Showing papers on "Stress relaxation published in 1973"

Investigation of domain structure in urethan elastomers by X-Ray and thermal methods

Abstract: Wide- and small-angle X-ray diffraction and differential scanning calorimetry have been used to gain some understanding of the extent of domain formation in polyurethans of varying urethan content. The process by which these hard domains and the rubbery polyester (or polyether) chains orient or crystallize upon stretching has been examined. It is shown that the hard-segment domains undergo continuous “restructuration” under stress, especially at elevated temperatures. This leads, in some cases, to extensive stress relaxation but results in highly oriented samples which have extremely high maximum tensile strengths.

High temperature creep of lithium zinc silicate glass-ceramics

Abstract: The creep and recovery behaviour in compression of two lithium zinc silicate glassceramics is established over the temperature range 590 to 750° C at stresses up to 91.4 MN m−2. It is shown that the transient creep obtained is linearly viscoelastic and obeys the Boltzmann superposition principle. The activation energy of the rate-controlling process is the same as that found for secondary creep and is due to viscous flow of the residual glass phase. A simple method of analysis of the strain-time curves is presented, which can be modified to apply to stress relaxation tests.

Non-elastic deformation of concrete under cyclic compression

Abstract: Synopsis It is shown that, in comparison with a static stress, cyclic loading accelerates the non-elastic deformation of concrete. The resulting creep can be expressed as the sum of a mean-stress component and of a component which is dependent both upon the range of stress and upon the value of the mean stress.

Stress relaxation and mechanical equation of state in austenitic stainless steels

Abstract: Stress relaxation experiments have been performed on types 304 and 316 stainless steel at room temperature. The stress-strain rate data show that these materials exhibit the behavior of mechanical equation of state. The experimental results are discussed using the concept of internal stress.

Creep and recovery of polycarbonate

Abstract: The creep behaviour of polycarbonate was investigated at high stresses in tension and compression. The creep curve was separated into a recoverable strain and persistent strain. The equation describing the recoverable strain was directly related to the kinetics of the recovery curve. The concept of an internal stress was useful in describing the recoverable part of the creep and recovery.

The work-hardening of copper-silica: III. Diffusional stress relaxation

Abstract: A method is described for the determination of the surface diffusion coefficient in a copper-silica interface The experimental values thus obtained are applied to a simple model for the relaxation of stresses produced at a silica particle, in a copper matrix, by applied plastic and elastic deformations The usefulness of the model is examined in terms of its application to the prediction of the conditions under which diffusion-aided relaxation becomes important

Interpretation of shift of relaxation time with deformation in glassy polymers in terms of excess enthalpy

Abstract: The mechanical relaxation time in a glassy polymer depends on the magnitude of strain. The stress relaxation modulus of a styrene acrylonitrile and polybutadiene composite system (ABS) was measured at strains ranging from 0.005 to 0.10. The relaxation time was observed to shorten by up to four orders of magnitude. In addition, there was observed a decrease in the elastic contribution to the modulus. These two aspects of nonlinear viscoelasticity are interpreted in terms of the excess enthalpy associated with dilatation under strain, a crucial factor for ductile behavior and the formation of crazes. Up to 0.9 cal/g of the excess enthalpy associated with the stress‐induced dilatation is obtained from the differential scanning calorimetry study.

Dynamic strain aging during creep of α-Zr

Abstract: In the temperature range 723 to 823 K (450° to 550°C) annealed, crystal bar α-Zr exhibits anomalous behavior with respect to both single stress and incrementally stressed creep tests. The nature and extent of the anomalous behavior depends on temperature, stress, and impurity content. Specimens with low oxygen content exhibit: 1) normal, three-stage creep behavior during single stress tests, and 2) normal transients during incremental stress and temperature tests. Specimens with higher oxygen contents exhibit: 1) multi-stage creep curves whose shapes depend on temperature and stress, 2) inverse transients following stress and temperature increments, and 3) peaks in activation energy-tempera-ture curves. The nature of the anomalous behavior is consistent with a model for strain aging in which the possibility of localized depletion of the strain aging species exists. In the material being studied oxygen is probably responsible for the observed effects.

Bending stress relaxation and recovery of wool, nylon 66, and terylene fibers

Abstract: The bending stress relaxation and subsequent recovery behavior were determined for merino wool, nylon, and Terylene fibers. The effect of four experimental parameters were investigated, viz., the level of bending strain (0.5–4%), the time of stress relaxation before release (1–1000 min), the relative humidity (0–85%), and the temperature (20°–60°C). For small strains the merino and nylon fibers displayed behavior characteristic of linear viscoelastic materials, while Terylene exhibited a degree of nonrecoverable set. It was possible to construct master recovery curves for fibers held bent for different times before release. These curves can be used as a more convenient means of presenting the results. A relationship was found, for each fiber type, between the percentage stress relaxation and the time taken to recover to a given level of set. This relationship appeared to be independent of the experimental conditions employed. Although the fibers were not linear viscoelastic under all conditions, recovery could be roughly predicted from their stress relaxation behavior at the particular test conditions using the Boltzmann superposition principle.

Dynamic strain ageing in some titanium-silicon alloys

Abstract: An investigation has been made of the mechanical properties of titanium and of Ti-05 wt% Si and Ti-1O wt% Si alloys in the temperature range 300–900K Dynamic strain ageing (DSA), characterised by serrated yielding, has been observed between 700 and 825K and has been studied by measurements of strain rate sensitivity and of stress relaxation The DSA is attributed to interstitial atoms, and is increased in magnitude by the presence of silicon, probably due to lattice distortions resulting from silicon-interstitial atom pairs; the temperature of dynamic strain ageing is reduced by increases in silicon or oxygen content The powerful solid solution strengthening role of silicon at elevated temperatures may arise from the intensification of the interstitial strain ageing effect by the silicon

Stress relaxation during force‐compression studies on foods with the instron universal testing machine and its implications

Abstract: . The time required to apply the force in force-compression studies with the Instron Universal Testing Machine is long enough for short time relaxation processes to operate, possibly via intermolecular attraction forces. A theoretical analysis is presented whereby stress relaxation can be determined for the stress-strain region in which the Boltzmann Superposition Principle is valid. Application of the analysis is demonstrated using force-compression data obtained on cheeses of different textural properties. After correction for stress relaxation, the calculated and experimental relaxation modulus-time curves agree satisfactorily over a much longer time interval for Edam cheese than for Cheddar cheese. The latter cheese becomes non-linear at much smaller compressions than does Edam cheese, possibly by the propagation of cracks through the sample during compression. Firmness is one of the textural properties evaluated during the first bite of mastication. Since the time associated with the first bite is longer than that required for application of force during the first compression of a sample with the Instron in this study, it is possible that the sensory evaluation of firmness is made under conditions involving stress relaxation.

Creep and stress relaxation of rubbers—the effects of stress history and temperature changes

Abstract: Two factors which affect the creep and stress relaxation of rubbers at room temperature are discussed. The rate of relaxation in filled rubbers is dependent on the previous mechanical history of the specimen. Pre-stressing a rubber reduces the subsequent relaxation rate and this reduction is for most purposes permanent. Recovery can, however, be induced by swelling and deswelling the specimen. A rise in temperature during a relaxation experiment is accompanied by a marked, short term, acceleration of the relaxation. The creep which this produces is not recovered when the temperature is reduced to its original level. The magnitude of the extra relaxation from this effect is greatest if the cycle occurs in the early stages of relaxation and also greatest during the first such cycle. Both the effects are of importance in any consideration of the molecular nature of relaxation, and indicate that a simple network theory of viscoelasticity is inadequate. The effects are also of practical significance in that both of these factors should be well understood if laboratory testing is to be used for predicting the service behaviour of rubber components.

Dislocation mobility and work hardening in sodium chloride

Abstract: Stress relaxation has been used in an indirect determination of the velocity—stress exponent for dislocation motion in NaCl at room temperature. The long-range internal stress arising from plastic deformation has been estimated and it is shown to be the principal source of hardening in both stage I and stage II of plastic deformation.

Strain and temperature dependence of relaxation phenomena in polycarbonate

Abstract: The mechanical behavior of glassy PC was investigated as a function of strain and temperature by stress relaxation measurements and by dynamic mechanical measurements For stress relaxation, the stress decay, measured by an Instron tensile tester, linearized vs logarithmic time from 1 sec to up to 16 h From these data a plot of the relaxation time vs strain was obtained showing that the relaxation time decreased with increased strain To separate the effect of the elastic and viscous components, E′ and E″ were investigated by the Rheovibron At a constant strain level, E′ showed a slight increase with time while E″ decreased considerably With stepwise increase in strain, E′ showed a slight decrease while E″ increased considerably Furthermore, the dynamic mechanical spectra showed that the peak did not shift with strain, but intensified and broadened considerably The stress relaxation can be related to the Rheovibron data

Acoustical determination of stress relaxation functions for polymers

Abstract: An analysis of acoustic waves in viscoelastic materials reveals that the stress relaxation function for such materials can be evaluated from acoustic dispersion data. In this paper we illustrate this method by evaluating the longitudinal stress relaxation function for the solid polymer, polymethyl methacrylate (PMMA) using previously reported acoustic data. From this result, we deduce the relaxation function appropriate for describing the shock wave response of PMMA by specifying the time‐scale characteristic of plate impact experiments. This relaxation function compares well with the function determined from the experimental observations of steady shock waves in PMMA.

Creep of Polyurethane under Varying Temperature for Nonlinear Uniaxial Stress

Abstract: Two methods are described to account for varying temperature during creep. Both employ the modified superposition principle. One uses a reduced time involving a shift factor which is a function of both stress and temperature history. The other considers the strain to be a function of the current values of stress and temperature. Experiments on polyurethane include constant stress creep and recovery at several temperatures in the nonlinear range and an experiment in which the stress was held constant while the temperature increased at a constant rate, then the stress was removed and the temperature decreased at a constant rate. The strain in this experiment was predicted by means of the theories from the results of the constant temperature creep tests. The strain in the constant temperature creep and recovery tests were described by means of the multiple integral representation and the modified superposition principle. Most of the nonlinearity and temperature effect were found in the coefficient of the time‐dependent term.

Process for treating copper alloys to improve creep resistance

Abstract: Improving the creep resistance and stress relaxation resistance of copper base alloys having a low stacking fault energy by cold working from about 10 to 90 percent; heating from about 25* to 360*C and cooling to room temperature.

A model of strain hardening during high-temperature creep

Abstract: A model of strain hardening is developed which shows that the large values of hardening coefficient measured during high-temperature creep are a consequence of the limited slip distances available in a well-developed 3-D dislocation network. It is predicted that the hardening coefficient should increase rapidly to a constant value during the early stages of primary creep and that this constant value should be independent of applied stress. These predictions are largely in agreement with previously published data.