Showing papers on "Creep published in 2001"
TL;DR: In this article, the authors reviewed the creep deformation resistance and rupture life of high Cr ferritic steel with a tempered martensitic lath structure, and focused on the following three subjects: creep mechanism of the ferritic steels, its alloy design for further strengthening, and loss of its creep rupture strength after long-term use.
Abstract: The creep deformation resistance and rupture life of high Cr ferritic steel with a tempered martensitic lath structure are critically reviewed on the basis of experimental data. Special attention is directed to the following three subjects: creep mechanism of the ferritic steel, its alloy design for further strengthening, and loss of its creep rupture strength after long-term use. The high Cr ferritic steel is characterized by its fine subgrain structure with a high density of free dislocations within the subgrains. The dislocation substructure is the most densely distributed obstacle to dislocation motion in the steel. Its recovery controls creep rate and rupture life at elevated temperatures. Improvement of creep strength of the steel requires a fine subgrain structure with a high density of free dislocations. A sufficient number of pinning particles (MX particles in subgrain interior and M 23 C 6 particles on sub-boundaries) are necessary to cancel a large driving force for recovery due to the high dislocation density. Coarsening and agglomeration of the pinning particles have to be delayed by an appropriate alloy design of the steel. Creep rupture strength of the high Cr ferritic steel decreases quickly after long-term use. A significant improvement of creep rupture strength can be achieved if we can prevent the loss of rupture strength. In the steel tempered at high temperature, enhanced recovery of the subgrain structure along grain boundaries is the cause of the premature failure and the consequent loss of rupture strength. However, the scenario is not always applicable. Further studies are needed to solve this important problem of high Cr ferritic steel. MX particles are necessary to retain a fine subgrain structure and to achieve the excellent creep strength of the high Cr ferritic steel. Strengthening mechanism of the MX particles is another important problem left unsolved.
610 citations
TL;DR: In this article, a constitutive equation based on these experimental results that includes flow laws for these four creep mechanisms is described. But this equation is in excellent agreement with published laboratory creep data for coarse-grained samples at high temperatures.
Abstract: Creep experiments on fine-grained ice reveal the existence of three creep regimes: (1) a dislocation creep regime; (2) a superplastic flow regime in which grain boundary sliding is an important deformation process; and (3) a basal slip creep regime in which the strain rate is limited by basal slip. Dislocation creep in ice is likely climb-limited, is characterized by a stress exponent of 4.0, and is independent of grain size. Superplastic flow is characterized by a stress exponent of 1.8 and depends inversely on grain size to the 1.4 power. Basal slip limited creep is characterized by a stress exponent of 2.4 and is independent of grain size. A fourth creep mechanism, diffusional flow, which usually occurs at very low stresses, is inaccessible at practical laboratory strain rates even for our finest grain sizes of approximately 3 micrometers. A constitutive equation based on these experimental results that includes flow laws for these four creep mechanisms is described. This equation is in excellent agreement with published laboratory creep data for coarse-grained samples at high temperatures. Superplastic flow of ice is the rate-limiting creep mechanism over a wide range of temperatures and grain sizes at stresses less than or equal to 0.1 MPa, conditions which overlap those occurring in glaciers, ice sheets, and icy planetary interiors.
608 citations
TL;DR: In this paper, a review focusing on precipitation in creep resistant austenitic stainless steels, in particular wrought heat resistant grades containing niobium and titanium additions, is presented.
Abstract: Austenitic stainless steels have for some time been used as superheater tubes in the electricity generation industries in harsh environments with temperatures as high as 650°C at pressures of some 200 atm; they are expected to provide reliable service for 30 years or more. Their detailed mechanical properties are dependent on the stability of the microstructure, particularly the formation, dissolution, and coarsening of precipitates. Although the precipitation processes have been studied extensively, there remain important discrepancies. It is known that small changes in the chemical composition or thermomechanical processing can profoundly influence the evolution of the microstructure. This review focuses on precipitation in creep resistant austenitic stainless steels, in particular wrought heat resistant grades containing niobium and titanium additions. Conventional alloys such as 18–8 and 16–10 are included together with the new NF709 (20–25) and other recent variants. Precipitates forming in a...
583 citations
01 May 2001
TL;DR: In this article, the authors enlarge the operator-based methodology of hysteresis operators by elements that allow the description of systems with hysteretic and creep effects, and use this concept for real-time inverse feedforward controller for piezo-electric actuators.
Abstract: Since the beginning of the 1990s, hysteresis operators have been employed on a larger scale for the linearisation of hysteretic transducers. One reason for this is the increasing number of mechatronic applications that use solid-state actuators based on magnetostrictive or piezo-electric material or shape memory alloys. All of these actuator types show strong hysteretic effects. In addition to hysteresis, piezo-electric actuators show strong creep effects. Thus, the objective of the paper is to enlarge the operator-based methodology of hysteresis operators by elements that allow the description of systems with hysteresis and creep. To reach this objective, following the procedure used for hysteretic systems, creep operators are introduced to form, together with the hysteresis operators, a system operator for the simultaneous consideration of both phenomena. With regard to applications in control and measurement technology, the existence, uniqueness, Lipschitz continuity and thus input-output stability of its inverse operator are theoretically supported by functional analytical methods. Subsequently, the efficiency of this new concept is demonstrated, in practice, by a real-time inverse feedforward controller for piezo-electric actuators. Using this control concept, the tracking errors caused by hysteretic and creep effects are reduced by approximately one order of magnitude.
483 citations
385 citations
TL;DR: In this article, the brittle failure of polycrystalline ice at temperatures > 0.8Tmp and strain rates (∼10−7−10−1 s−1) is discussed.
298 citations
TL;DR: An experimental and numerical study on mechanical properties, such as strength, elastic modulus, creep and shrinkage, of Self-Compacting Concrete, SCC, and the corresponding properties of Normal Compacting concrete, NC, is outlined in this paper.
291 citations
TL;DR: In this article, a detailed analysis of the creep behaviour for different film and substrate materials is done, and the influence of loading time and hold period at maximum load on the hardness and modulus results is investigated.
Abstract: To improve the accuracy and comparability of hardness and modulus results from nanoindentation experiments an evaluation of the creep behaviour is required. Creep depends on the material and normally diminishes to very low values within some seconds. Nevertheless, it influences the maximum depth and the upper part of the unloading curve in a way that measurement errors of more than 20% may occur. In this work, a detailed analysis of the creep behaviour for different film and substrate materials is done. In addition, the influence of loading time and hold period at maximum load on the hardness and modulus results is investigated. The results show that especially for materials with low hardness-to-modulus ratio (mostly metals), the modulus results are not reliable if the hold period is chosen too low. Hold periods are proposed in dependence on the material type that should be kept for high accuracy measurements.
282 citations
TL;DR: In this paper, the effect of metakaolin (MK) on the creep and shrinkage of concrete mixes containing 0, 5, 10, and 15% MK has been investigated, and the results showed that the early age autogenous shrinkage measured from the time of initial set of the concrete was reduced with the inclusion of MK, but the long-term autogeneous shrinkage measuring from the age of 24 h was increased.
Abstract: The effect of metakaolin (MK) on the creep and shrinkage of concrete mixes containing 0%, 5%, 10%, and 15% MK has been investigated. The results showed that the early age autogenous shrinkage measured from the time of initial set of the concrete was reduced with the inclusion of MK, but the long-term autogenous shrinkage measured from the age of 24 h was increased. At 5% replacement level, the effect of MK was to increase the total autogenous shrinkage considered from the time of initial set. While at replacement levels of 10% and 15%, it reduced the total autogenous shrinkage. The total shrinkage (autogenous plus drying shrinkage) measured from 24 h was reduced by the use of MK, while drying shrinkage was significantly less for the MK concretes than for the control concrete. The total creep, basic creep as well as drying creep were significantly reduced particularly at higher MK replacement levels. Compared with estimated values by the CEB 90 model, total creep of all concretes was overestimated, especially in the mixes containing the higher levels of MK. For basic creep, estimates for low levels of MK were acceptable but, for the higher levels, creep was overestimated.
271 citations
TL;DR: Non-linear viscoelasticity of ligament requires a description more general than the separable quasilinear viscoELasticity (QLV) formulation commonly used, and results from rat MCLs consistently show a nonlinear behavior in which the rate of creep is dependent upon stress level and the rates of relaxation are dependent upon strain level.
Abstract: Ligaments display time-dependent behavior, characteristic of a viscoelastic solid, and are nonlinear in their stress-strain response. Recent experiments (25) reveal that stress relaxation proceeds more rapidly than creep in medial collateral ligaments, a fact not explained by linear viscoelastic theory but shown by Lakes and Vanderby (17) to be consistent with nonlinear theory. This study tests the following hypothesis: nonlinear viscoelasticity of ligament requires a description more general than the separable quasilinear viscoelasticity (QLV) formulation commonly used. The experimental test for this hypothesis involves performing both creep and relaxation studies at various loads and deformations below the damage threshold. Freshly harvested, rat medial collateral ligaments (MCLs) were used as a model. Results consistently show a nonlinear behavior in which the rate of creep is dependent upon stress level and the rate of relaxation is dependent upon strain level. Furthermore, relaxation proceeds faster than creep; consistent with the experimental observations of Thornton et al. (25) The above results from rat MCLs are not consistent with a separable QLV theory. Inclusion of these nonlinearities would require a more general formulation.
TL;DR: In this paper, uniaxial restrained shrinkage tests were conducted on plain and fiber-reinforced concrete (FRC) samples to provide data on shrinkage stresses, shrinkage strain, and tensile creep at early age.
Abstract: In this study, uniaxial restrained shrinkage tests were conducted on plain and fiber-reinforced concrete (FRC) samples to provide data on shrinkage stresses, shrinkage strain, and tensile creep at early age. The influences of water-cement ratio (w/c), fiber reinforcement, and curing conditions on restrained shrinkage behavior of concrete were investigated. It was found that tensile creep relaxed shrinkage stresses by 50% and doubled the failure strain capacity. Both the magnitude and time history of the shrinkage stress influence the time of cracking, which in this study occurred at approximately 80% of the static tensile strength. Steel fibers substantially delay the shrinkage cracking, but without influencing the stress at failure. Finally, it was found that sealing of the concrete specimens did not eliminate the early age shrinkage, and that wet curing effectively relaxed shrinkage stresses.
TL;DR: It is reported here that even the particles in layers deep in the bulk exhibit very slow flow and that such motion can be detected at an arbitrary depth and that the creep motion is observable in all sheared granular systems.
Abstract: We investigate experimentally granular piles exhibiting steady surface flow. Below the surface flow, it has been believed that a ``frozen'' bulk region exists, but our results show no such frozen bulk. We report here that even the particles in layers deep in the bulk exhibit very slow flow and that such motion can be detected at an arbitrary depth. The mean velocity of the creep motion decays exponentially with depth, and the characteristic decay length is approximately equal to the particle size and is independent of the flow rate. It is expected that the creep motion we have seen is observable in all sheared granular systems.
TL;DR: In this paper, the effects of ultrafine ground granulated blast-furnace slag (GGBS) and silica fume (SF) on creep and drying shrinkage of HPC were compared and the mechanism was analyzed.
TL;DR: In this article, the authors summarized various aspects of the prediction of concrete creep and shrinkage to be discussed in the conference lecture, including theories of physical mechanism, prediction models, constitutive equations, computational approaches, probabilistic aspects, and research directions.
TL;DR: In this article, a coupled oxidation-constitutive approach is proposed to describe the effect of the phase transformations caused by local internal and external oxidation processes on the constitutive behav- iour of the metallic coating.
TL;DR: In this article, high temperature creep properties of Mg-Gd-Y-Zr alloys have been evaluated quantitatively under a stress range of 50-100 MPa and a temperature range of 250-300°C.
Abstract: High temperature creep properties of Mg-Gd-Y-Zr alloys have been evaluated quantitatively. Creep test was carried out under a stress range of 50-100 MPa and a temperature range of 250-300°C. Within the limits of the creep test conditions used in this study, the activation energy for creep of investigated alloys is in the range of 160-240 kJ/mol, and the stress exponent is in the range of 3.7-5.2. Accordingly, the creep mechanism of the investigated alloys is considered to be power law creep. Creep resistance of the investigated alloys depends on chemical composition, it improves with increase in gadolinium content, which implies an increase in the quantity of precipitates. The creep resistance of the high gadolinium-containing alloys exceeds that of the existing heat resistant magnesium alloy, WE54A alloy.
TL;DR: In this article, a methodology is described for the calculation of complex precipitation reactions over periods extending many tens of years. And the estimation of the creep rupture stress using a neural network technique is described.
Abstract: Creep resistant steels must be reliable over very long periods of time in severe environments. Their microstructures have to be very stable, both in the wrought and in the welded states. This paper reviews the quantitative methods for the design of steels for elevated temperature applications. A methodology is described for the calculation of complex precipitation reactions over periods extending many tens of years. However, microstructure alone is not enough in the design of alloys. The estimation of the creep rupture stress using a neural network technique is described in the second part of this review. The calculation of the influence of solute-elements on the self-diffusivity of iron, which features in many creep equations, is an emerging area in alloy design. The methodology for such calculations is reviewed in the final section of the paper.
TL;DR: In this article, a phenomenalogical viscoelasto-plastic constitutive model is presented to represent the observed time dependent mechanical behavior of polytetrafluoroethylene.
TL;DR: In this article, a family of oxide fibers, Nextel™ 610, 650 and 720, was developed specifically for the reinforcement of metal and ceramic matrix composites, and the strength of both single filaments and multi-filament rovings of Nextel610, 650, and 720 fibers was determined between 25 and 250mm gauge length.
Abstract: A family of oxide fibers, Nextel™ 610 Ceramic Oxide Fiber, Nextel™ 720 Ceramic Oxide Fiber and a new fiber, Nextel™ 650 Ceramic Oxide Fiber, has been developed specifically for the reinforcement of metal and ceramic matrix composites. This paper summarizes room and high temperature properties for these fibers. The strength of both single filaments and multi-filament rovings of Nextel 610, 650 and 720 fibers was determined between 25 and 250 mm gauge length. Weibull analysis was used to compare the statistical fracture distribution and gauge length dependence of strength. Fiber fracture statistics were in accord with Weibull theory; the effect of diameter variability on the statistical analysis was found to be small. Fractographic analysis on Nextel 610 fiber was used to identify primary fracture-causing defects; defect size was correlated with Griffith fracture predictions. High temperature single filament strength measurements were performed on Nextel 610, 650 and 720 fibers between 800 and 1400°C. High temperature strength varied inversely with strain rate. In combination with tensile creep tests at 1100 and 1200°C, these were used to compare the elevated temperature capability of each fiber and determine maximum use temperatures. The development of crystalline yttrium aluminum garnet fibers that demonstrate further improvements in creep performance relative to Nextel 720 fibers is also discussed.
01 Dec 2001-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the growth behavior of carbonitrides during aging and creep was investigated for 9Cr-0.5Mo-1.8W-VNb steel (ASME-P92).
Abstract: The growth behavior of MX carbonitrides during aging and creep was investigated for 9Cr–0.5Mo–1.8W–VNb steel (ASME-P92). The stress exponent of minimum creep rate decreases with increasing testing temperature. The value of stress exponent is 5.7 at 1023 K over a wide range of stress examined, while the value is 12.7 at 923 K. The low stress exponent at 1023 K may be due to the growth of MX carbonitrides during creep. The MX carbonitrides grow to a size of 60 nm during aging at 1023 K for 400 h. The size is much larger than the critical value over which the coherent strain between the MX and matrix decreases. The growth rate of the MX carbonitrides is larger in gauge portion than in head portion of crept specimens, implying a stress or strain effect.
TL;DR: In this paper, the authors investigated different methods of implementing thermal cycling analysis, namely using the "dwell creep" and "full creep" methods based on a phenomenological approach to modeling time independent plastic and time dependent creep deformations.
Abstract: The reliability concern in flip-chip-on-board (FCOB) technology is the high thermal mismatch deformation between the silicon die and the printed circuit board that results in large solder joint stresses and strains causing fatigue failure. Accelerated thermal cycling (ATC) test is one of the reliability tests performed to evaluate the fatigue strength of the solder interconnects. Finite element analysis (FEA) was employed to simulate thermal cycling loading for solder joint reliability in electronic assemblies. This study investigates different methods of implementing thermal cycling analysis, namely using the "dwell creep" and "full creep" methods based on a phenomenological approach to modeling time independent plastic and time dependent creep deformations. There are significant differences between the "dwell creep" and "full creep" analysis results for the flip chip solder joint strain responses and the predicted fatigue life. Comparison was made with a rate dependent viscoplastic analysis approach. Investigations on thermal cycling analysis of the temperature range, (/spl Delta/T) effects on the predicted fatigue lives of solder joints are reported.
TL;DR: In this paper, the effect of varying the relative amounts of ingredients in a brake friction material containing 12 ingredients was investigated by measuring the difference between kinetic (μ k ) and static (μ s ) coefficients of friction for each formulation.
30 Jun 2001-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effects of bismuth and antimony additions on the microstructure and mechanical properties of AZ91 alloy have been studied, and it was shown that a small amount of antimony adds to AZ91 increases the yield strength and creep resistance significantly at elevated temperatures up to 200°C.
Abstract: The effects of bismuth and antimony additions on the microstructure and mechanical properties of AZ91 alloy have been studied. Results show that a small amount of bismuth or antimony additions to AZ91 increases the yield strength and creep resistance significantly at elevated temperatures up to 200°C. The highest creep resistance has been obtained from the alloy with combined additions of bismuth and antimony. The activation energies of the steady-state creep for AZ91-based alloys studied were close to that of pure magnesium self-diffusion, indicating that dislocation climb is responsible for the creep mechanism under the present conditions. Microstructural observations reveal that the additions of bismuth or antimony have the effect of refining the b (Mg17Al12) precipitates in as-cast alloys and suppressing discontinuous precipitation of the b phase effectively during the aging process. Some rod-shaped Mg3Bi2 or Mg3Sb2 particles distributed mainly at grain boundaries have been observed in the alloys with bismuth or antimony additions. Both Mg3Bi2 and Mg3Sb2 have a high thermal stability and play important roles in improving creep resistance of the alloys at elevated temperatures. © 2001 Elsevier Science B.V. All rights reserved.
TL;DR: In this article, the authors investigated the early-age behavior of normal and high-strength concrete and showed that concrete exhibits high tensile creep strain if loaded at an age less than or equal to 1 day.
TL;DR: In this article, the development of highly creep resistant magnesium alloys is presented and operating temperatures of 300°C are envisaged, where the alloy systems studied are binary Mg−Sc, Mg-Gd and ternary Mg•Sc−Mn.
TL;DR: In this paper, the topological inversion of the ordered γ-phase of the superalloy SRR99 at 980°C and 200 MPa has been investigated quantitatively by analysis of scanning electron microscope images.
01 Dec 2001-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the authors investigated the creep deformation behavior of simple 9Cr-W and solute modified 9CrWVTa steels containing high W, where Fe 2 W Laves phase precipitates during creep, and found that fine MC carbides are significantly effective for the stabilization of lath martensitic microstructure.
Abstract: The creep deformation behavior has been investigated for simple 9Cr–W and solute modified 9Cr–WVTa steels containing high W, where Fe 2 W Laves phase precipitates during creep. Creep tests were carried out at 823, 873 and 923 K for up to 15 000 h. The precipitation of Fe 2 W effectively decreases a minimum creep rate, but the large coarsening of Fe 2 W promotes the acceleration of creep rate after reaching a minimum creep rate. As a result, the effect of Fe 2 W on the extension of creep rupture time is rather small. The fine MC carbides are significantly effective for the stabilization of lath martensitic microstructure.