Showing papers on "Creep published in 2003"

Effects of pressure on high-temperature dislocation creep in olivine

Abstract: Effects of pressure on high-temperature, dislocation creep in olivine ((Mg, Fe) 2 SiO 4 ) aggregates have been determined under both water-poor ('dry') and water-saturated ('wet') conditions. New experimental data were obtained at pressures of 1-2 GPa under 'dry' and 'wet' conditions using a newly developed high-resolution dislocation density measurement technique to estimate the creep strength. These data are compared with previous data at lower and higher pressures to determine the pressure dependence of high-temperature dislocation creep in olivine aggregates. We find that the creep strength †under 'dry' conditions increases monotonically with increasing pressure, whereas the creep strength under 'wet' conditions changes with pressure in a non-monotonic fashion: it first decreases rapidly with increasing pressure and then becomes less sensitive to pressure at above 1 GPa. Such behaviour can be described by the following formula: where the subscripts d and w refer to parameters for 'dry' and 'wet' condi...

Creep Resistant Magnesium Alloys for Powertrain Applications

Abstract: Creep resistant magnesium alloys are candidate materials for automotive powertrain applications. Since the 90’s, a number of new creep-resistant magnesium alloy systems have been investigated and developed. These are for the most part based on rare-earth, alkaline earth, and silicon additions. This paper gives an overview of creep resistance in magnesium and a review of creep resistant magnesium alloys for power-train applications.

Nanomechanical characterisation of solid surfaces and thin films

Abstract: To measure nanomechanical properties of surface layers of bulk materials and thin films, depth-sensing nanoindentation measurement techniques are used commonly. The nanoindentation apparatus continuously monitors the load and the position of the indenter relative to the surface of the specimen (depth of an indent or displacement) during the indentation process. Indentation experiments can be performed at a penetration depth of as low as about 5 nm. This chapter presents an overview of various nanoindentation techniques, various measurement options, and data analysis. Data on elastic-plastic deformation behavior, hardness, elastic modulus, scratch resistance, film-substrate adhesion, residual stresses, time-dependent creep and relaxation properties, fracture toughness, and fatigue are presented.

Using JMatPro to model materials properties and behavior

Abstract: This article describes the development of a new multi-platform software program called JMatPro for calculating the properties and behavior of multi-component alloys. These properties are wide ranging, including thermophysical and physical properties (from room temperature to the liquid state), time-temperature-transformation/continuous-cooling transformation diagrams, stress/strain diagrams, proof and tensile stress, hardness, coarsening of γ′ and γ″, and creep. A feature of the new program is that the calculations are based on sound physical principles rather than purely statistical methods. Thus, many of the shortcomings of methods such as regression analysis can be overcome. With this program, sensitivity to microstructure can be included for many of the properties and the true inter-relationship between properties can be developed, for example in the modeling of creep and precipitation hardening.

Creep-strengthening of steel at high temperatures using nano-sized carbonitride dispersions

Abstract: Creep is a time-dependent mechanism of plastic deformation, which takes place in a range of materials under low stress-that is, under stresses lower than the yield stress. Metals and alloys can be designed to withstand creep at high temperatures, usually by a process called dispersion strengthening, in which fine particles are evenly distributed throughout the matrix. For example, high-temperature creep-resistant ferritic steels achieve optimal creep strength (at 923 K) through the dispersion of yttrium oxide nanoparticles. However, the oxide particles are introduced by complicated mechanical alloying techniques and, as a result, the production of large-scale industrial components is economically unfeasible. Here we report the production of a 9 per cent Cr martensitic steel dispersed with nanometre-scale carbonitride particles using conventional processing techniques. At 923 K, our dispersion-strengthened material exhibits a time-to-rupture that is increased by two orders of magnitude relative to the current strongest creep-resistant steels. This improvement in creep resistance is attributed to a mechanism of boundary pinning by the thermally stable carbonitride precipitates. The material also demonstrates enough fracture toughness. Our results should lead to improved grades of creep-resistant steels and to the economical manufacture of large-scale steel components for high-temperature applications.

Failure forecast for large rock slides by surface displacement measurements

Abstract: Forecasting the failure of large rock slides is difficult because of nonlinear time dependency and seasonal effects, which affect the displacements. Starting from the accelerating creep theory prop...

Measurement of Creep Compliance of Solid Polymers by Nanoindentation

Abstract: Methods to measure the local surface creep compliance of time-dependent materials are proposedand validated in the regime of linear viscoelasticity using nanoindentation. Two different bulkpolymers, Polymethyl Methacrylate (PMMA) and Polycarbonate (PC), were employed in thevalidation study; though it is expected that the methods developed herein can be applied for verysmall amounts of materials and heterogeneous materials. Both Berkovich and sphericalnanoindenters were used to indent into the material in nanoindentation tests. Two loading historieswere used: (1) a ramp loading history, in which the indentation load and displacement wererecorded; and (2) a step loading history, in which the indentation displacement was recorded as afunction of time. Analysis of the linearly viscoelastic material response was performed to measurethe creep compliance functions for the two materials under two different loading histories. The limitof linearly viscoelastic behavior for each of the two materials was determined through theobservation of the indent impression recovery after complete unloading; it is postulated that linearityis achieved if indentation impression is fully recovered after unloading. Results fromnanoindentation tests generally agree well with data from conventional tension and shear tests. It hasthus validated the techniques of measuring linear creep compliance in the glassy state usingnanoindentation with the Berkovich and spherical indenter tips.

Load–displacement behavior during sharp indentation of viscous–elastic–plastic materials

Abstract: A model is developed that describes the sharp indentation behavior of time-dependent materials. The model constitutive equation is constructed from a series of quadratic mechanical elements, with independent viscous (dashpot), elastic (spring), and plastic (slider) responses. Solutions to this equation describe features observed under load-controlled indentation of polymers, including creep, negative unloading tangents, and loading-rate dependence. The model describes a full range of viscous–elastic–plastic responses and includes as bounding behaviors time-independent elastic–plastic indentation (appropriate to metals and ceramics) and time-dependent viscous–elastic indentation (appropriate to elastomers). Experimental indentation traces for a range of olymers with different material properties (elastic modulus, hardness, viscosity) are econvoluted and ranked by calculated time constant. Material properties for these polymers, deconvoluted from single load–unload cycles, are used to predict the indentation load–displacement behavior at loading rates three times slower and faster, as well as the steady-state creep rate under fixed load.

Recent Advances in Creep Resistant Steels for Power Plant Applications

Abstract: The higher steam temperatures and pressures required to achieve increase in thermal efficiency of fossil fuel-fired power-generation plants necessitate the use of steels with improved creep rupture strength. The 9% chromium steels developed during the last three decades are of great interest in such applications. In this report, the development of steels P91, P92 and E911 is described. It is shown that the martensitic transformation in these three steels produces high dislocation density that confers significant transient hardening. However, the dislocation density decreases during exposure at service temperatures due to recovery effects and for long-term creep strength the sub-grain structure produced under different conditions is most important. The changes in the microstructure mean that great care is needed in the extrapolation of experimental data to obtain design values. Only data from tests with rupture times above 3,000 h provide reasonable extrapolated values. It is further shown that for the 9% chromium steels, oxidation resistance in steam is not sufficiently high for their use as thin-walled components at temperatures of 600°C and above. The potential for the development of steels of higher chromium contents (above 11%) to give an improvement in steam oxidation resistance whilst maintaining creep resistance to the 9% chromium steels is discussed.

Effects of creep and cyclic loading on the mechanical properties and failure of human Achilles tendons.

Abstract: The Achilles tendon is one of the most frequently injured tendons in humans, and yet the mechanisms underlying its injury are not well understood. This study examines the ex vivo mechanical behavior of excised human Achilles tendons to elucidate the relationships between mechanical loading and Achilles tendon injury. Eighteen tendons underwent creep test- ing at constant stresses from 35 to 75 MPa. Another 25 tendons underwent sinusoidal cyclic loading at 1 Hz between a mini- mum stress of 10 MPa and maximum stresses of 30- 80 MPa. For the creep specimens, there was no significant relationship between applied stress and time to failure, but time to failure decreased exponentially with increasing initial strain ~strain when target stress is first reached! and decreasing failure strain. For the cyclically loaded specimens, secant modulus decreased and cyclic energy dissipation increased over time. Time and cycles to failure decreased exponentially with increasing ap- plied stress, increasing initial strain ~peak strain from first load- ing cycle!, and decreasing failure strain. For both creep and cyclic loading, initial strain was the best predictor of time or cycles to failure, supporting the hypothesis that strain is the primary mechanical parameter governing tendon damage accu- mulation and injury. The cyclically loaded specimens failed faster than would be expected if only time-dependent damage occurred, suggesting that repetitive loading also contributes to Achilles tendon injuries. © 2003 Biomedical Engineering So- ciety. @DOI: 10.1114/1.1569267#

Flow as an effective promotor of nucleation in polymer melts: a quantitative evaluation

Abstract: An apparatus is presented which enables the application of defined portions of mechanical work to the polymer sample in its state of undercooled melt. For the purpose mainly intermittent shear creep is used. Results are presented for an industrial grade of polypropylene. A three-dimensional picture is presented, in which the resulting numbers of nuclei (per unit volume) are plotted against two responsible parameters: crystallization temperature and mechanical work. With decreasing temperature and with increasing mechanical work the number of nuclei increases by many decades. At sufficiently high mechanical loads a transition to thread-like precursors ("shishs") has been observed previously. Oriented structures (kind of "shish-kebabs") are formed in this way. The periods of shearing applied have always been extremely short compared with the time until crystallization becomes observable. In this way an accumulation of various processes could be avoided. The description of shear induced crystallization, as previously given, is modified in the light of the present results.

Precipitate Stability in Creep Resistant Ferritic Steels-Experimental Investigations and Modelling

Abstract: Predictions of long-term microstructure stability of creep resistant ferritic 9-12 % Cr steels up to 200 000-300 000 h at temperatures up to 600-650°C are highly interesting for safe power plant operation. At technically interesting creep conditions the microstructure stability is mainly controlled by the stability of precipitate particles. Predictions of precipitate stability have to rely on i) Microstructure characterisation methods to measure volume fractions and mean particle sizes of individual precipitate types, and ii) Microstructure models to predict the evolution of precipitate volume fractions and mean sizes as functions of temperature, time and applied stress. Characterisation methods, which allow on-line particle type discrimination in 9-12 % Cr steels include energy filtered transmission electron microscopy (EFTEM) and scanning electron microscopy (SEM) with atomic number contrast. Modelling of precipitate stability based on thermodynamic equilibrium calculations and multicomponent diffusion databases is demonstrated. A multi-component coarsening model gives accurate predictions of coarsening rates for MX and Laves phase precipitates in steel P92 with fit values for the interfacial energy in the expected range. For M 23 C 6 carbides in steel P92 the model results in unexpectedly low apparent values for the interfacial energy. Modelling of published data for steel P91 indicate much higher coarsening rates for M 23 C 6 carbides, and the fit value for the interfacial energy is as expected. A possible explanation for the low apparent value of the interfacial energy for M 23 C 6 carbides in steel P92 is the content of boron in the steel.

Microstructure and mechanical properties of Mg–Al based alloy with calcium and rare earth additions

Abstract: Effects of calcium and rare earth additions to alloy AZ91 on the microstructure and mechanical properties were investigated. The results indicated that small amounts of calcium addition to AZ91 did not cause the formation of any new phases in the microstructure, but refined the as-cast microstructure and increased the thermal stability of the β phase so that the yield strength and creep resistance of the alloy were significantly improved. Additions of lanthanum-rich misch metal (MM) resulted in the formation of needle-shaped particles, which showed very high thermal stability and did not dissolve into the matrix after the solution treatment at 420 °C for 20 h. The strength as well as creep resistance of the alloy at elevated temperatures was remarkably increased when MM was added combined with calcium. The highest creep resistance was obtained from the alloy with 3% of MM and 0.3% of calcium addition and its steady state creep rate reached as low as 2.69×10 −8 s −1 , one order of magnitude lower than that of alloy AZ91 without MM and calcium additions.

Accurate measurement of tip–sample contact size during nanoindentation of viscoelastic materials

Abstract: Polypropylene (PP) and amorphous selenium (a-Se) were used as prototype materials at room temperature to explore the problems that may exist in the accurate measurement of the reduced modulus of viscoelastic materials using depth-sensing nanoindentation. As has been reported previously by others, we observed that a “nose” in the load-displacement curve may occur during unloading, indicating significant creep effects at the onset of unloading. To accurately measure the elastic modulus in viscoelastic materials like PP or a-Se, both the contact stiffness and the contact area at the onset of unloading must be determined accurately. The issue of removing the influence of creep on the measurement of the contact stiffness using the Oliver-Pharr method has been addressed in a previous paper by Feng and Ngan. In this work, the effect of creep on contact-depth measurement is considered. Removal of creep effects in both contact stiffness and contact-area measurement leads to satisfactory prediction of the reduced moduli in PP and a-Se.

Measured mechanical properties of LIGA Ni structures

Abstract: Room and elevated temperature tensile, creep and high-cycle fatigue properties of electrodeposited LIGA Ni microsamples have been measured and are being used to predict the reliability of LIGA Ni MEMS structures Tensile specimens with dimensions of hundreds of microns have been LIGA fabricated and characterized in terms of their underlying microstructure, elevated temperature tensile and creep strength and their high-cycle fatigue performance The stiffness of these LIGA Ni structures was found to be reduced by the introduction of porosity during the plating process The strength of these structures was observed to decrease dramatically at temperatures above 200 °C At stresses significantly below the yield strength, substantial creep deformation was observed at moderately elevated temperatures The fatigue life of the LIGA Ni microsamples increased with decreasing stress amplitude in a manner comparable to what has been reported for wrought Ni An apparent fatigue limit was observed for the LIGA Ni microsamples, but the importance of underlying microstructure and component geometry on the fatigue life was also highlighted

Creep indentation of single cells.

Abstract: An apparatus for creep indentation of individual adherent cells was designed, developed, and experimentally validated. The creep cytoindentation apparatus (CCA) can perform stress-controlled experiments and measure the corresponding deformation of single anchorage-dependent cells. The apparatus can resolve forces on the order of 1 nN and cellular deformations on the order of 0.1 micron. Experiments were conducted on bovine articular chondrocytes using loads on the order of 10 nN. The experimentally observed viscoelastic behavior of these cells was modeled using the punch problem and standard linear solid. The punch problem yielded a Young's modulus of 1.11 +/- 0.48 kPa. The standard linear solid model yielded an instantaneous elastic modulus of 8.00 +/- 4.41 kPa, a relaxed modulus of 1.09 +/- 0.54 kPa, an apparent viscosity of 1.50 +/- 0.92 kPa-s, and a time constant of 1.32 +/- 0.65 s. To our knowledge, this is the first time that stress-controlled indentation testing has been applied at the single cell level. This methodology represents a new tool in understanding the mechanical nature of anchorage-dependent cells and mechanotransductional pathways.

Shrinkage cracking characteristics of concrete using ring specimens

Abstract: This paper outlines a testing and analysis procedure to quantify behavior of concrete under restrained shrinkage ring specimens. Cracking resistance is found to depend not only on shrinkage potential and tensile strength, but also on shrinkage rate and tensile creep characteristics of the concrete. The creep strain obtained for the ring specimens can be as high as 50% of the free shrinkage strain, indicating that tensile creep is significant and must be considered in evaluating the risk of cracking under restrained conditions. Shrinkage reducing admixtures greatly enhance the cracking resistance of concrete by reducing both the shrinkage potential and the shrinkage rate. A separate study was conducted on the concrete mixtures to determine the tensile creep characteristics under a constant stress. Results from the 2 studies are compared, and findings are discussed.

Viscoelastic Properties of a Mixed Culture Biofilm from Rheometer Creep Analysis

Abstract: The mechanical properties of mixed culture biofilms were determined by creep analysis using an AR1000 rotating disk rheometer. The biofilms were grown directly on the rheometer disks which were rotated in a chemostat for 12 d. The resulting biofilms were heterogeneous and ranged from 35 microns to 50 microns in thickness. The creep curves were all viscoelastic in nature. The close agreement between stress and strain ratio of a sample tested at 0.1 and 0.5 Pa suggested that the biofilms were tested in the linear viscoelastic range and supported the use of linear viscoelastic theory in the development of a constitutive law. The experimental data was fit to a 4-element Burger spring and dashpot model. The shear modulus (G) ranged from 0.2 to 24 Pa and the viscous coefficient (eta) from 10 to 3000 Pa. These values were in the same range as those previously estimated from fluid shear deformation of biofilms in flow cells. A viscoelastic biofilm model will help to predict shear related biofilm phenomena such as elevated pressure drop, detachment, and the flow of biofilms over solid surfaces.