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To improve the stress relaxation resistance of the Cu-5Ni-S8n alloy, the effects of Co or P addition on its stress relaxation resistance were investigated by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction), X-ray diffraction, and high-temperature creep durability tester. The results revealed that the relaxation rate increased dramatically as the temperature rose. The thermal stress relaxation resistance of Cu-15Ni-8Sn alloy was significantly improved with Co addition; After 100 h of stress relaxation tests at 200°C or 250°C, the residual stress value of the Cu-15Ni-8Sn alloy increased by 23 MPa or 22 MPa with Co addition, which was associated with the consumption of vacancies by Co atoms and the pinning effect of Co atoms on dislocation. However, at 250°C, the stress relaxation resistance of the Cu-15Ni-8Sn-0.2P alloy was lower than that of the Cu-15Ni-8Sn alloy. This difference was due to the inclusion of P, which promoted the recrystallization reaction. Article highlights Recrystallization and the decrease of dislocation density are the major reasons for the stress relaxation of the samples. The stress relaxation resistance of Cu-15Ni-8Sn alloy was significantly improved with Co addition. The addition of the P element is not conducive to the improvement of thermal stress relaxation resistance. [ABSTRACT FROM AUTHOR]

In the injection-molded parts, prediction of accurate warpage at initial level becomes mandatory to avoid iterative work of mold modifications. Simulation teams of many organizations are using existing commercial programs for process simulations. Material models in existing simulation technologies are having certain limitations and assumptions, which can regularly result in up to 50% variation of warpage results as compared to the actual physical warpage measurement. The commonly used Moldflow simulation model, for example, ignores temperature-dependent mechanical properties and the stress relaxation spectrum for viscoelastic materials. These assumptions affect the accuracy of the warpage prediction results significantly. To decrease these kinds of variations, BASF extended its Ultrasim® tool which is based on integrative simulation technology. Recently, a newly developed thermomechanical material model with temperature-dependent nonlinear mechanical properties and stress relaxation behavior was added in the Ultrasim. This model has been used in this work to consider the complete transient description of the warpage, which starts at packing phase of the part inside the mold, followed by actual cooling and ejection. In this article, unreinforced semicrystalline polybutylene terephthalate polymer material (Ultradur® B4520) is considered for warpage correlational study. The accuracy of the warpage prediction is compared between the integrative simulation approach, existing warpage simulation method, and the actual experimental inspection results. The result exhibits that the accuracy of the integrative simulation (Ultrasim)-based warpage simulation is relatively better than existing simulation technologies and closer to the actual measurement. [ABSTRACT FROM AUTHOR]

An externally restrained stress relief cracking test was developed and demonstrated in testing susceptible and resistant to cracking welds in Cr–Mo steels. Compared to other externally restrained tests, it simultaneously applies stress and compensates thermal expansion during heating to post-weld heat treatment temperature and utilises digital image correlation for quantification of key characteristics of the stress relaxation and stress relief cracking phenomena. In contrast with resistant to stress relief cracking materials, susceptible materials experienced lower levels of stress relaxation, strain absorption, and sustained mechanical energy, with accelerated kinetics of strain accumulation and strain localisation leading to failure. The processes of stress relief cracking and stress relaxation were quantified as low strain – slow strain rate – low energy phenomena. [ABSTRACT FROM AUTHOR]

In this note, we study the response of a viscoelastic body whose stress relaxation modulus and creep compliance depend on the density of the body in such a manner that the stress and strain appear linearly in the constitutive equation. Such models would be useful to study the response of porous viscoelastic bodies undergoing small deformations, as the moduli depend on the porosity, and hence the density. We study the problem of tension–torsion of cylinders of arbitrary cross-section within the context of this constitutive relation. [ABSTRACT FROM AUTHOR]

The present work deals with the microwave-assisted compression moulding of high-density polyethylene (HDPE)-based composites. In the present work, 20 wt% of reinforcement in the form of kenaf and multi-walled carbon nanotube (MWCNT) was used to fabricate HDPE/kenaf and HDPE/MWCNT polymer composites. The mechanical characterizations of the microwave-processed composites were carried out in terms of uniaxial tensile test with different strain rate, multistep stress relaxation, flexural and impact test. The uniaxial tensile test revealed that the tensile modulus of microwave-processed four-layered HDPE/kenaf polymer composite was 35.2% higher than that of HDPE/MWCNT polymer composite. The HDPE/MWCNT polymer composite showed a minimum of 1.25 GPa and a maximum of 4.7 GPa of elastic modulus when tested at different strain rate. The impact energy absorbed by the HDPE/kenaf polymer composite (1.055 J) was 81.12% higher than the HDPE/MWCNT polymer composite (0.582 J). [ABSTRACT FROM AUTHOR]

Background: Nonunion following hindfoot arthrodesis may be caused by failure to maintain compression at the arthrodesis site. The ability of lag screws, commonly used in arthrodesis, to maintain compression in hindfoot bones has not been well characterized. The aim of this work was to quantify the stress relaxation response of hindfoot bone with initial and repeated compression with a lag screw. Methods: Ten sets of 25-mm-diameter bone cylinders were cut from the talus and calcaneus in fresh-thawed cadaveric feet. A load cell was compressed between cylinders with an 8.0-mm partially threaded cannulated lag screw simulating arthrodesis. For 7 sets, screws were tightened by 3 quarter-turns, rested for 3 minutes, retightened 1 quarter-turn, and rested for 30 minutes. Three sets served as controls in which screws were not retightened. Results: Maximum compression after initial screw tightening and retightening averaged 275 and 337 N (P = .07), respectively. Compression 3 minutes after initial screw tightening and retightening averaged 199 and 278 N (P = .027), respectively. The compression recorded 3 minutes after screw retightening was an average of 40% higher than that recorded 3 minutes after initial tightening. The average compression 30 minutes after screw retightening was 255 N, a compression loss of 25% from the average maximum compression after retightening. Eighty percent of this compression loss happened in an average of 5.5 minutes. Conclusion: Hindfoot bones exhibit compression loss over time during simulated arthrodesis. Compression maintenance in bone is improved with screw retightening. Further work is needed to understand the mechanism of action and determine optimum time for recompression. Clinical Relevance: Retightening lag screws before wound closure may improve compression at the arthrodesis site and thereby decrease the chance of nonunion. [ABSTRACT FROM AUTHOR]

The stress relaxation behaviour of Cu–0.23Cr–0.08Ag alloy was studied at different deformations (10%, 20%, 30%). The high percentage of low ΣCSL (Coincidence Site Lattice) grain boundary could improve the stress relaxation resistance of the alloy. There are two main factors affecting the stress relaxation resistance: one is the pinning effect of Cr to movable dislocations; the other is the obstacle of twins to the movement of movable dislocations. [ABSTRACT FROM AUTHOR]

Temperature and stretching are important factors in the high-temperature treatment of carbon fiber. The axial stress during carbon-fiber high-temperature treatment affects its ability to stretch. The high-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber was studied through in situ tension tests, Raman spectroscopy, X-ray diffractometry, elemental analysis, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, thermal expansion coefficient tests, and density methods. The high-temperature axial stress evolution of polyacrylonitrile-based carbon fiber involved three stages: rapid increase, rapid decrease, and relaxation. The highest stress and relaxation temperatures of the polyacrylonitrile-based carbon fiber were 1600°C and 1950°C, respectively. The main factors that affected the fiber axial stress included carbon-structure rearrangement and the effect of thermal expansion and cold shrinkage on fiber length. During the first stage (T < 1600°C), carbon-structure rearrangement after nitrogen atom removal increased the fiber axial stress. In the second stage (1600 ⩽ T ⩽ 1950°C), the difference in the thermal expansion of fibers that entered the graphite furnace and the cold shrinkage of fibers that exited the graphite furnace increased gradually, which resulted in a decrease in fiber axial stress by up to 1950°C, where the fiber relaxed and the third stage (T > 1950°C) began. The difference between expansion and shrinkage increased significantly, which increased fiber relaxation. Carbon fibers with fewer nitrogen atoms and more regular structures had a lower axial stress during high-temperature treatment, but the trend and characteristic temperature remained unchanged. The corresponding fiber high-temperature maximum stretching ratio and axial stress showed opposite trends below 1950°C. The ability to stretch the carbon fiber increased above 1950°C, which differed from the axial stress relaxation. [ABSTRACT FROM AUTHOR]

Floor stress distribution is the main index for evaluating the mining effect of upper protective coal seam. However, parameters currently used in theoretical analysis of mining-induced stress lack practicality. Therefore, in this article, a model for calculating floor stress during mining of upper protective coal seams was established based on the theory of elastic mechanics. Subsequently, the stress induced by the abutment pressure at any point in the five parts of the floor was derived. Moreover, the distribution characteristics of the horizontal, vertical, and shear stresses of the floor during mining of the upper protective coal seam were elaborated. The results show that with the continuous mining of the working face of the protective coal seam, the vertical stress of the floor strata experiences three stages, that is, rapid increase, abrupt stress relaxation, and gradual recovery to the in situ stress. With regard to the morphology, the floor strata recompress or expand in the vertical direction. Vertical and horizontal stress are relieved in the shallow part of the floor in the goaf behind the working face, and there is an abrupt reduction in the increase of concentration degree of vertical and horizontal stress in the floor strata in front of the working face. High shear stress occurs underneath the goaf near the working face. The isoline of the shear stress is distributed in a bubble shape and is oblique to the goaf. These research achievements can provide some theoretical basis for understanding the gas drainage during the mining of the upper protective coal seam. [ABSTRACT FROM AUTHOR]

Temperature and stretching are important factors in the high-temperature treatment of carbon fiber. The axial stress during carbon-fiber high-temperature treatment affects its ability to stretch. The high-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber was studied through in situ tension tests, Raman spectroscopy, X-ray diffractometry, elemental analysis, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, thermal expansion coefficient tests, and density methods. The high-temperature axial stress evolution of polyacrylonitrile-based carbon fiber involved three stages: rapid increase, rapid decrease, and relaxation. The highest stress and relaxation temperatures of the polyacrylonitrile-based carbon fiber were 1600°C and 1950°C, respectively. The main factors that affected the fiber axial stress included carbon-structure rearrangement and the effect of thermal expansion and cold shrinkage on fiber length. During the first stage (T < 1600°C), carbon-structure rearrangement after nitrogen atom removal increased the fiber axial stress. In the second stage (1600 ≤ T ≤ 1950°C), the difference in the thermal expansion of fibers that entered the graphite furnace and the cold shrinkage of fibers that exited the graphite furnace increased gradually, which resulted in a decrease in fiber axial stress by up to 1950°C, where the fiber relaxed and the third stage (T > 1950°C) began. The difference between expansion and shrinkage increased significantly, which increased fiber relaxation. Carbon fibers with fewer nitrogen atoms and more regular structures had a lower axial stress during high-temperature treatment, but the trend and characteristic temperature remained unchanged. The corresponding fiber high-temperature maximum stretching ratio and axial stress showed opposite trends below 1950°C. The ability to stretch the carbon fiber increased above 1950°C, which differed from the axial stress relaxation. [ABSTRACT FROM AUTHOR]

Elastomers at elevated temperatures can undergo microstructural changes owing to the scission, re-coiling, and re-crosslinking in their macromolecular network. These result in chemically based stress relaxation and permanent set. In previous work, a constitutive equation was developed for the thermo-mechanical response of such elastomers that generalized the Tobolsky two-network theory and incorporated results from an experimental program. One experimental result was that the scission process appeared to be unaffected by stretch, to within moderate stretches. There are indications that larger deformations affect the scission process. The purpose of the present work is to modify the previously developed constitutive theory to allow for this. The constitutive theory contains material properties, representing the chemical kinetics of scission, re-coiling, and re-crosslinking, that are expressed in terms of activation energies. The premise considered here, motivated by the mechano-chemical discussion in the literature, is that an increasing deformation decreases the activation energy and results in faster chemical stress relaxation. A method for accounting for this is introduced. The resultant constitutive theory is then used in the simulation of the radial planar deformation of a planar annular sheet by tractions at its outer boundary. This particular geometry is chosen for two reasons: (1) its convenience for experimental study; (2) the stretch concentration near the inner circular boundary will cause faster chemically based stress relaxation there, which could be readily observable. [ABSTRACT FROM AUTHOR]

This paper presents rate dependent uniaxial tensile behaviour and relaxation response of high strength acrylonitrile butadiene rubber (NBR) with moderate carbon black content. The rate dependent uniaxial tensile response is investigated for large deformations over moderate range of elongation rates (50–500 mm min−1). Effects of elongation rate on the overall deformation behaviour in terms of stress–strain response, total elongation at break as well as on the breaking stress of the material are investigated. Experimental results revealed that the material behaviour, including elongation at break and breaking stress, is rate dependent. Relaxation behaviour over a temperature range of 30–90 C, investigated using uniaxial compression experiments, shows drop of stress over the duration of the tests. Additionally, a generic rate dependent hyperviscoelastic model to predict the rate dependent response of the material and simulations of the relaxation tests are presented. Single-step stress relaxation tests from an inbuilt experimental setup and tensile response obtained from test conducted at 200 mm/min elongation rate were used in calibration. The numerical results obtained are seen to be in good agreement with the experimental observations for the range of deformation rate and temperature considered in the uniaxial tensile and relaxation tests, respectively. [ABSTRACT FROM AUTHOR]

Impression creep behaviour of the powder metallurgy processed steel matrix composites was investigated under constant stress at different temperatures in the range of 873-973 K. By using the power-law relationship, the estimated activation energy for unreinforced steel was found to be 149 kJ mol−1 and steel reinforced with 2 and 4 vol.-% TiB2 was found to be 298 and 338 kJ mol−1, respectively indicating better creep resistance of the reinforced steel matrix composites. Dislocation creep is the dominant creep mechanism based on the calculated values of stress exponent and activation energy. Hence, this method can be used to assess the potential of steel matrix composites for use as structural materials for high-temperature application. [ABSTRACT FROM AUTHOR]

Quasi-static mechanical properties of aramid fibres reinforced natural rubber/SBR composites are comprehensively investigated via scanning electron microscopy (SEM), uniaxial tensile tests, multi-step stress relaxation, and the Mullins experiment. The strength, stiffness, viscoelasticity, and Mullins effect of composites are analysed, and the micro failure mechanism is also explored. It is shown that the samples appear similar to a laminate plate with random short fibres in a plane. The results of the uniaxial tensile tests indicate that the tensile strength and elongation at failure decrease, while the stiffness increases following the addition of a small amount of aramid fibres into the rubber matrix. The multi-step relaxation and Mullins experiments reveal that the aramid-short-fibres weaken the viscoelasticity of rubber composites. Finally, the strain energy of the composite is divided into four parts, and one part is considered with respect to its application to the study of interface destruction between the matrix and fibre. [ABSTRACT FROM AUTHOR]

Textural attributes are a manifestation of the rheological properties and physical structure of foods, cheeses among these. In order to describe these physical properties, the objective of this work was to analyse the effect of deformation history on the stress relaxation behaviour of Colombian Caribbean coastal cheese made from goat milk with 3.75% (F1), 4.00% (F2) and 4.25% (F3) fat content, through prediction made by a four-term Prony series based on Chen's model. For this, stress relaxation data and stress relaxation spectra were analysed. Moreover, textural attributes by texture profile analysis were measured. Physicochemical results were similar to those published by other authors, and all samples meet national and international standards. Results from this work showed that Chen's model could be successfully used to describe the effect of deformation history on the stress relaxation behaviour of Colombian Caribbean coastal cheese made from goat milk. F1 had the highest elastic response, with the most significant residual modules (P0) and relaxation times (τ1, τ2 and τ3). On the other hand, residual modules and relaxation times (τ1, τ2 and τ3) for cheeses F2 and F3 did not present statistically significant differences (p > 0.05). Besides, by interpretation of the stress relaxation spectra, F1 presented the firmest structure (greatest distribution function and relaxation time) which was characterised by the highest elastic behaviour. Finally, according to texture profile analysis test, F1 had the highest hardness, cohesiveness and chewiness, whereas F2 and F3 did not present statistically significant differences (p > 0.05) between them. [ABSTRACT FROM AUTHOR]

Nanocomposites based on polyepoxy isocyanurate crosslinked systems of different chemical composition were produced in order to investigate the specific effect of technical nanocarbon on each individual polymer network. Monodispersed carbon particles were produced by methane chlorination conducted in a continuous reactor in a non-equilibrium regime with a chlorine deficit. The dependences of the forced elasticity limit, stress relaxation, etc., on the degree of filling were analysed. The possibility of interphase interaction between a specific polyepoxy isocyanurate network and 'nanofiller' was investigated. It was shown that a fairly low concentration (1 wt%) of monodispersed carbon particles of 20-25 nm size leads to a 25-30% increase in the elastic modulus and forced elasticity limit; similarly, the relaxation characteristics are also increased. It was established that improvement in a number of such properties is connected with the greater influence of interphase interactions between the polymer and filler on account of the large surface layer area. [ABSTRACT FROM AUTHOR]

The effect of nanotubes and nanofibres on the stress relaxation of nanocomposites based on high-density polyethylene (HDPE) was studied. It was found that a very low concentration of multiwall nanotubes (0.1 wt%) leads to a significant increase in the relaxing stresses over the entire period of relaxation. Generalised curves of the relaxation process were plotted, and it was found that the greatest reinforcing effect is rendered by the multiwall nanotubes with the greatest specific surface. [ABSTRACT FROM AUTHOR]

A specially designed relaxometer was used to measure the stress relaxation under 30% elongation for tread rubber compounds based on a blend of three rubbers: natural rubber, neodymium butadiene rubber, and oil-extended styrene butadiene rubber. The rubber compounds were filled with carbon black and silica in different ratios: with 80 parts carbon black only, with 80 parts silica only, and with different ratios of carbon black and silica. The method proposed by Bartenev was used to calculate the relaxation specta. The maxima of the spectra at different relaxation times were assigned to different types of interaction: rubber- filler interaction, carbon black-carbon black interaction, silica-silica interaction, carbon black-silica interaction. New maxima were obtained for specimens containing roughly equal amounts of carbon black and silica. These results were compared with data obtained using transmission electron microscopy. The existence of three levels of structure of filled elastomers and the presence of 'rigid' rubber around filler particles are suggested. [ABSTRACT FROM AUTHOR]

It is estimated that 2% to 4% of the US population will seek treatment for temporomandibular joint (TMJ) symptoms, typically occurring with anterior disc displacement. The temporomandibular retrodiscal tissue (RDT) has been postulated to restrict pathologic disc displacement. To elucidate RDT function, understanding regional RDT biomechanics and ultrastructure is required. No prior biomechanical analysis has determined regional variations in RDT properties or associated biomechanical outcomes with regional variations in collagen and elastin organization. The purpose of this study was to determine direction- and region-dependent tensile biomechanical characteristics and regional fibrillar arrangement of porcine RDT. Incremental stress relaxation experiments were performed on 20 porcine RDT specimens, with strain increments from 5% to 50%, a ramp-strain rate of 2% per second, and relaxation periods of 2.5 min. Tensile characteristics were determined between temporal and condylar regions and anteroposterior and mediolateral directions. RDT preparations were imaged using second-harmonic generation (SHG) microscopy for both collagen and elastin. Young's modulus showed significant differences by region ( P < 0.001) and strain ( P < 0.001). Young's modulus was <1 MPa from 5% to 20% strain, before increasing from 20% to 50% strain to a maximum of 2.9 MPa. Young's modulus trended higher in the temporal region and mediolateral direction. Instantaneous and relaxed moduli showed no significant difference by region or direction. Collagen arrangement was most organized near the disc boundary, with disorganization increasing posteriorly. Elastin was present at the disc boundary and RDT mid-body. Porcine RDT demonstrated region- and strain-dependent variations in tensile moduli, associated with regional differences in collagen and elastin. The small tensile moduli suggest that the RDT is not resistive to pathologic disc displacement. Further biomechanical analysis of the RDT is required to fully define RDT functional roles. Understanding regional variations in tissue stiffness and ultrastructure for TMJ components is critical to understanding joint function and for the long-term goal of improving TMJ disorder treatment strategies. [ABSTRACT FROM AUTHOR]

This work considers the inflation and extension of an elastomeric tubular membrane when its material exhibits a time-dependent response. Three different models for time-dependent response are considered: finite linear viscoelasticity, Pipkin–Rogers non-linear viscoelasticity, and thermally induced chemorheological degradation. The first two are based on different assumptions about stress relaxation effects while the third accounts for time-dependent microstructural changes due to simultaneous scission and re-cross-linking of macromolecular network junctions. Each of these models describes a material response that softens with time. It is shown that the constitutive equations for all three models are included in a general non-linear single-integral constitutive equation.In previous work, for elastic membranes, the material is fixed and a localized bulge may form as the load increases. In this work, the load is specified, and a localized bulge may form as the membrane material undergoes a time-dependent response. It is assumed that the extension and inflation histories are initially uniform, but there may be a time when a localized bulge-like deformation starts to form. This is treated as branching from the uniform extension and inflation history. For times beyond this ‘branching time’, the governing equations are satisfied by both the continuation of the initial uniform deformation history and the branched deformation history for the bulge. A unified condition for determining this branching time, applicable to all three models, is derived in terms of the general non-linear single-integral constitutive equation. Post-branching response is not considered here. [ABSTRACT FROM AUTHOR]

Dynamically cured 60/40 epoxidized natural rubber (ENR)/polyamide 12 (PA-12) and unmodified natural rubber (NR)/PA-12 blends with different types of curing systems (i.e. sulfur- and peroxide-cured systems) were prepared. It was found that mixing torque, shear viscosity, tensile strength, hardness, stress relaxation, thermal, and oil resistance properties of the ENR/PA-12 blends were higher than those of the NR/PA-12 blends. This is attributed to the chemical interaction between polar functional groups in ENR and PA-12 molecules which caused the formation of ENR-grafted PA-12. Smaller vulcanized rubber domains dispersed in the PA-12 matrix was observed in the dynamically sulfur-cured ENR/PA-12 blends. This is attributed to higher shear and elongational viscosities during mixing operation at high temperature. However, the peroxide-cured blends exhibited higher relaxation property, oil resistance, and cross-link density than those of the sulfur-cured blends due to strong reversion effect observed in the sulfur-cured system. [ABSTRACT FROM AUTHOR]

The article discusses the study regarding the absorption of sound by tyre rubbers. Topics discussed include the impact of stress relaxation time on the sound absorption of rubber, the dependence of sound absorption on acoustic signal frequencies and intensities, and the calculation of the viscoelastic properties.

The article examines the behaviour of rubber compounds using the viscoelastic mathematical model. It mentions the importance to conduct testing on rubbers to obtain the relaxation curves under the stress-strain and tension curves. Among the parameters that were being assessed include the bulk modulus, load curve, stress relaxation curve and the Poisson's ratio.

Experimental observation and numerical modelling were employed to investigate the solidification cracking behaviour during fibre laser welding of 6013 aluminium alloy. The solidification cracking initiation location and propagation path were studied using a high speed camera system and via metallurgical analysis. A three-dimensional thermomechanical finite element model of fibre laser welding of aluminium alloys was developed, which considered cylindrical volumetric heat source, temperature dependent material properties, solidification shrinkage and stress relaxation in the weld molten pool. The transient evolution and distribution of mechanical strain in the brittle temperature range (BTR) were analysed in detail to find the factors which drove the crack initiation and propagation. The results showed that the solidification cracking initiated near the fusion line and then propagated along the centreline of the weld, which was the result of the strain distribution characteristic in BTR. [ABSTRACT FROM AUTHOR]

Ultrasonic impact treatment (UIT) is a relatively novel technique applied to the toe of welded joints to improve the fatigue life by changing the weld geometry and the residual stress state. In this study, the stress relaxation due to ultrasonic impact treatment is investigated on a six pass welded high strength quenched and tempered steel section. Stress measurements in two orthogonal directions were conducted by energy dispersive synchrotron X-ray diffraction. Results show that the application of only ultrasound to a welded component re-distributes the residual stresses more uniformly, while mechanical impacts in combination with ultrasound is an effective way to release the residual stresses. After welding, diffraction peak broadening due to the lattice distortion, characterised by the full width at half maximum (FWHM), is observed in the region of the weld toes. Ultrasonic impact treatment reduces the FWHM at these locations. [ABSTRACT FROM AUTHOR]

The morphology of the microcellular ethylene-octene copolymer (EOC) of both unfilled and conducting carbon black (Ensaco 250G) filled compounds with different blowing agent and carbon black loading was studied using SEM photomicrographs. The systematic variation in the average, minimum and maximum cell size and cell density with different blowing agent and filler loading was observed. Some physical properties such as relative density, hardness, tensile strength, elongation at break, modulus and tear strength decreases with increase in blowing agent and filler loading. However, the stress relaxation behaviour is found to be independent of blowing agent loading. [ABSTRACT FROM AUTHOR]

The dynamic recrystallisation (DRX) and static recrystallisation (SRX) behaviours of three V-Ti microalloyed steels were studied by the analysis of the true stress-strain curves and the stress relaxation curves under different deformation conditions. The results of DRX showed that deformation activation energy Qdef, peak stress and peak strain increased, as a result of the solute strengthening and dragging effect due to Si. The results of SRX showed that Si increased the SRX activation energy QSRX. The solute retardation parameter for static recrystallisation of Si was calculated. Based on the SRX results, to quantify the drag effect of Si and V, a new model was proposed to describe the time for 50% recrystallisation (t0·5), which was tested and verified by previously published data on similar steels. Precipitation during recrystallisation could lead to a lower value of the Avrami exponent. [ABSTRACT FROM AUTHOR]

The results of tests of the stress relaxation in crosslinked polyethylene under different conditions are presented. With the use of relaxation kernels for describing the experimental data, the mechanism of the relaxation process has been determined, and the dependences of the stresses and Young's modulus over time at different temperatures and strains have been obtained. [ABSTRACT FROM AUTHOR]

The deformation of reinforced concrete columns and beams is controlled by the variation of the flexural rigidity (EI) both along the member and with applied loads and time. Currently, the moment-curvature (M/χ) approach is used to quantify EI. Prior to cracking, the M/χ approach provides a pure mechanics based solution for EI; that is, the only components of the model that have to be determined empirically are the material stress-strain relationships. However after cracking, the M/χ approach has to be semi-empirical, that is EI has to be determined empirically because the M/χ approach cannot simulate the mechanics of tension-stiffening. An alternative approach for quantifying EI using a moment-rotation (M/θ) approach is described in this paper. It is shown that the M/θ approach gives exactly the same results as the M/χ approach prior to cracking but after cracking has an advantage over the M/χ approach in that it can quantify the mechanics of tension-stiffening, that is allow for bond slip and its effect on crack spacing and crack widths. This paper deals with the mechanics of incorporating creep, shrinkage, prestress, relaxation and thermal gradients (broadly referred to as residual strains) on the flexural rigidity of RC beams and columns at all levels of loading prior to concrete softening. [ABSTRACT FROM AUTHOR]

Binary and ternary composites composed of high-density polyethylene (HDPE), boehmite alumina (BA) and different kinds of natural and animal fibers, such as flax, sponge gourd, palm and pig hair (PH), were produced by hot press technique. Aqueous BA suspensions were sprayed on the HDPE/flax mat to prepare nanoparticle/natural fiber–reinforced ternary polymer composites followed by drying. The dispersion of the natural and animal fibers and BA particles in the composites was studied by scanning electron microscopy and discussed. The thermomechanical and stress relaxation properties of the composites were determined by the thermogravimetric analysis, dynamic-mechanical thermal analysis and short-time stress relaxation tests (performed at various temperatures), respectively. The HDPE-based composites were subjected to water absorption and instrumented falling weight impact tests. It was found that all the composite systems increased the stiffness and stress relaxation and reduced the impact toughness. The stress relaxation modulus of natural and animal fiber composites was higher compared with that of the neat HDPE. This modulus increased greatly with incorporation of BA. The relaxation master curves were constructed by applying the time–temperature superposition principle. The inverse of Findley power law could be fairly applicable to describe the relaxation modulus versus time traces for all systems studied. Incorporation of BA particles enhanced the thermal resistance, which started to degrade at higher temperature compared with the HDPE/flax mat composite. The HDPE/flax mat/BA composite could reduce the water uptake. [ABSTRACT FROM AUTHOR]

Residual stress relaxation during post-weld heat treatment (PWHT) is a thermodynamic process, which is affected not only by the heat treatment process, but also by the welding residual stress. In this study, the residual stresses in as welded and heat treated surfacing metal were measured using blind hole and X-ray method. The results reveal that the welding residual stresses are compressive at the surface of the weld and tensile at inner weld. However, after PWHT, the residual stresses at surface and inner weld change to the opposite state. Finite element simulations show that the differences of expansion coefficients between base metal and filler material are the main factor to the changes of stress state. The experimental results verify that the expansion coefficients of base metal and filler materials have been changed greatly after long soaking at high temperature. [ABSTRACT FROM AUTHOR]

A residual facesheet dent on an indented or impacted foam-core sandwich structure significantly affects the residual mechanical properties and the detectability of the damage. However, previous studies have not sufficiently addressed residual dent formation during unloading and the subsequent relaxation. This study quantitatively clarifies the mechanism underlying the formation and the relaxation of a facesheet dent using a segment-wise modeling approach that was formulated for honeycomb sandwich structures in our previous study. We utilize fully backed foam-core sandwich beams under static indentation loading, focusing on the interaction between local deformation of the facesheet and crushing/stretching of the core. This study begins by evaluating the indentation response using quasi-static tests. The indentation behavior is then predicted by extending our previous model. The underlying mechanism of facesheet dent formation and relaxation is quantitatively discussed from the viewpoint of a reaction force from the core to the facesheet. [ABSTRACT FROM AUTHOR]

School principals experience high levels of stress that hamper their self-efficacy and inhibit their executive control capacities. The main stressors that school principals encounter, their reactions to stress and the influence of controlled breathing on their stress relief were investigated by means of a mixed-methods research approach. With a structured questionnaire, participants’ levels of stress were compared before and after attendance of a workshop based on controlled breathing. Participants’ main stressors, their reaction to stress and the influence of controlled breathing on their stress relief were investigated through individual interviewing. It was found that school principals’ main stressors related to extensive workloads carried out in an environment of resource constraints. The regular practising of controlled breathing resulted in a decrease of the levels of stress experienced with main improvements related to revitalized energy levels, restored clarity of thinking and improved interpersonal relationships. The findings contribute to research on school principals’ main stressors and constructive ways of stress relief within the school environment. [ABSTRACT FROM PUBLISHER]

The article presents a study on the effect of anionic surfactant on the ozone resistance of vulcanisates, based from butadiene-acrylonitrile rubbers. The study involved butadiene-acrylonitrile rubbers with different contents of acrylonitrile units, infrared (IR) spectroscopy, and stress relaxation in an ozone-containing medium. The result showed surfactant introduced in the same quantity has a dissimilar effect on the ozone resistance of vulcanisates of butadiene-acrylonitrile copolymer (SKN).

An approximation to the viscoelastic Maxwell model is developed and combined with a Cross-WLF shear- and temperature-dependent model as a means of introducing aspects of viscoelasticity into the Cross-WLF model at a low computational cost. The main objective of the model is to simulate the gas-assisted injection moulding (GAIM) process with the aspect of a material's strain history included. It is shown that the model gives a transient and steady response comparable to the Doi–Edwards viscoelastic model in constant rate shear and uniaxial deformations, and follows WLF temperature dependence. The model is implemented in a three-dimensional finite element code using the ‘pseudo-concentration’ method to model the polymer and gas phases. The ordinary Cross-WLF model had demonstrated a consistent under-prediction of residual wall thickness (RWT) measurements in comparison to experimental results. It is shown that the viscoelastic extension to the Cross-WLF model gives a marked increase in RWT and exhibits aspects of stress relaxation and history dependence. The model is tested against variations of other process control parameters. It is shown that the simulation gives the correct qualitative response for all control parameters assessed, with quantitative prediction within a factor of 2. [ABSTRACT FROM AUTHOR]

A finite strain model for the mechanical degradation of composite materials with multiple families of fine reinforcing fibers is developed and studied. At any instant of time the matrix material may or may not be degrading with all, some, or none of the interpenetrating fibers also undergoing degradation. This multi-component description of damage is governed by coupled differential equations when more than one damage mechanism is active. These differential equations, and the threshold values of the strain invariants that activate the damage process, emerge naturally from a general framework that describes the response of dissipative systems under a maximum rate of dissipation postulate. In this context we then study uniaxial loadings when either a constant stretch or a constant force is suddenly applied to the composite. It is found that the initial type of degradation (e.g. degrading fibers in a non-degrading matrix) may transition to an alternative type of degradation (e.g. the degradation of all constituents) at some finite time into the process. A rich variety of material and load-dependent transition possibilities are systematically uncovered using a combination of asymptotic and numerical techniques. The resulting macroscopic behavior as the material weakens involves relaxation and creep phenomena that are formally similar to viscoelastic material behavior in solids even though the underlying processes are significantly different. Describing implants and tissue constructs containing biodegradable polymers is one possible area of application. [ABSTRACT FROM PUBLISHER]

The dynamic behavior of oral soft tissues supporting removable prostheses is not well understood. We hypothesized that the stress and strain of the mucosa exhibited time-dependent behavior under masticatory loadings. Displacement of the mucosa on the maxillary residual ridge was measured in vivo by means of a magnetic actuator/sensor under vertical loading in partially edentulous individuals. Subject-specific finite element models of homogeneous bone and mucosa were constructed based on computed tomography images. A mean initial elastic modulus of 8.0 × 10—5 GPa and relaxation time of 494 sec were obtained from the curve adaptation of the finite element output to the in vivo time-displacement relationship. Delayed increase of the maximum compressive strain on the surface of the mucosa was observed under sustained load, while the maximum strain inside the mucosa was relatively low and uninfluenced by the duration of the load. The compressive stress showed a slight decrease with sustained load, due to stress relaxation of the mucosa. On simulation of cyclic load, the increment of the maximum strain and the evidence of residual strain were revealed after each loading. The results support our hypothesis, and suggest that sustained and repetitive loads accumulate as surface strain on the mucosa. [ABSTRACT FROM PUBLISHER]

The current study focuses on manufacturing, recycling and mechanical testing of thin (1.5 mm) sisal-PP extruded composite sheets. An 3-factor two-level experimental design based on the Taguchi method was applied in the manufacturing of the composite sheets to maximise their mechanical properties. The sisal-PP composite sheets obtained by setting the factors predicted by the Taguchi analysis were mechanically recycled by pelletising the sheets and feeding through the extruder. The effects of recycling on crystallinity, fibre length, mechanical properties and stress-relaxation were evaluated. By selecting a fibre mass of 30%, polymer MFI of 1.3 g/10 min and 1% of lubricant, ~12%, ~20% and >100% increases in tensile strength, impact strength and tensile modulus respectively were observed. The fibre lengths dropped from 7 mm before extrusion to 6mm and under after extrusion, and under 5 mm after recycling. A marginal change (± 5%) in modulus values was observed in both directions after recycling, the ultimate tensile strength of the recycled sisal-PP specimens dropped down from 41.4 ± 2.5 to 36.4 ± 0.75 MPa along the machine direction, and increased from 19.2 ± 0.5 MPa to 21.4 ± 0.1 MPa transverse to the machine direction mainly due to the decrease in the reinforcing fibre lengths. The recycled composites exhibited greater relaxation compared to the sisal-PP composites; this is because, at higher temperatures, the polymer matrix is in a softened state and the bonding between the fibre and matrix is expected to be weaker and the short fibres behave like polymer rich areas and fail to share the imposed load, thus exhibiting greater relaxation at elevated temperatures. [ABSTRACT FROM AUTHOR]

The relaxation of residual stress in a shot peened high strength low alloy steel in response to applied loads has been investigated for both single shot and cyclic loading. The near surface residual stress was measured predominantly by X-ray diffraction (first 10 μm) but supplemented for some conditions by measurements using high resolution, fine increment hole drilling to yield depth profiling. The compressive residual stress showed a peak at ∼50 μm below the surface. Under single shot compressive loading, relaxation became significant when the combined surface and applied compressive stresses exceeded the proportional limit of the unpeened surface. The steel showed good resistance to residual stress relaxation under cyclic loading conditions, with only a 13% decrease despite cyclic load amplitudes at 50% of the 02% proof stress in compression. Significant relaxation did occur at applied stress amplitudes of ∼80% of the 02% proof stress in compression. While large single shot or cyclic stresses would represent rare occurrences, they could lead to a marked loss in beneficial compressive stress and should be accounted for in structural integrity assessment. [ABSTRACT FROM AUTHOR]

Perioperative low-dose ethanol infusion is a feasible option for stress reduction and prophylaxis of alcoholism-associated complications. Because alcohol has inhibitory effects on hemostasis, our study focused on the effect of perioperative low-dose ethanol infusion on bleeding complications, defined as transfused blood units and reoperations, in alcoholic patients undergoing major surgery. We included 44 long-term alcoholic patients scheduled for tumor resection of the aerodigestive and gastrointestinal tract. Patients were randomly assigned to the ethanol or control group. Ethanol infusion (0.5 g ethanol/kg body weight/24 hours) started before surgery and was continued until the postoperative Day 3. Regarding all patients, there was no statistically significant difference in the amount of transfused blood between the ethanol and control groups. However, the effect of ethanol infusion on bleeding complications depended on the site of surgery. Ethanol infusion resulted in an increased number of transfused blood units in gastrointestinal patients and a decreased number of transfused units in patients undergoing tumor resection of the aerodigestive tract. In conclusion, perioperative ethanol infusion in long-term alcoholic patients with tumor resections of the aerodigestive tract is an option for stress reduction without increased risk for blood transfusion. In contrast, ethanol infusion in patients with tumor resections in the gastrointestinal tract could increase the risk for bleeding complications. [ABSTRACT FROM AUTHOR]

Introduction/Purpose: Non-union and delayed union are common complications following hindfoot arthrodesis. The failure to obtain and maintain compression at the arthrodesis site may be a causative factor. Partially threaded cannulated 'lag' screws are commonly used for fixation in hindfoot arthrodesis; however, the ability of these screws to maintain compression in the hindfoot bones has not been well-characterized. The goals of this work were to: 1) quantify the stress relaxation response of hindfoot bones when compressed with a lag screw, particularly compression change upon screw retightening, and 2) compare the results to prior studies on stress relaxation in direct bone compression. Methods: 12 pairs of calcaneus, talus and tibia bone cylinders, 25mm in diameter, were cut with a keyhole saw from fresh thawed cadaveric feet. The bone cylinders were prepared for simulated arthrodesis by removing the cartilage and flattening the subchondral surface with an oscillating saw. A Futek LTH 300 donut load cell and two metal washers were sandwiched between the two bone cylinders. An 8.0mm partially threaded cannulated lag screw (Smith and Nephew) was placed from the posteriorinferior aspect of the calcaneus cylinder to the anterior-superior aspect of the talus cylinder to simulate subtalar arthrodesis, or from the superior aspect of the tibia cylinder to the inferior aspect of the talus cylinder. Compression was recorded continuously as the screws were tightened by three-quarters of a turn and left untouched for 3 minutes. Recording continued as the screws were retightened one quarter-turn and then left untouched for 30 additional minutes. Results: Both initial and subsequent compression with a lag screw demonstrated a stress relaxation pattern, with increased maximum compression and slowed decay upon retightening. Maximum compression after three quarter-turns and retightening averaged 284 N and 351 N, respectively (n=12; t=-2.55; p=0.0136). Time to decay to 80% of maximum compression after three quarter-turns and retightening averaged 34 and 528 (n=9; t=-2.59; p=0.0159) seconds, respectively, compared to 21 seconds for direct compression. Compression loss 30 minutes after retightening averaged 25.5% (SD=8.8%), compared to 34.3% for direct compression. Conclusion: The maximum compression attained following retightening was significantly greater than the maximum compression attained after initial tightening. The time required to reach 80% of maximum compression was significantly longer after subsequent compared to initial maximum compression. These findings suggest that screw retightening before surgical wound closure may slow compression loss, which could increase the likelihood of successful arthrodesis. Lag screw compression resulted in a lower percent compression loss and loss rate compared to direct compression, which was previously used to describe bone viscoelasticity. [ABSTRACT FROM AUTHOR]

The objective of this project is to investigate the application influences of stitching techniques on the compaction and relaxation behavior of preforms made of three different semi-finished glass fabric types. Particular attention is directed to the impact of stitching parameters (i.e., thread tension and seam spacing) and the compaction procedure (single-stage vs multi-stage) on the experimental results. The experiments prove that there is no difference between single-stage and multi-stage compaction. Furthermore, it is shown that the stitching of the preforms leads to an increase in compaction force due to the additional volume of thread to be compacted. Comparing high and low thread tension reveals that an increasing thread tension causes pre-compaction of the preform, thus reducing the required compaction force, though the degree of pre-compaction seems to be confined to a limit above which no reduction in compaction force can be observed any more. Regarding the spacing of the seams, it turns out that only the narrow spacing (5 mm) has a significant influence on compaction, whereas the wide spacing (20 mm) does not lead to a notable change in compaction behavior compared to the unstitched preforms. Finally, visual inspection of the preforms during compaction and relaxation shows that the morphologic change of the preform surfaces is a very complex phenomenon which cannot easily be modeled. No visual change of the preform surfaces are observed during relaxation. Therefore, it is assumed that relaxation is caused by invisible microstructural reorientation of glass filaments within single rovings. [ABSTRACT FROM AUTHOR]

This article deals with experimental and analytical analysis of the mechanical behavior of crushed foam cores. Three rigid cellular core materials are tested in a uniaxial compression-tension cycle. An analytical model is proposed describing the stress-strain curve in tension and secondary compression. A special emphasis is laid on the relaxation behavior of a crushed foam core in tension. [ABSTRACT FROM AUTHOR]

Control of the sequential orientation (tenter) process for optical films is one of the most important factors in developing the retardation films which are needed for large sized liquid crystal displays (LCD). Analysis to predict the thickness distributions and bowing phenomenon during the tentering process is performed by finite element analysis (FEA). In addition, this study provides a new calculation method to predict the optical axes of stretched films from elastoplastic FEA results. The optical axes have to components. One is a stress-dependent term and the other is a strain-dependent term. To predict the molecular orientation, time-dependent effects are concerned with both. Moreover, polymer relaxation processes are examined. The FEA and optical property calculation are independent in the methods. This method enables one to easily calculate optical axes of films during the tentering process without a change of the conventional FEA calculation process. [ABSTRACT FROM AUTHOR]

This work discusses the elastic-viscoelastic correspondence principle and the related issues in linear viscoelasticity of functionally graded materials (FGMs). It is clarified that the correspondence principle is directly applicable to certain boundary value problems of viscoelastic FGMs with closed-form solutions, and for general boundary value problems, the correspondence principle holds only for FGMs with variables separable relaxation moduli (moduli in separable form in space and time). For the relaxation moduli of an FGM to be variables separable, the time-dependance of the relaxation moduli in shear and dilatation should be identical. This imposes a rather severe restriction on the application of the correspondence principle to FGMs because engineering materials usually exhibit significantly different relaxation behavior in shear and dilatation. As an application of the correspondence principle, the stress and displacement fields near a stationary crack tip in a viscoelastic FGM are obtained. The paper also presents general stress intensity factor formulae for the crack problems of FGMs with separation of variables solutions. [ABSTRACT FROM AUTHOR]

Stress relaxation is relevant to the design of both civil and mining excavations. While many authors refer to the adverse effect of stress relaxation on excavation stability, some present compelling empirical evidence indicating that stress relaxation does not have a significant effect. Establishing clear definitions of stress relaxation was critical to understanding and quantifying stress relaxation of the various types that have been referred to in the literature. This paper defines three types of stress relaxation – partial relaxation, full relaxation and tangential relaxation. Once clear definitions were determined, it became clear that the theoretical arguments and empirical evidence presented by various authors to support their respective cases are not contradictory; rather, the different conclusions can be attributed to different types of stress relaxation. In particular, when the minor principal stress is negative the intermediate principal stress has been identified as significantly affecting jointed rock mass behaviour. The aim of the study was to review and evaluate existing methods of quantifying the effect of stress relaxation around underground excavations and, if necessary, propose a new set of recommendations. An empirical stope stability model, that has been termed the Extended Mathews stability chart, was considered to be the most appropriate method of quantifying the effects of stress relaxation. A new set of guidelines to account for the effect of stress relaxation on excavation stability in the Extended Mathews stability chart has been proposed from a back-analysis of 55 case histories of stress relaxation. [ABSTRACT FROM AUTHOR]

Creep and stress relaxation behaviors of unidirectional nylon6-rubber composite under biaxial tensile loading are studied in this paper. Experiments of creep and stress relaxation are performed at room temperature at different stress levels and different strain levels on both longitudinal and transverse directions respectively. The influence of different stress levels on creep strain and the interaction between longitudinal and transverse stresses are studied. Similar analyses are conducted to assess stress relaxation when specimens are subjected to biaxial tensile loading. These analyses provide solid experimental foundation for understanding nylon6-rubber composite properties. A new relaxation-type constitutive model is presented for predicting stress relaxation behaviors under biaxial tensile condition. In this model, octahedral shear strain is introduced as a variable parameter, which is related to longitudinal strain and transverse strain respectively. Schapery unidirectional integral model and the least square method are applied to determine parameters in the constitutive model. Comparisons of experimental data with theoretical curves are made and good agreements are obtained for rubber composite subjected to biaxial tensile loads. [ABSTRACT FROM AUTHOR]

Using stress relaxation measurements, we describe the kinetics of the physical aging process for two semi-crystalline, biaxially stretched polyester films, PET and PEN, within their respective a transition regimes. By applying the principles of time-aging time and time-aging temperature equivalence, we establish the temperature dependence of the corresponding aging acceleration factors over a temperature range spanning from ca. Tg - 40°C to Tg. The resuits for both materials show significant increase in the rate of physical aging at ca. 20°C below the glass transition temperature. However, the increase in aging rate is approximately tenfold higher for PET, implying that the segmental mobility of this polymer is significantly higher at the corresponding temperature. [ABSTRACT FROM AUTHOR]

Dynamic creep tests on specimens of dental amalgam gave a relationship between creep (ε), applied dynamic stress (σ), and number of applied stress cycles (N) as ε = AN (mσ + c) where A, m, and c are constants. This equation can be used to predict creep of amalgam at low levels of stress and extended times. The pattern of results produced for different types of amalgam indicates that the creep process is multifactorial in nature. [ABSTRACT FROM AUTHOR]

Stress-relaxation measurements were performed on specimens of radicular human dentin. The relaxation modulus showed a linear dependence on the logarithm of time and the approximation to the logarithmic distribution function of relaxation times was used to predict the behavior of other viscoelastic properties. This experimental technique provides significant criteria for the design of polymeric restorative and prosthetic materials. [ABSTRACT FROM AUTHOR]