Showing papers on "Stress relaxation published in 2010"

New Insights on the strain-induced mesophase of poly(D,L-lactide): in situ WAXS and DSC study of the thermo-mechanical stability

Abstract: This work deals with the study of the mesomorphic form or mesophase induced by tensile drawing from the amorphous state of a polylactide material containing 4 mol % of d-stereoisomer units. Investigations have been carried out over the draw temperature domain 45−90 °C, i.e. an interval spanning roughly ±20 °C about the glass transition temperature. In situ WAXS experiments during drawing, stress relaxation, and/or heating of stretched samples invariably showed the strain-induced occurrence of the mesophase as far as temperature did not exceed 70 °C. This seems to be the upper stability temperature of the mesophase identified in a previous study. DSC traces upon heating of drawn samples exhibit a post glass transition endothermic peak similar to the enthalpy relaxation phenomenon observed for aged polymers. The amplitude of this strain-induced endotherm proved to be strongly dependent on draw temperature and draw ratio. Draw ratio also appeared to strongly influence the temperature domain of cold crystalli...

Misfit dislocations in nanowire heterostructures

Abstract: The development of strain relaxation in lattice-mismatched semiconductor nanowire heterostructures is reviewed. Theoretical predictions for critical geometries for axial and core–shell structures are summarized and compared to experimental reports. All agree that nanowires can accommodate a greater elastic strain than is commonly seen with planar interfaces. Large mismatch as high as 10% has been elastically accommodated consistent with theoretical predictions. Elastically strained nanowire examples predominate, likely since nucleation is otherwise inhibited. A (maximum) critical radius is observed for epitaxial growth directly onto lattice-mismatched substrates. The few examples where strain relaxation via dislocations or roughening has been observed have been reported for core–shell geometries.This paper is dedicated to the late Professor Ulrich Gosele whose innovative leadership and insights in this field will not be forgotten.

Modeling the relaxation mechanisms of amorphous shape memory polymers.

Abstract: In this progress report, we review two common approaches to constitutive modeling of thermally activated shape memory polymers, then focus on a recent thermoviscoelastic model that incorporates the time-dependent effects of structural and stress relaxation mechanisms of amorphous networks. An extension of the model is presented that incorporates the effects of multiple discrete structural and stress relaxation processes to more accurately describe the time-dependent behavior. In addition, a procedure is developed to determine the model parameters from standard thermomechanical experiments. The thermoviscoelastic model was applied to simulate the unconstrained recovery response of a family of (meth)acrylate-based networks with different weight fractions of the crosslinking agent. Results showed significant improvement in predicting the temperature-dependent strain recovery response.

Stress relaxation and creep on living cells with the atomic force microscope: a means to calculate elastic moduli and viscosities of cell components

Abstract: In this work we present a unified method to study the mechanical properties of cells using the atomic force microscope Stress relaxation and creep compliance measurements permitted us to determine, the relaxation times, the Young moduli and the viscosity of breast cancer cells (MCF-7) The results show that the mechanical behaviour of MCF-7 cells responds to a two-layered model of similar elasticity but differing viscosity Treatment of MCF-7 cells with an actin-depolymerising agent results in an overall decrease in both cell elasticity and viscosity, however to a different extent for each layer The layer that undergoes the smaller decrease (36-38%) is assigned to the cell membrane/cortex while the layer that experiences the larger decrease (70-80%) is attributed to the cell cytoplasm The combination of the method presented in this work, together with the approach based on stress relaxation microscopy (Moreno-Flores et al 2010 J Biomech 43 349-54), constitutes a unique AFM-based experimental framework to study cell mechanics This methodology can also be extended to study the mechanical properties of biomaterials in general

On the correlation between nanoscale structure and magnetic properties in ordered mesoporous cobalt ferrite (CoFe2O4) thin films.

Abstract: In this work, we report the synthesis of periodic nanoporous cobalt ferrite (CFO) that exhibits tunable room temperature ferrimagnetism. The porous cubic CFO frameworks are fabricated by coassembly of inorganic precursors with a large amphiphilic diblock copolymer, referred to as KLE. The inverse spinel framework boasts an ordered open network of pores averaging 14 nm in diameter. The domain sizes of the crystallites are tunable from 6 to 15 nm, a control which comes at little cost to the ordering of the mesostructure. Increases in crystalline domain size directly correlate with increases in room temperature coercivity. In addition, these materials show a strong preference for out-of-plane oriented magnetization, which is unique in a thin film system. The preference is explained by in-plane tensile strain, combined with relaxation of the out-of-plane strain through flexing of the mesopores. It is envisioned that the pores of this ferrimagnet could facilitate the formation of a diverse range of exchange coupled composite materials.

Fatigue Behavior of Stainless Steel 304L Including Strain Hardening, Prestraining, and Mean Stress Effects

Abstract: This paper discusses cyclic deformation and fatigue behaviors of stainless steel 304L and aluminum 7075-T6. Effects of loading sequence, mean strain or stress, and prestraining were investigated. The behavior of aluminum is shown not to be affected by preloading, whereas the behavior of stainless steel is greatly influenced by prior loading. Mean stress relaxation in strain control and ratcheting in load control and their influence on fatigue life are discussed. Some unusual mean strain test results are presented for SS304L, where in spite of mean stress relaxation fatigue lives were significantly longer than fully-reversed tests. Prestraining indicated no effect on either deformation or fatigue behavior of aluminum, while it induced considerable hardening in SS304L and led to different results on fatigue life, depending on the test control mode. Possible mechanisms for secondary hardening observed in some tests, characterized by a continuous increase in the stress response and leading to runout fatigue life, are also discussed. The Smith-Watson-Topper parameter was shown to correlate most of the experimental data for both materials under different loading condition.

Stress relaxation and creep experiments with the atomic force microscope: a unified method to calculate elastic moduli and viscosities of biomaterials (and cells)

Abstract: We show that the atomic force microscope can perform stress relaxation and creep compliance measurements on living cells. We propose a method to obtain the mechanical properties of the studied biomaterial: the relaxation time, the elastic moduli and the viscosity.

Optical properties of InGaN/GaN nanopillars fabricated by postgrowth chemically assisted ion beam etching

Abstract: The optical properties of InGaN/GaN quantum wells, which were nanopatterned into cylindrical shapes with diameters of 2 μm, 1 μm, or 500 nm by chemically assisted ion beam etching, were investigated. Photoluminescence (PL) and time-resolved PL measurements suggest inhomogeneous relaxation of the lattice-mismatch induced strain in the InGaN layers. By comparing to a strain distribution simulation, we found that partial stain relaxation occurs at the free side wall, but strain remains in the middle of the pillar structures. The strain relaxation leads to an enhanced radiative recombination rate by a factor of 4–8. On the other hand, nonradiative recombination processes are not strongly affected, even by postgrowth etching. Those characteristics are clearly reflected in the doughnut-shape emission patterns observed by optical microscopy.

Stress Relaxation by Addition−Fragmentation Chain Transfer in Highly Cross-Linked Thiol−Yne Networks

Abstract: Radical mediated addition-fragmentation chain transfer of mid-chain allyl sulfide functional groups was utilized to reduce polymerization-induced shrinkage stress in thiol-yne step-growth photopolymerization reactions. In previous studies, the addition-fragmentation of allyl sulfide during the polymerization of a step-growth thiol-ene network demonstrated reduced polymerization stress; however, the glass transition temperature of the material was well below room temperature (~ -20°C). Many applications require super-ambient glass transition temperatures, such as microelectronics and dental materials. Polymerization reactions utilizing thiol-yne functional groups have many of the advantageous attributes of the thiol-ene-based materials, such as possessing a delayed gel-point, resistant to oxygen inhibition, and fast reaction kinetics, while also possessing a high glass transition temperature. Here we incorporate allyl sulfide functional groups into a highly crosslinked thiol-yne network to reduce polymerization-induced shrinkage stress. Simultaneous shrinkage stress and functional group conversion measurements were performed during polymerization using a cantilever-type tensometer coupled with a FTIR spectrometer. The resulting networks were highly crosslinked, possessed super-ambient glass transition temperatures, and exhibited significantly reduced polymerization-induced shrinkage stress when compared with analogous propyl sulfide-containing materials that are incapable of addition-fragmentation.

The microstructure and impression creep behavior of cast, Mg–5Sn–xCa alloys

Abstract: The effects of 0.7, 1.4 and 2 wt.% Ca additions on the microstructure and creep behavior of a cast Mg–5Sn alloy were investigated by impression tests. Impression creep tests were carried out in the temperature range 423–523 K and under punching stresses in the range 150–475 MPa for dwell times up to 3600 s. Analysis of the data showed that for all loads and temperatures, the Mg–5Sn–2Ca alloy had the lowest creep rates, and thus the highest creep resistance among all materials tested. This is attributed to the diminishing of the less stable Mg 2 Sn particles and formation of the more thermally stable CaMgSn phase which strengthens both matrix and grain boundaries during creep deformation in the investigated system. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.012 σ imp / G )

Effects of Particle Crushing in Stress Drop-Relaxation Experiments on Crushed Coral Sand

Abstract: Stress relaxation and stress drop-relaxation tests have been performed to complement a test series performed to study strain rate, creep, and stress drop-creep effects on crushed coral sand. Drained experiments with constant effective confining pressure of 200 kPa were performed in which triaxial specimens of crushed coral sand were loaded to initial stress differences of 500, 700, and 900 kPa, followed by stress drops of 0, 100, 200, 300, and 400 kPa at which points the axial strains were kept constant while the axial stress relaxation and the volumetric strains were observed. The stress drops produced delays in initiation of stress relaxation that were proportional with the magnitudes of the stress drops. The experiments show that sands do not exhibit classic viscous effects, and their behavior is indicated as "nonisotach," while the typical viscous behavior of clay is termed "isotach." Thus, there are significant differences in the time-dependent behavior patterns of sands and clay. A mechanistic picture of time effects in sands is proposed.

Viscoelastic nature of calcium silicate hydrate

Abstract: The origin of the time-dependent response of cement-based materials to applied stress has not been clearly resolved. The role of interlayer water in the mechanical behavior of calcium silicate hydrate (C–S–H) is still debated. In order to better understand the pertinent mechanisms, the stress relaxation tests were conducted on thin rectangular beams of compacted synthetic C–S–H powder and hydrated Portland cement subjected to three-point bending. C–S–H specimens of variable composition (C/S = 0.8, 1.2 and 1.5) were prepared at various moisture content levels from saturation to the dry state. A special drying procedure was applied in order to remove the adsorbed and interlayer water incrementally from C–S–H conditioned at 11%RH. It was shown that a significant part of the relaxation at saturation is attributed to the hydrodynamic component associated with the pore water. It was demonstrated that the viscoelastic performance of C–S–H depends considerably on the presence of interlayer water. It was argued that the results support the validity of the theory of sliding of C–S–H sheets as a time-dependent deformation mechanism responsible for the creep and stress relaxation of cement-based materials. This concept was illustrated in a proposed model for the viscoelastic response of C–S–H.

DEM simulations of thermally activated creep in soils

Abstract: Discrete element modelling (DEM) has been used to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory. Soil stiffness is represented by a pair of values of linear spring stiffness normal and tangential to each inter-granular contact, and the limiting coefficient of contact friction is described as varying linearly with the logarithm of sliding velocity. DEM simulations of an assembly of 3451 spheres reproduce a number of significant phenomena including: creep rate as a function of the mobilisation of deviatoric stress; initially linear decay of creep strain rate with time plotted on log-log axes and with a slope m in the range −0·8 to −1; and ultimate creep failure in triaxial simulations at high deviatoric stress ratios. Creep-induced failure is shown to occur at a unique axial strain for a given state of initial packing, and to be linked with dilatancy. The numerical results are compared qua...

Time-dependent rheology of colloidal star glasses

Abstract: Suspensions of multiarm star polymers are studied as models for soft colloidal glasses. Using an established pre-shearing protocol which ensures a reproducible initial state (the “rejuvenation” of the system), we report here the time evolution of the stress upon startup of simple shear flow for a range of shear rates. We show the existence of critical shear rates, γc(c) which are functions of the concentration, c. When the suspensions are sheared at rates below γc(c), the stress rises to a common value σc(c) which is also a function of the concentration. The system thus develops a yield stress. This behavior manifests itself as an evolution from a monotonic slightly shear-thinning flow curve to a flow curve dominated by a stress plateau. We relate this bulk evolution to spatially resolved velocity profiles. Hence, yield stress is linked to shear banding in this class of soft colloids.

Crystal plasticity modeling of cyclic deformation for a polycrystalline nickel-based superalloy at high temperature

Abstract: Cyclic deformation at elevated temperature has been modeled for a polycrystalline nickel-based superalloy using the crystal-plasticity constitutive formulations. Finite element analyses were carried out for a representative volume element (RVE), consisting of randomly oriented grains and subjected to periodic boundary constraints. Model parameters were determined by fitting the strain-controlled cyclic test data at 650 °C for three different loading rates. Simulated results are in good agreement with the experimental data for both stress–strain loops and cyclic hardening behavior. The model was utilized to predict the stress relaxation behavior during the hold periods at the maximum and minimum strain levels, and the prediction compares well with the experimental results. Localized stress and strain concentrations were observed due to the heterogeneous nature of grain microstructure and the mismatch of the mechanical properties of individual grains.

Atomistic simulation of creep in a nanocrystal.

Abstract: We describe a method to simulate on macroscopic time scales the stress relaxation in an atomistic nanocrystal model under an imposed strain. Using a metadynamics algorithm for transition state pathway sampling we follow the full evolution of a classical anelastic relaxation event, with relaxation times governed by the nanoscale microstructure imperfections in the solid. We show that probing this sensitive variation leads to mechanistic insights that reveal a direct correlation between system-level relaxation behavior and localized atomic displacements in the vicinity of the nanostructured defects, in turn implying a unit mechanism for self-organized plastic response. This suggests a new class of measurements in which the microstructure imperfections are characterized and matched to predictive simulations enabled by the present method.

Stress, Sheet Resistance, and Microstructure Evolution of Electroplated Cu Films During Self-Annealing

Abstract: Electroplated copper films are known to change their microstructure due to the self-annealing effect. The self-annealing effect of electroplated copper films was investigated by measuring the time dependence of the film stress and sheet resistance for different layer thicknesses between 1.5 and 20 ?m. While the sheet resistance was found to decrease as time elapsed, a size-dependent change in film stress was observed. Films with the thickness of 5 ?m and below decrease in stress, while thicker films initially reveal an increase in film stress followed by a stress relaxation at a later stage. This behavior is explained by the superposition of grain growth and grain-size-dependent yielding.

Stress relaxation in entangled melts of unlinked ring polymers.

Abstract: Stress relaxation in unlinked ring polymer melts poses an important challenge to our theoretical understanding of entangled polymer dynamics. Recent experiments on entangled unlinked ring melts show power-law stress relaxation with no hint of a rubbery plateau, usually the hallmark of entangled polymers. Here we present a theory for stress relaxation in rings analogous to the successful approach for star polymers. We augment our theory with mesoscale Monte Carlo dynamics simulations of equivalent "lattice animal" configurations. We find a stress relaxation function G(t)∼t(-α) with α≈1/2 consistent with experiment, emerging ultimately from the disparate relaxation times of more- and less-central portions of ring conformations.