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This study is aimed to define the dependence of the mechanical properties of ferrogel (FG), chosen as a prospective scaffold for cellular technologies, on the concentration of magnetic particles (MPs) embedded into composite. FGs based on polyacrylamide hydrogel filled with the strontium hexaferrite (SrFe12O19) MPs with weight fraction in the range of 0 – 11 % were investigated with the use of two independent approaches: stress-strain testing of mechanical deformations in step-wise and dynamic modes and by medical ultrasound imaging. Stress-strain test showed that the increase of MPs concentration in FG resulted in significant increase of elastic storage modulus (G'), loss modulus (G") and dynamic viscosity coefficient (η') of the magnetic composite. According to the ultrasonography test, the increase of MPs concentration in FG provided the enhancement of FG visualization. The larger was the MPs concentration in FG, the higher was brightness of the ultrasound image at the surface of FG and inside its bulk. Close correlation was found between the elastic storage modulus (G') and the brightness at water/FG interface for full range of MPs concentrations. [ABSTRACT FROM AUTHOR]

The purpose of this work is to determine experimentally and computationally the coefficients of the Williams series expansion in the vicinity of two interacting cracks or notches. In this study, the multi-parameter description of the crack-tip fields for digital processing of the experimental data obtained from photoelastic experiments is complied. The special programming tool for digital processing of the experimental data is elaborated. The Williams expansion is introduced for approximation of the stress/displacement distribution. The over-deterministic method is used for calculation of the coefficients of the power series terms because it requires only a conventional FE analysis. Results determined via finite element analysis are compared to those calculated by means of the stress distribution expressed via the Williams expansion under consideration of various numbers of initial terms of the series. It is shown that the higher-order terms of the Williams expansion can be significant when the extent of the isochromatic fringe is far enough in comparison to the crack length. It is shown that the use of higher order terms leads to increase of the domain where the Williams series expansion can be used. [ABSTRACT FROM AUTHOR]

Mixed mode I+II fracture of triangular shape specimens is studied experimentally and numerically. The investigated fracture results have been obtained for a brittle polycarbonate by testing an inclined edge crack triangular shape specimen subjected to symmetrical three-point bend loading in the entire range of mode mixities from pure mode I to pure mode II. Stress intensity factors, T-stresses and higher-order terms have been found by the digital photoelasticity and finite element methods. It is shown that the experimental and numerical results are in good agreement. [ABSTRACT FROM AUTHOR]

This report presents research of polarization-optical tomography (integral photoelasticity) possibilities for non-destructive diagnostics of mechanical stresses in dielectric cylindrical structures of hexagonal single crystal used in electrooptic and nonlinear optical devices, high-power laser, fiber optics and other photo optic applications. The polarization of light is determined in a plane orthogonal to the optical axis of the object. It is supposed a linear dependence of the stresses are determined within the permittivity tensor on stresses (the Maxwell piezo-optic law) and weak rotation of quasi – principal directions of the stress tensor. The report generalizes the application of polarization-optical tomography with the small curving beam of light in case of radial refractive index gradient. [ABSTRACT FROM AUTHOR]

The multi-parameter description of the stress field in the neighborhood of the crack tips of two interacting cracks based on the photoelastic study and finite element analyses is given. Digital photoelasticity method is used for obtaining the isochromatic and isoclinic patterns in the plate with two collinear horizontal and two inclined cracks in anisotropic linear elastic material. Stress intensity factors, T-stresses and coefficients of the higher order terms in the multi-parameter Williams series expansion are experimentally determined. The finite element analysis for the cracked plates with the same configurations has been performed. Stress intensity factors, T-stress and coefficients of nine-term asymptotic expansion for the stress field are numerically obtained and are compared with the experimental results. The comparison shows good agreement of experimental and numerical estimations of these fracture mechanics parameters. Very good agreement is shown to exist between the digital photoelasticity method and finite element results confirming the effectiveness of the photoelasticity technique in obtaining the coefficients of higher order terms of the Williams series expansion from the experimental stress field around the crack tip [ABSTRACT FROM AUTHOR]

The paper proposes the algorithm for the determination of the coefficients of the Williams series expansion in notched semidisks with different angles of the notch. The algorithm is based on the experimental procedure of the photoelasticity method and the finite element analysis. The large series of experiments for semidiscs was performed. Digital photoelasticity method is used to analyze experimentally the complete Williams series expansion of the stress and displacement fields in the vicinity of the crack tip in isotropic linear elastic plates under Mixed Mode loading. The distribution of the isochromatic fridge patterns is employed for obtaining the stress field near the crack tip by the use of the complete Williams asymptotic expansion for various classes of the experimental specimens (plates with two collinear cracks under tensile loading and under mixed mode loading conditions). The higher order terms of the Williams series expansion are taken into account and the coefficients of the higher order terms are experimentally obtained. The stress field equation of Williams up to fifty terms in each in mode I and mode II has been considered. The comparison of the experimental results and the calculations performed with finite element analysis has shown the importance and significant advantages of photoelastic observations for the multi-parameter description of the stress field in the neighborhood of the crack tip. [ABSTRACT FROM AUTHOR]

In the paper we present the results of an experimental investigation of photoelasticity of polycarbonate which is subject to compressive stress. For investigation, we used a homemade plane polariscope and determined the residual birefringence of unloaded samples, prior to the investigation itself. After that we gradually applied the load on the samples under investigation and recorded the spectra of light transmitted through the polariscope as function of load and the displacement of the head of the loading apparatus, as well. As the result we obtained the normalized intensity of light as function of engineering strain which provided information on the photoelastic birefringence as function of the strain. [ABSTRACT FROM AUTHOR]

In the paper we present the results of an experimental investigation of birefringence of polycarbonate specimens that are subject to compressive stress. A plane polariscope setup is used for investigation and the so called residual birefringence of unloaded samples is determined in VIS, first. After that a load within the range of elastic deformation is gradually applied to the specimen under investigation and the spectra of light transmitted through the polarizer - compressed sample - analyzer are recorded and the loading head displacement is measured. Normalized intensity of light as function of strain is obtained and analyzed in order to get the photoelastic birefringence as function of strain. The obtained results are discussed in terms of suitability of polycarbonate material for constructing optical sensors of load. [ABSTRACT FROM AUTHOR]

We discuss the use of metamaterials for the creation and enhancement of opto-acoustic interactions. We show that both photo-elastic and the roto-optic components of the opto-acoustic response can be greatly enhanced as a result of artificial terms that arise directly from the interaction of the sub-elements of the structure. This work has important implications for the creation of new photonic devices that are able to efficiently harness Stimulated Brillouin Scattering. [ABSTRACT FROM AUTHOR]

Calcite and sapphire were shock compressed along the <1 0 1 0> direction (a axis) in a plate impact configuration. Polarimetery and Photonic Doppler Velocimetery (PDV) were used to measure the change in birefringence with particle velocity in the shock direction. Results for sapphire agree well with linear photoelastic theory and current literature showing a linear relationship between birefringence and particle velocity up to 310 m s-1. A maximum change in birefringence of 5% was observed. Calcite however showed anomolous behaviour with no detectable change in birefringence (less than 0.1%) over the range of particle velocities studied (up to 75 m s-1). [ABSTRACT FROM AUTHOR]

The quantum analysis of the polar atom under the action of a mechanical load is presented in this work. The obtained solution can be used to describe the optical properties of the Cosserat medium in the form of the gyrobirefringence. Rayleigh scattering of laser light coming towards the Cosserat medium head is analysed to measure the rotation of the azimuth of polarisation associated with couple stresses. [ABSTRACT FROM AUTHOR]

A commercially available grey-field polariscope (GFP) instrument for photoelastic examination is used to assess impact damage inflicted upon the outer-most pane of Space Shuttle windows made from fused silica. A method and apparatus for calibration of the stress-optic coefficient using four-point bending is discussed. The results are validated on known material (acrylic) and are found to agree with literature values to within 6%. The calibration procedure is then applied to fused-silica specimens and the stress-optic coefficient is determined to be 2.43 ± 0.54 x 10-12 Pa-1. Fused silica specimens containing impacts artificially made at NASA's Hypervelocity Impact Technology Facility (HIT-F), to simulate damage typical during space flight, are examined. The damage sites are cored from fused silica window carcasses and examined with the GFP. The calibrated GFP measurements of residual stress patterns surrounding the damage sites are presented. [ABSTRACT FROM AUTHOR]

New instrumentation and a widening range of phased array transducer options are affording the industry a greater potential. Visualization of the complex wave components using the photoelastic system can greatly enhance understanding of the generated signals. Diffraction, mode conversion and wave front interaction, together with beam forming for linear, sectorial and matrix arrays, will be viewed using the photoelastic system. Beam focus and steering performance will be shown with a range of embedded and surface targets within glass samples. This paper will present principles and sound field images using this visualization system. [ABSTRACT FROM AUTHOR]

Several recent studies on granular impact have used a macroscopic force law which describes the slow (mean) dynamics. However, these force-law models must be modified to capture large fluctuations at short time-scales. Here, we discuss granular impact experiments using photoelastic particles, where high-speed video captures both the intruder dynamics and the local granular force response. We show how to systematically separate the fluctuations from the mean dynamics. We also show that these fluctuations are multiplicative with the mean force, and otherwise decoupled from the dynamics. These observations are instructive in connecting to microscopic processes which generate the fluctuations. [ABSTRACT FROM AUTHOR]

In granular materials, it is not so simple to assess experimentally the contact forces. Photoelasticity is generally used for this purpose but this technique involves some constraints that may limit its use. We propose a different solution, which implements both the digital image correlation (DIC) technique and the non-smooth contact dynamics (NSCD) formalism. In a nutshell, the technique aims to find a set of contact forces mechanically admissible given a set of measured contact velocities. We used photographs of a simple shear test of a two-dimensional analogue granular material (about 1000 aluminum rods) to apply the solution, and we showed that valuable information about the contact forces can be extracted from the kinematic field provided that no major rearrangement occurs for at least five image shots. [ABSTRACT FROM AUTHOR]

We have designed a novel experimental apparatus to probe the mechanics of sheared quasi two-dimensional frictional granular materials with tunable friction from the supporting base of the apparatus. The experiment consists of a floating layer of photoelastic disks, which is subject to deformation. Forces on the particles are measured at the particle scale, using their photoelastic properties. This novel setup makes the study of the role of basal friction on sheared granular media possible. [ABSTRACT FROM AUTHOR]

Parallel program systems for numerical solution of 2D and 3D problems of the dynamics of deformable media with constitutive relationships of rather general form on the basis of universal mathematical model describing small strains of elastic, elastic-plastic and granular materials are worked out. Some computations of dynamic problems with and without taking into account the moment properties of a material were performed on supercomputers with cluster architecture. [ABSTRACT FROM AUTHOR]

Photoelastic visualization can be used to establish vibrational modes of solid transparent materials having complicated longitudinal and shear strains. On the other hand, determining the sign of a stress field by the photoelastic visualization is difficult. Color visualization of resonance vibrational modes of a glass plate by using stroboscopic photoelasticity with a sensitive tint plate is described. This technique enables to determine the sign of the stress in acoustic fields. [ABSTRACT FROM AUTHOR]

In order to visualize ultrasound in steel the photoelastic imager is a complementary technique to simulations, with the advantage that real transducers are used. In this contribution we show the linear behavior of the derived photoelastic signal and compare the results to FEM calculations. We use the result for quantitative analysis of sound-fields of immersion and phased array transducers. Interesting results are derived by analysis of the influence of missing elements on the shape and intensity of phased array (PA) ultrasonic pulses. Furthermore the formation of grating lobes from PA excitation is demonstrated and the plurality of waves generated from a small notch is displayed exemplarily. [ABSTRACT FROM AUTHOR]

This work presents the results from an analysis which considers the interaction between a crack and surrounding microcracks. The method of complex variables is used to derive explicit expressions for the stress intensity factors. The results are computed and discussed for a variety of geometrical configurations of microcracks, with the intent of developing an understanding of the shielding and amplification phenomena. A qualitative experimental technique of photoelasticity is used to validate the patterns of the iso-stress. [ABSTRACT FROM AUTHOR]

Glass beads, when immersed in a liquid having the same refractive index, provide an idealised granular material which can be used both for full-field determination of displacements and hence strains and also, by viewing under polarised light, information concerning the internal stresses within the granular assembly. A simple test box has been used to subject this granular material to rotation of principal strain axes and to encourage the development of a shear mechanism. Results from image analysis are presented to show detail of shear strains and volumetric strains. The images obtained using polarised light have also been subjected to digital manipulation. The nature of the patterns that are revealed is subtle and appears to occur at a scale quite different from the strain patterns. [ABSTRACT FROM AUTHOR]

By means of photoelasticity, we success in visualizing in real time the propagation of acoustic waves in a granular packing of cylinders. As previously mentioned by Nesterenko [1] for the case of spherical grains, the nonlinearity of the contact law between grains induces a dependence of the wave velocity both on its amplitude and on the confinement force. Our experimental procedure allows an access to the local state of stress of individual grains as a function of time with a good accuracy. Our results concerning the wave velocity as a function of the amplitude, and of the confinement force, are compared to theoretical predic tions and to the spherical beads case. [ABSTRACT FROM AUTHOR]

The effect of the force chain network on sound propagation in a granular material is poorly understood. To quantitatively study these effects, we perform acoustics experiments in a two dimensional photoelastic granular material in which force chains are visible. We send acoustic pulses into the material from a point source and measure the effects of this pulse via two methods: accelerometers within individual grains and movies which produce spatiotemporally resolved measurements of the acoustic propagation. [ABSTRACT FROM AUTHOR]

Using photoelastic experimental techniques, we probe the nature of jamming in dense 2D granular systems. This approach allows a complete characterization of the system, including contact forces, and particle motion. Key findings for systems of disks include: force distributions that are sensitive to the stress state of the system (e.g. isotropic or not), The distributions have a roughly exponential tail following pure shear, but transition to a more gaussian-like distribution for isotropic compression. Near jamming, we find that the contact number per particle, Z, rises sharply as a function of packing fraction, [lowercase_phi_synonym], at jamming, and then continues to rise above critical with an exponent that is consistent with 1/2. The pressure also rises as a function of [lowercase_phi_synonym] above jamming with an exponent slightly larger than 1.0, which is consistent with predictions. We also find that under cyclic shear, a dense granular material undergoes a different type of jamming transition, and we present initial results showing that P is a function of Z above this transition. [ABSTRACT FROM AUTHOR]

In this work, we describe the spectral properties of fiber Bragg gratings (FBGs) in Hi-Bi PANDA fibers subjected to diametrical compression and fibers with internal electrodes, as two examples of the facilities offered by specialty single-mode fibers for processing optical signals. Due to the complex structure of the fibers, the induced effects on the fiber effective birefringence are presented and discussed, based on numerical analysis with the finite element method. [ABSTRACT FROM AUTHOR]

Several studies can be found where the mechanical strength of fiber composites is evaluated, but not much can be found regarding the influence of voids, as a second phase, in the same mechanical behavior of materials. In the present work, the influence of bubble presence into translucid polymeric bars is investigated. For this purpose, cubic probes with an anisotropic distribution of bubbles were casted, and other were casted controlling the manufacture process to avoid the bubble formation. Each probe was placed in a He-Ne laser-photoelastic arrangement and subjected to compression by an static load. The transmitted 630 nm, He-Ne laser beam was captured by a single mode fiber optic sensor and then transmitted to a spectrometer and PC. Analysis of the spectrometer signals showed that the maximum stresses are concentrated on the bubbles reducing the surface stresses and inducing damping on the probe. The amount of damping depends on the size of the bubbles formed within the probe and their arrangement, thus it can be anticipated that stresses can be directed for specific arrangements in size of bubbles. A white light was also employed in substitution of the laser to investigated the effect of wave length on the results previously obtained. The effect of damping was also experienced but although more information could be obtained caused of the non-coherent nature of the light, it was no possible to obtain full information of the structure of the probes. [ABSTRACT FROM AUTHOR]

We propose a closed-form scheme that reproduces a wide class of nonlinear and hysteretic effects exhibited by sedimentary rocks in longitudinal bar resonance. In particular, we correctly describe: hysteretic behavior of a resonance curve on both its upward and downward slopes; linear softening of resonant frequency with increase of driving level; gradual (almost logarithmic) recovery (increase) of resonance frequency after large dynamical strains; and temporal relaxation of response amplitude at fixed frequency. Further, we are able to describe how water saturation enhances hysteresis and simultaneously decreases quality factor. The basic ingredients of the original bar system are assumed to be two coupled subsystems, namely, an elastic subsystem sensitive to the concentration of intergrain defects, and a kinetic subsystem of intergrain defects supporting an asymmetric response to an alternating internal stress. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Optical, vibrational and structural properties of II-VI ternary alloys, ZnSxSe1-x and CdSxSe1-x are presented in this work. The estimates of elastic constants and vibrational frequencies are provided following the empirical pseudopotential method. The alloying effects are modeled through the virtual and modified virtual crystal approximation. Elastic constants are computed by combining both the VCA and MVCA with the Harrison bond orbital model. The band gaps for a number of alloys are computed to evaluate the refractive index (n) from Moss formula. We give the longitudinal ωLO(0), transverse ωTO(0) optical frequency, static frequency ([variant_greek_epsilon]0), high frequency ([variant_greek_epsilon]∞) dielectric constant and pressure derivatives of [variant_greek_epsilon]0, [variant_greek_epsilon]∞, ωLO (0) and ωTO(0). Moreover, photo-elastic constants are also computed applying the Keating model and prescription of Davydov and Tikhonov. [ABSTRACT FROM AUTHOR]

We recently developed a novel extension of MR Elastograpy (MRE) to image the force chain structure within a dense 3D quasi-static granular assembly. Subsequently, computer codes and algorithms to determine the distribution of the force chain lengths, where a chain is taken to be a relatively straight section between branching points, were developed. Similar tools were used previously to analyze 2D photoelastic data and now have been expanded to analyze our most current 3D MRE force chain data. These investigations reveal that the distribution of the chain lengths in 3D decays exponentially, as was observed in 2D. The exponential decay of the length distribution is consistent with DEM simulation results of Peters, et al. We conclude that the decay length of this distribution is a meaningful quantitative measure that characterizes granular assemblies. [ABSTRACT FROM AUTHOR]

We report on an experimental study of creep and stick slip in a 2D granular medium. In particular, we study how the accumulation of material in front of the slider after slider motion affects its stick slip dynamics. To do so, we probe the position and acceleration of a slider resting on a 2D layer of photoelastic particles. We either apply a constant force to the slider, or we drag the slider across the particles at a constant rate. The resulting motion of the slider is creep like with stick-slip features. The photoelastic particles beneath the slider allow us to record the structure and dynamics of the forces inside the granular layer that resist motion of the slider. Our goal is to identify features in both internal stress and structure of the granular layer the can serve as precursors for slip events. [ABSTRACT FROM AUTHOR]