Showing papers on "Stress relaxation published in 1982"
TL;DR: In this paper, a three dimensional constitutive equation is derived based on the Perzyna's elasto-viscoplastic theory and Cambridge theory, which can explain the behavior not only such timedependent behavior as creep, stress relaxation and strain rate effect, but also as secondary compression, aging and isotropic stress relaxation.
263 citations
TL;DR: In this paper, the authors examined the competition between shear deformation and crazing in thin films of polystyrene-acrylonitrile co-polymers and polymer blends.
Abstract: Whereas thin films of some polymers such as polystyrene readily form crazes when strained in tension, thin films of other polymers such as polycarbonate rarely exhibit crazing under the same testing conditions; the polymers that rarely craze tend to form regions of shear deformation instead. Polymers such as polystyrene-acrylonitrile which lie between these two extremes of behaviour may exhibit both modes of deformation. Thin films suitable for optical and transmission electron microscopy (TEM) of six such co-polymers and polymer blends have been prepared. After straining, the nature of the competition between shear deformation and crazing is examined by TEM. It is found that in these polymers many crazes have tips which are blunted by shear deformation. This process leads to stress relaxation at the craze tip, preventing further tip advance. In this way short, but broad, cigar-shaped crazes are formed. Examination of the deformation at crack tips in the same polymers shows more complex structures, the initial high stress levels lead to chain scission and fibrillation but as the stress drops, shear becomes the dominant mechanism of deformation and the stress is relieved further. Finally, at long times under stress, chain disentanglement may become important leading to fibrillation and craze formation again. The nature of the competition is thus seen to be both stress and time dependent. Physical ageing of these polymers, via annealing below Tg, suppresses shear leading to the generation of more simple craze structures.
151 citations
TL;DR: In this article, the authors measured the stress due to thermal expansion anisotropy in polycrystalline Al2O3 and found that the maximum stress varied from 80 to 100 MPa, depending on the thermal history of the sample.
Abstract: The stress due to thermal expansion anisotropy in polycrystalline Al2O3 was measured. The broadening of spectroscopic R lines (692 and 693 nm, due to Cr3+ impurities) was used to measure the stresses (at 77 K) in samples with grain sizes of 50 to 150 μm that had been cooled, from 2150 K, at constant rates from 0.1 to 100 K/min. The maximum stress was found to vary from 80 to 100 MPa, depending on the thermal history of the sample. The results are compared to the predictions of a model based on stress relaxation by diffusional creep and are in good agreement for the dependence on cooling rate. No effect due to grain-size changes was observed due to the limited range of grain sizes accessible in this study.
138 citations
TL;DR: In this article, a simple model based on the theory of interaction between electrical double layers was proposed to compare the shear-modulus data for polystyrene latices at various volume fractions and sodium chloride concentrations.
Abstract: Shear-modulus data for polystyrene latices at various volume fractions and sodium chloride concentrations are compared with a simple model based on the theory of interaction between electrical double layers. Stress relaxation and low shear-rate viscosity are also discussed in the context of particle interaction forces.
136 citations
TL;DR: In this article, the authors investigated the role of grain boundaries at grain boundaries in primary creep and strain recovery in Si3N4/MgO alloys which were frozen under stress.
Abstract: Highly localized strain fields are observed at grain boundaries in crept specimens of Si3N4/MgO alloys which were frozen under stress. These fields disappear upon annealing. Unresolved asperities between the grain pairs appear to give rise to the strain field during deformation. Viscoelastic effects responsible for primary creep and strain recovery are explained in terms of grain-boundary sliding on the glassy interphase which is accommodated by the elastic strain arising at the asperities. Each boundary containing an asperity can be modelled as a simple Kelvin element. The spectrum of these boundaries within the bulk gives rise to a spectrum of relaxation times that is observed for the strain recovery effect. The highly stressed region at the asperity also gives rise to the higher chemical potential required to drive diffusional creep. Although the source of the asperities was not observed, the possibility of opposing ledges of either single or multiple interplanar height is discussed.
126 citations
TL;DR: In this article, a comparison of different methods of measuring the yield stress of liquids was made using an Instron 3250 Rheometer in several geometries (cone-plate, parallel plate and eccentric disk) in shear flow and stress relaxation.
Abstract: An analysis has been made of different methods of measuring the yield stress of liquids. In the experimental program, a comparison is made of measurements of the yield stress using an Instron 3250 Rheometer in several geometries (cone-plate, parallel plate and eccentric disk) in shear flow and stress relaxation, a laboratory vane and a cone penetrometer. Good agreement has been obtained between the shear flow data and the laboratory vane, while stress relaxation appears to underestimate the yield stress.
123 citations
TL;DR: In this article, the relaxation of thermal stress in a thin film adhering to a substrate of differing expansion coefficient is discussed, and the implications of this for long-range material transport in the film are discussed.
87 citations
TL;DR: In this article, the authors analyzed the time to fracture by coupled diffusion and power-law creep, using approximate methods, both for uniaxial tension and for axial tension plus a superimposed hydrostatic tension.
Abstract: Creep fracture by coupled diffusion and power-law creep is analysed, using approximate methods, both for uniaxial tension and for axial tension plus a superimposed hydrostatic tension. Over a wide range of stress and temperature voids grow predominantly by diffusion when they are small and by power-law creep when they are large, so that the time to fracture is determined by a contribution from each mechanism. A change of temperature, stress, or stress state changes the relative contribution of each mechanism. The equations for void growth are integrated to give both the times and strains to fracture.
83 citations
TL;DR: In this article, a constitutive equation of creep for polycrystalline materials is developed by introducing a concept of a creep-hardening surface corresponding to the loading surfaces of plastic deformation.
81 citations
Book•
01 Oct 1982
TL;DR: A review of the state-of-the-art results can be found in this article, where the authors discuss the nature of residual stress and its measurement, as well as the effect of temperature on stress relaxation.
Abstract: Session I.- The Nature of Residual Stress and Its Measurement.- Session II.- Thermal and Transformation Stresses.- Load Induced Residual Stress Changes in Carburized Steel Computations and Experiments.- Residual Stresses and Distortion in Weldments-A Review of the Present State-of-the-Art-.- Thermal Modifications of Welding Residual Stresses.- Shot Peening and Residual Stresses.- Session III.- Measurement of Throughwall Residual Stresses in Large-Diameter Piping Butt Weldments Using Strain-Gauge Techniques.- X-Ray Methods for Measuring Residual Stress.- Measurements of Stress in the Interior of Solids with Neutrons.- Session IV.- Mechanical Relaxation Response of 21/4 Cr-1 Mo Steel.- Effect of Temperature on Stress Relaxation of Several Metallic Materials.- Thermal Relaxation in Autofrettaged Cylinders.- Session V.- The Numerical Simulation of Crack Growth in Weld-Induced Residual Stress Fields.- Linear Elastic Fracture Mechanics and Fatigue Crack Growth-Residual Stress Effects.- Stress Corrosion Crack Growth in the Presence of Residual Stresses.- The Relaxation of Residual Stresses During Fatigue.- Session VI.- The Generation of Residual Stresses in Metal-Forming Processes.- Stress Relaxation in Spring Materials.- Problems with Residual Stress in the Railroad Industry.- Measurement of Residual Stresses in Ammunition.- Session VII.- X-Ray Residual Stress Measurement Systems for Army Material Problems.- Residual Stresses and Stress Relieving in Uranium Alloys.- Residual Stresses in Resin Matrix Composites.- Residual Stress Measurement in Metal Matrix Composites.- Residual Stress Development in Ceramics.- Session VIII.- Determination of Residual Stress Distributions in the Interior of Technical Parts by Means of Neutron Diffraction.- Calculation of Quenching Stresses with and Without Transformation Effects.- X-Ray Measurements of Long-Range Strains: A Bridge Between Micromechanics and Macromechanics.- Stress Relaxation Characteristics and Data Utilization.
80 citations
TL;DR: In this article, shear stress relaxation data were measured for a range of shear strains on several entangled polymer liquids in order to test time-strain factorability and the Doi-Edwards predictions about strain dependence.
Abstract: Shear stress relaxation data were measured for a range of shear strains on several entangled polymer liquids in order to test time‐strain factorability and the Doi‐Edwards predictions about strain dependence. Undiluted samples and concentrated solutions of well‐characterized linear and star polybutadienes with narrow molecular weight distributions were used. Some departures from time‐strain factorability were found, but these appear to be nonsystematic, i.e., they do not correlate in any obvious way with the polymer structure. The strain dependence, however, follows the pattern observed by Osaki and Kurata for polystyrene solutions. The Doi‐Edwards prediction gives a good account of the results at intermediate entanglement densities (35,000≲cM≲150,000 for linear and star polybutadienes), but systematic departures are found at higher values. The causes of the latter are unknown.
TL;DR: In this article, a new technique is proposed for determining a measure σi of the long range internal back stresses in a deformed material, based on the inelastic back strain measured as the difference between the maximum observable and the instantaneous back strain accompanying a reduction in stress.
TL;DR: The Laplace transform method of viscoelastic stress analysis is reviewed and applied to the sandwich seal and bead seal in this article, where the concept of thermorheological simplicity, the visco-elastic analogy, and the role of dilatational relaxation in uniaxial, biaaxial, and triaxial stress are discussed.
Abstract: The Laplace transform method of viscoelastic stress analysis is reviewed and applied to the sandwich seal and bead seal. The concept of thermorheological simplicity, the viscoelastic-elastic analogy, and the role of dilatational relaxation in uniaxial, biaxial, and triaxial stress are discussed. Methods of incorporating measured stress relaxation data into the analysis to provide a realistic description of glass behavior are detailed and closed-form solutions are derived for the stresses in seals.
TL;DR: In this article, the authors used Raman scattering measurements of the Si optical phonon spectrum to map the stress in silicon-on-sapphire device structures, and showed that devices defined by isolated island etch technique exhibit a stress relaxation extending about 1.5 μm from the edges of a 6μm wide Si stripe.
Abstract: Spatially resolved (∼1 μm) Raman scattering measurements of the Si optical phonon spectrum have been used to map the stress in silicon‐on‐sapphire device structures. Devices defined by an isolated island etch technique exhibit a stress relaxation extending about 1.5 μm from the edges of a 6‐μm‐wide Si stripe. Devices defined by a local oxidation of the Si exhibit a more uniform stress profile.
TL;DR: In this article, a closed-loop servo-hydraulic test system inside a cold room was used to investigate the unconfined compressive strength of transversely isotropic columnar-grained ice.
Abstract: Unconfined compressive strength of transversely isotropic columnar-grained ice has been investigated for loads applied normal to the longitudinal axis of the columns at the high homologous temperature of 0.96 Tm (Tm is the melting temperature) under truly constant strain and stress rates. A closed-loop, servo-hydraulic test system inside a cold room was used. Both the strain- and stress-rate dependences of upper yield stress can be expressed in terms of power laws. The observed strain-rate dependence of strength was found to be numerically the same as the dependence of viscous-flow rate on stress in constant stress creep tests at the same temperature. It is shown that the strain-rate sensitivity of yield strength compares well with previous results (obtained under constant cross-head rates using a conventional machine) only if the average strain rate to yield is used as the independent variable instead of the conventional nominal strain rate. The paper also discusses the strain and time aspects of the tests. It shows interdependence among values for compressive yield strength, strain rate, failure strain and time very similar to the interdependence among the corresponding values in tensile creep failures in metals, alloys and other polycrystalline materials at high temperatures. It is emphasized that the splitting type of brittle-like premature failure depends on the stiffness of the test system and should not be considered to be a fundamental material property. The concept of failure modulus is proposed for examining the ductile to brittle transition.
TL;DR: In this article, a generalised damage parameter for tertiary creep is proposed for remanent creep life prediction based on in situ strain assessment, which is shown that for relatively ductile materials the rupture life can be accurately related to a single strain or strain rate measurement without a knowledge of the rupture strain.
TL;DR: Room temperature compression creep and stress relaxation behaviour of normal as well as carbon fibre reinforced bone cement was studied and their properties compared and it was observed that increased deformation due to creep in 24 h was about 70% of the initial strain for normal PMMA and this could be reduced to 45% by carbon fibre reinforcement.
TL;DR: In this paper, the authors investigated the stress/creep rate relationship for single and polycrystalline lime at 1473 K and found that the change from dislocation to diffusional creep can be interpreted in terms of a change in dislocation.
Abstract: When the variation of the creep rate, ∊s, with stress, [sgrave], is described as ∊s α [sgrave]n, the stress/creep rate relationships are found to be similar for both single and polycrystalline lime at 1473 K. Since creep of single crystals can only occur by the generation and movement of dislocations, the observed transition in stress exponent, n, from ∼ 5 at high stresses to almost unity at low-stress levels cannot be interpreted in terms of a change from dislocation to diffusional creep processes. Moreover, the forms of the primary creep curves are similar, the primary stage can be restored by a recovery anneal, and incubation periods of zero creep rate are invariably recorded before creep recommences after small stress reductions during creep over the entire stress range studied for both the single and polycrystalline materials. The similarity of the behaviour patterns exhibited at all stress levels suggests that the recovery-controlled dislocation processes governing the creep properties diff...
TL;DR: In this article, an extensive fundamental investigation of the rheological properties and solid wall boundary condition shear stress of elastomers and elastomer-carbon black compounds has been carried out.
Abstract: An extensive fundamental investigation of the rheological properties and solid wall boundary condition shear stress of elastomers and elastomer–carbon black compounds has been carried out. The elastomers were an emulsion butadiene–styrene copolymer (SBR 1500) and a polybutadiene. Shear flow rheological properties were measured using a newly designed sandwich rheometer, in both constant shear rate and creep modes as well as in a capillary rheometer. A constant elongation rate rheometer for elastomers was developed. Stress relaxation measurements were also carried out in the sandwich rheometer. The shear viscosity of the gum elastomers exhibits a constant very high shear zero viscosity (8 × 108 Pa.s for SBR 1500 at 100°C) and decreases with increasing shear rate. The compounds exhibit yield values of similar magnitude to carbon black compounds of molten plastics. Only the SBR 1500 and its compounds were studied in the elongational flow mode. It was not possible to achieve a steady state in these experiments. An apparatus for measurement of shear stress as a function of velocity (shear rate) at a specified pressure was developed. The instrument, which we call a friction tester, was used not only to determine wall shear stress but to investigate the regime of flow and potentially determine conditions for the onset of slip. Evidence of changing flow regimes were found, and the implications discussed.
TL;DR: In this article, the creep of β-Co was studied at stresses of up to 5.0 MN m−2 over the temperature range 973 − 1176 K (0.55 − 0.67 Tm) and for grain sizes from 55 to 379 μm, conditions favorable for viscous creep mechanisms to dominate.
TL;DR: In this paper, the velocity of the swelling front is related to the stress exerted by the penetrant on the glassy matrix at the moving boundary and solvent stress is considered as the sum of the osmotic tension and of the differential swelling stress.
Abstract: The sorption of swelling penetrants in glassy polymers is modeled as a diffusion problem with moving boundary The velocity of the swelling front is related to the stress exerted by the penetrant on the glassy matrix at the moving boundary The solvent stress is considered as the sum of the osmotic tension and of the differential swelling stress The latter increases as the residual thickness of the glassy core is reduced, giving rise to an acceleration of the front velocity (Super Case II) It is shown that the stress relaxation properties in the swollen region must be taken into account in order to obtain a thickness-dependent Super Case II effect, which disappears in thick samples, according to experimental evidence All the observed sorption behaviors, including Fickian diffusion, Case II, and Super Case II sorption, are properly accommodated in the theory
TL;DR: In this article, an analytical model was developed to predict creep-fatigue endurance for constant-strain tensile dwell tests by the application of cavity growth mechanisms to periods of stress relaxation.
TL;DR: In this article, the redistribution of calculated welding stresses during different post weld heat treatments (PWHT) is numerically investigated for a girth-butt welded thin-walled pipe made of a micro-alloyed carbon manganese steel.
Abstract: The redistribution of calculated welding stresses during different post weld heat treatments (PWHT) is numerically investigated for a girth-butt welded thin-walled pipe made of a micro-alloyed carbon manganese steel. A finite element analysis with assumed rotational symmetry is performed. The influence of creep, relaxation and phase transformations is included. The amount of stress reduction is found to be mainly controlled by the holding temperature. Most of this reduction was observed to occur during the end of the heating period where the transient stresses followed the fall of the yield stress with temperature.
TL;DR: Measurements of stress relaxation in uniaxial extension and associated time‐dependent birefringence have been made on bovine fibrin film, prepared by gentle compaction of coarsefibrin clots, containing 13–22% fibr in plasticized with either aqueous buffer or glycerol.
Abstract: Measurements of stress relaxation in uniaxial extension and associated time-dependent birefringence have been made on bovine fibrin film, prepared by gentle compaction of coarse fibrin clots, containing 13–22% fibrin plasticized with either aqueous buffer or glycerol. Both unligated and ligated (i.e., with α-α and γ-γ ligation by fibrinoligase, factor XIIIa) films were studied. Both types showed two stages of stress relaxation, with time scales of approximately 10 and 103–104 s, respectively, with a plateau region between. In the plateau, the nominal (engineering) stress for ligated glycerol-plasticized film is proportional to In λ, where λ is the stretch ratio, up to λ ≅ 2, and it decreases with increasing temperature. For unligated glycerol-plasticized film, the stresses are smaller by a factor of one-half to one-third. For ligated film, the second stage of relaxation is relatively slight, and recovery after release of stress is often nearly complete. For unligated film, the second stage involves a substantial drop in stress, and after recovery there is a significant permanent set. A second relaxation for ligated film reproduces the first, but for unligated film it reproduces the first only if the initial relaxation is terminated before the second stage; otherwise, the second relaxation shows a weaker structure. The behavior of water-plasticized film is similar to that of glycerol-plasticized except that the second stage of relaxation occurs at shorter times. During the first stage of stress relaxation, up to about 100 s, the birefringence and the stress-optical coefficient increase; during the plateau zone of stress relaxation, the birefringence of ligated films is approximately constant and is proportional to 2λ2/(λ2 + 1) − 1, where λ is the stretch ratio. This dependence is predicted by a two-dimensional model in which rodlike elements in the plane of the film are oriented with independent alignment. During the final stage of stress relaxation, the birefringence of ligated films decreases slightly; that of unligated films decreases substantially, but less rapidly than the stress, corresponding to a further increase in the stress-optical coefficient. With additional information from small-angle x-ray scattering reported in an accompanying paper, the first stage of relaxation is attributed to partial release of bending forces in the fibers by orientation, accompanied by increased birefringence. The second stage is attributed, for ligated films, to an internal transition in the fibrin units accompanied by elongation of some of the fibers; and in the unligated films, to a combination of the latter transition with slippage of protofibrils lengthwise within the fiber bundles that causes some loss of orientation, which diminishes the birefringence.
TL;DR: In this article, the authors show that a midrib of higher fibrils forms by drawing fibril from the craze-matrix interface in the high stress region just behind the tip of a craze tip, which is the analogue of the bulge remaining on a macroscopic fibre.
TL;DR: The transition from linear stress-strain rate behavior (m = 1) to a nonlinear relationship (m > 1) at high stress has previously been observed and explained by transition state theory.
TL;DR: In this article, it was shown that the fractional volume increase due to cavity growth is equal to the measured creep strain in support with theoretical models, with increasing stress and as the power law creep regime is entered, the creep strain becomes progressively greater than the volume change.
TL;DR: In this paper, the effects of prior stretching and of stress relaxation on the velocity of sound and free retraction of stretched strips of vulcanized natural rubber have been investigated and it has been shown that these velocities increase with increasing strain, in agreement with earlier work.
Abstract: Measurements have been made of the velocity of sound and the velocity of free retraction for stretched strips of vulcanized natural rubber. Both of these velocities are found to increase markedly with increasing strain, in agreement with earlier work. The velocity of sound is shown to be related to the appropriate modulus of elasticity, defined by the slope of the curve relating true stress to tensile strain. Values obtained range from about 50 to about 800 m/s. The effects of prior stretching and of stress relaxation on the velocity of sound are shown to arise from corresponding changes in the modulus of elasticity at a given strain. The velocity of free retraction is shown to be directly related to the velocity of sound in the stretched strip and to the imposed tensile strain, increasing from zero up to about 100 m/s at high strains.
TL;DR: In this article, the authors studied the stress relaxation of a two-dimensional solid with the assumption that defects have been trapped in the sample and calculated the effective shear modulus and stress-strain relaxation rate for a variety of defect configurations, including large and small-angle grain boundaries and dislocation pairs.
Abstract: The stress relaxation of a two-dimensional solid is studied, with the assumption that defects have been trapped in the sample. The effective shear modulus and stress-strain relaxation rate are calculated for a variety of defect configurations, including large- and small-angle grain boundaries and dislocation pairs. Effects of dislocation climb on the long-term stability of the configurations are considered. The viscosity resulting from moving free dislocations and/or flow at the boundaries is described in a particular geometry. The response to a finite applied shear is discussed, in particular, nucleation of free dislocations and sliding of grain boundaries. The theory is applied to free-standing smectic-B films, and it is suggested that stress relaxation observed in these films may result from a dilute ''random neutral array'' of dislocations or from small-angle grain boundaries.
TL;DR: In this paper, the surface stress component of a moving uniformly distributed isolated heat input source is investigated by restricting the analysis to the two-dimensional plane stress case, and the thermal compressive stress is found to be roughly ten times the normal load for the assumed contact conditions.