Stress Relaxation Technique Research Articles

Occupational Burnout among Correctional Health Workers

1. Approximately one third of the sample study fit Maslach's burnout profile, with low personal accomplishment, high depersonalization, and high emotional exhaustion. Perceived stress was a significant predictor of the sense of personal accomplishment and emotional exhaustion. 2. Both male and female subjects had low to moderate mean scores on the social support scale. Social support as measured in this study was limited to that received from coworkers and supervisors. 3. The results of this study point to a need for intervention in this setting. Specific interventions for decreasing stress and thus decreasing chances for burnout are discussed. 4. The development of a broad employee health care program is an example of a primary prevention intervention to decrease stress in this setting. Such a program would include instruction in methods of stress reduction and stress management, including exercise and relaxation techniques.

EFFECTS OF TEMPERATURE AND MOISTURE CONTENT ON THE VISCOELASTIC BEHAVIOR OF COWPEAS

The viscoelastic parameters of cowpeas (Vigna unguiculata) were studied at different temperature and moisture content combinations. Cowpeas were tested at temperatures of 25°, 45° and 70°C and moisture contents of 13.9, 19.6 and 22.4% dry basis on an Instron Universal Testing Machine using stress relaxation techniques. The response of cowpeas was characterized by two Maxwell bodies in parallel without the incorporation of a residual spring. The relaxation curves (uniaxial modulus versus time) were used to calculate temperature and moisture shift factors, which subsequently led to the construction of a single master response curve. Cowpeas are both thermo‐rheologically and hydro‐rheologically simple biological materials.

Growth inhibition, turgor maintenance, and changes in yield threshold after cessation of solute import in pea epicotyls.

The dependence of stem elongation on solute import was investigated in etiolated pea seedlings (Pisum sativum L. var Alaska) by excising the cotyledons. Stem elongation was inhibited by 60% within 5 hours of excision. Dry weight accumulation into the growing region stopped and osmotic pressure of the cell sap declined by 0.14 megapascal over 5 hours. Attempts to assay phloem transport via ethylenediaminetetraacetate-enhanced exudation from cut stems revealed no effect of cotyledon excision, indicating that the technique measured artifactual leakage from cells. Despite the drop in cell osmotic pressure, turgor pressure (measured directly via a pressure probe) did not decline. Turgor maintenance is postulated to occur via uptake of solutes from the free space, thereby maintaining the osmotic pressure difference across the cell membrane. Cell wall properties were measured by the pressure-block stress relaxation technique. Results indicate that growth inhibition after cotyledon excision was mediated primarily via an increase in the wall yield threshold.

Effect of crosslink density on physical ageing of epoxy networks

Physical ageing of polypropylene oxide/DGEBA networks with different crosslink densities was investigated using the small-strain stress relaxation technique in simple extension. The effects of crosslink density on the glass transition temperature, T g, and the change in specific heat ΔC p at T g were measured using a differential scanning calorimeter (d.s.c.) in heating. Although we observed an increase in T g as crosslink density increased, contrary to other studies of crosslinked polymers, the ΔC p did not change as the crosslink density changed. Ageing was studied at several temperatures below T g after quenching from above T g. It was possible to superimpose the stress relaxation curves at different ageing times, temperatures and crosslink densities to form a single master curve, demonstrating the applicability of a time-ageing time-temperature-crosslink density superposition principle to this type of network. Under all ageing conditions, the double logarithmic shift rate was found to decrease with increasing crosslink density while being independent of temperature for a given network. Furthermore, at temperatures of 10 and 5°C below T g, we were able to age the network glasses into structural equilibrium, thus obtaining t∗, the time required to reach structural equilibrium. At a constant temperature ΔT below T g, we observed increases in t∗ as the crosslink density increased.

A stress relaxation method for following carbonitride precipitation in austenite at hot working temperatures

Stress relaxation measurements were carried out on a plain carbon and four solution-treated Ti steels over the temperature range 850 to 1050 °C. The results show that the stress relaxation of plain carbon austenite after a 5 pct prestrain (i.e., in the absence of precipitation) can be described by the relation σ = σ0 -α ln(l + βt). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is completed. As a result, this mechanical method is particularly suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures. A new model regarding the effect of precipitation on dislocation motion is proposed, on the basis of which, the phenomenology of the stress relaxation technique is clarified. Precipitation-time-temperature (PTT) diagrams were determined for the Ti bearing steels containing 0.05, 0.11, 0.18, and 0.25 pct Ti. The PTT curves obtained are C-shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the compositions of the steels tested.

Experimental “sytectonic” dehydration of serpentinite under conditions of controlled pore water pressure

We have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine + talc + water, at temperatures ranging between 300° and 600° and at total pressures from 100 to 300 MPa. Constant stress, stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed. Effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system. Stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction. At 500° and 600°C, however, the strength was linearly dependent on the inverse of the deformation rate, and independent of effective confining pressure over the range investigated. Microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine‐grained (0.1 µm olivine produced in the breakdown reaction. The enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion‐accommodated grain boundary sliding. These experimental results suggest that metamorphic transformations affect the deformability of rocks, and demonstrate enhancement of deformability through the transient existence of fine‐grained reaction products. The results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain‐size reduction by dynamic recrystallization in plastically deforming rocks.

Foam Rheology: III. Measurement of Shear Flow Properties

The experimental study of foam is complicated by its inherently unstable nature and by the presence of a liquid film slip layer at the wall. A method is described for generating reproducible and stable foam capable of retaining its structure for prolonged times. An experimental technique has also been devised which eliminates problems associated with wall slippage and allows measurement of material functions without use of any empiricisms for the wall region. In a steady‐shear flow, foam behaves like a Bingham plastic with a viscosity inversely proportional to shear rate indicating the presence of a yield stress τ0. The value of the viscosity, which is significantly higher than the parent liquid viscosity, is an increasing function of gas volume fraction φ. Yield stress values obtained by extrapolating viscosity versus shear stress data agree with direct τ0 measurements obtained by using a stress relaxation technique. The yield stress is also found to increase with φ. Small amplitude oscillatory shearing experiments show foam to behave as an elastic solid for small deformations. This is evident from its frequency independent moduli, very small phase shift between strain input and stress output, and the elastic modulus's being much larger than the loss modulus. Stress growth experiments at the start‐up of steady‐shear flow reveal stress overshoots not larger than 30%. The transient viscosity divided by the steady‐state viscosity is found to be insensitive to shear rate, but is an increasing function of φ.

Characterization of Wheat Varieties Based on Rheological Behaviour during Stress-relaxation

Rheological measurements of wheat flour doughs, using the stress-relaxation technique, can give information that is of fundamental interest for the baking industry. The measurements carried out in ...

Application of chemorheology to radiation damage of elastomer

The technique of chemical stress relaxation was applied to investigating radiation damage of elastomers. The interests center of this paper are as follows. 1. 1. The efficiency (G value) of chain scission and cross-linking obtained by the measurement of continuous and intermittent chemical stress relaxation. 2. 2. Dose rate dependence of chain scission and cross-linking. 3. 3. A method which expresses the magnitude of synergistic relationship between heat and radiation on the degradation of elastomers. 4. 4. A quantitative expression on the decrease in heat resistant properties of elastomer by pre-irradiation.

Chemorheological Studies of a γ‐Irradiated Dimethylsiloxane Elastomer

The degradation of a peroxide‐cured polydimethylsiloxane (PDMS) elastomer subjected to γ‐irradiation at various temperatures has been studied by chemorheological techniques. Preliminary investigations of the thermal degradation of mechanisms operative in the elastomer utilising continuous and intermittent stress relaxation measurements revealed that degradation took place by a chain interchange reaction. The cleavage reaction occurred randomly along the main chain and was a hydrolytic scission reaction catalyzed by the presence of peroxide decomposition products in the material—namely, 2, 4‐dichlorobenzoic acid. Subsequent chain reformation took place by the condensation of silanol chain end groups. The reaction was found to have an activation energy of 15±4 kJ mol−1. Intermittent stress relaxation techniques revealed that the net crosslink density of the elastomer increased as thermal degradation proceeded. This phenomenon has not been reported in any previous studies of the thermal degradation of PDMS elastomers. The effect has been attributed to the reformation of network chains cleaved during the post‐cure operation prior to the commencement of the thermal degradation studies. This reaction which is also believed to take place by the condensation of silanol groups, occurs simultaneously with the conventional interchange reaction. Continuous stress relaxation studies during γ‐irradiation indicated that the rate of the radiation induced scission reaction increased with increasing temperature. The temperature dependence of this rate did not obey the Arrhenius relationship suggesting a possible change in the scission kinetics at higher temperatures. For the particular sample geometry in the investigations, it was discovered that there was no significant difference between the response of elastomers tested in air and in an inert atmosphere.

Stress relaxation technique for thermally grown SiO2

A corona discharge technique has been utilized at 800 °C to fully relax thermal oxides grown on Si wafers. In an experiment involving two oxidation steps, the corona-relaxed oxides are shown to be equivalent to ∼1200 °C thermally grown or thermally relaxed oxides, both with respect to index of refraction and oxygen diffusion coefficient. The activation energy for oxygen diffusion through a completely relaxed thermal oxide is found to be 0.8 eV.

A study on gelation of soybean globulin solutions. Part 4. Effect of gelation conditions on the visco-elastic properties and structure of thermotropic gels of soybean globulin fraction.

To the study of the structure and properties of the thermotropic soybean globulin fraction (SGF) gels the stress relaxation and electron microscopy techniques has been applied. The value of slope of normalized relaxation function was used as a characteristic of relaxation properties of SGF gels. This value characterizes the velocity of the gels relaxation processes. It has been found that this characteristic is determined only by the gels' heating temperatures. Using the differential scanning microcalorimetric technique it was shown that the changes of the slope of normalized relaxation function of SGF gels are controlled only by heating temperature. The reason for this lies in the changes of the composition of the denaturation product of SGF gels. The increase is shown of the shear moduli of gels prepared at other than optimum temperatures by additional heating under optimal gelation conditions. That is a result of existence of a sol-fraction. The most significant increase of shear moduli is found for the SGF gels, sol-fraction of which is native 11S globulin. A correlation has been established between the changes of SGF gels' structure obtained from the dates of rheological measurements and electron microscopy technique.

Photomechanical effects in PZT

A photomechanical effect has been observed in the piezoelectric sonar transducer ceramic, lead zirconate titanate (PZT) in which the material becomes measurably softer when illuminated with visible light. The phenomenon has been studied by stress relaxation techniques and attempts have been made to analyze the data in terms of a thermal activation analysis that had been successfully applied to PZT in the absence of illumination. The effect of light intensity on the stress relaxation was determined. The results are discussed in terms of the effect of illumination on the migration of non-180 degree domain boundaries.

Internal stress measurments during the saturation fatigue of polycrystalline aluminum

The internal stress and the effective stress were measured using stress relaxation techniques for 99.988% purity polycrystalline aluminum containing a well-defined cellular dislocation microstructure characteristic of large amplitude low cycle fatigue. Room temperature stress relaxation measurements were carried out after cyclic saturation at constant plastic strain amplitude and constant total strain rate as a function of location within a particular hysteresis loop. Measurements of stress relaxation at the tensile and compressive hysteresis loop tips show the dependence of internal and effective stress on plastic strain amplitude and strain rate. Both the internal stress and the effective stress were found to exhibit cyclic behavior representable by a hysteresis loop on each strain reversal. The internal stress was insensitive to strain rate while the saturation stress and the effective stress showed significant rate dependences. Both the internal stress and the effective stress increased with strain amplitude. The stress sensitivity exponent of the strain rate was determined from the variation in the effective stress with rate both during a given hysteresis loop and during strain rate changes and was found to decrease with increasing strain amplitude. The findings are discussed in the context of dislocation velocity and mobile dislocation density contributions to the strain rate sensitivity from which it is suggested that the mobile dislocation density decreases with increasing strain amplitude and strain rate. Plastic strain changes during stress relaxation suggest that the mobile density during saturation fatigue is equivalent to one mobile dislocation segment per dislocation cell.

Structure-property relationships in injection-molded PVC

Abstract Injection-molded plaques were prepared from a rigid PVC dry blend using a variety of processing conditions. Test specimens cut from the moldings were examined to determine their structure as a function of these processing conditions, by measurement of density and heat shrinkage. Microscopic observations through the thickness of the molding revealed oriented skins, which varied in thickness. Thinner skins were observed under conditions where there was more available heat in the mold, enabling more molecular relaxation to occur. Percentage heat shrinkage was found to be directly proportional to measured skin thickness. Residual internal stresses in the moldings were also measured using a stress relaxation technique, and attributed to skin and core morphologies. X-ray diffraction was used in an attempt to account for observed density results, which could not be explained solely by skin/core ratios.

Deformation modeling of aluminum: Stress relaxation, transient behavior, and search for microstructural correlations

The inelastic behavior of high purity aluminum was measured by stress relaxation techniques following three mechanical histories: monotonie tensile loading, reversed cyclic loading, and elastic reloadings subsequent to an initial relaxation event. The data are used to test the validity of a model which had found previous success in describing simple stress relaxation of aluminum, stainless steels and solder alloys. The model is found to apply, in this work, to monotonie histories for strains greater than 0.6% and for the cyclic histories where total accumulated strains are greater than 0.5%. However, the model failed to describe relaxation after either small monotonie straining (<0.10%) or after elastic reloadings subsequent to an initial relaxation. Transmission electron microscopy of specimens supposedly in different material ‘states’ failed to show any difference in dislocation sub-grain size or subboundary character. It is concluded that the model under discussion does not represent a complete deformation law for aluminum, and further, that searches for correlation between deformation law state variables and microstructural elements may be futile. General considerations for deformation law development are discussed in light of the experimental results.

Localized stress relaxation by impression testing

A small flat-ended cylindrical punch was impressed onto the surface of an LiF single crystal. The impression was made in an Instron at a constant cross-head speed. At a certain depth of the impression the crosshead was stopped and the load relaxation with time was recorded. It was found that the velocity-stress exponent for dislocation motion obtained in this way was identical with that obtained by conventional stress relaxation techniques using bulk samples. However, this new method caused only local deformation of the specimen and used very little material in the process.

Fracture and deformation of PZT

Abstract Subcritical crack propagation and time dependent deformation in the transducer ceramic, PZT, have been studied using the double torsion and stress relaxation techniques. Slow crack growth in PZT is sensitive to both chemical and electrical testing environments. Moisture and high ac or dc fields applied perpendicular to the crack plane enhance crack propagation in unpoled PZT. The stress relaxation data can be treated in terms of an analysis of thermally activated deformation. A dislocation model of the domain processes responsible for deformation is consistent with the experimental data.

Stress-relaxation technique for deformation studies in four-point bend tests: application to polycrystalline ceramics at elevated temperatures

The advantages of the stress-relaxation technique can be effectively realized by applying the procedure to conventional four-point bend tests usually employed in the deformation studies of ceramic materials. A test system and procedure for determining plastic strain rate-stress relationships at elevated temperatures (up to 1600° C) by the stress-relaxation method is described. An analysis to calculate true plastic strains and stresses from measured deflections and loads is presented and it is shown that such an analysis requires minimum assumptions regarding the materials behaviour. Preliminary results obtained on an iron-doped MgO specimen are discussed and compared with the constant load test results obtained on identical specimens. Sources of error in the four-point bend stressrelaxation tests and the methods to minimize them are also discussed.

Viscoelastic Behavior of Soybeans Due to Temperature and Moisture Content

ABSTRACT THE mechanical properties of biological products, because they vary with both temperature and moisture content, have been difficult to describe by single mathematical models of stress or strain versus time. This paper describes the determination of temperature and moisture shift factors to include both parameters in the single equation of mechanical response. Soybeans were studied as a viscoelastic material at four temperatures and at four moisture levels employing stress relaxation techniques. Product response was characterized by a generalized Maxwell model. The uniaxial relaxation modulus function was plotted against time, and, from this, temperature and moisture shift factors determined. Soybean mechanical response was found to be both thermo-rheologically and hydro-rheologically simple.