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Experimental data are reported on poly(lactic acid) (PLA) in tensile loading-unloading tests and relaxation tests under stretching and retraction at temperatures ranging from room temperature up to 50[degrees]C. Two characteristic features of the time-dependent response of PLA are revealed: (i) with a decrease in minimum stress under retraction at a fixed temperature, relaxation curves change their shape from monotonically decaying with time (simple relaxation), to non-monotonic (mixed relaxation) to monotonically increasing (inverse relaxation) and (ii) with an increase in temperature, inverse relaxation after unloading down to the zero stress evolves into mixed relaxation with a pronounced shift of the peak position to smaller relaxation times. Constitutive equations are derived for the mechanical behavior of PLA, and adjustable parameters in the stress-strain relations are found by fitting the observations. Ability of the model to predict the time-dependent response under cyclic deformation is confirmed by numerical simulation., INTRODUCTION This article deals with experimental investigation and constitutive modeling of the time-dependent behavior of poly(lactic acid) (PLA) at temperatures T ranging from room temperature up to 50[degrees]C (slightly below [...]

Renin-expressing cells appear early in the embryo and are distributed broadly throughout the body as organogenesis ensues. Their appearance in the metanephric kidney is a relatively late event in comparison with other organs such as the fetal adrenal gland. The functions of renin cells in extra renal tissues remain to be investigated. In the kidney, they participate locally in the assembly and branching of the renal arterial tree and later in the endocrine control of blood pressure and fluid-electrolyte homeostasis. Interestingly, this endocrine function is accomplished by the remarkable plasticity of renin cell descendants along the kidney arterioles and glomeruli which are capable of reacquiring the renin phenotype in response to physiological demands, increasing circulating renin and maintaining homeostasis. Given that renin cells are sensors of the status of the extracellular fluid and perfusion pressure, several signaling mechanisms (β-adrenergic receptors, Notch pathway, gap junctions and the renal baroreceptor) must be coordinated to ensure the maintenance of renin phenotype--and ultimately the availability of renin--during basal conditions and in response to homeostatic threats. Notably, key transcriptional (Creb/ CBP/p300, RBP-J) and posttranscriptional (miR-330, miR125b-5p) effectors of those signaling pathways are prominent in the regulation of renin cell identity. The next challenge, it seems, would be to understand how those factors coordinate their efforts to control the endocrine and contractile phenotypes of the myoepithelioid granulated renin-expressing cell. Keywords Juxtaglomerular cell * Cell identity * Cyclic AMP * MicroRNAs * Cell-cell communication * Gap junctions * RBP-J, Introduction Renin cells are necessary for the maintenance of blood pressure and fluid/electrolyte homeostasis. Perhaps because they synthesize a hormone, their rarity (they constitute 0.01% of the total kidney cell [...]

Early sensory experience instructs the maturation of neural circuitry in the cortex (1,2). This has been studied extensively in the primary visual cortex, in which loss of vision to one eye permanently degrades cortical responsiveness to that eye (3,4), a phenomenon known as ocular dominance plasticity (ODP). Cortical inhibition mediates this process (4-6), but the precise role of specific classes of inhibitory neurons in ODP is controversial. Here we report that evoked firing rates of binocular excitatory neurons in the primary visual cortex immediately drop by half when vision is restricted to one eye, but gradually return to normal over the following twenty-four hours, despite the fact that vision remains restricted to one eye. This restoration of binocular-like excitatory firing rates after monocular deprivation results from a rapid, although transient, reduction in the firing rates of fast-spiking, parvalbumin-positive (PV) interneurons, which in turn can be attributed to a decrease in local excitatory circuit input onto PV interneurons. This reduction in PV-cell-evoked responses after monocular lid suture is restricted to the critical period for ODP and appears to be necessary for subsequent shifts in excitatory ODP. Pharmacologically enhancing inhibition at the time of sight deprivation blocks ODP and, conversely, pharmacogenetic reduction of PV cell firing rates can extend the critical period for ODP. These findings define the microcircuit changes initiating competitive plasticity during critical periods of cortical development. Moreover, they show that the restoration of evoked firing rates of layer 2/3 pyramidal neurons by PV-specific disinhibition is a key step in the progression of ODP., Throughout the neocortex, form and function of neural circuitry are shaped by experience (1,2,7,8). This sensitivity to experience is most pronounced during adolescence, and has been studied most extensively in [...]

A successive initiation finite element modeling approach is presented in which an empirical continuum damage model, energy partitioning damage evolution model, developed by the author is used to update state of damage and constitutive properties of the material under thermomechanical cyclic loading and accumulate damage in the elements. Plastic and viscoplastic damages are evaluated based on the plastic and viscoplastic work densities obtained through finite element. Constitutive properties are updated elementwise at each step of the process based on the state of damage in each element. The elements that have reached the damage threshold are removed from the structure to initiate and propagate fatigue crack. This successive initiation approach is used to model crack initiation and propagation in Pb-free solder material under thermomechanical loading. A case study is presented, damage propagation path and pattern are compared with typical experimental results, and the accuracy of the model was verified. [DOI: 10.1115/1.4002722]

Finite-element analysis of strain localization based on classical theory of continuum mechanics suffers from pathological mesh dependence when strain softening models are used. For quasistatic problems, the mesh dependence is demonstrated through an analysis of the tangent stiffness matrix of a one-dimensional problem. To regularize the mesh dependence, a nonlocal strain softening model is proposed, which is based on the nonlocal plasticity theory and the representative line element. Both analytical and numerical solutions of strain localization with the proposed model are developed and compared with each other. The model is also applied in the numerical simulation of a direct tensile test of a concrete specimen in the existing literature, and reasonable agreement is achieved between numerical solutions and the experimental response. DOI: 10.1061/(ASCE)EM.1943-7889.0000184 CE Database subject headings: Strain softening; Localization; Plasticity; Finite element method. Author keywords: Strain softening; Localization; Plasticity; Finite-element method.

The multiple normal loading-unloading process of an elastic-plastic sphere by a rigid flat is analyzed using finite element method for stick contact condition and both kinematic and isotropic hardening models. The behavior of the global contact parameters as well as the stress field within the sphere tip is presented for several loading cycles. It was found that under stick contact condition, secondary plastification occurs even after the second loading cycle and that the hardening model used has little effect on the loading-unloading process. The cyclic loading process gradually converges into elastic shakedown. [DOI: 10.1115/1.4002103] Keywords: spherical contact, elastic-plastic contact, contact condition, multiple loadings, hardening

A contact mechanics (CM) based model of a fixed-length rigid cylinder impacting a homogeneous elastic-plastic homogeneous body was developed and includes an improved method of estimating the residual depth after impact. The nonlinear elastic behavior during unloading was accounted for to develop an improved coefficient of restitution model The impact model was applied to study a practical case of a cylindrical feature on the slider of a magnetic storage hard disk drive impacting the disk to predict various critical impact contact parameters. The CM model was validated using a plane strain finite element model and it was found that a cylindrical feature with a longer length results in a substantial alleviation of impact damage. [DOI: 10.1115/1.4002331] Keywords: contact mechanics, oblique impact, elastic-plastic impact, coefficient of restitution, magnetic storage

This paper aims to deal with plastic collapse assessment for thick vessels under internal pressure, thick tubes in plane strain conditions, and thick spheres, taking into consideration various strain hardening effects and large deformation aspect. In the framework of von Mises' criterion, strain hardening manifestation is described by various rules such as isotropic and/or kinematic laws. To predict plastic collapse, sequential limit analysis, which is based on the upper bound formulation, is used. The sequential limit analysis consists in solving sequentially the problem of the plastic collapse, step by step. In the first sequence, the plastic collapse of the vessel corresponds to the classical limit state of the rigid perfectly plastic behavior. At the end of each sequence, the yield stress and/or back-stresses are updated with or without geometry updating via displacement velocity and strain rates. The updating of all these quantities (geometry and strain hardening variables) is adopted to conduct the next sequence. As a result of this proposal, we get the limit pressure evolution, which could cause the plastic collapse of the device for different levels of hardening and also hardening variables such as back-stresses with respect to the geometry change. [DOI: 10.1115/1.4001272] Keywords: plastic collapse, isotropic hardening, kinematic hardening, thick vessels

A defect in the maternal copy of a ubiqutin ligase gene Ube3a can produce a neurodevelopmental defect in human children known as Angelman syndrome. We investigated the role of the maternally expressed Ube3a gene in experience-dependent development and plasticity of the mouse visual system. As demonstrated by optical imaging, rapid ocular dominance (OD) plasticity after brief monocular deprivation (MD) was severely impaired during the critical period (CP) in the visual cortex (VC) of Ube3a maternal-deficient (m-/p+) mice. Prolonged MD elicited significant plasticity in m-/ p+ mice that never matched the level seen in control animals. In older animals after the CP, 7-day MD elicited mild OD shifts in both control and m-/p+ mice; however, the OD shifts in m-/p+ mice lacked the strengthening of visual responses to the two eyes characteristic of normal adult plasticity. Anatomic effects of the maternal deficiency include reduced spine density on basal, but not apical, dendrites of pyramidal neurons in the binocular region of the VC. Imprinting of Ube3a expression was not fully established in the early postnatal period, consistent with the normal development of cortical retinotopy and visual acuity that we observed in m-/p+ mice, but was fully established by the onset of the CP. These results demonstrate that paternal and maternal genomes are not functionally equivalent for cortical plasticity, and that maternally expressed Ube3a is required for normal experience-dependent modification of cortical circuits during and after the CP. Angelman syndrome | critical period | epigenetics | ocular dominance plasticity | visual cortex www.pnas.org/cgi/doi/10.1073/pnas.1001281107

On the basis of the special tensile test results under various loading histories, the rate-dependent behaviors of three polymer geosynthetics due to their viscous properties have been investigated. All the investigated polymer geosynthetics show significant loading rate effects, creep deformation, and stress relaxation. Except for the polyester geogrid showing the combined viscosity, all the investigated polymer geosynthetics exhibit the isotach viscosity. An elasto-viscoplastic constitutive model described in a nonlinear three-component model framework is developed to simulate the rate-dependent behaviors of polymer geosynthetics. The developed constitutive model is verified by comparing its simulated results with the experimental data of polymer geosynthetics presented in this study and those available from the literature. The comparison indicates that the developed model can reasonably interpret the rate-dependent behaviors of polymer geosynthetics under arbitrary loading histories, including the step-changed strain rate loading, creep, and stress relaxation applied during otherwise mono-tonic loading (ML). POLYM. ENG. SCI., 50:550-560, 2010. [C] 2009 Society of Plastics Engineers, INTRODUCTION Polymer geosynthetics are increasingly used as reinforcements in many civil, geotechnical, and traffic engineering including retaining structures, roads, railroads, and airfields. The viscosity is viewed as a significant property [...]

In 1946, L. A. Galin proposed an ingenious solution to the problem of analytically determining the plastic region completely surrounding a circular hole in a thin initially elastic plate subject to tensile stress at infinity. The results of Galin's analysis are recovered by a simple collocation, provided an elliptical elastic-plastic boundary is assumed. This elementary procedure is then applied to treat the case, unresolvable by Galin's method, in which the plastic domain does not completely surround the hole. DOI: 10.1061/(ASCE)EM.1943-7889.0000062 CE Database subject headings: Plastics; Holes; Elastoplasticity.

Temperature sensing is essential for the survival of living cells. A major challenge is to understand how a biological thermometer processes thermal information to optimize cellular functions. Using structural and biochemical approaches, we show that the thermosensitive histidine kinase, DesK, from Bacillus subtilis is cold-activated through specific interhelical rearrangements in its central four-helix bundle domain. As revealed by the crystal structures of DesK in different functional states, the plasticity of this helical domain influences the catalytic activities of the protein, either by modifying the mobility of the ATP-binding domains for autokinase activity or by modulating binding of the cognate response regulator to sustain the phosphotransferase and phosphatase activities. The structural and biochemical data suggest a model in which the transmembrane sensor domain of DesK promotes these structural changes through conformational signals transmitted by the membrane-connecting two-helical coiled-coil, ultimately controlling the alternation between output autokinase and phosphatase activities. The structural comparison of the different DesK variants indicates that incoming signals can take the form of helix rotations and asymmetric helical bends similar to those reported for other sensing systems, suggesting that a similar switching mechanism could be operational in a wide range of sensor histidine kinases. coiled-coil | conformational rearrangement | crystallography | signal transduction

Actin filaments and microtubules polymerize and depolymerize by adding and removing subunits at polymer ends, and these dynamics drive cytoplasmic organization, cell division, and cell motility. Since Wegner proposed the treadmilling theory for actin in 1976, it has largely been assumed that the chemical state of the bound nucleotide determines the rates of subunit addition and removal. This chemical kinetics view is difficult to reconcile with observations revealing multiple structural states of the polymer that influence polymerization dynamics but that are not strictly coupled to the bound nucleotide state. We refer to these phenomena as 'structural plasticity' and discuss emerging evidence that they play a central role in polymer dynamics and function.

The technology of large scale hydrogen transmission from central production facilities to refueling stations and stationary power sites is at present undeveloped. Among the problems that confront the implementation of this technology is the deleterious effect of hydrogen on structural material properties, in particular, at gas pressures of the order of 15 MPa, which are the suggested magnitudes by economic studies.for efficient transport. In order to understand the hydrogen embrittlement conditions of the pipeline materials, we simulate hydrogen diffusion through the surfaces of an axial crack on the internal wall of vessel coupled with material deformation under plane strain small scale yielding conditions. The calculation of the hydrogen accumulation ahead of the crack tip accounts for stress-driven transient diffusion of hydrogen and trapping at microstructural defects whose density evolves dynamically with deformation. The results are analyzed to correlate for a given material system the time after which hydrogen transport takes place under steady state conditions with the level of load in terms of the applied stress intensity factor at the crack tip and the size of the domain used for the simulation of the diffusion. [DOI: 10.1115/1.3027497] Keywords: hydrogen embrittlement, diffusion, plasticity, low- or medium-strength steels, pipeline

The characteristics of dynamic strength and fracture in structural steels and their welded joints particularly for pipelines should be evaluated based on the effects of the strain rate and service temperature. The temperature, however, rises so rapidly in structures due to the plastic work under the high strain rate such as ground sliding by earthquake when the effect of the temperature cannot be negligible for the dynamic fracture. It is difficult m predict or measure the temperature rise history' with the corresponding stress-strain behavior, including the region beyond the uniform elongation, though the behavior at the large strain region after the maximum loading point is very important for the evaluation of fracture. In this paper, the coupling phenomena of the temperature and stress-strain fields under dynamic loading were simulated by using the finite element method. A modified rate-temperature parameter was defined by accounting for the effect of the temperature rise under rapid plastic deformation, and it was applied to the fully coupled analysis between the heat conduction and thermal elastic-plastic behavior. The temperature rise and stress-strain behavior, including the coupling phenomena, were studied including the region beyond the maximum loading point in structural steels and their undermatched joints, and then compared with the measured values. [DOI: 10.1115/1.3027477] Keywords: welded joint, undermatched joint, strength heterogeneity, dynamic loading, plastic deformation, temperature rise, thermal elastic-plastic analysis

This paper presents an investigation about the plasticity correction factor, [K.sub.e], proposed in the Part 3-Clause 18 of the EN13445 standard, for correction of the elastic stress ranges exceeding twice the yield stress, resulting from mechanical loading. The plasticity correction factors are analyzed using a calculation strategy based on a comparison between results from linear elastic and elastoplastic analyses. Several materials, elastoplastic models, and geometries are considered in the study. The performed analyses revealed, for an important number of the situations, an underestimation of the [K.sub.e] factor when calculated according to the standard procedures. A greater emphasis should be dedicated in the standard to the elastoplastic analysis, since the fatigue resistance data presented in the standard are strain-life type data, overcoming the need for the plasticity correction factor. The elastoplastic analysis should be the preferred approach for evaluation of the strains/stresses for fatigue analysis. [DOI: 10.1115/1.3027455]

This paper is concerned with infinitesimally constrained equilibrium states, which are nonequilibrium states and infinitesimally close to equilibrium states. The corresponding thermodynamics is established in this paper within the thermodynamic framework of Rice (1971, 'Inelastic Constitutive Relations for Solids: An Internal Variable Theory and Its Application to Metal Plasticity,' J. Mech. Phys. Solids, 19, pp. 433-455). It is shown that the thermodynamics of infinitesimally constrained equilibrium states belongs to linear irreversible thermodynamics. The coefficient matrix is the Hessian matrix of the flow potential function at the equilibrium state. The process of a state change induced by an infinitesimal stress increment in time-independent plasticity can be viewed as a sequence of infinitesimally constrained equilibrium states. The thermodynamic counterpart of yield functions are flow potential functions, and their convexity is required by intrinsic dissipation inequality. Drucker and Il'yushin's inequalities are not essential thermodynamic requirements. [DOI: 10.1115/1.2998484] Keywords: thermodynamics, internal variables, equilibrium states, Drucker's inequality, plasticity

Rigorous scale-dependent bounds on the constitutive response of random polycrystalline aggregates are obtained by setting up two stochastic boundary value problems (Dirichlet and Neumann type) consistent with the Hill condition. This methodology enables one to estimate the size of the representative volume element (RVE), the cornerstone of the separation of scales in continuum mechanics. The method is illustrated on the single-phase and multiphase aggregates, and, generally, it turns out that the RVE is attained with about eight crystals in a 3D system. From a thermodynamic perspective, one can also estimate the scale dependencies of the dissipation potential in the velocity space and its complementary potential in the force space. The viscoplastic material, being a purely dissipative material is ideally suited for this purpose. [DOI: 10.1115/1.2912999] Keywords: homogenization, random polycrystals, representative volume element (RVE), bounds, plasticity

Abstract: This paper presents two new and fast methods of determining product ratios for kinetic schemes leading to more than one product on which the Acree--Curtin--Hammett (ACH) principle is based. [...]

In this paper, a dislocatlon-based plasticity model is formulated in the framework of continuum mechanics and nonequllibrium thermodynamics for solids under high strain rates. Specifically, the formation of a dislocation network behind the shock front, associated plastic flow, resulting heating due to plastic work, and evolution of the morphology of the polycrystalline structure are modeled. T he associated changes of dynamic strength or yield stress with the increasing shock strength due to nucleation of dislocations at the shock front are modeled. The microstructural characteristics of the dislocation network are modeled by selected internal state variables. The irreversible nonequilibrium processes at high strain rates are modeled by selected nonequilibrium state variables and their evolution equations. The results clearly show the increased localized heating due to increased dynamic strength and the associated plastic flow.

Byline: Anurag Gupta (Department of Mechanical Engineering University of California, Berkeley, CA 94720, USA); David J. Steigmann (Department of Mechanical Engineering University of California, Berkeley, CA 94720, USA); James S. [...]

The mutagenesis experiments, designed to understand the nature of the packing constraints within the amyloid motif are reviewed. The plastic nature of amyloid appears to involve the malleability of both side chain interactions and H-bonded secondary structure and at its most basic level could be attributed to an apparently significant stability contribution from the noncovalent multimeric nature of the amyloid fibril.

Following a lesion, the adult central nervous system undergoes dramatic structural and physiological reorganization in diverse subcortical and cortical areas. Our knowledge of the events that parallel recovery within the [...]

In this work the geometrical method for the assessment of discontinuous bifurcation conditions is extended to encompass gradient-dependent plasticity. To this end, the gradient-dependent localization condition is cast in the form of an elliptical envelope condition in the coordinates of Mohr. The results in this work demonstrate the capability of thermodynamically consistent gradient-dependent elastoplastic model formulations to suppress the localized failure modes of the classical plasticity that take place when the hardening/softening modulus [bar.H] equals the critical value for localization [[bar.H].sub.c], provided the characteristic length l remains positive. [DOI: 10.1115/1.2202348]

Abstract: Plasticity is a characteristic feature that all plastic soils possess. It is shown that the soil plasticity is mainly due to undrained cohesion. Soil plasticity characteristics obtained from laboratory [...]

In biological interactions, phenotypic change in interacting organisms induced by their interaction partners causes a substantial shift in some environmental factor of the partners, which may subsequently change their phenotype in response to that modified environmental factor. Few examples of such arms-race-like plastic responses, known as reciprocal phenotypic plasticity, have been identified in predator-prey interactions. We experimentally identified a reciprocal defensive plastic response of a prey species against a predator with a predaceous phenotype using a model system of Close predator-prey interaction. Rana pirica tadpoles (the prey species) were reared with larvae of the salamander Hynobius retardatus (the predator species) having either a predaceous or a typical, nonpredaceous phenotype. The H. retardatus larvae with the predaceous phenotype, which is known to be induced by the presence of R. pirica tadpoles, induced a more defensive phenotype in the tadpoles than did larvae with the typical phenotype. The result suggests that the reciprocal phenotypic plasticity of R. pirica tadpoles is in response to a phenotype-specific signal under a close-signal recognition process. Key words: arms race; bulgy phenotype; coevolution; Hynobius retardatus; inducible defense; inducible offense; predator-induced plasticity; Rana pirica.

The magnitude, integration, and pattern of plastic character responses in external body form induced by shallow versus open water condition between two sunfish ecomorphis that coexist in four postglacial lakes are discussed. Results reveal that plastic responses of local sunfish ecomorphs could diverge through changes in the magnitude and coordination of plastic responses.

The scale invariance approach from micro-plasticity by Aifantis and Ning and Aifantis is investigated within rice's normality structure. The scale invariance approach is justified by the hypothesis of generalized scale invariance from a multiscale thermodynamic viewpoint.

A semi-analytical thermo-elastic-plastic contact model has been recently developed and presented in a companion paper. The main advantage of this approach over the classical finite element method (FEM) is the treatment of transient problems with the use of fine meshing and the possibility of studying the effect of a surface defect on the surface deflection as well as on subsurface stress state. A return-mapping algorithm with an elastic predictor/plastic corrector scheme and avon Mises criterion is now used, which improves the plasticity loop. This improvement in the numerical algorithm increases the computing speed significantly and shows a much better convergence and accuracy. The contact model is validated through a comparison with the FEM results of Kogut and Etsion (2002, J. Appl. Mech., 69, pp. 657-662) which correspond to the axisymmetric contact between an elastic-perfectly plastic sphere and a rigid fiat. A model for wear prediction based on the material removal during cyclic loading is then proposed. Results are presented, first, for initially smooth surfaces and, second, for rough surfaces. The effects of surface shear stress and hardening law are underlined. [DOI: 10.1115/1.2163360]

The writers introduce a design approach for braced excavations based directly on the data of carefully chosen soil tests, conceived within the framework of plasticity theory, but allowing for strain hardening. Mobilized shear stresses beneath and around braced excavations are found by a stability calculation based on a proposed plastic deformation mechanism. Strains required to mobilize these stresses are deduced from a direct simple shear test on a representative sample taken from a selected location in the plastic zone of influence. These strains are entered into a simple plastic deformation mechanism to predict boundary displacements. Hence, the proposed Mobilizable Strength Design (MSD) method can satisfy both safety and serviceability in a single step of calculation, without the need for finite element analyses. In this method, design parameters can be chosen rationally with regard to the initial state of soil, the stiffness following the appropriate stress path, and the level of acceptable deformations under working conditions. Examples demonstrating the success of the MSD method are given for a variety wall and soil conditions. Comparisons are made both with previously published field studies and with comprehensive nonlinear finite element analyses. CE Database subject headings: Excavation; Clays; Plasticity; Displacement.

NMR spectroscopy and molecular modeling are used to characterize the solution Ca(super 2+)-dependent structural and molecular features of the C-terminal domain of centrin (C-HsCen2) in complex with a peptide from the xeroderma pigmentosum group C protein ( Pl-XPC). Investigation of the centrin interactions show that the N-terminal domain of the protein is not involved in the complex formation and is structurally independent from the peptide-bound C-terminal domain.

Local protein synthesis in the synapse is required for synaptic plasticity and has been implicated in learning and memory, however, direct evidence that behavioral training induces local protein synthesis is lacking. This protein synthesis is regulated by the RNA-induced silencing complex (RISC).

During polymer foaming with physical blowing agents, plasticization affects the melt viscosity, gas diffusivity in the melt, and the gas-melt interfacial tension. In this paper, we propose a model for plasticization during bubble growth, and estimate its effects under typical foaming conditions. The theoretical model incorporates well-established mixture theories into a recent model for diffusion-induced bubble growth. These include the free-volume theories for the viscosity and diffusivity in polymer-blowing agent mixtures and the density gradient theory for the interfacial tension. The viscoelasticity of the melt is represented by an Oldroyd-B constitutive equation. We study the radial growth of a single bubble in an infinite expanse of melt, using parameter values based on experiments on polystyrene-C[O.sub.2] systems. Our results show that even at relatively low gas concentrations, plasticization increases the blowing-agent diffusivity markedly and thus boosts the rate of bubble growth. In contrast, the reduction in melt viscosity and interfacial tension has little effect on bubble growth. Though not intended as quantitative guidelines for process design, these results are expected to apply qualitatively to typical foaming conditions and common polymer-blowing agent combinations. POLYM. ENG. SCI., 46:97-107, 2006. © 2005 Society of Plastics Engineers, INTRODUCTION The foaming of polymer melts has been studied by many authors, both theoretically [e.g., 1-8] and experimentally [e.g., 9-12]. Extensive reviews and references can be found in a handbook [...]

The branching patterns and fitness responses of tall wormseed mustard (Erysimun strictum) to simulated browsing, soil nutrients, and competition in common garden are explored. Simulated browsing increased lateral branching and had little impact on plant fitness.

Abstract: The path-independent J-integral is used as a fracture prediction criterion for loading beyond the elastic limit as linear elastic fracture mechanics (LEFM) can not be applied. While J was [...]

The dislocation-indenter interaction is described using contact mechanics in linear, isotropic elasticity. The size dependent behavior is expected to be important during incipient plasticity wherein, dislocation-indenter interactions are expected to play a role in determining plastic flow.

A computational material modeling technique for analyzing intragranular microplasticity and intergranular microdamage in polycrystalline ceramics subjected to dynamic compression and tension is developed. The failure process and the resulting spall strength are affected strongly by the intensity of local release heterogeneity induced by heterogeneous microplasticity, and by the grain-boundary shear damage during compression.

The plasticity of any given trait, which has a genetic basis and which may or may not be adaptive, can intensify or attenuate evolved responses, and can itself evolve in response to selection depending on the scale of spatial or temporal heterogeneity. To investigate the complex function and evolution of plastic traits, an appealing yet challenging approach is assessing responses to artificial selection. Here, I review how artificial selection has been employed to explore four botanical research themes: (1) relationships between plastic and evolved responses to multiple stresses, (2) integration of cellular, leaf-level, and whole-plant responses to altered C[O.sub.2] concentrations, (3) photomorphogenic and photoperiodic development, both mediated by phytochrome photoreceptors, and (4) the evolution of the pest-induced myrosinase-glucosinolate system in cruciferous plants. These diverse topics are unified not only because they have been studied using artificial selection experiments, but also because they have considered variability in multiple traits affected by multiple factors in the external environment. Limitations of such research include a dearth of long-term studies; a surprising but often logistically necessary omission of control or replicate lines; and numerous issues relating to assessing impacts of inbreeding and drift. In addition to discussing options for circumventing such limitations, I draw attention to strategies for integrating the results of artificial selection studies with progress in functional and evolutionary genomics.

The alkylation of LZnSPh complexes designed to mimic the tetrahedral binding site found in all zinc thiolate proteins is studied. It is shown in model complexes designed to mimic the binding site of zinc-thiolate proteins that a hydrogen bond between an amide N-H and a Zn-coordinated thiolate reduces its reactivity towards electrophiles by up to 2 orders of magnitude.

A variational formulation in plasticity has been used to develop improved limit load estimation technique, such as the m-alpha method. Lower bound limit load estimates are especially germane to design and fitness-for-service assessments. The concept of 'integral mean of yield' has been applied to problems involving locally thinned areas (LTA) and local hot spots in the context of industrial pressure vessels and piping. Simplified procedures for 'fitness-for-service' assessment, suitable for use by plant engineers, have been developed. The results are compared with the corresponding inelastic finite elastic analyses. [DOI: 10.1115/1.1849230]

Abstract: Some fundamental problems of revealing micromechanisms of hydrogen plastification, superplasticity, embrittlement, cracking, blistering and delayed fracture of some technologically important industrial metallic materials are formulated. The ways are considered [...]

A superposition technique is introduced that allows for the application of discrete dislocation (DD) plasticity to a wide range of thermomechanical problems with reduced computational effort. Problems involving regions of differing elastic and/or plastic behavior are solved by superposing the solutions to i) DD models only for those regions of the structure where dislocation phenomena are permitted subject to either zero traction or displacement at every point on the boundary and ii) an elastic (EL) (or elastic/cohesivezone) model of the entire structure subject to all desired loading and boundary conditions. The DD subproblem is solved with standard DD machinery for an elastically homogeneous material. The EL subproblem requires only a standard elastic or elastic/cohesivezone finite element (FE) calculation. The subproblems are coupled: the negative of the tractions developed at the boundaries of the DD subproblem are applied as body forces in the EL subproblem, while the stress field of the EL subproblem contributes a driving force to the dislocations in the DD subproblem structure. This decomposition and the generic boundary, conditions of the DD subproblem permit the DD machinerry to be easily applied as a 'black-box' constitutive material description in an otherwise elastic FE formulation and to be used in a broader scope of applications due to the overall enhanced computational efficiency. The method is validated against prior results for crack growth along a plastic/rigid bimaterial interface. Preliminary results for crack growth along a metal/ ceramic bimaterial interface are presented. [DOI: 10.1115/1.1794167]

While numerous studies have been conducted on the ecology and evolution of phenotypic plasticity, to really understand plasticity we need to expose organisms to different environments over several ontogenetic stages. In this way, we can examine whether organisms change their phenotypic strategy over ontogeny, whether there are developmental windows that constrain the development of plastic traits, and whether behavior is more reversible than morphology if the environment reverts back to its original state. I addressed these questions by examining predator-induced plasticity in gray treefrog tadpoles (Hyla versicolor). Using aquatic mesocosms, I reared tadpoles with a constant absence of predators, a constant presence of predators, and the addition or removal of predators at three different times during their larval period. Tadpoles changed their phenotypic strategy over ontogeny, early in ontogeny they responded to predators by hiding, reducing their activity, and developing relatively deep tail fins. Later in ontogeny the tadpoles no longer employed behavioral defenses but relied on a combination of greater mass, deeper tails, and shorter bodies. The phenotypic changes were inducible throughout most of ontogeny, suggesting that there were few developmental windows. Activity, tail depth, and body depth were highly reversible early in ontogeny but less reversible later in ontogeny; because hiding was only used early in ontogeny, I could not assess its reversibility. This reversibility should affect not only the induced species, but will likely transmit the effects throughout the larger ecological community. Key words: anuran; Hyla versicolor; phenotypic plasticity; predator-induced plasticity; tadpole.

Artificial selection is used by quantitative geneticists to examine the genetic variances and covariances that underlie the evolution of traits that show continuous phenotypic variation. Such traits are crucial for adaptive evolution in natural populations. Here, I discuss the extent to which artificial selection experiments can provide insights about both the proximate and ultimate causes of adaptive evolution. In particular, such experiments can explore the basis of interactions among traits, and the potential consequences of genetic coupling of traits for the paths taken during evolution. They can also yield phenotypes of relevance to examining the reasons why differences in reproductive success and fitness occur. I focus here on the methodology, with particular reference to examples from investigations of the evolution of the development of morphologies and of phenotypic plasticity. Key words: adaptive evolution; artificial selection; development; evolutionary constraints; genetic variances; life history; morphology; natural selection; phenotypic plasticity.

Many plants display a characteristic suite of developmental 'shade avoidance' responses, such as stem elongation and accelerated reproduction, to the low ratio of red to far-red wavelengths (R:FR) reflected or transmitted from green vegetation. This R:FR cue of crowding and vegetation shade is perceived by the phytochrome family of photoreceptors. Phytochrome-mediated responses provide an ideal system for investigating the adaptive evolution of phenotypic plasticity in natural environments. The molecular and developmental mechanisms underlying shade avoidance responses are well studied, and testable ecological hypotheses exist for their adaptive significance. Experimental manipulation of phenotypes demonstrates that shade avoidance responses may be adaptive, resulting in phenotypes with high relative fitness in the environments that induce those phenotypes. The adaptive value of shade avoidance depends upon the competitive environment, resource availability, and the reliability of the R:FR cue for predicting the selective environment experienced by an induced phenotype. Comparative studies and a reciprocal transplant experiment with Impatiens capensis provide evidence of adaptive divergence in shade avoidance responses between woodland and clearing habitats, which may result from population differences in the frequency of selection on shade avoidance traits, as well as differences in the reliability of the R:FR cue. Recent rapid progress in elucidating phytochrome signaling pathways in the genetic model Arabidopsis thaliana and other species now provides the opportunity for studying how selection on shade avoidance traits in natural environments acts upon the molecular mechanisms underlying natural phenotypic variation.