Showing papers on "Stress relaxation published in 2004"

Effect of substrate-induced strain on the structural, electrical, and optical properties of polycrystalline ZnO thin films

Abstract: The effect of substrate-induced strain in polycrystalline ZnO thin films on different substrate, e.g., GaN epilayer, sapphire (0001), quartz glass, Si(111)∕SiO2, and glass deposited by sol-gel process, has been investigated by x-ray diffraction, scanning electron microscope, electrical resistivity, and photoluminescence measurements. A strong dependence of orientation, crystallite size, and electrical resistivity upon the substrate-induced strain along the c axis has been found. The results of structural and morphological studies indicate that relatively larger tensile strain exists in ZnO deposited on sapphire and glass, while a smaller compressive strain appears in film deposited on GaN and the strain is relaxed in larger crystallite size. The electrical resistivity of the films increases exponentially with increasing strain. The excitonic peak positions are found to shift slightly towards lower energy side with increasing strain. The analysis shows that GaN being a closely lattice-matched substrate produces ZnO films of better crystallinity with a lower resistivity.

Stress-induced transformation behavior of a polycrystalline NiTi shape memory alloy: micro and macromechanical investigations via in situ optical microscopy

Abstract: An experimental investigation of the micro and macromechanical transformation behavior of polycrystalline NiTi shape memory alloys was undertaken. Special attention was paid to macroscopic banding, variant microstructure, effects of cyclic loading, strain rate and temperature effects. Use of an interference filter on the microscope enabled observation of grain boundaries and martensitic plate formation and growth without recourse to etching or other chemical surface preparation. Key results of the experiments on the NiTi include observation of localized plastic deformation after only a few cycles, excellent temperature and stress relaxation correlation, a refined definition of “full transformation” for polycrystalline materials, and strain rate dependent effects. Several of these findings have critical implications for understanding and modeling of shape memory alloy behavior.

Assessing residual stress development and stress relaxation in restrained concrete ring specimens

Abstract: Recently, the American Association of State Highway and Transportation Officials implemented a provisional standard that uses the `ring test' to help quantify a materials' propensity for cracking. While this test may provide qualitative information that enables different mixtures to be compared, it does not provide quantitative information to describe how close a specimen is to failure. This paper will describe how the ring test may be used to provide quantitative information about stress development that may be used to assess the potential for cracking in concrete. An analytical stress formulation is presented to compute the actual residual stress level in the concrete using only the measured strain from the steel ring. The theoretical elastic stress is computed using the free shrinkage, ring deformation, and elastic modulus of the concrete. A comparison of the residual and theoretical elastic stress levels provides information about the extent of stress relaxation in a material. Continuously monitoring the strain that develops in the steel ring from the time of casting enables the effects of autogenous shrinkage to be determined as well as the effects of drying shrinkage.

Shear Properties of Brain Tissue over a Frequency Range Relevant for Automotive Impact Situations: New Experimental Results

Abstract: This research aims at improving the definition of the shear linear material properties of brain tissue. A comparison between human and porcine white and gray matter samples was carried out over a new large frequency range associated with both traffic road and non-penetrating ballistic impacts. Oscillatory experiments were performed by using an original custom-designed oscillatory shear testing device. The findings revealed that no significant difference occured between the linear viscoelastic behavior of the porcine and the human brain tissue. On the average, the storage modulus (G') and the loss modulus (G") of the white matter increased respectively from 2.1 +/- 0.9 kPa to 16.8 +/- 2.0 kPa and from 0.4 +/- 0.2 kPa to 18.7 +/- 2.3 kPa between 0.1 and 6300 Hz at 37 degrees C. In addition, the gray and white matter behaviors seemed to be similar at small strains. The reliability of the data and the robustness of the experimental protocol were checked using a standard rheometer (Bohlin C-VOR 150). A good agreement was found between the data obtained in the frequency and time field. As a result, the linear relaxation modulus was determined over an extensive time range (from 10(-5) s to 300 s). In a first approach, the nonlinear behavior of brain tissue was studied using stress relaxation tests. Brain tissue showed significant shear softening for strains above 1% and the time relaxation behavior was independent of the applied strain. On this basis, a visco-hyperelastic model was proposed using the generalized Maxwell model and the Ogden hyperelastic model. These models respectively describe the linear relaxation modulus and the strain dependence of the shear stress.

Viscoelastic measurement techniques

Abstract: Methods for measuring viscoelastic properties of solids are reviewed The nature of viscoelastic response is first presented This is followed by a survey of time and frequency-domain considerations as they apply to mechanical measurements Subresonant, resonant, and wave methods are discussed, with applications

Characterization of Models for Time-Dependent Behavior of Soils

Abstract: Different classes of constitutive models have been developed to capture the time-dependent viscous phenomena (creep, stress relaxation, and rate effects) observed in soils. Models based on empirica...

An improved method to analyze the stress relaxation of ligaments following a finite ramp time based on the quasi-linear viscoelastic theory.

Abstract: The quasi-linear viscoelastic (QLV) theory proposed by Fung (1972) has been frequently used to model the nonlinear time- and history-dependent viscoelastic behavior of many soft tissues. It is common to use five constants to describe the instantaneous elastic response (constants A and B) and reduced relaxation function (constants C, tau 1, and tau 2) on experiments with finite ramp times followed by stress relaxation to equilibrium. However, a limitation is that the theory is based on a step change in strain which is not possible to perform experimentally. Accounting for this limitation may result in regression algorithms that converge poorly and yield nonunique solutions with highly variable constants, especially for long ramp times (Kwan et al. 1993). The goal of the present study was to introduce an improved approach to obtain the constants for QLV theory that converges to a unique solution with minimal variability. Six goat femur-medial collateral ligament-tibia complexes were subjected to a uniaxial tension test (ramp time of 18.4 s) followed by one hour of stress relaxation. The convoluted QLV constitutive equation was simultaneously curve-fit to the ramping and relaxation portions of the data (r2 > 0.99). Confidence intervals of the constants were generated from a bootstrapping analysis and revealed that constants were distributed within 1% of their median values. For validation, the determined constants were used to predict peak stresses from a separate cyclic stress relaxation test with averaged errors across all specimens measuring less than 6.3 +/- 6.0% of the experimental values. For comparison, an analysis that assumed an instantaneous ramp time was also performed and the constants obtained for the two approaches were compared. Significant differences were observed for constants B, C, tau 1, and tau 2, with tau 1 differing by an order of magnitude. By taking into account the ramping phase of the experiment, the approach allows for viscoelastic properties to be determined independent of the strain rate applied. Thus, the results obtained from different laboratories and from different tissues may be compared.

A Model Treating Tensile Deformation of Semicrystalline Polymers: Quasi-Static Stress−Strain Relationship and Viscous Stress Determined for a Sample of Polyethylene

Abstract: Tensile deformation of semicrystalline polymers follows a common scheme with changes in the mechanism at critical strains. Choosing a poly(ethylene-co-12% vinyl acetate) (PEVA12) as an example, we measured true stress−strain relationships at constant strain rates, determined the elastic and plastic part of imposed strain in step-cycle experiments, and followed the stress relaxation at fixed strains. On the basis of the general observations, a model was constructed and then used for a description of the properties of PEVA12. The model treats the stress as arising from three contributions: quasi-static stresses originating from the stretched network of entangled chains in the fluid regions and from the force-transmitting skeleton of crystallites, plus the viscous forces described by Eyring's equation. Adjustment of the measured data to the model provides a decomposition of the stress in the three parts. With increasing strain the dominance shifts from the crystal- to network-transmitted stress, while the v...

Strain measurements by convergent-beam electron diffraction: The importance of stress relaxation in lamella preparations

Abstract: Local convergent-beam electron diffraction (CBED) patterns have been acquired on focus ion beam prepared samples in order to determine the strain field generated by a NiSi layer in a n-MOS transistor A broadening of the high order Laue zone lines in the transmitted disk of CBED patterns is observed when approaching the NiSi∕Si interface We show that this broadening is mainly due to the atomic plane bending that occurs as a result of the stress relaxation during the preparation of the thin lamella From the analysis of this relaxation, we are able to determine the initial stress state of the bulk structure The presented CBED procedure appears to be a promising tool to measure the strain and stress in any layer or structure deposited on a crystalline substrate

Three-dimensional multiparticle cell simulations of deformation and damage in sphere-reinforced composites

Abstract: The tensile loading of an elasto–plastic matrix reinforced with elastic spheres was studied by means of the three-dimensional finite element analysis of periodic multiparticle cells containing a random dispersion of thirty identical spheres. Damage in the form of void nucleation, growth and coalescence in the matrix was included with the aid of the modified Gurson model. It was found that damage was early localized in regions between spheres closely packed along the deformation axis: the strain concentration in these regions nucleates voids, which grew driven by the tensile hydrostatic stresses. The stress relaxation induced by void nucleation and growth reduced significantly the flow stress of the composite, and eventually a maximum in the stress–strain curves was observed. The multiparticle cell results were compared with those obtained from single-particle axisymmetric simulations, and the effect of reinforcement volume fraction was analyzed as well as that of the matrix damage parameters on the composite behavior.

Yielding and flow of sheared colloidal glasses

Abstract: We have studied some of the rheological properties of suspensions of hard-sphere colloids with particular reference to behaviour near the concentration of the glass transition. First we monitored the strain on the samples during and after a transient step stress. We find that, at all values of applied step stress, colloidal glasses show a rapid, apparently elastic, recovery of strain after the stress is removed. This recovery is found even in samples which have flowed significantly during stressing. We attribute this behaviour to 'cage elasticity', the recovery of the stress-induced distorted environment of any particle to a more isotropic state when the stress is removed. Second, we monitored the stress as the strain rate of flowing samples was slowly decreased. Suspensions which are glassy at rest show a stress which becomes independent of as . This limiting stress can be interpreted as the yield stress of the glass and agrees well both with the yield stress deduced from the step stress and recovery measurements and that predicted by a recent mode coupling theory of sheared suspensions. Thus, the behaviours under steady shearing and transient step stress both support the idea that colloidal glasses have a finite yield stress. We note however that the samples do exhibit a slow accumulation of strain due to creep at stresses below the yield stress.

Atomistic Simulations of End-Linked Poly(dimethylsiloxane) Networks: Structure and Relaxation

Abstract: The structure and elastic moduli of end-cross-linked poly(dimethylsiloxane) (PDMS) networks are studied using molecular dynamics (MD). The systems consist of 2000 PDMS chains of length 20 monomers and 1000 chains of length 40 monomers with varying amounts of cross-linker molecules. The networks are formed dynamically within the MD simulations. Starting from an equilibrated melt, tetrakis(dimethylsiloxy)silane cross-linkers are attached randomly to a fraction of the chain ends. When a free end comes within a short capture distance from an unsaturated cross-linker, a bond is formed between the chain and the cross-linker. The kinetics of the cross-linking process are studied as a function of the stoichiometry of the number of cross-linkers and are found to agree with earlier predictions. Stress relaxation simulations are performed on the fully formed networks by straining each system at four different rates and recording the tensile stress during the relaxation period. Results for the elastic moduli of the n...

Misfit dislocation formation in the AlGaN∕GaN heterointerface

Abstract: Heteroepitaxial growth of AlxGa1−xN alloy films on GaN results in large tensile strain due to the lattice mismatch. During growth, this strain is partially relieved both by crack formation and by the coupled introduction of dense misfit dislocation arrays. Extensive transmission electron microscopy measurements show that the misfit dislocations enter the film by pyramidal glide of half loops on the 1∕3⟨1123⟩∕{1122} slip system, which is a well-known secondary slip system in hcp metals. Unlike the hcp case, however, where shuffle-type dislocations must be invoked for this slip plane, we show that glide-type dislocations are also possible. Comparisons of measured and theoretical critical thicknesses show that fully strained films can be grown into the metastable regime, which we attribute to limitations on defect nucleation. At advanced stages of relaxation, interfacial multiplication of dislocations dominates the strain relaxation process. This work demonstrates that misfit dislocations are important mec...

Cyclic deformation behavior of an epoxy polymer. Part I: Experimental investigation

Abstract: A series of uniaxial cyclic tests were carried out on solid cylindrical specimens of an epoxy resin, Epon 826/Epi-Cure Curing Agent 9551. The focus of the study was to investigate time-dependent viscoelastic behavior of this thermosetting polymer material under cyclic loading and to develop a constitutive model with the capabilities to simulate the observed deformation response. The tests include stress-controlled or strain-controlled cyclic loading with/without mean stress or mean strain at various amplitudes and loading rates. It was found that the cyclic stress-strain response of this material is amplitude-dependent and rate-dependent, and the response to axial tension is different from that in compression. The stress-strain loops exhibit more pronounced nonlinearity with high amplitudes or low loading rates. For stress-controlled cyclic loading with mean stress, ratcheting strain is accumulated, which is of viscoelastic nature, and this is confirmed by its full recovery after load removal. For strain-controlled cyclic loading with mean strain, the mean stress relaxation occurs, which contributes to the observed longer life in comparison to the stress-controlled cyclic loading with mean stress. Polym. Eng. Sci. 44:2240–2246, 2004. © 2004 Society of Plastics Engineers.

In situ measurements of the critical thickness for strain relaxation in AlGaN∕GaN heterostructures

Abstract: Using in situ wafer-curvature measurements of thin-film stress, we determine the critical thickness for strain relaxation in AlxGa1−xN∕GaN heterostructures with 0.14⩽x⩽1. The surface morphology of selected films is examined by atomic force microscopy. Comparison of these measurements with critical-thickness models for brittle fracture and dislocation glide suggests that the onset of strain relaxation occurs by surface fracture for all compositions. Misfit-dislocations follow initial fracture, with slip-system selection occurring under the influence of composition-dependent changes in surface morphology.

Dynamics of glass-forming liquids. IX. Structural versus dielectric relaxation in monohydroxy alcohols.

Abstract: The prominent Debye-type but non-Arrhenius dielectric relaxation is a feature common to many monohydroxy alcohols in their liquid state. Although this exponential process is often considered to reflect the primary structuralrelaxation, only a faster, smaller, and nonexponential relaxation peak correlates with viscous flow and mechanical relaxation. We provide dielectric relaxation data for 2-methyl-1-butanol, 2-ethyl-1-hexanol, and 3,7-dimethyl-1-octanol across ten decades in time. Based on these and previous results, we show that there exists a variety of dielectric to mechanical relaxation time ratios in the viscous regime, but a universal value of 100 for that ratio appears to evolve in the high temperature limit. The temperature dependence for both the relaxation time and strength of the Debye peak differs from the typical behavior of structural dynamics in terms of the α process. The implications of these findings for rationalizing the Debye-type dielectric process of hydrogen-bonded liquids are discussed.

Elastic-plastic wave profiles in cyclotetramethylene tetranitramine crystals

Abstract: The explosive molecular crystal cyclotetramethylene tetranitramine was studied in three orientations in a set of plate impact experiments; the orientations studied were {110}, {011}, and {010} in P21/n space group. The elastic–plastic shock response was measured using laser interferometry. The measured particle velocity profiles showed elastic precursor decay typical of a stress relaxing material. There is anisotropy in elastic shock strength and decay. The amount of precursor decay with propagation distance and stress relaxation behind the elastic shock varied among the orientations. The {010} orientation had larger elastic precursors than did the other two orientations; the {010} crystal does not have the regular plastic deformation mechanisms available to it. Elastic Hugoniots were obtained from the measurements. The inelastic deformation mechanisms may vary with orientation.

Nonlinear viscoelasticity in rabbit medial collateral ligament.

Abstract: The goal of this study was to characterize the vis- coelastic behavior of the rabbit medial collateral ligament (MCL) at multiple levels of strain (between 0% and»5%) and their corre- sponding stresses (between 0 and»55 MPa) for stress relaxation and creep, respectively. We hypothesized that in the rabbit MCL the rate of stress relaxation would be strain dependent and the rate of creep would be stress dependent. Thirty MCLs from 15 rabbits were tested ex vivo for this study. Results show that within the physiologically relevant region of ligament behavior, the rate of stress relaxation is strain dependent in the rabbit MCL, with the rate of relaxation decreasing with increasing tissue strain. The rate of creep is stress dependent in the rabbit MCL, with the rate of creep decreasing with increasing stress. These results support our hypothesis, with the greatest nonlinearities in a physiologically relevant region of loading. As such, these nonlinearities should be considered when quantifying ligament viscoelastic behavior with a rabbit model.