Showing papers on "Stress relaxation published in 2005"

Photoinduced Plasticity in Cross-Linked Polymers

Abstract: Chemically cross-linked polymers are inherently limited by stresses that are introduced by post-gelation volume changes during polymerization. It is also difficult to change a cross-linked polymer's shape without a corresponding loss of material properties or substantial stress development. We demonstrate a cross-linked polymer that, upon exposure to light, exhibits stress and/or strain relaxation without any concomitant change in material properties. This result is achieved by introducing radicals via photocleavage of residual photoinitiator in the polymer matrix, which then diffuse via addition-fragmentation chain transfer of midchain functional groups. These processes lead to photoinduced plasticity, actuation, and equilibrium shape changes without residual stress. Such polymeric materials are critical to the development of microdevices, biomaterials, and polymeric coatings.

Strain gradients in epitaxial ferroelectrics

Abstract: X-ray analysis of ferroelectric thin layers of Ba1/2Sr1/2TiO3 with different thicknesses reveals the presence of strain gradients across the films and allows us to propose a functional form for the internal strain profile. We use this to calculate the influence of strain gradient, through flexoelectric coupling, on the degradation of the ferroelectric properties of films with decreasing thickness, in excellent agreement with the observed behavior. This paper shows that strain relaxation can lead to smooth, continuous gradients across hundreds of nanometers, and it highlights the pressing need to avoid such strain gradients in order to obtain ferroelectric films with bulklike properties.

Viscoelastic characterization of polymers using instrumented indentation. I. Quasi-static testing

Abstract: The use of instrumented indentation to characterize the mechanical response of polymeric materials was studied. A model based on contact between a rigid probe and a linear viscoelastic material was used to calculate values for the creep compliance and stress relaxation modulus for two glassy polymeric materials, epoxy and poly(methyl methacrylate), and two poly(dimethyl siloxane) (PDMS) elastomers. Results from bulk rheometry studies were used for comparison with the indentation stress relaxation results. For the two glassy polymers, the use of sharp pyramidal tips produced responses that were considerably more compliant (less stiff) than the rheometry values. Additional study of the deformation remaining in epoxy after indentation creep testing as a function of the creep hold time revealed that a large portion of the creep displacement measured was due to postyield flow. Indentation creep measurements of the epoxy with a rounded conical tip also produced nonlinear responses, but the creep compliance values appeared to approach linear viscoelastic values with decreasing creep force. Responses measured for the unfilled PDMS were mainly linear elastic, with the filled PDMS exhibiting some time-dependent and slight nonlinear responses in both rheometry and indentation measurements. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1794–1811, 2005

The shear-induced solid–liquid transition in yield stress materials with chemically different structures

Abstract: The shear-induced solid–liquid transition of viscoplastic materials has been studied extensively through various steady-shearing experiments including steady stress sweep, stress ramp, creep and strain recovery. The results are consistent in showing a clear change in behaviour of the materials from solid-like to liquid-like. A transition stress plateau separates the regions of solid behaviour and liquid behaviour. The slope of the plateau reflects the uniformity of the structure, and hence the distribution of bonding strength within this structure. Depending on the structure of the material, the yielding process of viscoplastic materials can occur over a wide or a narrow range of stress, which represents “ductile-type” or “brittle-type” failure. Altering the concentration and extent of particle flocculation (for suspensions) or polymer chain entanglements (for polymer gels) can vary the bonding strength and strength distribution, and therefore change the slope of the stress plateau. The continuous solid structure exhibits creep at stresses well below the yield stress and fails at a critical strain. The yielding of the material seems to be characterised by a critical strain rather than by a critical stress or a critical shear rate. The deformation of viscoplastic materials can be recovered fully or partially in the solid-like region once the stress is removed. This is a significant difference from the behaviour of purely viscoelastic materials. Strain recovery tests result in two characteristic strain values that can be used to define the two commonly used “yield stress” values, the higher one of which is in good agreement with the traditional value of yield stress as measured by vane torsion, for example.

A mechanical model for creep, recovery and stress relaxation in polymeric materials

Abstract: A mechanical model is presented, in which viscoelastic response is described by the action of time-dependent latch elements. The model represents viscoelastic changes occurring through incremental jumps as opposed to continuous motion. This is supported by the observation that polymeric creep, recovery and stress relaxation can be correlated with stretched exponential functions, i.e. Weibull and Kohlrausch-Williams-Watts, since (i) the former is also used in reliability engineering to represent the failure of discrete elements and (ii) there is evidence of the latter being an approximation to the Eyring potential energy barrier relationship, which describes motion in terms of molecular jumps.

Mathematical descriptions for the behaviour of ice-rich frozen soils at temperatures close to 0 °C

Abstract: With the use of creep and constant strain rate (CSR) tests, mathematical formulations were found that describe the thermomechanical behaviour of ice-rich frozen soils. A Glen-type relationship was ...

Role of inclined threading dislocations in stress relaxation in mismatched layers

Abstract: (0001)-oriented epitaxial wurtzite III-nitride layers grown on mismatched substrates have no resolved shear stress on the natural basal and prismatic slip planes; however, strained III-nitride layers may gradually relax. We report on the stress relaxation of Al0.49Ga0.51N layers grown on nominally relaxed Al0.62Ga0.38N buffer layers on sapphire. The reduction in elastic strain of the Al0.49Ga0.51N was enhanced by Si doping which caused an increased surface roughness. Despite the Si doping, the films always sustained step-flow growth. The extent of relaxation of the Al0.49Ga0.51N layer was determined by on-axis ω‐2θ scans of (000l) peaks and reciprocal space maps of inclined (off-axis) peaks. Cross-section and plan-view transmission electron microscopy studies showed that the threading dislocations in the Al0.49Ga0.51N layer inclined from the [0001] direction towards ⟨11¯00⟩ directions by ∼15–25°, perpendicular to their Burgers vector (13⟨112¯0⟩). These inclined threading dislocations have a misfit disloca...

Deformation mechanisms in superplastic AA5083 materials

Abstract: The plastic deformation of seven 5083 commercial aluminum materials, produced from five different alloy heats, are evaluated under conditions of interest for superplastic and quick-plastic forming. Two mechanisms are shown to govern plastic deformation in AA5083 over the strain rates, strains, and temperatures of interest for these forming technologies: grain-boundary-sliding (GBS) creep and solutedrag (SD) creep. Quantitative analysis of stress transients following rate changes clearly differentiates between GBS and SD creep and offers conclusive proof that SD creep dominates deformation at fast strain rates and low temperature. Furthermore, stress transients following strain-rate changes under SD creep are observed to decay exponentially with strain. A new graphical construction is proposed for the analysis and prediction of creep transients. This construction predicts the relative size of creep transients under SD creep from the relative size of changes in an applied strain rate or stress. This construction reveals the relative size of creep transients under SD creep to be independent of temperature; temperature dependence resides in the “steady-state” creep behavior to which transients are related.

Modeling of Shape Memory Induction and Recovery in Heat-Shrinkable Polymers

Abstract: A mechanical model was developed to describe qualitatively and quantitatively the stress-strain-time behavior of a prepared shape memory crosslinked polyethylene during hot stretching, stress relaxation under 200% strain at high temperature and strain recovery of the heat shrinkable polymer The stress-strain, the stress relaxation and the irrecoverable strain behavior of the model were established by driving the constitutive equation, which could qualitatively represent the behavior of the real material By choosing significant values for the parameters of the proposed model, an excellent fit was obtained between the experimental behavior of the polymer and that predicted by the model It was also revealed that the main source responsible for the imperfect recovery of the induced strain observed was the stress relaxation occurring during the stretch holding-cooling time step

Investigation of optical properties of nanoporous GaN films

Abstract: In this study, we have investigated the optical properties of nanoporous GaN films on sapphire (0 0 0 1) prepared by UV-assisted electrochemical and electroless etching. Using various etching conditions, we can control the average pore size, pore depth and the density of pores on the GaN surface. Scanning electron microscopy (SEM) measurements reveal the nature of the pore morphology and microstructures. Optical properties of these nanoporous films have been studied using micro-photoluminescence and micro-Raman scattering. As compared to the as-grown GaN films, nanoporous layers exhibit a substantial photoluminescence (PL) intensity enhancement with red-shifted band-edge PL peaks associated with the relaxation of compressive stress. The red shifted E 2 phonon peak in the Raman spectra of the porous GaN films further confirms such a stress relaxation. Microscopic optical measurements also suggest that light extraction from porous GaN surface is enhanced by such nanopatterning.

Strain relaxation due to V-pit formation in InxGa1−xN∕GaN epilayers grown on sapphire

Abstract: Strain relaxation in semiconductor heterostructures generally occurs through the motion of dislocations that generates misfit dislocations above a critical thickness. However, majority of the threading dislocations in GaN-related materials have no driving force to glide, and those with a driving force are kinetically impeded even at a temperature of 1000 °C. In spite of this, the strain in InxGa1−xN∕GaN epilayers grown on c-plane sapphire substrates was observed to decrease as the InxGa1−xN layer becomes thicker. We have explored the possibility of V-pit formation at terminated dislocations as the predominant relaxation mechanism in highly mismatched systems such as InxGa1−xN∕GaN. We demonstrate that a driving force exists to nucleate V pits for strain relief. The formation of V pits was modeled through the energy balance between the strain energy in the InxGa1−xN epilayer, the destruction of dislocation energy to form V pits and the strain that is relieved due to the formation of edges during the process...

Nonlinear viscoelastic, thermodynamically consistent, models for biological soft tissue.

Abstract: The mechanical behavior of most biological soft tissue is nonlinear viscoelastic rather than elastic. Many of the models previously proposed for soft tissue involve ad hoc systems of springs and dashpots or require measurement of time-dependent constitutive coefficient functions. The model proposed here is a system of evolution differential equations, which are determined by the long-term behavior of the material as represented by an energy function of the type used for elasticity. The necessary empirical data is time independent and therefore easier to obtain. These evolution equations, which represent non-equilibrium, transient responses such as creep, stress relaxation, or variable loading, are derived from a maximum energy dissipation principle, which supplements the second law of thermodynamics. The evolution model can represent both creep and stress relaxation, depending on the choice of control variables, because of the assumption that a unique long-term manifold exists for both processes. It succeeds, with one set of material constants, in reproducing the loading–unloading hysteresis for soft tissue. The models are thermodynamically consistent so that, given data, they may be extended to the temperature-dependent behavior of biological tissue, such as the change in temperature during uniaxial loading. The Holzapfel et al. three-dimensional two-layer elastic model for healthy artery tissue is shown to generate evolution equations by this construction for biaxial loading of a flat specimen. A simplified version of the Shah–Humphrey model for the elastodynamical behavior of a saccular aneurysm is extended to viscoelastic behavior.

Theoretical and experimental investigation of stress redistribution in open hole composite laminates due to damage accumulation

Abstract: Two methods were examined for the prediction of stress redistribution due to subcritical damage accumulation near open holes in composite laminates. A finite element analysis in conjunction with the method of material property degradation was performed to assess the accuracy of the fiber direction stress redistribution prediction due to formation of longitudinal splitting. A simple case of a unidirectional composite with an open hole was considered. The method was unable to accurately predict the fiber stress relaxation due to longitudinal splitting, which in the case of laminates with holes is of paramount importance for the accurate prediction of ultimate strength. Three-dimensional ply level modeling of discrete damage near an open hole in a quasi-isotropic composite laminate was subsequently considered. Normally the mesh configuration is dictated by the boundaries of the specimen, such as the presence of a hole, creating formidable difficulties to modeling matrix cracking, which is aligned with the fiber direction in a given ply. A mesh independent displacement discontinuity modeling method based on higher order shape functions was constructed for this purpose. The surface of the displacement jump associated with matrix cracking was defined in terms of the domain Heaviside function approximated by using higher order polynomial B-splines. Moire interferometry was used to determine the strain and displacement fields in the surface layers of a quasi-isotropic composite, previously prestressed beyond the damage initiation load. Good correlation between the experimental data and the stress redistribution predicted by the mesh independent damage modeling technique was observed.

Effect of the Presence of Diblock Copolymer on the Nonlinear Elastic and Viscoelastic Properties of Elastomeric Triblock Copolymers

Abstract: The mechanical properties of blends of triblock and diblock copolymers of polystyrene (PS) and polyisoprene (PI) containing 15 wt % PS have been investigated with tensile tests, relaxation tests, dynamic mechanical tests, and hysteresis tests. The properties of binary triblock/diblock blends have been investigated in parallel with those of the same blends diluted with 60 wt % of a low molecular weight but high-Tg solvent, miscible with the PI component but not with the PS domains. Both binary and ternary blends show a nonlinear elastic behavior at large strains, which can be analyzed with rubber elasticity models, and a pronounced nonlinear viscoelastic behavior at intermediate strains. The analysis of the viscoelastic behavior beyond the linear regime shows that the proportion of PI blocks effectively bridging two PS domains controls the deformation of the blends at large strains, while the small and intermediate strain behavior is controlled by the density of entanglements in the rubbery component and b...

Residual stress state and hardness depth in electric discharge machining: de-ionized water as dielectric liquid

Abstract: Procedures and results of experimental work to measure residual stresses and hardness depth in electric discharge machined surfaces are presented. Layer removal method is used to express the residual stress profile as a function of depth caused by a die sinking type EDM. Thin stressed layers are removed from machined samples by electrochemical machining. Corresponding deformations due to stress relaxation are recorded for each removal to determine the stress profile from elasticity theory. The relational dependence of the machining parameters with residual stresses is obtained and a semi-empirical model is proposed for plastic mold steel for de-ionized water as dielectric liquid. These stresses are found to be increasing rapidly with respect to depth, attaining to its maximum value, around the yield strength, and then fall rapidly to compressive residual stresses in the core of the material since the stresses within plastically deformed layers are equilibrated with elastic stresses.

Shear stress relaxation and physical aging study on simple glass-forming materials

Abstract: Relaxation and aging behaviors in three supercooled liquids: m-toluidine, glycerol, and sucrose benzoate have been studied by shear stress relaxation experiments in the time domain above and below their nominal glass transition temperatures. For the equilibrium state, the current study provides new data on the behavior of organic complex fluids. The shape of the relaxation function as characterized by the stretching exponent β is discussed considering that a time-temperature master curve can be constructed even though the β’s for the individual response curves at each temperature vary systematically. In the nonequilibrium state, isothermal physical aging experiments at different glassy structures reveal that the effect of the aging process on the mechanical shear relaxation in these simple glass formers is similar to that observed in polymeric and other systems. Departure from the Vogel-Fulcher-Tamman behavior after the samples have aged back to equilibrium in the glassy state is observed for m-toluidine ...

Sorbitan tristearate layers at the air/water interface studied by shear and dilatational interfacial rheology.

Abstract: The shear and dilatational rheology of condensed interfacial layers of the water-insoluble surfactant sorbitan tristearate at the air/water interface is investigated. A new interfacial shear rheometer allows measurements in both stress- and strain-controlled modes, providing comprehensive interfacial rheological information such as the interfacial dynamic shear moduli, the creep response to a stress pulse, the stress relaxation response to a strain step, or steady shear curves. Our experiments show that the interfacial films are both viscoelastic and brittle in nature and subject to fracture at small deformations, as was supported by in-situ Brewster angle microscopy performed during the rheological experiments. Although any large-deformation test is destructive to the sample, it is still possible to study the linear viscoelastic regime if the deformations involved are controlled carefully. Complementary results for the dilatational rheology in area step compression/expansion experiments are reported. The dilatational behavior is predominantly elastic throughout the frequency spectrum measured, whereas the layers exhibit generalized Maxwell behavior in shear mode within a deformation frequency regime as narrow as two decades, indicating the presence of additional relaxation mechanisms in shear as opposed to expansion/compression. If the transient rheological response from stress relaxation experiments is considered, then the data can be described well with a stretched exponential model both in the shear and dilatational deformations.

Relaxation of compressively-strained AlGaN by inclined threading dislocations

Abstract: Transmission electron microscopy and x-ray diffraction were used to assess the microstructure and strain of AlxGa1−xN(x=0.61–0.64) layers grown on AlN. The compressively-strained AlGaN is partially relaxed by inclined threading dislocations, similar to observations on Si-doped AlGaN by P. Cantu, F. Wu, P. Waltereit, S. Keller, A. E. Romanov, U. K. Mishra, S. P. DenBaars, and J. S. Speck [Appl. Phys. Lett. 83, 674 (2003)]; however, in our material, the dislocations bend before the introduction of any Si. The bending may be initiated by the greater lattice mismatch or the lower dislocation density of our material, but the presence of Si is not necessarily required. The relaxation by inclined dislocations is quantitatively accounted for with the model of A. E. Romanov and J. S. Speck [Appl. Phys. Lett. 83, 2569 (2003)], and we demonstrate the predicted linear dependence of relaxation on layer thickness. Notably, such relaxation was not found in tensile strained AlGaN grown on GaN [J. A. Floro, D. M. Follstae...

Moisture effects on FM300 structural film adhesive: Stress relaxation, fracture toughness, and dynamic mechanical analysis

Abstract: The behavior of cured FM300 epoxy, a structural film adhesive, subjected to partial and full moisture saturation has been evaluated. Three separate but interrelated test methods were used: stress relaxation, fracture toughness, and dynamic mechanical testing. The mechanical response of the epoxy due to increasing moisture content was dependent on the testing method. In stress relaxation testing, the epoxy was plasticized when partially saturated with moisture, but it became more rigid when fully saturated. The plasticization-to-stiffening transition was not observed in the other two test methods. Fracture testing showed that the material toughness increased with increasing moisture concentration: plasticization effects were dominant. Similar changes in the loss modulus were found in dynamic mechanical analysis. We propose that the differences in behavior have been due to differences in load levels and loading rates used in these probing techniques. Stress relaxation testing, at a relatively lower load and loading rate, appeared to be more sensitive to the localized interactions between the absorbed water molecules and the crosslinked structure. Higher loads and loading rates tended to reveal the bulk effects of plasticization only. Nevertheless, there was also strong evidence from glass-transition temperature measurements that these moisture effects were mostly reversible. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95:1285–1294, 2005

High-resolution strain mapping in heteroepitaxial thin-film features

Abstract: Heteroepitaxial thin-film features that are lattice matched to the underlying substrate undergo elastic relaxation at the free edges of the feature. To characterize the degree of elastic relaxation, we employed synchrotron-based x-ray diffraction techniques to map the change in lattice spacing in the thin film at a submicron resolution. Measurements were conducted on 0.24‐μm thick, heteroepitaxially grown SiGe strips of various widths on Si (001). A comparison of the SiGe diffraction peak positions across the features provides a real-space mapping of the extent of elastic relaxation as a function of linewidth. The resultant in-plane normal film stress measurements were compared to calculated values from several elastic mechanical models to assess their validity in predicting stress distributions within the features.