Crack growth induced by thermal-mechanical loading

Advanced aerospace structures are often subjected to combined thermal and mechanical loads. The fracture-mechanics behavior of these structures may be altered by the thermal state existing around the crack. Hence, design of critical structural elements requires the knowledge of stress-intensity factors under both thermal and mechanical loads. This paper describes the development of an experimental technique to verity the thermal-stress-intensity factor generated by a temperature gradient around the crack. Thin plate specimens of a model material (AISI-SAE 1095 steel) were used for the heart transfer and thermal-mechanical fracture tests. Rapid thermal loading was achieved using high-intensity focussed infrared spot heaters. These heaters were also used to generate controlled temperature rates for heat-transfer vertification tests. The experimental results indicate that thermal loads can generate stress-intensity factors large enough to induce crack growth. The proposed thermal-stress-intensity factors appear to have the same effect as the conventional mechanical-stress-intensity factors with respect to fracture.     

Analysis of fluctuations induced by a scale hierarchy of interacting particles

We investigate the two-point averaging over space of fluctuations arising from a multiscale hierarchy of interacting particles. We assume this will satisfy a condition of homogeneity with respect to scale. We consider the second-order correlation of fluctuations arising from particles of a single scale in the hierarchy; we then form an average over the set of such single-scale correlations. As the hierarchy is refined, a condition of scale continuum is approached. We use the limiting value of this procedure to define a two-point correlation function for the multiscale system as a whole, and identify this with the experimental measurement of correlation in such a multiscale context. In the energy spectrum which emerges in this limit, one term comes to dominate the spectrum for large k; this term has the form ‘k ⁻² ln k’. In fact, a variety of different shape functions (intended to represent correlation functions) leads to this energy spectrum, which bears a qualitative resemblance to a Kolmogorov power-law. In this sense a degree of universality is exhibited. The ideas are illustrated for two simple one-dimensional test cases before a more general treatment in one and three dimensions is developed.     

Deformation of FCC nanowires by twinning and slip

We present atomistic simulations of the tensile and compressive loading of single crystal face-centered cubic (FCC) nanowires with and orientations to study the propensity of the nanowires to deform via twinning or slip. By studying the deformation characteristics of three FCC materials with disparate stacking fault energies (gold, copper and nickel), we find that the deformation mechanisms in the nanowires are a function of the intrinsic material properties, applied stress state, axial crystallographic orientation and exposed transverse surfaces. The key finding of this work is the first order effect that side surface orientation has on the operant mode of inelastic deformation in both and nanowires. Comparisons to expected deformation modes, as calculated using crystallographic Schmid factors for tension and compression, are provided to illustrate how transverse surface orientations can directly alter the deformation mechanisms in materials with nanometer scale dimensions.     

Controlled growth of silver nanoparticles in a hydrothermal process

A two-step synthesis was used to control the shape of silver nanoparticles prepared via reduction of Ag^＋ ions in aqueous Ag（NH3）2NO3 by poly（N-vinyl-2 First, a few spherical silver nanoparticles,-10 nm in size, were pyrrolidone） （PVP）. Then, in a subsequent hydrothermal treatment, the remaining Ag^＋ ions were reduced by PVP into polyhedral nanoparticles, or larger spherical nanoparticles formed from the small spherical seed silver nanoparticles in the first step. The morphology and size of the resultant particles depend on the hydrothermal temperature, PVP/Ag molar ratio and concentration of Ag^＋ ions. By using UV-visible spectroscopy （UV-vis）, transmission electron microscopy （TEM） and powder X-ray diffraction （XRD）, the possible growth mechanism of the silver nanoparticles was discussed. 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V.     

Stop flow lithography synthesis of non-spherical metal oxide particles

Non-spherical micron and nano-sized particles and their composites have become essential in select application areas of optics,wear resistance,personnel protection,chemical mechanical polishing,and biomedicine.In this paper,the synthesis of composite and ceramic non-spherical particles using stop flow lithography is reported.Precursor suspensions of poly(ethylene glycol) diacrylate,2-hydroxy-2-methylpropiophenone and SiO_2 or Al_2O_3 are prepared.The precursor suspension flows through a microfluidic device mounted on an upright microscope and is polymerized in an automated process.A photomask patterned with transparent geometric features,which define the cross-sectional shapes of the particles,masks the UV light to synthesize micron sized particles.Particles with axial dimensions ranging from 35 to 167 μm were synthesized.Control of device channel depth and objective lens magnification enables the manipulation of the particle size.Composite particles in triangular,square,pentagonal,hexagonal,and circular cross sections were synthesized.Subsequently,the transformation of the composite particles into the corresponding metal oxide particles was achieved through polymer burn-off and sintering.     

Fluid flows induced by focused ultrasound and their use in single-crystal growth

Particle image velocimetry was used to study the structure of stationary acoustic flows on a solid surface subjected to acoustic radiation along the normal to the prefocal and postfocal planes of a spherical concentrator. The results of model experiments were used for rapid growth of water-soluble single crystals. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 4, pp. 11–17, July–August, 2009.     

Diffusional growth of cloud particles: existence and uniqueness of solutions

Diffusional growth of cloud particles is commonly described by a coupled system of parabolic equations and ordinary differential equations. The Dirichlet boundary condition for the parabolic equation is obtained from the solution of the ordinary differential equations, but this solution itself depends on the solution of the parabolic equations. We first present the governing equations describing diffusional growth of cloud particles. In a second step, we consider a simplified model problem, motivated by the diffusional growth equations. The main difference between the simplified model problem and the diffusional growth equations consists in neglecting the dependence of the domain for the parabolic equations on the solution. For the model problem, we show unique solvability using a fixed point method. Finally, we discuss application of the main result for the model problem to the diffusional growth equations and illustrate these equations with the help of a numerical solution.     

Surface wrinkling of mucosa induced by volumetric growth: Theory, simulation and experiment

Mechanics of living tissues focusing on the relationships between growth, morphology and function is not only of theoretical interest but can also be useful for diagnosis of certain diseases. In this paper, we model the surface wrinkling morphology of mucosa, the moist tissue that commonly lines organs and cavities throughout the body, induced by either physiological or pathological volumetric growth. A theoretical framework of finite deformation is adopted to analyze the deformation of a cylindrical cavity covered by mucosal and submucosal layers. It is shown that compressive residual stresses induced by the confined growth of mucosa can destabilize the tissue into various surface wrinkling patterns. A linear stability analysis of the critical condition and characteristic buckling patterns indicates that the wrinkling mode is sensitive to the thicknesses of the mucosal and submucosal layers, as well as the properties of the tissues. The thinner the mucosal layer and the lower its elastic modulus, the shorter the buckling wavelength. A series of finite element simulations are performed to validate the theoretical predictions and to study local wrinkling or non-uniform patterns associated with inhomogeneous growth. Our postbuckling analysis shows that the surface pattern may evolve towards a period-doubling morphology due to continuous growth of mucosa or submucosa beyond the critical state. Finally, the theoretical predictions and numerical simulations are compared to experimental observations.     

Experimental investigations and modeling of volume change induced by void growth in polyamide 11

Polymers are known to be sensitive to hydrostatic pressure. The influence of stress triaxiality ratio on cavitation and damage has been highlighted in numerous studies. This paper proposes experimental investigations allowing the control of both the stress triaxiality ratio and the void distribution via microscopic observations of microtome-cut surfaces from interrupted tests. With the help of a finite element code, the Gurson–Tvergaard–Needleman model was calibrated by using these multi-scale experimental data. Then comparison between both numerical and analytical models and experimental data was performed. Bridgman formulae were reported to be valid up to the peak load. Moreover, a better understanding of the time evolution of significant parameters such as the porosity (volume change) and the stress triaxiality ratio (hydrostatic pressure), was highlighted.     

Capuli cherry-mediated green synthesis of silver nanoparticles under white solar and blue LED light

In this article, the Capuli (Prunus serotina Ehrh. var. Capuli) cherry extract was used for the synthesis of silver nanoparticles (AgNPs) in the presence of white/visible solar and blue light-emitting diode (LED) light. For the characterization of the extract and the AgNPs, Fourier transform infrared spectroscopy and ultraviolet–visible spectroscopy were employed, along with hydrodynamic particle size analysis, transmission electron microscopy and X-ray diffraction. The Ag nanospheres obtained using white light were 40–100 nm in diameter and exhibited an absorption peak at λ_max = 445 nm, whereas those obtained using blue LED light were 20–80 nm in diameter with an absorption peak at λ_max = 425 nm. Thermal analysis revealed that the content of biomolecules surrounding the AgNPs was about 55–65%, and it was also found that blue LED light AgNPs (56.28%, 0.05 mM) had a higher antioxidant efficacy than the white solar light AgNPs (33.42%, 0.05 mM) against 1,1-diphenyl-2-picrylhydrazyl. The results indicate that obtaining AgNPs using a blue LED light may prove to be a simple, cost-effective and easily reproducible method for creating future nanopharmaceuticals.     

Influence of seed layers on the vertical growth of ZnO nanowires by low-temperature wet chemical bath deposition on ITO-coated glass substrate

Experimental Techniques - Aligned ZnO nanowires (NWs) were synthesized by low-temperature wet chemical bath deposition (CBD) technique on indium tin oxide (ITO)-coated glass substrates using c-axis...     

Atmospheric pressure plasma-assisted green synthesis of amphiphilic SiO2 Janus particles

A facile green approach for the synthesis of amphiphilic SiO₂ Janus particles using low temperature atmospheric pressure plasma is reported in this study. Monodispersed SiO₂ particles were masked by embedding half of their surface inside a polystyrene film. The exposed surfaces of the SiO₂ particles were readily modified using He/CF₃CFH₂ low-temperature atmospheric pressure plasma to obtain amphiphilic Janus particles. Their amphiphilic nature was confirmed using fluorescent microscopy by tagging their hydrophilic part with a fluorescent dye. The present method can be used to generate amphiphilic Janus particles with a variety of functionalities, which may find applications as surfactants and surface modifying agents.     

Fatigue crack growth induced by domain switching under electromechanical load in ferroelectrics

Reliability calls for a better understanding of the failure of ferroelectric ceramics. The fracture and fatigue of ferroelectric ceramics under an electric field or a combined electric and mechanical loading are investigated. The small-scale domain-switching model is modified to analyze failure due to fracture and fatigue. Effects of anisotropy and electromechanical load coupling are taken into account. Analytical expressions are obtained for domain-switching regions near the crack tip such that of 90° domain switching can be distinguished from 180° domain switching in addition to different initial poling directions. The crack tip stress intensity variation of ferroelectric ceramics due to the domain switching is analyzed. A positive electric field tends to enhance the propagation of an insulating crack perpendicular to the poling direction, while a negative field impedes it. Fatigue crack growth under various coupling loads and effects of the stress field and electric field on near field stress intensity variation are analyzed. Predicted crack growth versus cyclic electric field agrees well with experiment.     

Evaluation of particle growth systems for sampling and analysis of atmospheric fine particles

Three types of water-based condensational growth systems, which can enable particles to grow in size to facilitate sampling and subsequent chemical analysis, were evaluated. The first one is a mixing type growth system where aerosols are mixed with saturated water vapor, the second one is a thermal diffusive growth system where warm flow enters cold-walled tube, and the third one is a laminar flow type where cold flow enters a warm wet-wall tube. Hygroscopic sodium chloride (NaCl), ammonium sulfate ((NH₄)₂SO₄) and ammonium nitrate (NH₄NO₃), and non-hygroscopic polystyrene latex (PSL) particles, in the size range of 50–400 nm, were used to determine their growth factors through the growth systems. Our data showed that the third-type growth system could enable particles to grow most efficiently regardless of their hygroscopic property. Collection efficiency of particles in the size range of 0.05–2.5 μm, in a continuous aerosol sampler after they passed through the third-type growth system was about 100%, suggesting that the third-type growth system would be the most useful among the tested growth systems for sampling and subsequent chemical analysis of fine and ultrafine particles.     

Numerical investigation of magnetic-field induced self-assembly of nonmagnetic particles in magnetic fluids

The underlying mechanisms of controlling the self-assembly of micro-size nonmagnetic particles (NPs) in magnetic fluids are essential for the manufacture, process and exploitation of nano-magnetic material. In this study, a multi-physical numerical model, which couples a distribution function correction-based immersed boundary lattice Boltzmann method (DFCIB-LBM) for fluid–structure interaction (FSI) and a self-correcting procedure of Poisson equation solver for magnetic field, is carried out to investigate such self-assembling behaviors and mechanisms. The interactions of two neighboring micro-size NPs placed in different distance and orientation, and the self-assembling behaviors of numerous micro-size NPs under uniform magnetic field are studied in details. The results demonstrate that the self-assembling behaviors are caused by the inverse magnetic effect, which can be adjusted by varying the concentration and size of NPs, permeability ratio, and the strength of external magnetic field. On the contrary, NPs in magnetic fluid affect their surrounding magnetic field and hinder the magnetization near the NP boundary region. These findings can provide a better understanding of the bottom-up fabrication of magnetic functional materials and devices.