Stress relaxation

About: Stress relaxation is a research topic. Over the lifetime, 12959 publications have been published within this topic receiving 270815 citations.

Modeling of Residual Stress in Oxide Scales

Abstract: The magnitude of the residual stress in an oxidescale, and how this varies with temperature, is of majorimportance in understanding the failure mechanisms ofoxide scales. This stress encompasses both growth stresses introduced at the oxidationtemperature and thermal-expansion-mismatch stressesinduced on heating and cooling, as well as anyexternally applied stresses or stress relaxation whichtakes place in the scale/substrate system. Althoughsome of these components are reasonably well understood(e.g., thermal stresses), growth stresses and therelaxation of the total scale stress by creep orfracture processes are much less well understood. Inthis study a model has been developed to predict stressgeneration and relaxation in oxide scales as a functionof time and temperature for both isothermal exposure and cooling to room temperature. The modeldetermines growth stress and thermal-stress generationin the scale and how this is balanced by stresses in thesubstrate. The substrate stresses are then allowed to relax by creep and the scale stressesrecalculated. This model accurately predicts theroom-temperature scale stresses for a range ofscale/alloy systems. The model can be used to show howthe scale stress depends on oxidation temperature,cooling rate, substrate, and scale thickness. The modelpredictions are discussed in light of experimentalobservations for alumina scales on FeCrAlY.

Stress Relaxation by Addition−Fragmentation Chain Transfer in Highly Cross-Linked Thiol−Yne Networks

Abstract: Radical mediated addition-fragmentation chain transfer of mid-chain allyl sulfide functional groups was utilized to reduce polymerization-induced shrinkage stress in thiol-yne step-growth photopolymerization reactions. In previous studies, the addition-fragmentation of allyl sulfide during the polymerization of a step-growth thiol-ene network demonstrated reduced polymerization stress; however, the glass transition temperature of the material was well below room temperature (~ -20°C). Many applications require super-ambient glass transition temperatures, such as microelectronics and dental materials. Polymerization reactions utilizing thiol-yne functional groups have many of the advantageous attributes of the thiol-ene-based materials, such as possessing a delayed gel-point, resistant to oxygen inhibition, and fast reaction kinetics, while also possessing a high glass transition temperature. Here we incorporate allyl sulfide functional groups into a highly crosslinked thiol-yne network to reduce polymerization-induced shrinkage stress. Simultaneous shrinkage stress and functional group conversion measurements were performed during polymerization using a cantilever-type tensometer coupled with a FTIR spectrometer. The resulting networks were highly crosslinked, possessed super-ambient glass transition temperatures, and exhibited significantly reduced polymerization-induced shrinkage stress when compared with analogous propyl sulfide-containing materials that are incapable of addition-fragmentation.

Effects of Strain Level and Proteoglycan Depletion on Preconditioning and Viscoelastic Responses of Rat Dorsal Skin

Abstract: The mechanical response of rat dorsal skin was experimentally studied under cyclic uniaxial ramp stretches to various strain levels. Special emphasis was paid to the effects of the preconditioning protocol on the stress-strain relationship, and to the effects of ramp strain level and proteoglycan (PG) depletion, on viscoelasticity and preconditioning responses. The results show that preconditioning significantly reduced both the slope of the low strain stress-strain relationship, and the stress levels at consecutive stretch cycles. Following a short rest there was a significant partial recovery. Stress decay due to preconditioning was significant at all strain levels, and increased with strain. Stress relaxation was significant at all strain levels, but varied little with strain. Recovery following a 10 min rest was minor at all strain levels and varied little with strain. PG-depleted samples manifested similar response patterns. These results are consistent with the following notion: (1) skin consists of three mechanical components: elastin and proteoglycan which dominate the low strain response and are effected by preconditioning and (PG) depletion, and collagen which dominates the high strain response and is unaffected by preconditioning and PG depletion; (2) that the viscoelasticity of elastin and PG vs that of collagen are similar, so that rat dorsal skin can be regarded quasilinear viscoelastic.

Impression creep behavior of lead-free Sn–5Sb solder alloy

Abstract: Creep behavior of the lead-free Sn–5Sb solder alloy in the cast and wrought conditions was investigated by impression testing. The tests were carried out under constant stress in the range of 18–135 MPa and at temperatures in the range of 298–403 K. Assuming a power law relationship between the impression rate and the punching stress, stress exponents of 2.8 and activation energies of 41.3 kJ mol −1 were determined for the wrought material over the whole stress and temperature ranges studied. For the cast condition, however, stress exponents of 5.4 and 11.4 and activation energies of 53.8 and 75.8 kJ mol −1 were obtained at low and high stresses, respectively. The n value of 2.8 and the activation energy of 41.3 kJ mol −1 , which is very close to the activation energy for grain boundary diffusion of β-Sn, together with a very fine grain size of 4.5 μm and a uniform distribution of fine SnSb particles, may suggest that grain boundary sliding is the dominant creep mechanism in the wrought condition. For the cast material with a coarse grain size of 280 μm, the low stress regime activation energy of 53.8 kJ mol −1 , which is close to that of the self diffusion of pure Sn, and a stress exponent of 5.4 suggest that the operative creep mechanism is dislocation climb. This behavior is in contrast to the high stress regime in which, the n = 11.4 and Q = 75.8 kJ mol −1 are indicative of a dislocation creep mechanism.

Time-dependent compaction band formation in sandstone

Abstract: Compaction bands in sandstone are laterally extensive planar deformation features that are characterized by lower porosity and permeability than the surrounding host rock. As a result, this form of localization has important implications for both strain partitioning and fluid flow in the Earth's upper crust. To better understand the time dependency of compaction band growth, we performed triaxial deformation experiments on water-saturated Bleurswiller sandstone (initial porosity = 0.24) under constant stress (creep) conditions in the compactant regime. Our experiments show that inelastic strain accumulates at a constant stress in the compactant regime, manifest as compaction bands. While creep in the dilatant regime is characterized by an increase in porosity and, ultimately, an acceleration in axial strain rate to shear failure, compaction creep is characterized by a reduction in porosity and a gradual deceleration in axial strain rate. The global decrease in the rates of axial strain, acoustic emission energy, and porosity change during creep compaction is punctuated at intervals by higher rate excursions, interpreted as the formation of compaction bands. The growth rate of compaction bands formed during creep is lower as the applied differential stress, and hence, background creep strain rate, is decreased. However, the inelastic strain associated with the growth of a compaction band remains constant over strain rates spanning several orders of magnitude (from 10−8 to 10−5 s−1). We find that despite the large differences in strain rate and growth rate (from both creep and constant strain rate experiments), the characteristics (geometry and thickness) of the compaction bands remain essentially the same. Several lines of evidence, notably the similarity between the differential stress dependence of creep strain rate in the dilatant and compactant regimes, suggest that as for dilatant creep, subcritical stress corrosion cracking is the mechanism responsible for compactant creep in our experiments. Our study highlights that stress corrosion is an important mechanism in the time-dependent porosity loss, subsidence, and permeability reduction of sandstone reservoirs.