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Stress relaxation
About: Stress relaxation is a research topic. Over the lifetime, 12959 publications have been published within this topic receiving 270815 citations.
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TL;DR: In this article, the authors reviewed the creep mechanisms in ceramics and compared experimental data, showing that there are three major types of creep behavior: a stress exponent close to 5 due to control by dislocation climb and fully ductile behaviour; a stress expander close to 3 due to controlling by climb from Bardeen-Herring sources and less than jive interpenetrating independent slip systems; and a stress extraction close to 1 due to diffusion creep.
Abstract: The creep mechanisms in ceramics are reviewed briefly and then compared with experimental data. It is shown that there are three major types of creep behaviour: a stress exponent close to 5 due to control by dislocation climb and fully ductile behaviour; a stress exponent close to 3 due to control by climb from Bardeen–Herring sources and less than jive interpenetrating independent slip systems; and a stress exponent close to 1 due to diffusion creep. The role of interface reaction control and the transitions from diffusion to power law creep are also examined.MST/1389
86 citations
TL;DR: This paper examined the tensile viscoelastic properties of circumferential and radial strips of porcine aortic valve leaflets following fixation in glutaraldehyde and formaldehyde, with or without pressure.
Abstract: We have examined the tensile viscoelastic properties of circumferential and radial strips of porcine aortic valve leaflets following fixation in glutaraldehyde and formaldehyde, with or without pressure. After aldehyde treatment, the radial strips remained weaker and less stiff than circumferential strips and responded slightly differently to the treatment. After fixation, with or without pressure, the radial strips showed large changes in stress-strain and hysteresis responses due to initial loading, and there was a twofold reduction in tensile strength and final stiffness. For strips in both directions, fixation without pressure produced doubled extensibility and a ramping stress-strain curve. Permanent (plastic) deformation of 5-20% occurred as a result of cyclic loading, stress relaxation, and creep experiments. Pressure fixation, however, produced little change in stress-strain results other than a simple shift to lower strain and produced no plasticity. Both methods of fixation reduced stress relaxation and creep. Mechanical test results are consistent with a loss of ground substance matrix during fixation. Reductions in tensile strength after fixation may be due to "riveting" of collagen geometry, producing local stress concentrations.
86 citations
TL;DR: In this article, the application of stress intensity factors derived from linear elastic fracture mechanics (LEFM) to fracture at creep temperatures has been considered and it is shown that a LEFM approach is inapplicable to predicting creep crack growth rates, whilst the net section stress is found to correlate well with the crack growth rate.
Abstract: The application of stress intensity factors derived from linear elastic fracture mechanics (LEFM) to fracture at creep temperatures has been considered. From tensile creep rupture tests on single edge notched and notched centre hole specimens of solution treated A.I.S.I. type 316 stainless steel, it is shown that a LEFM approach is inapplicable to predicting creep crack growth rates, whilst the net section stress is found to correlate well with the crack growth rates. These observations have been explained by considering the creep relaxation that takes place at the notch root, smoothing out the local stresses and thus making the LEFM stress distribution inapplicable. The resulting stress distribution supports the observation that the net section stress is a successful criterion on which to predict creep rupture in stainless steel.
86 citations
TL;DR: In this article, the isotropic residual stresses in a composite subjected to three-dimensional constraints are calculated by extending a thermo-viscoelastic model developed previously by Simon et al. to describe the time, temperature, and conversion dependence of the shear modulus for a commercial thermosetting material during cure.
Abstract: The isotropic residual stresses in a composite subjected to three-dimensional constraints are calculated by extending a thermo-viscoelastic model developed previously by Simon et al. [1] to describe the time, temperature, and conversion dependence of the shear modulus for a commercial thermosetting material during cure. Experimental residual stress data as a function of cure time are fit to obtain limiting values for the rubbery and glassy bulk moduli. The effects of cure history on isotropic residual stresses are then investigated via simulations using the bulk moduli, a model of the cure kinetics, the relationship beween T g and conversion, and the stress relaxation function obtained in the thermo-viscoelastic model which includes the dependence of the shift factor on temperature and conversion. The isotropic residual stresses at room temperature can be directly related to the cure temperature at which gelation occurred for cases when vitrification does not occur during cure.
86 citations
TL;DR: In this paper, a finite element model including a thermal analysis and a stress-strain analysis of the deposition was developed to model the origin of the thermal stresses and to verify the measured residual stresses.
Abstract: Thermal barrier coatings consisting of a NiCrAlY bond coating and a 1.4 mm thick partially stabilised zirconia top coating were air plasma sprayed onto grit blasted nickel-base substrates. Two samples were produced using different amounts of external cooling during spraying of the top coatings. The residual stress profiles in the samples were measured after each manufacturing process step with a layer removal technique. A finite element model including a thermal analysis and a stress-strain analysis of the deposition was developed to model the origin of the thermal stresses and to verify the measured residual stresses. The main components for the residual stresses in the sprayed coatings were identified as stresses developing during the rapid cooling of individual droplets (quenching stresses) and stresses formed during cooling from deposition temperature to room temperature. The quenching stresses were predicted to be low and tensile in the top coating, due to stress relaxation by formation of vertical microcracks. During cooling to room temperature, compressive stresses were superimposed on the top coating quenching stresses. The final residual stresses were predicted to be compressive in the top coating. This was confirmed by measurements (−15 MPa). In the bond coating, no stress relaxation by microcracking was observed and the residual stresses were found to be tensile (~100 MPa). In the substrate, compressive residual stresses reaching −200 MPa were found in a zone to a depth of 0.3, mm into the substrate. The stresses were found to have originated during the grit blasting of the substrates prior to bond coating deposition. A correlation between modelled inelastic strain and measured densities of vertical microcracks in the top coating was obtained. High values were found close to the bond coating, which were correlated to a low substrate temperature during spraying of the top coating material.
86 citations