Mechanical Response (mechanical + response)

Distribution by Scientific Domains
Distribution within Polymers and Materials Science

Selected Abstracts

Strain Rate Effects in the Mechanical Response of Polymer-Anchored Carbon Nanotube Foams,

Abha Misra
Strain rate effects on the mechanical properties of carbon nanotube forests are studied, and several related interesting new phenomena are reported. Dense vertically aligned foam-like forests of carbon nanotubes are anchored on a thin, flexible polymer layer to provide structural stability, particularly at the higher strain rates. Permanent deformation and for the first time the delamination and crumbling of carbon nanotube walls is observed. [source]

Mechanical Response of Single Plant Cells to Cell Poking: A Numerical Simulation Model

Rong Wang
Abstract Cell poking is an experimental technique that is widely used to study the mechanical properties of plant cells. A full understanding of the mechanical responses of plant cells to poking force is helpful for experimental work. The aim of this study was to numerically investigate the stress distribution of the cell wall, cell turgor, and deformation of plant cells in response to applied poking force. Furthermore, the locations damaged during poking were analyzed. The model simulates cell poking, with the cell treated as a spherical, homogeneous, isotropic elastic membrane, filled with incompressible, highly viscous liquid. Equilibrium equations for the contact region and the non-contact regions were determined by using membrane theory. The boundary conditions and continuity conditions for the solution of the problem were found. The force-deformation curve, turgor pressure and tension of the cell wall under cell poking conditions were obtained. The tension of the cell wall circumference was larger than that of the meridian. In general, maximal stress occurred at the equator around. When cell deformation increased to a certain level, the tension at the poker tip exceeded that of the equator. Breakage of the cell wall may start from the equator or the poker tip, depending on the deformation. A nonlinear model is suitable for estimating turgor, stress, and stiffness, and numerical simulation is a powerful method for determining plant cell mechanical properties. (Managing editor: Wei Wang) [source]

Models of Irregular Hyperbranched Polymers: Topological Disorder and Mechanical Response

Thorsten Koslowski
Abstract Summary: We study the impact of topological disorder on the mechanical response of hyperbranched macromolecules from a theoretical and numerical perspective. The polymer models are generated using a bond switching algorithm, and the emerging systems are described within the Zimm and Rouse pictures of macromolecular dynamics. The topological disorder is manifest in the frequency-dependent dynamic moduli, . These are clearly distinct from that of regular hyperbranched fractals of the same size, and they do not obey simple scaling rules. The dynamic moduli reflect the short-range order inherent in the model, and we thus suggest that the extent of disorder in branched tree-like polymers may be well-estimated experimentally using . Model of an irregular hyperbranched polymer. [source]

Large Mechanical Response of Single Dendronized Polymers Induced by Ionic Strength,

Ionel Popa Dr.
Die mechanischen Eigenschaften kationischer dendronisierter Einzelpolymere variieren stark in wässrigen Lösungen unterschiedlicher Ionenstärke. Indem es gelingt, ihre Elastizität über die Lösungseigenschaften einzustellen, bieten sich dendronisierte Polymere als Bausteine für molekulare Maschinen wie Aktuatoren oder Motoren an. [source]

Mechanical response of a jointed rock beam,numerical study of centrifuge models

Michael Tsesarsky
Abstract In this paper we present a comparison between a set of benchmark centrifuge models of a jointed beam and the predictions of two numerical models: fast Lagrangian analysis of continua (FLAC) and discontinuous deformation analysis (DDA). The primary objective of this paper is a comparison between the measured deformation profiles and thrust evolution to predictions of the numerical methods employed. A secondary objective is an attempt to clarify the issue of compressive arch geometry which is still in controversy among researchers. It is found that both FLAC and DDA result in insufficiently accurate predictions to the measured displacements. The mode of deformation is only partially captured and is dependent on the aspect ratio of the individual blocks which made up the beam. It is shown that the accuracy of the predicted displacements is a function of the assigned interface stiffness. The thrust predicted by both methods is found to be considerably lower than that measured in the model; however, the linear evolution of thrust and equilibrium conditions are correctly captured. The geometry of the compressive arch as predicted by FLAC compares extremely well with the data measured in the physical model. Based on the FLAC analysis it is found that for a beam composed of equidimensional blocks the thickness of the compressive arch varies from 0.8t at the abutment interface to the entire beam thickness (1t) at a distance of a half block width from the abutment face, extending across the interface separating the block and its neighbour, and attains a value of 0.5t at the beam mid span. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Measurement of the force and torque produced in the calcium response of reactivated rat sperm flagella

CYTOSKELETON, Issue 1 2001
Mark J. Moritz
Abstract Rat sperm that are demembranated with Triton X-100 and reactivated with Mg-ATP show a strong mechanical response to the presence of free calcium ion. At pCa < 4, the midpiece region of the flagellum develops a strong and sustained curvature that gives the cell the overall appearance of a fishhook [Lindemann and Goltz, 1988: Cell Motil. Cytoskeleton 10:420,431]. In the present study, the force and torque that maintain the calcium-induced hook have been examined quantitatively. In addition, full-length and shortened flagella were manipulated to evaluate the plasticity of the hooks and determined the critical length necessary for maintaining the curvature. The hooks were found to be highly resilient, returning to their original configuration (>95%) after being straightened and released. The results from manipulating the shortened flagella suggest that the force holding the hook in the curved configuration is generated in the basal 60 ,m of the flagellum. The force required to straighten the calcium-induced hooks was measured with force-calibrated glass microprobes, and the bending torque was calculated from the measured force. The force and torque required to straighten the flagellum were found to be proportional to the change in curvature of the hooked region of the flagellum, suggesting an elastic-like behavior. The average torque to open the hooks to a straight position was 2.6 (±1.4) × 10 -7 dyne × cm (2.6 × 10 -14 N × m) and the apparent stiffness was 4.3 (±1.3) × 10 -10 dyne × cm2 (4.3 × 10 -19 N × m2). The stiffness of the hook was determined to be approximately one quarter the rigor stiffness of a rat sperm flagellum measured under comparable conditions. Cell Motil. Cytoskeleton 49:33,40, 2001. © 2001 Wiley-Liss, Inc. [source]

Mechanical behavior of bovine periodontal ligament under tension-compression cyclic displacements

Tatsuya Shibata
In the present study, the mechanical response of bovine periodontal ligament (PDL) subjected to displacement-controlled tension,compression harmonic oscillations and subsequent rupture was examined. Specimens including dentine, cementum, PDL, and alveolar bone were extracted from different depths and locations of bovine first molars. They were immersed in a saline solution at room temperature and clamped on their bone and dentine extremities. The samples were tested at ±35% of the PDL's width, with a frequency of 1 Hz. The mechanical parameters evaluated were hysteresis, phase lag, and the modulus of the stress,stretch ratio curves in tension and compression. The tensile strength and the corresponding stretch ratio were also recorded. Stress,stretch ratio curves indicated a non-linear, time-dependent response with hysteresis and preconditioning effects. The hysteresis and phase lag in compression were much higher than in tension, suggesting that the dissipated energy was higher in compression than in tension. The root depth and location did not play essential roles for the tension or compression data, with the exception of limited statistical differences for tensile strength and corresponding stretch ratio. Thus, biological variability in the specimens, as a result of different bone contours, PDL width, and fiber orientation, did not affect the energy-absorbing capacity of the PDL. The evolution of the stress rate with stress demonstrated a constant increase of stiffness with stress. The stiffness values were twofold higher in tension than in compression. The data also showed that the stiffness of the PDL was comparable with data reported for other soft tissues. [source]

Mechanical Behaviour of Internal Reinforced Aluminium Foams,

E. Solórzano
The paper presents a new way to improve the mechanical behaviour of aluminum based foams produced by the powder metallurgical (PM) route; the method is based on the use of internal reinforcements. These reinforcements allow an excellent improvement of the mechanical response in compression, tension and bending all at the same time. The produced samples have showed an excellent reproducibly in their mechanical response. [source]

Determination of Mechanical Properties of Copper at the Micron Scale,

D. Kiener
Using a focused ion beam workstation, micron-sized bending and compression samples were fabricated from a pure copper single crystal. The bending and compression experiments exhibited a strong size effect on the flow stress of copper, reaching values in the order of 1,GPa for the smallest test structures. Conventional strain gradient plasticity approaches are not capable of explaining this behaviour. The surface damage introduced by Ga+ ion implantation during focused ion beam preparation was investigated using Auger electron spectroscopy and its consequence on the mechanical response of the miniaturized test samples is addressed. [source]

Plastic Dissipation Mechanisms in Periodic Microframe-Structured Polymers

Lifeng Wang
Abstract Novel lightweight micro- and nanostructured materials are being used as constituents in hierarchically structured composites for providing high stiffness, high strength, and energy absorbing capability at low weight. Three dimensional SU-8 periodic microframe materials with submicrometer elements exhibit unusual large plastic deformations. Here, the plastic dissipation and mechanical response of polymeric microframe structures is investigated using micromechanical modeling of large deformations. Finite element analysis shows that multiple deformation domains initiate, stabilize, and then spread plasticity through the structure; simulated deformation mechanisms and deformation progression are found to be in excellent agreement with experimental observation. Furthermore, the geometry can be used to tailor aspects of 3D behavior such as effective lateral contraction ratios (elastic and plastic) during tensile loading as well as negative normal stress during simple shear deformation. The effects of structural geometry on mechanical response are also studied to tailor and optimize mechanical performance at a given density. These quantitative investigations enable simulation-based design of optimal lightweight material microstructures for dissipating energy. [source]

On the use of dislocations to model interseismic strain and stress build-up at intracontinental thrust faults

J. Vergne
Summary Creeping dislocations in an elastic half-space are commonly used to model interseismic deformation at subduction zones, and might also apply to major intracontinental thrust faults such as the Main Himalayan Thrust. Here, we compare such models with a more realistic 2-D finite element model that accounts for the mechanical layering of the continental lithosphere and surface processes, and that was found to fit all available constraints on interseismic and long-term surface displacements. These can also be fitted satisfactorily from dislocation models. The conventional back-slip model, commonly used for subduction zones, may, however, lead to a biased inference about the geometry of the locked portion of the thrust fault. We therefore favour the use of a creeping buried dislocation that simulates the ductile shear zone in the lower crust. A limitation of dislocation models is that the mechanical response of the lithosphere to the growth of the topography by bending of the elastic cores and ductile flow in the lower crust cannot be easily introduced. Fortunately these effects can be neglected because we may assume, to first order, a stationary topography. Moreover, we show that not only can dislocation models be used to adjust surface displacements but, with some caution, they can also provide a physically sound rationale to interpret interseismic microseismicity in terms of stress variations. [source]

Revealing the Design Principles of High-Performance Biological Composites Using Ab initio and Multiscale Simulations: The Example of Lobster Cuticle

Svetoslav Nikolov
Natural materials are hierarchically structured nanocomposites. A bottom-up multiscale approach to model the mechanical response of the chitin-based mineralized cuticle material of Homarus americanus is presented, by combining quantum-mechanical ab initio calculations with hierarchical homogenization. The simulations show how the mechanical properties are transferred from the atomic scale through a sequence of specifically designed microstructures to realize optimal stiffness. [source]

Bearing capacity of shallow foundations in transversely isotropic granular media

A. Azami
Abstract The main focus in this work is on the assessment of bearing capacity of a shallow foundation in an inherently anisotropic particulate medium. Both the experimental and numerical investigations are carried out using a crushed limestone with elongated angular-shaped aggregates. The experimental study involves small-scale model tests aimed at examining the variation of bearing capacity as a function of the angle of deposition of the material. In addition, the results of a series of triaxial and direct shear tests are presented and later employed to identify the material functions/parameters. The numerical part of this work is associated with the development and implementation of a constitutive framework that describes the mechanical response of transversely isotropic frictional materials. The framework is based on the elastoplasticity and accounts for the effects of strain localization and inherent anisotropy of both the deformation and strength characteristics. The results of numerical simulations are compared withthe experimental data. A parametric study is also carried out aimed at examining the influence of various simplifications in the mathematical framework on its predictive abilities. Copyright © 2009 John Wiley & Sons, Ltd. [source]

A hypoplastic model for mechanical response of unsaturated soils

David Ma
Abstract A new constitutive model is developed for the mechanical behaviour of unsaturated soils based on the theory of hypoplasticity and the effective stress principle. The governing constitutive relations are presented and their application is demonstrated using several experimental data from the literature. Attention is given to the stiffening effect of suction on the mechanical response of unsaturated soils and the phenomenon of wetting-induced collapse. All model parameters have direct physical interpretation, procedures for their quantification from test data are highlighted. Quantitative predictions of the model are presented for wetting, drying and constant suction tests. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Implicit integration of a mixed isotropic,kinematic hardening plasticity model for structured clays

Angelo Amorosi
Abstract In recent years, a number of constitutive models have been proposed to describe mathematically the mechanical response of natural clays. Some of these models are characterized by complex formulations, often leading to non-trivial problems in their numerical integration in finite elements codes. The paper describes a fully implicit stress-point algorithm for the numerical integration of a single-surface mixed isotropic,kinematic hardening plasticity model for structured clays. The formulation of the model stems from a compromise between its capability of reproducing the larger number of features characterizing the behaviour of structured clays and the possibility of developing a robust integration algorithm for its implementation in a finite elements code. The model is characterized by an ellipsoid-shaped yield function, inside which a stress-dependent reversible stiffness is accounted for by a non-linear hyperelastic formulation. The isotropic part of the hardening law extends the standard Cam-Clay one to include plastic strain-driven softening due to bond degradation, while the kinematic hardening part controls the evolution of the position of the yield surface in the stress space. The proposed algorithm allows the consistent linearization of the constitutive equations guaranteeing the quadratic rate of asymptotic convergence in the global-level Newton,Raphson iterative procedure. The accuracy and the convergence properties of the proposed algorithm are evaluated with reference to the numerical simulations of single element tests and the analysis of a typical geotechnical boundary value problem. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Nonlinear SEM numerical analyses of dry dense sand specimens under rapid and dynamic loading

C. di Prisco
Abstract The paper mainly concerns the mechanical response of 2D dry dense sand specimens under shock loading. The problem is numerically analysed by means of a SEM dynamic code, within which an already conceived non-local viscoplastic constitutive model characterized by a non-associated flow rule and by an anisotropic strain hardening has been implemented. In particular the strain localization and time dependency of the material mechanical response are taken into consideration. Both rapid/static loading and dynamic histories are numerically simulated. In the first case, the time dependency of the material mechanical response can be captured by neglecting inertial effects, while in the second one the two factors are superimposed and the propagation of the stress waves within the specimen is discussed. The interest of these analyses derives from the fact that the diffusion phenomenon takes place within a specimen already localized. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Microstructure and physical properties of open-cell polyolefin foams

M. A. Rodriguez-Perez
Abstract The cellular structure, physical properties, and structure,property relationships of novel open-cell polyolefin foams produced by compression molding and based on blends of an ethylene/vinyl acetate copolymer and a low-density polyethylene have been studied and compared with those of closed-cell polyolefin foams of similar chemical compositions and densities and with those of open-cell polyurethane foams. Properties such as the elastic modulus, collapse stress, energy absorbed in mechanical tests, thermal expansion, dynamic mechanical response, and acoustic absorption have been measured. The experimental results show that the cellular structure of the analyzed materials has interconnected cells due to the presence of large and small holes in the cell walls, and this structure is clearly different from the typical structure of open-cell polyurethane foams. The open-cell polyolefin foams under study, in comparison with closed-cell foams of similar densities and chemical compositions, are good acoustic absorbers; they have a significant loss factor and lower compressive strength and thermal stability. The physical reasons for this macroscopic behavior are analyzed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source]

Effect of accelerated aging on the structure and properties of monolayer and multilayer packaging films

P. A. Tarantili
Abstract The effect of accelerated aging on the structure and properties of single, metalized, and multilayer films used in food packaging was studied through the exposure of specimens of those films to repeated aging cycles in a weather meter under the combined action of ultraviolet, humidity, and heat. The aged specimens were tested for their mechanical properties and water vapor transmission characteristics, and the results were compared to those obtained from the original specimens. The property changes introduced into the films by aging were further explored by attenuated total reflectance spectroscopy and differential scanning calorimetry in an attempt to correlate the changes in the properties with structural characteristics. The results showed that the films made of polypropylene (PP) underwent severe chain scission upon irradiation and lost mechanical properties but still retained their impermeability to water vapor. The metallic coating could not prevent PP from degrading, as it seemed to oxidize under the aging conditions. Therefore, the metalized film showed the same mechanical response as PP, but its water impermeability dropped dramatically. Polyethylene (PE) and poly(ethylene terephthalate) (PET) films showed modest decreases in their mechanical properties, which could be attributed to crosslinking reactions taking place with PE and to the increased ultraviolet stability of PET, respectively. On the other hand, the multilayer films presented a decrease in their mechanical properties according to those of their weak component, which would be expected for a composite structure. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source]

Some aspects of the mechanical response of BMI 5250-4 neat resin at 191°C: Experiment and modeling,

M. B. Ruggles-Wrenn
Abstract The inelastic deformation behavior of BMI-5250-4 neat resin, a high-temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress,strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain-time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi-step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source]

Defibrillation Causes Immediate Cardiac Dilation in Humans

Erin Sylvester B.S.
Introduction: Prior studies in isolated heart tissue have shown both excitation and deexcitation to be the primary mechanism of defibrillation. This article presents the first evidence in man of deexcitation immediately following defibrillation by tracking the heart's mechanical response. Methods and Results: The geometric changes of the ventricular chambers were measured before and after defibrillation in seven human subjects receiving an implantable cardioverter defibrillator (ICD). The ICD was used to produce approximately three episodes of ventricular fibrillation and defibrillation in each subject. Twenty-two two-dimensional echocardiographic images of the right ventricle (RV) and 11 images of the left ventricle (LV) were recorded and analyzed at 30 frames per second. Just over 2 seconds of each episode were digitized, beginning half a second before the defibrillation shock. Individual frames were analyzed to yield cross-sectional, ventricular chamber area as a function of time. Immediately following defibrillation, ventricular chambers dilated with significant fractional area increase (RV: 1.58 ± 0.25, LV: 1.10 ± 0.06), with peak dilation at 194 ± 114 msec. Conclusion: Defibrillation causes a rapid increase in ventricular chamber area due to relaxation of the myocardium, suggesting that defibrillation synchronizes the cardiac cells to the deexcited state in man. (J Cardiovasc Electrophysiol, Vol. 14, pp. 832-836, August 2003) [source]

Three-dimensional analysis of intermediate filament networks using SEM tomography

Summary We identified tomographic reconstruction of a scanning electron microscopy tilt series recording the secondary electron signal as a well-suited method to generate high-contrast three-dimensional data of intermediate filament (IF) networks in pancreatic cancer cells. Although the tilt series does not strictly conform to the projection requirement of tomographic reconstruction, this approach is possible due to specific properties of the detergent-extracted samples. We introduce an algorithm to extract the graph structure of the IF networks from the tomograms based on image analysis tools. This allows a high-resolution analysis of network morphology, which is known to control the mechanical response of the cells to large-scale deformations. Statistical analysis of the extracted network graphs is used to investigate principles of structural network organization which can be linked to the regulation of cell elasticity. [source]

Mitochondrial displacements in response to nanomechanical forces

Yaron R. Silberberg
Abstract Mechanical stress affects and regulates many aspects of the cell, including morphology, growth, differentiation, gene expression and apoptosis. In this study we show how mechanical stress perturbs the intracellular structures of the cell and induces mechanical responses. In order to correlate mechanical perturbations to cellular responses, we used a combined fluorescence-atomic force microscope (AFM) to produce well defined nanomechanical perturbations of 10,nN while simultaneously tracking the real-time motion of fluorescently labelled mitochondria in live cells. The spatial displacement of the organelles in response to applied loads demonstrates the highly dynamic mechanical response of mitochondria in fibroblast cells. The average displacement of all mitochondrial structures analysed showed an increase of ,40%, post-perturbation (,160,nm in comparison to basal displacements of ,110,nm). These results show that local forces can produce organelle displacements at locations far from the initial point of contact (up to ,40,µm). In order to examine the role of the cytoskeleton in force transmission and its effect on mitochondrial displacements, both the actin and microtubule cytoskeleton were disrupted using Cytochalasin D and Nocodazole, respectively. Our results show that there is no significant change in mitochondrial displacement following indentation after such treatments. These results demonstrate the role of the cytoskeleton in force transmission through the cell and on mitochondrial displacements. In addition, it is suggested that care must be taken when performing mechanical experiments on living cells with the AFM, as these local mechanical perturbations may have significant structural and even biochemical effects on the cell. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Parameters of drug antagonism: re-examination of two modes of functional competitive drug antagonism on intraocular muscles

Popat N. Patil
There are two distinct kinetic functional pharmacological procedures by which the equilibrium affinity constant, KB, of a competitive reversible blocker is obtained. The classical method on an organ system requires the study of the parallel displacement of the agonist concentration-response curve in the presence of the blocker. In the second method, the agonist-evoked functional mechanical response is reduced to half by the blocker IC50 (the concentration required for 50% inhibition). In relation to these parameters the role of the ionization constant pKa and liposolubility log Pc or log D of blockers was examined. On the ciliary muscle from human eye, IC50/KB ratios for (±)-atropine, its quaternary analogue (±)-methylatropine, (-)-scopolamine, (±)-cyclopentolate, (-)-tropicamide, (±)-oxybutynin and pirenzepine were 15, 23, 4.4, 2.6, 1.66, 1.46 and 1.71, respectively. The ratios on the iris sphincter were comparable with those of ciliary muscle. When compared with large proportions of ionized molecules with water soluble properties of (±)-atropine and (±)-methylatropine, relatively high amounts of un-ionized and/or with greater partitioning of all other blockers in the lipoid barrier co-related well to low IC50/KB ratios, as predicted by the classical theory of competitive drug antagonism. It was hypothesized that due to receptor biophase access, the reduction of the mechanical response of the agonist by the highly ionized water-soluble antagonist at IC50 represented time-distorted "pseudo-equilibrium" estimation, where a higher concentration of the blocker was needed. On the other cholinergic effectors, like that of rat anococcygeus muscle or frog rectus abdominus muscle, IC50/KB ratios of respective blockers atropine or (+)-tubocurarine and hexamethonium were close to 1. Thus physicochemical properties, which affect the distribution coefficient log D and the tissue morphology (where asymmetric distribution of receptors may occur), appeared to be a critical factor in the analysis of the affinity parameters of the competitive reversible blocker. On the intraocular muscles, two functional pharmacological procedures for obtaining KB and IC50 values were not kinetically equivalent. [source]


ABSTRACT The mechanism relating sensory perception of brittle food foams to their mechanical and acoustic properties during crushing was investigated. Cornstarch was extruded with four levels of whey protein isolate (0, 6, 12 and 18%) and two levels of in-barrel moisture (23 and 27%). Hardness, fracturability and roughness of mass were three main sensory attributes that varied substantially between products. High correlations (r = 0.841,0.998) were observed between sensory attributes and instrumentally determined mechanical properties, including crushing force (11.2,57.9 N) and crispness work (4.6,75.8 N·mm). Based on acoustic data obtained during instrumental crushing, time-domain signal processing and a novel voice recognition technique utilizing frequency spectrograms were successfully employed for understanding the differences in the sensory properties of various products. Microstructure features, including average cell diameter (1.00,2.94 mm), average wall thickness (0.04,0.27 mm) and cell number density (7,193 cell/cm3), were characterized noninvasively using X-ray microtomography, and proved to be critical in relating sensory perception of the cellular extrudates to their mechanical,acoustic signatures. PRACTICAL APPLICATIONS The sensory perception of crispy and crunchy food products is primarily a function of their mechanical response and emission of sounds during fracture. The current study was focused on understanding these relationships in the context of brittle extruded foods. The mechanical,acoustic techniques outlined in this study have the potential of reducing the time, costs and subjectivity involved in evaluation of new foods by human panels, and can be a useful tool in the overall product development cycle. These techniques need not be limited only to food systems, as properties of any rigid, fracturable material can be characterized based on its mechanical,acoustic signature. [source]

Effects of Thermal Aging on the Mechanical Properties of a Porous-Matrix Ceramic Composite

Eric A. V. Carelli
The present article focuses on changes in the mechanical properties of an all-oxide fiber-reinforced composite following long-term exposure (1000 h) at temperatures of 1000,1200°C in air. The composite of interest derives its damage tolerance from a highly porous matrix, precluding the need for an interphase at the fiber,matrix boundary. The key issue involves the stability of the porosity against densification and the associated implications for long-term durability of the composite at elevated temperatures. For this purpose, comparisons are made in the tensile properties and fracture characteristics of a 2D woven fiber composite both along the fiber direction and at 45° to the fiber axes before and after the aging treatments. Additionally, changes in the state of the matrix are probed through measurements of matrix hardness by Vickers indentation and through the determination of the matrix Young's modulus, using the measured composite moduli coupled with classical laminate theory. The study reveals that, despite evidence of some strengthening of the matrix and the fiber,matrix interfaces during aging, the key tensile properties in the 0°/90° orientation, including strength and failure strain, are unchanged. This strengthening is manifested to a more significant extent in the composite properties in the ±45° orientation, wherein the modulus and the tensile strength each exhibit a twofold increase after the 1200°C aging treatment. It also results in a change in the failure mechanism, from one involving predominantly matrix damage and interply delamination to one which is dominated by fiber fracture. Additionally, salient changes in the mechanical response beyond the maximum load suggest the existence of an optimum matrix strength at which the fracture energy in the ±45° orientation attains a maximum. The implications for long-term durability of this class of composite are discussed. [source]

Physical Properties of PBMA- b -PBA- b -PBMA Triblock Copolymers Synthesized by Atom Transfer Radical Polymerization

Luis Martín-Gomis
Abstract The physical properties of well-defined poly(butyl methacrylate)- block -poly(butyl acrylate)- block -poly(butyl methacrylate) (PBMA- b -PBA- b -PBMA) triblock copolymers synthesized by atom transfer radical polymerization (ATRP) are reported. The glass transition and the degradation temperature of copolymers were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC measurements showed phase separation for all of the copolymers with the exception of the one with the shortest length of either inner or outer blocks. TGA demonstrated that the thermal stability of triblock copolymers increased with decreasing BMA content. Dynamic mechanical analysis was used for a preceding evaluation of adhesive properties. In these block copolymers, the deformation process under tension can take place either homogeneously or by a neck formation depending on the molecular weight of the outer BMA blocks and on the length of the inner soft BA segments. Microindentation measurements were also performed for determining the superficial mechanical response and its correlation with the bulk behavior. Stress-strain curves for the different PBMA- b -PBA- b -PBMA specimens at room temperature and at 10 mm/min. [source]

On Processing and Impact Deformation Behavior of High Density Polyethylene (HDPE),Calcium Carbonate Nanocomposites

Qiang Yuan
Abstract Different processing approaches were adopted to obtain the best combination of strength and toughness. The approach that yielded superior properties was examined in detail to study the mechanical response of nanoscale calcium carbonate-reinforced high density polyethylene in conjunction with unreinforced high density polyethylene. The reinforcement of high density polyethylene with nanoscale calcium carbonate increases impact strength and is not accompanied by decrease in yield strength. The addition of nanoscale calcium carbonate to high density polyethylene alters the micromechanism of deformation from crazing-tearing in high density polyethylene to fibrillation in high density polyethylene,calcium carbonate nanocomposite. [source]

Polymer Morphology: A Guide to Macromolecular Self-Organization

D.C. Bassett
Abstract The study of polymer morphology continues to be the principal means of acquiring knowledge and understanding of macromolecular self-organization. Longstanding problems of the nature of melt-crystallized lamellae and spherulitic growth have been resolved, bringing understanding of how characteristic properties such as a broad melting range and spatially-varying mechanical response are inherent in spherulitic morphologies. This reflects the distinctive features of the crystallization of long molecules, i.e. that they impede each other and, for faster growth, form rough basal surfaces. Knowledge of morphology is an essential accompaniment to the informed development of advanced polymeric materials and a full understanding of their structure/property relations. [source]

Models of Irregular Hyperbranched Polymers: Topological Disorder and Mechanical Response

Thorsten Koslowski
Abstract Summary: We study the impact of topological disorder on the mechanical response of hyperbranched macromolecules from a theoretical and numerical perspective. The polymer models are generated using a bond switching algorithm, and the emerging systems are described within the Zimm and Rouse pictures of macromolecular dynamics. The topological disorder is manifest in the frequency-dependent dynamic moduli, . These are clearly distinct from that of regular hyperbranched fractals of the same size, and they do not obey simple scaling rules. The dynamic moduli reflect the short-range order inherent in the model, and we thus suggest that the extent of disorder in branched tree-like polymers may be well-estimated experimentally using . Model of an irregular hyperbranched polymer. [source]

Linear and Nonlinear Viscoelasticity of a Model Unentangled Polymer Melt: Molecular Dynamics and Rouse Modes Analysis

Mihail Vladkov
Abstract Summary: Using molecular dynamics simulations, we determine the linear and nonlinear viscoelastic properties of a model polymer melt in the unentangled regime. Several approaches are compared for the computation of linear moduli and viscosity, including Green-Kubo and nonequilibrium molecular dynamics (NEMD). An alternative approach, based on the use of the Rouse modes, is also discussed. This approach could be used to assess local viscoelastic properties in inhomogeneous systems. We also focus on the contributions of different interactions to the viscoelastic moduli and explain the microscopic mechanisms involved in the mechanical response of the melt to external solicitation. [source]