polypropylene functionalized with glycidyl methacrylate (PP-g-GMA). Morphology observation by using an atomic force microscope (AFM) and TEM revealed that the size of CNBR dispersed phase in CNBR/PP-g-GMA binary blends was much smaller than that of the corresponding CNBR/PP binary blends. Thermal behavior of CNBR/PP-g-GMA and CNBR/PP blends was studied by DSC. Comparing with the plain PP-g-GMA, T, of PP-g-GMA in CNBR/PP-g-GMA blends increased about 10degreesC. Both thermodynamic and kinetic effects would influence the crystallization behavior of PP-g-GMA in CNBR/PP-g-GMA blends. At a fixed content of CNBR, the apparent viscosity of the blending system increased with increasing the content of PP-g-GMA. FTIR spectrum verified that the improvement of miscibility of CNBR and PP-g-GMA was originated from the reaction between carboxy end groups of CNBR and epoxy groups of GMA grafted onto PP molecular chains. Comparing with CNBR/PP blends, the tensile strength, stress at 100% strain, and elongation at break of CNBR/PP-g-GMA blends were greatly improved.

Mechanical and thermal properties of high-density polyethylene toughened with glass beads

Glass beads were used to improve the mechanical and thermal properties of high-density polyethylene (HDPE). HDPE/glass-bead blends were prepared in a Brabender-like apparatus, and this was followed by press molding. Static tensile measurements showed that the modulus of the HDPE/glass-bead blends increased considerably with increasing glass-bead content, whereas the yield stress remained roughly unchanged at first and then decreased slowly with increasing glass-bead content. Izod impact tests at room temperature revealed that the impact strength changed very slowly with increasing glass-bead content up to a critical value; thereafter, it increased sharply with increasing glass-bead content. That is, the lzod impact strength of the blends underwent a sharp transition with increasing glass-bead content. It was calculated that the critical interparticle distance for the HDPE/glass-bead blends at room temperature (25degreesC) was 2.5 mum. Scanning electron microscopy observations indicated that the high impact strength of the HDPE/glass-bead blends resulted from the deformation of the HDPE matrix. Dynamic mechanical analyses and thermogravimetric measurements implied that the heat resistance and heat stability of the blends tended to increase considerably with increasing glass-bead content.

Interparticle distance-temperature-strain rate equivalence for the brittle-tough transition in polymer blends

From the angle of energy transformation an equation was obtained for the brittle transition in polymer blends. The effects of interparticle distance, temperature and strain rate on the brittle-tough transition in polymer blends were characterized by this equation. The calculations show that, for this transition: (1) increasing temperature and decreasing interparticle distance are equivalent and the shift factor increases with increasing temperature; (2) decreasing strain rate and decreasing interparticle distance have equivalent effects on the transition; (3) the strain rate must be optimum in order to find the brittle-tough transition phenomena for a given temperature region. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.

Flexural fatigue behavior of injection-molded composites based on poly(phenylene ether ketone)

Flexural fatigue tests were conducted on injection-molded short fiber composites, carbon fiber/poly(phenylene ether ketone) (PEK-C) and glass fiber/PEK-C (with addition of polyphenylene sulfide for improving adhesion between matrix and fibers), using four-point bending at stress ratio of 0.1. The fatigue behavior of these materials was presented. By comparing the S-N curves and analyzing the fracture surfaces of the two materials, the similarity and difference of the failure mechanisms in the two materials were discussed. It is shown that the flexural fatigue failure of the studied materials is governed by their respective tensile properties. The matrix yielding is main failure mechanism at high stress, while at lower stress the fatigue properties appear fiber and interface dominated. (C) 1997 John Wiley & Sons, Inc.

The crystal structure and drawing-induced polymorphism in poly(aryl ether ketone)s .2. Poly(ether ether ketone ketone), PEEKK

The crystal structure, morphology and polymorphism induced by uniaxial drawing of poly(ether ether ketone ketone) [PEEKK] have been studied by transmission electron microscopy (TEM), electron diffraction (ED) and wide angle X-ray diffraction (WAXD). On the basis of WAXD and ED patterns,the crystal structure of unoriented PEEKK is determined to have two-chain orthorhombic packing with unit cell parameters of a 0.772 nm, b = 0.600 nm, c = 1.004 nm (form I), A stress-induced crystal modification (form II) is identified and found to possess a two-chain orthorhombic lattice with unit cell dimensions of a = 0.461 nm, b = 1.074 nm, c = 1.080 nm. The 7.5% increase in c-axis dimension for form II is attributed to an overextended chain conformation, arising from extensional deformation during uniaxial drawing and fixed ''in-situ'' through strain-induced crystallization. The average ether-ketone bridge bond angles in form II crystal are determined to be 148.9 degrees by using standard bond lengths. The crystal morphology of PEEKK bears a great similarity to that of PEEK. The crystals grow in the form of spherulites and have the b-axis of unit cell radial. The effects of draw rate on strain-induced crystallization and induction of form II structure are also discussed.

TEMPERATURE AND STRAIN-RATE DEPENDENCE OF FRACTURE-TOUGHNESS OF PHENOLPHTHALEIN POLYETHER KETONE

A strong strain-rate and temperature dependence was observed for the fracture toughness of phenolphthalein polyether ketone (PEK-C). Two separate crack-blunting mechanisms have been proposed to account for the fracture-toughness data. The first mechanism involves thermal blunting due to adiabatic heating at the crack tip for the high temperatures studied. In the high-temperature range, thermal blunting increases the fracture toughness corresponding to an effectively higher test temperature. However, in the low-temperature range, the adiabatic temperature rise is insufficient to cause softening and Jic increases with increasing temperature owing to viscoelastic losses associated with the p-relaxation there. The second mechanism involves plastic blunting due to shear yield/flow processes at the crack tip and this takes place at slow strain testing of the single-edge notched bending (SENB) samples. The temperature and strain-rate dependence of the plastic zone size may also be responsible for the temperature and strain-rate dependence of fracture toughness.

MECHANICAL-PROPERTIES OF PHENOLPHTHALEIN POLYETHER KETONE - YIELD STRESS, YOUNGS MODULUS, AND FRACTURE-TOUGHNESS

A series of tensile and three-point bending studies was conducted at various temperatures and loading rates using phenolphthalein polyether ketone (PEK-C). Yield stress, Young's modulus, fracture toughness, and crack opening displacement data were obtained for various conditions. In general, both yield stress and Young's modulus increase with decreasing temperature. However, the relationships between fracture toughness, loading rate, and temperature are very complex. This behavior is due to the simultaneous intersection of viscoelasticity and localized plastic deformation. The increased yield stress is the main factor contributing to the reduction in fracture toughness and crack opening displacement. The relationship between fracture toughness and yield stress are discussed. (C) 1995 John Wiley and Sons, Inc.