Showing papers on "Stress relaxation published in 1990"

Cone-and-plate rheometry of yield stress fluids. Study of an aqueous gel

Abstract: This work particularly focuses on the rheometric study of a physical gel exhibiting a yield stress. The measurements were carried out in a cone—plate configuration using two different types of rheometer working under controlled torque or under controlled velocity. Shear creep, constant shear rate, and stress relaxation tests have been performed. Measurements of apparent viscometric properties were conducted at the same time as observation of the strain field in the sample. Observing the strain field enables us to confirm the reliability of the interpretation of the results and also to estimate the true shear rate in the fluid. It is shown how the determination of shear rheological properties can be affected by anomalous phenomena such as fracture and slip at the wall. The influence of roughness of the tool surfaces and of evaporation shows up. The results presented in this study show how some rheometrical measurements of the yeild stress and the microstructural interpretations given, may be erroneous. Some recommendations are made in order to improve current rheometrical tests and their interpretation. A log—log graph with typical shear stress-shear rate measurements and their corresponding strain fields is given: it should be used as a guideline in yield stress fluids rheometry. In addition it is made clear that visual observation of the sheared sample is a key technique. A protection which completely eliminates evaporation is suggested. It is shown that the measurement of residual stress in stress relaxation tests may be a convenient means of determining the value of the yield stress.

Relaxation of Polymerization Contraction Shear Stress by Hygroscopic Expansion

Abstract: We studied relaxation by hygroscopic expansion of the interfacial polymerization shear stress of bonded resin composites. In the experimental set-up, resin composite-cured-to-glass strips bent due to the polymerization shrinkage. The strips were stored wet or dry. The curvatures of the bent strips were recorded, by the scanning of the glass surfaces with a contact profilometer, periodically over a period of two months. From the measured deflections, we calculated the maximum tangential bending stress in the resins near the adhesive interface, which provided an estimation for the maximum shear stress values occurring at both ends of the strips. In the particular experimental set-up, the shear stresses in Bis-GMA/TEGDMA and urethane dimethacrylate-based resins were either fully relieved or converted into an "expansion stress" by hygroscopic expansion. The hydrophobic tricyclodecane dimethacrylate-based resins showed very little stress relaxation.

The onset of non-Newtonian rheology of silicate melts

Abstract: The viscoelastic behavior of silicate melts has been measured for a range of compositions (NaAlSi3O8, NaCaAlSi2O7, CaMgSi2O6, Li2Si4O9, Na2Si4O9, K2Si4O9, Na2Si3O7, K2Si3O7 and Na2Si2O5) using the fiber elongation method. A1l compositions exhibit Newtonian behavior at low strain-rates, but non-Newtonian behavior at higher strain-rates, with strain-rate increasing faster than the applied stress. The decrease in shear viscosity observed at the high strain-rates ranges from 0.3 to 1.6 log10 units (Pa s). The relaxation strain-rates, έrelax, of these melts have been estimated from the low strain-rate, Newtonian, shear viscosity, using the Maxwell relationship; έrelax=τ −1=(ηs/G∞)−1. For all compositions investigated, the onset of non-Newtonian rheology is observed at strain-rates 2.5+0.5 orders of magnitude less than the calculated relaxation strain-rate. This difference between the non-Newtonian onset and the relaxation strain-rate is larger than that predicted by the single relaxation time Maxwell model. Normalization of the experimental strain-rates to the relaxation strain-rate predicted from the Maxwell relation, eliminates the composition. and temperature-dependence of the onset of non-Newtonian behavior. The distribution of relaxation in the viscoelastic region appears to be unrelated to melt chemistry. This conclusion is consistent with the torsional, frequency domain study of Mills (1974) which illustrated a composition-invariance of the distribution of the imaginary component of the shear modulus in melts on the Na2O-SiO2 join. The present, time domain study of viscoelasticity contrasts with frequency domain studies in terms of the absolute strains employed. The present study employs relatively large total strains (up to 2). This compares with typical strains of 10−8 in ultrasonic (frequency domain) studies. The stresses used to achieve the strain-rates required to observe viscoelastic behavior in this study approach the tensile strength of the fibers with the result that some of our experiments were terminated by fiber breakage. Although the breakage is unrelated to the observation of non-Newtonian viscosity, their close proximity in this and earlier studies suggests that brittle failure of igneous melts, may, in general, be preceded by a period of non-Newtonian rheology.

Relaxation of strained InGaAs during molecular beam epitaxy

Abstract: Relaxation of strained InxGa1−xAs films grown on GaAs substrates has been measured in situ during molecular beam epitaxy growth by reflection high‐energy electron diffraction (RHEED). Growth is found to be layer by layer up to a strain‐dependent ‘‘critical’’ thickness where three‐dimensional clusters with {114} facets form. The onset of cluster growth is simultaneous with lattice relaxation as measured by RHEED. The relaxation during growth is compared with the Dodson–Tsao model for strained‐layer relaxation [Appl. Phys. Lett. 53, 1325 (1987)]. Two distinct mechanisms for relaxation were found depending on film strain. An activation energy for relaxation was measured to be 4.4 eV for a film strain of 2.3%. The relaxation deviated from the Dodson–Tsao model for nongrowth conditions.

Development and relaxation of stress in surface layers; Composition and residual stress profiles in γ′-Fe 4 N 1−x layers on α-Fe substrates

Abstract: Composition-depth and (residual) stress-depth profiles were investigated in two different γ′-Fe4N1−x surface layers on α-iron substrates. X-ray diffraction analysis was applied. Determination of the actual lattice-spacing depth profiles required correction for the effect of penetration of the X-rays. The dependence of the nitrogen content in γ′-Fe4N1−x surface layers on depth below the surface corresponds with that expected for diffusion-controlled layer growth. The formation of porous grain boundaries (channels) in the surface-adjacent part of the layers provides the possibility of nitrogen uptake at “internal” surfaces during nitriding, leading to the development of concave concentration-depth profiles in this part of the layers. Stress buildup in the massive interface-adjacent part of γ′-Fe4N1−x layers on a-Fe substrates originates from both the presence of a concentration-depth gradient and the difference in thermal contraction between layer and substrate on cooling from the nitriding temperature to the measurement temperature. Channel/pore development along grain boundaries in the surface-adjacent part of the layers provides a mechanism for stress relaxation during nitriding. On cooling, the channels can also accommodate part of the thermally induced misfit. Lateral strain gradients in the most severely porous part of the layers are revealed by the X-ray diffraction analysis.

Viscoelastic material characterization and modeling for polyethylene

Abstract: An extensive set of stress relaxation and constant strain rate tests for characterizing the mechanical responses of a medium density polyethylene and a high density polyethylene that are commonly used in natural gas distribution piping is described and analyzed. The development of coherent master curves for the relaxation modulus, maximum stress, and the time-to-failure for pressurized pipes through a combination of both horizontal and vertical shifting is presented. The relaxation data are used to develop a nonlinear Viscoelastic material model. The model is assessed by making comparisons of the predicted stress-strain response with the measured response in the constant strain rate tests.

Fundamentals of flakeboard manufacture: viscoelastic behavior of the wood component.

Abstract: Theories of the viscoelastic behavior of amorphous polymers are reviewed and are used to describe the density gradient formation in flakeboard. This technique utilizes measured temperature and gas pressure at discrete locations inside a flake mat during hot pressing to predict the glass transition temperature of wood as a function of press time. The difference between the flake temperature and the predicted glass transition temperature is a relative indicator of the amount of flake deformation and stress relaxation at a location in the mat. A knowledge of the stress history imposed in the mat is then used to relate flake deformation and stress relaxation to the formation of a density gradient. This analysis allows for a significant portion of the density gradient to develop after the hot press has closed. Experimental data for various density gradients support the theories presented here.

Enhanced strain relaxation in Si/GexSi1−x/Si heterostructures via point‐defect concentrations introduced by ion implantation

Abstract: It is shown that strain relaxation during annealing of Si/GexSi1−x/Si heterostructures is significantly enhanced if the strained GexSi1−x layers are implanted with p (B) or n (As) type dopants below the amorphization dose. Comparison of strain relaxation during in situ annealing studies in a transmission electron microscope between unimplanted and implanted structures reveals that the latter show residual strains substantialy below those for unimplanted structures. We propose that this enhanced relaxation is caused by increased dislocation nucleation probabilities due to the high point‐defect concentrations arising from implantation.

Plastic Deformation of Al-Pd-Mn Icosahedral Quasicrystal

Abstract: Compression experiments have been performed for coarse-grained Al72Pd20Mn8 icosahedral quasicrystals at high temperatures. Above 950 K, specimens became deformed due to localized slip, exhibiting pronounced yield drop. Analysis of the stress-relaxation curve at the lower yield point has shown that the internal stress accounts for about half of the lower yield stress. From the strain-rate sensitivity and the temperature dependence of the effective stress, the activation volume and the activation enthalpy of plastic deformation have been analyzed. The obtained results have been interpreted in terms of dislocation glide in quasicrystals.

Physical and chemical cross-linking effects in polyurethane elastomers

Abstract: A series of polyester-urethane block copolymers of various molecular weights was prepared via a two-step polymerization process. The prepolymer composition was kept constant in all the samples, while the NCO/OH ratio during the chain extension was varied from 0.9 to 1.2. Chemical and physical cross-linking effects were studied by means of F.T.I.R spectroscopy, swelling, and elastic behavior. Equilibrium stress-strain measurements and tensile-retraction tests were carried out to examine the elastomeric behavior of the materials tested. The extent of agreement between microscopic and macroscopic behavior was then evaluated.

Relaxation of internal stresses in optical fibers

Abstract: A method for measuring the relaxation of internal stresses of optical fibers is developed. The authors discuss a few principal previous publications on the subject of annealing and relaxation of glass. The internal stress measurement setup is explained, and useful relations are given. It is pointed out that the relaxation of internal stresses of optical fibers can be represented by the equations of J. De Bast and P. Gilard (1963) sigma = sigma /sub o/exp-(at)/sup b/ (b is constant in the annealing region and a depends on temperature). It is shown that the internal stress profile change is negligible at temperatures of use after 25 years. Hence, the initial internal stress profile, i.e. the refractive index profile, of a high-tension MCVD fiber is not modified during the optical-fiber cable lifetime. >

Mechanical properties of the soybean cotyledon and failure strength of soybean kernels

Abstract: The relaxation modulus, ultimate compressive and tensile strength of soybean cotyledon were determined using stress relaxation, compression, and bending tests. The master curve for the cotyledon relaxation modulus was derived using the time-moisture and time-temperature shifting technique. Both the relaxation modulus and the ultimate strength were lower at higher moisture contents during drying at higher temperatures. A two-term Maxwell model was derived to describe the behavior of the soybean cotyledon. The soybean cotyledon can be treated as a thermo-hydro-rheologically simple viscoelastic material.

Diffusional relaxation and void growth in an aluminum interconnect of very large scale integration

Abstract: Using the previously obtained stress distributions in an Al line after relaxation by plastic deformation, another possible relaxation process by diffusion was analyzed. Even after this relaxation occurs, some stresses still remain in the Al line. If these remaining stresses are large enough, they can be responsible for the growth of voids causing line failure. Using a theory for diffusional growth of grain‐boundary voids, the time to failure of the Al line was estimated analytically. The previous and present papers together constitute a full analysis of the so‐called ‘‘stress migration’’ phenomenon.

The high strain rate superplastic deformation mechanisms of mechanically alloyed aluminum IN90211

Abstract: The tensile behavior of mechanically alloyed (dispersion strengthened) IN90211 was investigated at strain rates between 0.0001 and 340 s−1 at temperatures between 425 and 475 °C. At strain rates above 0.1 s−1, superplastic elongations were obtained (maximum elongation of 525% at 475 °C and 2.5 s−1). Superplastic elongations were found to result from grain boundary sliding. The data were analyzed assuming that a threshold stress resists dislocation motion. The threshold stresses were obtained assuming n = 2 (grain boundary sliding) or n = 3 (solute drag) for the stress exponent. Both assumptions provided equally credible values for a temperature-dependent threshold stress between 1% and 20% of the Orowan looping stress. The n = 3 threshold stresses agreed with load relaxation data, but the n = 2 values corresponded to the lower limit of the superplastic deformation regime, as indicated by creep tests at lower strain rates. Based upon the threshold stress theories in the literature, the threshold stresses are suggested to arise from a combination of local and general climb of lattice dislocations over particles. Consideration of the activation energies, details of flow behavior, and stress relaxation experiments provided strong evidence for the n = 3 solute drag mechanism to be the rate-limiting process in the superplastic deformation regime.

Strain rate sensitivity and transient behaviour in an AlMgSi alloy

Abstract: The effect of strain rate changes on the flow stress of an AlMgSi alloy at strains prior to the onset of the Portevin-Le Chatelier effect has been studied. Measurements of the instantaneous and steady state strain rate sensitivity of the flow stress and the flow stress transient period were carried out as a function of strain and strain rate. Negative values of the steady state strain rate sensitivity were reached prior to the onset of localised yielding, which occurred by the growth of local strain rate oscillations. The results are interpreted in terms of a constitutive flow model which takes into account time dependent changes in the local solute composition at dislocations accompanying a strain rate change.

Observed stress-strain behaviour of remoulded saturated clay under plane strain conditions

Abstract: The stress-strain behaviour of saturated, remoulded clay has been investigated in a cuboidal plane strain apparatus. The tests covered a variety of strain paths ranging from proportional paths starting from a slurry state to complex paths with up to two sharp bends. Both normally consolidated and overconsolidated samples were investigated. As a result of these observations, a list of material properties has been formulated. The most important properties refer to: the dependence of the stress response on the stress level, the asymptotic property of clay, the influence of the history of straining on the stress response, and the ohservation that the stress paths reaching the state boundary surface may move along this surface. The list of material properties is thought to be a guideline for development of constitutive theories. It provides also a useful framework within which the behaviour of natural clays may be qualitatively described. Le comportement contrainte/deformation de l'argile saturee remanise a et...

A comparison of relaxation time distributions obtained from dynamic light scattering and dynamic mechanical measurements for high molecular weight polystyrene in entangled solutions

Abstract: With both techniques, the dynamical processes probed at long times are characterized by a similar wide range of relaxation times and that the later increases strongly with concentration and molecular weight. The distribution of relaxation times from dynamic light scatterint is much more sensitive in the slow part of the spectrum to changes in solvent quality

Strain relaxation of CdTe(100) layers grown by hot‐wall epitaxy on GaAs(100) substrates

Abstract: The strain relaxation of CdTe(100) layers grown on GaAs(100) substrates by hot‐wall epitaxy was investigated by measurement of optical properties, x‐ray analysis, and transmission electron microscopy. It is considered from transmission electron microscopy observation that relaxation of most of the strain due to lattice mismatch occurred at the interface. However, a small amount of strain, of the order of 10−3, remained in layers thicker than 0.7 μm, and it was relaxed as the layer thickness increased. The residual strain of 4×10−4, which exists in layers thicker than 10 μm, was due to the difference between the thermal expansion coefficients of the layer and the substrate. Moreover, for layers thicker than 17 μm, split ground (n=1) and first excited (n=2) free‐exciton states due to internal strain have for the first time been observed by photoluminescence and reflectance spectroscopy. The results show that CdTe layers with excellent crystallinity and homogeneity in strain are obtained.

Characterization of gellan gels by uniaxial compression, stress relaxation and creep

Abstract: The rheological properties of 0.5–2.5% gellan gels were evaluated by uniaxial compression, stress relaxation and creep tests. The gel's strength was in the range of 0.1–1 kg.cm-2 and their deformability modulus 1–6 kg.cm-2. The asymptotic modulus determined in relaxation tests at two strains and the asymptotic creep compliance determined under two loads indicated that gellan gels have a yielding structure. The strains sustained in creep were considerably higher than the failure strain in uniaxial compression. Weight loss due to syneresis was on the order of 4–38% depending on the gum concentration, the deformation level in relaxation or the load in the creep tests.

Relaxation of entangled polymers at the classical gel point

Abstract: We examine the relaxation behaviour of an entangled cross-linked polymer gel as it approaches the gel point in mean field (Flory-Stockmayer) percolation. The calculation is based on a tube model for the topological interactions in which stress is lost via hierarchical fluctuation of the primitive paths between cross-links. The decay time of a segment is calculated via a recursion relation which has an analytic solution near the gel point. The startling conclusion is that all clusters relax in a finite time T~ giving a relaxation modulus G (t ) = Go 03B3-2[03B1-1 ln (T~/t)]4, where 03B1 counts the number of entanglements between cross-links and 03B3 is a constant of order unity. For timescales much shorter than T~ this may resemble a weak power law. The unphysically rapid relaxation of the largest clusters is prevented by a transition to percolation statistics at long length scales. The timescale separating the two regimes is close to

Phase transformations and stress relaxation in γ′-Fe4N1-x surface layers during oxidation

Abstract: Compound layers composed of γ′-Fe4N1-x on α-Fe substrates were oxidized in oxygen-containing gas atmospheres at 603 K. The microstructural changes were analyzed applying X-ray diffraction, dilatometry, and light, scanning electron, and transmission electron microscopy. Compositional changes were traced, in particular, with scanning Auger electron spectroscopy. Dual-phase, Fe3O4 (magnetite) and α-Fe2O3 (hematite), oxide layers formed at the compound layer surface. At the interface between the oxide and nitride layers, e-Fe2N1=x nucleated because of a local nitrogen enrichment caused by the conversion of γ′ nitride into oxide. The volume of each of the three phases formed (hematite, magnetite, and e nitride) increased with the square root of oxidation time, indicating solid-state diffusion-controlled layer growth. No evidence was obtained for the existence of γ′ or e oxynitrides. Within the γ′ layer, ferrite precipitated during oxidation as a consequence of an iron supersaturation of γ′ nitride due to a production temperature higher than the oxidation temperature. During this iron precipitation, stress relaxation occurred, as was concluded from X-ray diffractometric and dilatometric analyses. The stress relaxation was rate-controlled by nitrogen diffusion in γ′ nitride. The present findings were also used to explain microstructural changes during (commercial) oxidation of e-Fe2(N,C)1-x compound layers.

Measurement of three dimensional stress and modeling of stress induced migration failure in aluminium interconnects

Abstract: Stress-induced migration failure in fine aluminium interconnects is explained by measurements of mechanical stress and a kinetic failure model. An advanced stress-measurement method using X-ray diffractometry revealed the actual three-dimensional mechanical stresses and open failure test results were well correlated in terms of the covering insulator, metal linewidth, test temperature, and time-dependent relaxation profile. A kinetic model, successfully reproduced the non-Arrhenius behavior of the failure rate under various parameters. An exponent of 4 and an activation energy of 1.0-1.4 eV were obtained. The microscopic mechanism of stress migration along with stress relaxation, and the practical advantage of the modeling are discussed. >

Step Strain Deformations for Viscoelastic Fluids: Experiment

Abstract: Shear stress and normal stress relaxation data were collected for a range of shear strains for a polystyrene–dibutyl phthalate solution. Data from single‐step shear strain experiments were used to test time–strain factorability and to check on the applicability of the Lodge–Meissner rule. Significant departures from time–strain factorability were found, but the shear and normal stress data were in excellent agreement with the Lodge–Meissner prediction. Data from double‐step shear strain experiments were used to check the predictions of the K–BKZ and Doi–Edwards theories. It was found that these theories do not generally provide adequate descriptions of double‐step shear strain deformations.

Improved Analysis for Flexural Creep with Application to Sialon Ceramics

Abstract: By using a statistical least-squares method to minimize the differences between predicted and measured load-point displacement rates from four-point bend specimens, power-law creep parameters for tension and compression were estimated. An alternative but simpler method of estimating power-law creep parameters from flexural creep data is also proposed. This method entails the direct measurements of steady-state creep strain rates at two stress levels by an indentation technique. Based on a closed-form solution, the power-law creep parameters could then be estimated from both the measured neutral axis locations and curvature rates. The results from these two methods compare favorably with one another, and with the simple compressive creep data. Both methods yield a high stress exponent of about 14 for tension and a stress exponent of about unity for compression. Cavitation-enhanced creep in tension and diffusional creep in compression are responsible for this asymmetric behavior. [Key words: sialons, flexure, creep, tensile, compression.]

Nucleation rate and glide velocity of misfit dislocations in Si1−xGex/(100) Si heterostructures

Abstract: Expressions are presented for the rate of strain relaxation, misfit dislocation nucleation, and propagation in strained Si1−xGex/(100)Si heterostructures. Independent measurements of misfit dislocation nucleation and 60° type a/2 〈110〉 dislocation glide velocity in the temperature range 450–1000 °C have led to a model which characterizes the kinetics of strain relaxation for 0