A MODEL FOR STRUCTURED FLUIDS

Steady-state and time-dependent non-Newtonian properties of structurally complex systems, such as suspensions and foodstuffs, are simulated by the extension of a recently developed fluid model. This model accommodates the concept of structure variation induced by flow via a kinetic approach similar to the treatment of reversible chemical reactions. Purely viscous and viscoelastic responses are considered, entailing different equations for the computation of their contribution to the overall stress sustained by the systems. The model successfully describes pseudoplastic and thixotropic behavior, with or without yield stress.     

Sharp indentation stress fields in fused silica: Finite element analysis and Yoffe analytic model

The closed-form Yoffe analytic model (1982), which superimposes two linear-elastic stress fields, was developed as a first approximation to quantify the mechanical response of silicate glasses to sharp indentation. However, a detailed study with both experimental validation and numerical verification has yet to be published. This paper presents such a study, wherein experimental observation of crack systems in fused silica and numerical results from finite element analysis (FEA) are used to demonstrate the limitations of this analytic superimposition model. Specifically, analytic stress fields deviate from FEA numerical results and are inconsistent with several commonly observed crack systems. In contrast, FEA generated stress fields are entirely consistent with experimentally observed crack systems. Upon further investigation, it was found that the analytic model, with one tunable parameter, could not be adjusted to correlate with FEA and experimental evidence.     

Modelluntersuchungen zur Partikelbeanspruchung in Reaktoren

This paper reports systematic studies on hydrodynamic stress in agitated vessels with baffles, reactors with predominantly laminar flow, SERALE viscosimeters, bubble columns, and gas-driven loop reactors. The stress on particles is determined by way of the destruction of model particle systems, with the kinetics of destruction, the equilibrium particle diameters, and the enzyme activity after a given time serving, respectively, for assessment of the stress. Thereof one obtaines an indication of the methodology necessary for determining the stress and it permits selection of appropriate reactor systems according to the criterion of particle stress. In the case of reactors with turbulent motion without dominant laminar flow or gas/liquid interfaces give analogous results, permitting the assumption that they are also applicable to other particle systems. Specifically for stirred reactors a geometric function is derived from the experimental results which permits prediction of stress caused by various types of impellers. Impellers with large blade areas relative to the dimensions of the tank produce less shearing forces owing to their uniform power input, in contrast to small and especially axial flow impellers. In laminar flow or reactors with gas/liquid interfaces the level of particle stress depends upon the proclivity of the particles to enter the boundary layers. It can therefore be deduced that particles with different surface properties, which lead to differing interactions with boundary layers and interfaces, will also experience different kinds of particle stress on laminar flow or in aeration processes. Thus the scope for application of the results obtained with model particle systems to other particle systems is limited in such reactors.     

Mechanism of low profile behavior in unsaturated polyester systems

The mechanism by which low profile character is achieved in unsaturated polyester systems has been investigated. Using techniques of microscopy, data are developed that support a model in which micro stress cracking is induced to relieve stress promoted by polymerization shrinkage. Thus the strain is accommodated internally in a molded part, rather than through macroscopic shrinkage of the article.     

New generalized Newtonian fluid models for quantitative description of complex viscous behavior in shear flows

Two semiempirical models of generalized Newtonian fluid are discussed. Special attention was focused on the stress dependent model based on the free volume theory. However, the strain‐rate dependent model in form of a modified viscosity function resulting from Oldroyd equation is also presented. Both models (along with specific cases) reflecting pseudoplastic or dilatant behavior of liquids in shear flows are generalized to multimode models (defined as products of two or more basic models), which are able to describe quantitatively the behavior of more complex systems, for example, systems with pseudoplastic and dilatant properties in different shear stress (shear rate) ranges. A number of practical examples for viscosity curves of non‐Newtonian fluids described by these models are given. The questions of inverse models and model efficiency are also discussed. POLYM. ENG. SCI., 58:1446–1455, 2018. © 2017 Society of Plastics Engineers     

Viscoelastic stresses in a composite system

A large number of composite materials are polyphase systems in which one phase exhibits time-dependent properties. This paper presents an analysis of the stress redistribution in such a viscoelastic material system under the influence of applied external loads. In order to perform this study an appropriate idealized model, a series of which would tend to approximate the composition of a polyphase material, is formulated. The solution of the time-dependent stress field is obtained by applying the “correspondence principle” to the elastic stress field solution which, for the selected model, has previously been obtained in numerical form. Application of the solution to a model of an actual polyphase composite material system yields results which clearly indicate that, under certain normally encountered conditions of external loading, the principal stresses in certain areas of such a system will increase significantly with time.     

A unified visco-plastic model for the stress analysis of adhesively bonded structures

Previous work has identified the need for a multiaxial visco-plastic model that can be used in the stress analysis of adhesively bonded structures. Such a model should also be capable of reflecting the hydrostatic sensitivity displayed by such polymeric systems. This paper outlines the development of an appropriate uniaxial model, discusses the role of the various parameters, and outlines methods of fitting actual experimental data derived from creep, relaxation, recovery, and constant strain rate tests on two different adhesive systems. Two different schemes are then discussed to extend the model for multiaxial conditions. These have been integrated into a finite element code as user routines and details of the tangential modulus matrix are briefly presented. Following benchmark testing, these models have been used to predict the rate-dependent response of an adhesively bonded structure with considerable success.     

Solvent stress cracking and failure mechanisms in polyetherimide composites

Solvent stress cracking studies have been carried out in o-xylene and other solvents on polyetherimide (PEI) based materials including neat resin, woven fabric composites, and adhesively bonded systems. The results show crack growth in solvents at very low G_I levels as compared with tests in air. The composite and adhesively bonded systems have sufficiently high residual thermal stresses to drive an array of intersecting matrix/adhesive cracks even without mechanical loading. The matrix/adhesive residual stress driven crack patterns in these systems are shown to retard main delamination crack growth relative to that in the neat resin, and to raise the applied threshold G₁ level for main crack growth by about a factor of ten, as predicted by an approximate model.     

Shear stress generated by radial flow impellers at bioreactor integrated membranes

The study reveals the hydrodynamics at the surface of a submerged tubular membrane module integrated in a stirred membrane bioreactor. The reactor is equipped with a conventional six flat-blade impeller imposing radial circulation across the membrane interface. Simulation and computer visualization of “real” flow using a Reynolds-averaged Navier-Stokes model and CFD methodology are employed. A variety of model solutions at various mixing intensity are obtained and the mixing conditions are assessed by delineation of the near-wall zones and identification of the zones' shear rate and shear stress values. Shear rate non-uniformity along the surface of the tubular module is visualized. Shear stress values as high as 160 Pa at the membrane module lower section and as low as 0.6 Pa at the module upper section has been determined. Referring to reported data for shear stress near flat plate stirred filtration cells and external narrow-channel cross-flow systems, the mixing conditions are expected to allow enhanced access of the retentate fluid to the membrane surface, as well as possible low membrane fouling potential related to microfiltration practice.     

A nonlinear theoretical model for prediction of mechanical behavior of particulate composites and experimental verification of the model predictions

A model for prediction the stress‐strain behavior of particulate composite over wide ranges of filler concentration and composite deformation has been developed through combination of Anderson's and Yilmizer's model. The constitutive equations are extracted from first law of thermodynamic and nonlinear dilatational effects which are produced by filler‐matrix debonding process. In addition to nonlinear behavior that has been resulted by filler‐matrix debonding and was presented by Yilmizer, the formation and growing of void or cavitations has been also introduced in this model, whereas Anderson's model, most important reason for deviation of linear behavior is filler‐matrix debonding and has been indicated by change of modulus. Model predictions for effects of the filler concentration and its particle size and particle size distribution for some matrix‐filler systems are compared with related experimental data from literature and some investigated systems in this work. An excellent agreement even better than prediction of Anderson's model between experimental data and model predictions can be observed in most cases especially for some concentrated systems. POLYM. COMPOS., 31:1150–1155, 2010. © 2009 Society of Plastics Engineers     

Plant Parasites under Pressure: Effects of Abiotic Stress on the Interactions between Parasitic Plants and Their Hosts

Parasitic angiosperms, comprising a diverse group of flowering plants, are partially or fully dependent on their hosts to acquire water, mineral nutrients and organic compounds. Some have detrimental effects on agriculturally important crop plants. They are also intriguing model systems to study adaptive mechanisms required for the transition from an autotrophic to a heterotrophic metabolism. No less than any other plant, parasitic plants are affected by abiotic stress factors such as drought and changes in temperature, saline soils or contamination with metals or herbicides. These effects may be attributed to the direct influence of the stress, but also to diminished host availability and suitability. Although several studies on abiotic stress response of parasitic plants are available, still little is known about how abiotic factors affect host preferences, defense mechanisms of both hosts and parasites and the effects of combinations of abiotic and biotic stress experienced by the host plants. The latter effects are of specific interest as parasitic plants pose additional pressure on contemporary agriculture in times of climate change. This review summarizes the existing literature on abiotic stress response of parasitic plants, highlighting knowledge gaps and discussing perspectives for future research and potential agricultural applications.     

Estimating the Stresses in Linear Viscoelastic Sealants Subjected to Thermally-Driven Deformations

A framework for linear viscoelastic analysis of sealants is presented for analyzing stresses resulting from thermally driven deformations. Assuming that the strains induced within the sealant are proportional to the change in temperature from the strain-free state, the nominal stress state within the sealant can be estimated. The analysis method is used to estimate the stress states resulting from assumed diurnal temperature profiles for two representative Dow Corning silicone glazing sealants: a conventional elastomer and a crosslinked hot melt adhesive containing a high volume fraction of a silicate-based nanoparticle filler. The latter exhibits considerably more rate- and temperature-dependence than conventional silicones. The viscoelastic analysis allows for comparisons of stresses resulting in these two sealant systems, which are presented for several sinusoidal thermal profiles. However, the pronounced yielding behavior exhibited by the hot melt appears to limit the stress buildup, resulting in stress states that are significantly below those predicted using the linear viscoelastic model. Estimates of the yielding envelope for a representative thermal cycle profile are provided, based on experimental results reported elsewhere for the time and rate dependent yielding of the hot melt.     

Evaluation of Damage Evolution in Ceramic-Matrix Composites Using Thermoelastic Stress Analysis

Thermoelastic stress analysis (TSA) has been used to monitor damage evolution in several composite systems. The method is used to measure full-field hydrostatic stress maps across the entire visible surface of a sample, to quantify the stress redistribution that is caused by damage and to image the existing damage state in composites. Stress maps and damage images are constructed by measuring the thermoelastic and dissipational thermal signatures during cyclic loading. To explore the general utility of the method, test samples of several ceramic-matrix and cement-matrix composites have been fabricated and tested according to a prescribed damage schedule. The model materials have been chosen to illustrate the effect of each of three damage mechanisms: a single crack that is bridged by fibers, multiple matrix cracking, and shear bands. It is shown that the TSA method can be used to quantify the effect of damage and identify the operative damage mechanism. Each mechanism is identified by a characteristic thermal signature, and each is shown to be effective at redistributing stress and diffusing stress concentrations. The proposed experimental method presents a new way to measure the current damage state of a composite material.     

Analysis of the nonlinear creep behavior of concrete/FRP-bonded assemblies

This paper investigates the creep behavior of adhesively bonded concrete/fiber-reinforced polymer (FRP) joints, through experimental and modeling approaches. The first part proposes a methodology for predicting the long-term creep response of the bulk epoxy adhesive; such a procedure consists of (1) performing short-term tensile creep experiments at various temperatures and stress levels, (2) building the creep compliance master curves according to the time–temperature superposition principle in order to assess the long-term evolution for each stress level, and (3) developing a rheological model whose parameters are identified by fitting the previous master curves. In our case, it was found that master curves (and, consequently, parameters of the rheological model) are dependent on the applied stress level, highlighting the nonlinear creep behavior of the bulk epoxy adhesive. Therefore, evolution laws of the model parameters were established to account for this stress dependence. The second part focuses on the creep response of the concrete/FRP assembly in the framework of a double lap joint shear test configuration. Experiments showed that creep of the adhesive layer leads to a progressive evolution of the strain profile along the lap joint, after only one month of sustained load at 30% of the ultimate strength. Besides, a finite element approach involving the abovementioned rheological model was used to predict the nonlinear creep behavior of the bonded assembly. It confirmed that creep modifies the stress distribution along the lap joint, especially the stress value at the loaded end, and leads to a slight increase in the effective load transfer length. This result is of paramount interest since the transfer length is a key parameter in the design of FRP-bonded strengthening systems. Moreover, instantaneous and long-term calculated strain profiles were found in fair agreement with experimental data, validating the modeling approach.     

Time-dependent morphologies and viscoelastic properties of block copolymers

Dynamic mechanical properties of block copolymers over a wide temperature range have been previously correlated with the phase-separated microstructure of these systems. In the present work, the morphology of the block copolymer is altered by large tensile deformation at various temperatures. Upon removal of the applied stress, the morphological features of such stretched-and-released systems become functions of time, as the nonequilibrium microstructure reverts to a thermodynamically stable state. This reformation process is monitored by dynamic mechanical measurements, with a modified torsion pendulum capable of applying both tensile and torsional deformation. Experimental results are analyzed using a modified Nielsen model to obtain information on the time-dependent structural state of the samples. These results are then compared with stress-strain curves to provide further insight into the structure breakdown-reformation mechanisms. Two competing mechanisms, domain fracture and block pull-out, are proposed to explain these experimental observations.     

Determination of thermal stress distribution in a model microelectronic device encapsulated with alumina filled epoxy resin using fluorescence spectroscopy

A method for measuring residual and applied stresses in particulate polymer systems, which utilizes the piezo-spectroscopic effect of the optical fluorescence of filled particles, is presented. Fluorescence piezo-spectroscopy (PS) is non-destructive and provides microscopic lateral resolution upon using an optical microprobe system. Epoxy resin filled with α-alumina particles is used in this study. Stress values are obtained by frequency shift measurement of the characteristic optical fluorescence lines produced by Cr³⁺ impurities in alumina. The relationships between peak frequency shift of these lines and stress are derived by using a 4-point-bending test. The peak frequency shift shows linear correlation with tensile stress, while a non-linear relation between peak frequency shift and stresses is found in the compressive stress region. To a first order approximation, residual stresses were calculated by the frequency shift divided by the linear correlation coefficient in the tensile stress region. As an application of the PS method, we determined micron order residual stress distributions in a model plastic encapsulated silicon substrate for microelectronic devices and compared the stress data with those calculated using a two-dimensional finite element analysis of the device. The experimental results were in good agreement with the tensile stress components that have been obtained by theoretical calculation. Therefore, PS techniques can be used to measure residual stresses in polymer compounds utilizing the information obtained from the fluorescence lines of a dispersed ceramic powder.     

交联键类型对硫化丁苯橡胶拉伸应力松弛行为的影响

利用两相模型讨论了不同硫化体系的硫化丁苯橡胶的应力松弛行为。通过平衡溶胀法求得不同硫化体系中存在的几种不同交联网络结构的含量。普通硫化体系(CV)含有较多的多硫交联键,有效硫化体系(EV)含有较多的单硫交联键,而半有效硫化体系(SEV)的单硫、多硫交联键含量在前两者之间。考察了交联键类型、拉伸速率以及应变对应力松弛行为的影响。结果表明,CV体系的拉伸强度、不可松弛分量都要高于其它2种体系。交联键类型对可松弛分量的影响较小。     

Using computational fluid dynamics modeling to study the mixing of pseudoplastic fluids with a Scaba 6SRGT impeller

The 3D flow field generated by a Scaba 6SRGT impeller in the agitation of xanthan gum, a pseudoplastic fluid with yield stress, was simulated using the commercial CFD package. The flow was modeled as laminar and a multiple reference frame (MRF) approach was used to solve the discretized equations of motion. The velocity profiles predicted by the simulation agreed well with those measured using ultrasonic Doppler velocimetry, a non-invasive fluid flow measurement technique for opaque systems. Using computed velocity profiles across the impeller, the effect of fluid rheology on the impeller flow number was investigated. The validated CFD model provided useful information regarding the formation of cavern around the impeller in the mixing of yield stress fluids and the size of cavern predicted by the CFD model was in good agreement with that calculated using Elson's model.     

Residual stresses in epoxy systems by 3-D photoelastic method

The new 3-D photoelastic method was applied to the studies of residual stresses around spherical inclusion in polymeric matrices. Full stress tensor for several model samples was measured. The extent of significant stresses is not greater than three radii of an inclusion. It was found that the stress follows the 1/R³ rule at distances far from the inclusion, while in the narrow zone at the interface a plateau is observed. The level of stress ranges from few MPa up to the plastic yield of the polymeric matrix. The radial stress component is usually twice as large as the tangential stress component. Radial negative stress and tangential positive stresses are found in configuration with a hard inclusion, while radial positive stress and tangential negative stresses are in the systems with soft inclusion. The pressure in the matrix at points around inclusions calculated from the stress tensor is always near zero MPa, which indicates the action of purely deviatoric stresses in the matrix.