Showing papers on "Creep published in 1996"

Concrete at High Temperatures: Material Properties and Mathematical Models

Abstract: Introduction. PART I (written by M K Kaplan) 1. Composition and structure of hardened Portland cement paste. 2. Temperature effects on chemical composition, pore water and physical structures of Portland cement structure. 3. Thermal expansion of Portland cement concrete at high temeratures. 4. Thermal properties of Portland cement concrete at high temperatures. 5. Strength and deformation of Portland cement concrete at high temperature. 6. Composition and properties of materials used for refractory concrete. 7. Mix design and manufacture of refractory concrete. 8. Physical, thermal and chemical properties of refractory concrete. 9. Mechanical properties of refractor concrete. PART II Mathematical Modelling (written by Z P Bazant) 10. Mathematical modelling of moisture diffusion and pore pressures at high temperatures. 11. Mathematical modelling of creep and shrinkage of concrete at high temperatures. 12. Mathematical models for cracking, failure and nonlinear stress-strain relations.

Thermomechanical properties in a thin film of shape memory polymer of polyurethane series

Abstract: The thermomechanical properties of a thin film of shape memory polymer of polyurethane series were investigated experimentally. Based on the experimental results, the dynamic mechanical properties, cyclic deformation properties at high temperature, thermomechanical cycling properties, creep and stress relaxation are discussed. The shape fixity with loading above the glass transition temperature followed by unloading below does not change under thermomechanical cycling. The residual strain is recovered in the vicinity of during the heating process. Several applications of the polymer are introduced.

The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-based in situ composite

Abstract: This article describes room-temperature and high-temperature mechanical properties, as well as oxidation behavior, of a niobium-niobium silicide basedin situ composite directionally solidified from a Nb-Ti-Hf-Cr-Al-Si alloy. Room-temperature fracture toughness, high-temperature tensile strength (up to 1200 °C), and tensile creep rupture (1100 °C) data are described. The composite shows an excellent balance of high- and low-temperature mechanical properties with promising environmental resistance at temperatures above 1000 °C. The composite microstructures and phase chemistries are also described. Samples were prepared using directional solidification in order to generate an aligned composite of a Nb-based solid solution with Nb3Si- and Nb5Si3-type silicides. The high-temperature mechanical properties and oxidation behavior are also compared with the most recent Ni-based superalloys. This composite represents an excellent basis for the development of advanced Nb-based intermetallic matrix composites that offer improved properties over Ni-based superalloys at temperatures in excess of 1000 °C.

Rafting in superalloys

Abstract: The phenomenon of rafting in superalloys is described, with particular reference to modern superalloys with a high volume fraction of the particulate γ’ phase. It is shown that in the elastic regime, the thermodynamic driving force for rafting is proportional to the applied stress, to the difference between the lattice parameters of the γ matrix and the γ’ particles, and to the difference of their elastic constants. A qualitative argument gives the sign of this driving force, which agrees with that determined by Pineau for a single isolated particle. Drawing on the work of Pollock and Argon and of Socrate and Parks, it is shown that after a plastic strain of the sample of order 2 × 10-4, the driving force is proportional to the product of the applied stress and the lattice misfit, in agreement with the results of the calculations of Socrate and Parks. The rate of rafting is controlled by the diffusion of alloying elements. Here, the tendency of large atoms to move from regions of high hydrostatic pressure to those of low may outweigh the influence of concentration gradients. The deformation of the sample directly produced by rafting is small, of order 4.5 × 10-4. The rafted structure is resistant to creep under low stresses at high temperatures. Under most experimental conditions at relatively high stresses, rafting accelerates creep; this effect may be less pronounced at the small strains acceptable under operational conditions.

Mechanical behaviour of laminated metal composites

Abstract: Laminated metal composites (LMCs) are a unique form of composite material in which alternating metal or metal containing layers are bonded together with discrete interfaces. The mechanical properties of these materials are reviewed. The tensile properties at low and high temperatures are described. At low temperature, very high tensile strengths can be achieved in deposition processed laminates and very high tensile ductilities can be achieved in roll bonded laminates by layer thickness refinement. At high temperature, superplasticity has been observed and agrees with predictions from constitutive creep relations. Damage critical properties (such as fracture toughness, fatigue, and impact behaviour) and damping can be superior to those exhibited by the component materials. The damage critical properties are strongly influenced by local delaminations at layer interfaces. Mechanisms responsible for many of the unique properties of LMCs have been proposed. The influence of processing, laminate archit...

Sustained loading generates stress concentrations in lumbar intervertebral discs.

Abstract: STUDY DESIGN: Cadaveric motion segment experiment. Measurements on each specimen were compared before and after creep loading. OBJECTIVES: To show how sustained "creep" loading affects stress distributions inside intervertebral discs. SUMMARY OF BACKGROUND DATA: The central region of an intervertebral disc acts like a hydrostatic "cushion" between adjacent vertebrae. However, this property depends on the water content of the tissues and may be lost or diminished after creep. METHODS: Twenty-seven lumbar motion segments consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate erect standing postures in life. The distribution of compressive stress in the disc matrix was measured by pulling a miniature pressure transducer through the disc in the midsagittal plane. Profiles of vertical and horizontal compressive stress were repeated after each specimen had been creep loaded in compression for 2-6 hours. RESULTS: Creep reduced the hydrostatic pressure in the nucleus by 13-36%. Compressive stresses in the anulus were little affected when the profiles were measured at 1 kN, but at 2 kN, localized peaks of compressive stress appeared (or grew in size) in the posterior anulus after creep. CONCLUSIONS: Increased loading of the apophysial joints causes an overall reduction in intradiscal stresses after creep. In addition, water loss from the nucleus causes a transfer of load from nucleus to anulus. Stress concentrations may lead to pain, structural disruption, and alterations in chondrocyte metabolism. Disc mechanics depend on loading history as well as applied load.

Transverse cracking in newly constructed bridge decks

Abstract: Many concrete bridge decks develop transverse cracks soon after construction. The aims of this project were to investigate the causes of such cracking and to specify methods to reduce its incidence. Literature, research reports, and current practices concerning transverse cracking in bridge decks were reviewed to learn the extent of and the perceived causes of cracking. All departments of transportation in the United States and many foreign transportation departments were surveyed to learn the extent of deck cracking, and standard design and construction practices. The survey response revealed that transverse bridge deck cracking is a severe problem, since about half the bridges in the United States develop early cracking. Construction methods vary widely, as do techniques used in attempts to prevent cracking. The Portland-Columbia Bridge was instrumented and monitored during its redecking. The comprehensive recorded data provide important insight into early bridge behavior and cracking, influenced by thermal loading and concrete shrinkage. Systems of equations to predict shrinkage and thermal stresses in a composite bridge were developed. Behavior predicted by these equations compared favorably with actual measured behavior of the Portland-Columbia Bridge. The equations allow bridge designers to predict thermal and shrinkage stresses that may develop in their designs. Designers can evaluate and compare the thermal and shrinkage stresses of various designs. Analytical studies using the derived equations evaluated the influence of various material and geometry factors on deck stresses and cracking. The analyses examined the effects of various concrete properties, including modulus of elasticity, creep, drying shrinkage, and coefficient of thermal expansion. Girders were either steel or concrete, small or large. Simply-supported and continuous spans were analyzed. The effects of deck reinforcement were also studied. Three different temperature conditions and two different deck drying shrinkage profile conditions were analyzed. About 18,000 combinations of system and conditions were analyzed. The analytical study determined the material properties and geometries most likely to cause transverse deck cracking. These are deck restraint, the concrete modulus of elasticity and creep, and shrinkage and thermal strains. Because decks are restrained primarily by their supporting girders, and span length limits girder stiffness, often little can be done to reduce restraint. However, concrete properties have a large effect on stresses, and concrete properties can be easily changed. Construction techniques can also be easily improved to reduce early thermal strains and shrinkage. A test method to predict cracking tendency of concrete mixes was developed. This project identified and ranked the factors or combinations of factors that contribute to transverse cracking of newly constructed bridge decks.

Inelastic Deformation of Metals: Models, Mechanical Properties, and Metallurgy

Abstract: RELATIONSHIPS BETWEEN MATERIAL AND MECHANICAL PROPERTIES. Physical Basis of Inelasticity. Tensile, Compressive, and Cyclic Characteristics of Metals. Creep of Metals. MULTIAXIAL PLASTICITY AND CREEP. Principles of Mechanics. Yield Surface Plasticity and Classical Creep Modeling. STATE-VARIABLE APPROACH. Foundation of State-Variable Modeling. Multiaxial and Thermomechanical Modeling. Single-Crystal Superalloys. Finite-Element Methods. Appendices. Index.

Microstructure of a pressure die cast magnesium—4wt.% aluminium alloy modified with rare earth additions

Abstract: Addition of cerium-rich mixtures of rare earth (RE) elements to aluminium-containing magnesium pressure die cast alloys is known to improve the creep properties at elevated temperatures. In the present investigation, a detailed description of the microstructure of a magnesium-4 wt.% aluminium alloy containing 1.4 wt.% of a cerium-rich mixture of RE elements is presented. Particle types occurring and their distribution in the microstructure, as well as the distribution of elements in solid solution, are described. Heat treatment is carried out to verify if solid state precipitation occurs. Some qualitative arguments for the beneficial effect of RE elements on the creep properties are presented.

Rheology of partially molten mantle rocks

Abstract: ▪ Abstract Over the past decade, significant progress has been made in understanding the rheological properties of partially molten mantle rocks. Laboratory experiments demonstrate that a few percent of melt can have an unexpectedly large effect on viscosity both in the diffusional creep regime and in the dislocation creep regime. In both cases, the enhancement in creep rate is much larger than anticipated based on deformation models because melt wets at least a fraction of the grain boundaries. For diffusion creep, the wetted interfaces provide a rapid diffusion path that is not included in analyses based on melt distribution in isotropic melt-crystal systems. For dislocation creep, two points require consideration. First, even without a melt phase present, fine-grained samples deformed in the dislocation creep field flow a factor of ∼10 faster than coarse-grained rocks due to contributions from grain boundary mechanisms to the deformation process. Second, melt has only a small effect on creep rate for c...

Creep constitutive equations for a 0.5Cr–0.5Mo–0.25V ferritic steel in the temperature range 600–675°C

Abstract: Constitutive equations have been developed to describe the creep behaviour of a 0.5Cr-0.5Mo-0.25V ferritic steel under uniaxial and multi-axial states of stress, as well as over the temperature range of 600–675°C. This material has widespread application as the parent steam pipe in the power generation industry. An accurate description of creep behaviour has been achieved by formulating constitutive equations that describe the physical mechanisms of deformation and rupture. To this end, two state variables were introduced to model intergranular creep cavitation and the coarsening of carbide precipitates respectively. The influence of temperature on the constitutive equations is also considered and activation energies are identified for each of the material constants that show a notable temperature dependence.Numerical optimization methods are discussed for the determination of the material constants and of the corresponding activation energies. The constitutive equations provide a good representat...

Shrinkage cracking of high-strength concrete

Abstract: Pozzolanic materials and high-range water-reducing admixtures have improved the strength and durability of concrete construction. Since compressive strength is the most commonly used parameter to describe the quality of concrete, high-strength or high-performance concrete, has been used in many structures that are sensitive to shrinkage cracking. No systematic study has been conducted to evaluate the restrained shrinkage cracking behavior of high-strength concrete. This paper presents research on the restrained shrinkage cracking on several strength levels of concrete. Higher strength levels were derived by partial substitution of cement with silica fume and by reducing the water content. Ring-type specimens were used for restrained shrinkage cracking tests. Free shrinkage, creep, weight loss, compressive, and splitting tensile strength were also examined. Findings reveal that the free shrinkage for different concretes does not depend on the unit water content or weight loss and that high-strength silica fume concrete shows higher shrinkage and lower creep. Cracking for high-strength silica fume concrete develops much faster and is significantly wider than that of normal-strength concrete.

Metallurgy and creep properties of new 9-12%Cr steels

Abstract: Current understanding of the creep strengthening mechanisms in newly developed 9-12%Cr creep resistant steels is discussed. It is demonstrated how new computer programs for calculation of equilibrium phases in the steels can contribute to increased understanding of the microstructure of these steels. The calculations are useful for, e.g. estimation of the risk of δ-ferrite formation, and of balancing of the addition of V, Nb and N so M2X formation can be avoided. Combination of the equilibrium calculations with kinetic models and experiment has led to a description of the precipitation of Laves phase in W-alloyed 9%Cr steels. Combination of these results with creep tests indicated that Laves phases have a creep strengthening effect through precipitation hardening, and that any solid solution strengthening effect of W is small.

Effects of chemical environment on dislocation creep of quartzite

Abstract: The water-related chemical parameter that affects dislocation creep in quartzite has been determined from variations in sample strength and microstructure with chemical environment in buffered deformation and hydrostatic annealing experiments. Samples were weld-sealed in double capsules;, , and were buffered using solid oxygen buffers, AgCl or CO2. Black Hills quartzite was deformed at 900°C and 1.5 × 10−5s−1. Two samples were deformed at ∼1700 MPa confining pressure at constant and , with and varying over 8 and 15 orders of magnitude, respectively. Both samples deformed by climb-accommodated dislocation creep with flow stresses of 300 MPa. Two additional samples were deformed at ∼700 MPa at constant lower than for the 1700-MPa samples, with varying over 2 orders of magnitude. Both samples faulted with a peak strength of ∼800 MPa. These four experiments suggest no dependence of dislocation creep strength on , or ; instead, a strong dependence of strength on is inferred. Previously deformed samples of Heavitree quartzite were hydrostatically annealed for 4 days at 800°C and 1200 or 500 MPa confining pressure, varying and over 2.5 and 1 order of magnitude, respectively. The microstructures of these samples show increased rates of dislocation climb and grain boundary migration with increasing but no dependence on . These buffered experiments indicate that dislocation creep is affected by alone and suggest that the exponent for the term in the power law creep flow law is >2.

Rarefied gas flow through a long tube at any temperature ratio

Abstract: The mass flow rate of a rarefied gas through a long capillary caused by small pressure and temperature gradients has been calculated based on the s‐model for the diffuse specular gas‐surface interaction in the range of the rarefaction parameter from 0.005 to 50. A simple method of calculation of the thermomolecular pressure effect and the thermal creep caused by a large temperature ratio have been elaborated. Numerical calculations of both phenomena for the temperature ratio T2/T1=3.8—usually realized in experiments—have been carried out. It has been determined that the nonlinear thermomolecular pressure effect does not depend on the temperature distribution along the capillary. The nonlinear thermal creep has been calculated for two different temperature distributions along the capillary. A dependence of the creep on the temperature distribution has been found. It has been shown that the application of the linear theory based on the average rarefaction parameter to the gas flow at a large temperature rat...

Particle/Matrix Interface and Its Role in Creep Inhibition in Alumina/Silicon Carbide Nanocomposites

Abstract: This study focuses on interfacial bonding between intergranular silicon carbide particles and an alumina matrix, to determine the creep inhibition mechanism of alumina/ silicon carbide nanocomposites. It is revealed that the silicon carbide/alumina interface possesses much stronger bonding than the alumina/alumina interface through three approaches: investigation of fracture toughness and fracture mode and consideration of internal thermal stresses acting at grain boundaries, estimation of equilibrium thickness of intergranular glassy films by force balance, and direct observation of grain boundaries by TEM. The rigid bonding of alumina/silicon carbide interfaces causes inhibition of vacancy nucleation and annihilation at the interfaces, causing remarkably improved creep resistance of the nanocomposite.

Thermomechanical properties in a thin film of shape memory polymer of polyurethane series

Abstract: The thermomechanical properties of a thin film of shape memory polymer of polyurethane series were investigated experimentally. The results were summarized as follows: (1) Modulus of elasticity and yield stress are high below glass transition temperature Tg and low above Tg. The value of loss tangent is large in the vicinity of Tg. (2) The stress-strain curves vary significantly in the early cycles but slightly thereafter under cyclic deformation above Tg. (3) During the heating process after loading above Tg followed by unloading below Tg, strain is recovered in the vicinity of Tg. (4) Shape fixity with loading above Tg followed by unloading below Tg does not vary under thermomechanical cycling. (5) Creep strain is recovered after unloading above Tg. Creep residual strain below Tg is recovered in the vicinity of Tg during the heating process. (6) About a half of initial stress relaxes after a certain duration of time. Several applications of the polymers were introduced.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

On the Constitutive Response of 63/37 Sn/Pb Eutectic Solder

Abstract: The constitutive response of 63/37 Sn/Pb eutectic solder is studied under uniaxial tension. Stress-strain tests were performed under a wide range of applied strain rate, and creep tests were performed at several levels of applied load. Temperature dependence was ascertained by running the stress-strain and creep tests at various temperatures. With the goal of providing a constitutive description of the eutectic suitable for use in electronics packaging engineering finite element analyses, the material parameters of the phenomenological viscoplasticity model of Bodner and Partom were fit to these experiments as functions of temperature. The experiments described herein also yield important information concerning how many microstructural units must be present in a sample in order for repeatable, isotropic response to be obtained.

Elevated temperature behaviour of rapidly solidified magnesium alloys containing rare earths

Abstract: The microstructure and mechanical properties of bars of six magnesium alloys (AZ91, AZ151, ACa54, ZE41, WE43 and WE54) extruded from melt spun ribbons, were investigated. At room temperature, the 0.2% proof stress of all the alloys (300–500 MPa) is much higher than that of commercial cast magnesium alloys. As far as the effect of temperature is concerned, the alloys can be separated into two classes: whereas in the case of AZ91, AZ151, ZE41 and ACa54, the yield stress of the bars decreases rapidly as soon as the temperature increases, it remains very high up to about 150°C in the case of WE43 and WE54, before rapidly decreasing. Furthermore, the creep resistance of WE alloys is far better than that of AZ alloys. The high yield stress at low temperature is ascribed to a strong Hall-Petch effect (mean grain size ∼0.5 μm). The medium and high temperature behaviours are discussed, with emphasis on the particular case of WE alloys.

Time dependent mechanical behaviour of loose sands

Abstract: The time dependency of loose sands mechanical behaviour has been experimentally analysed and theoretically interpreted. A series of load controlled triaxial tests, by imposing finite load increments, has been performed. The single load increments are followed by variable time periods, in order to carry out many classical creep tests. According to the authors, the considered time dependency is due to the internal fabric rearrangement of the granular assembly, i.e. to the plastic strain development with time. This mechanical peculiarity is theoretically interpreted by means of an elastoviscoplastic constitutive model. This is a very simple extension of a previous incremental elastoplastic constitutive model and appears to be capable to reproduce experimental data quite well. Finally, the importance of the considered time dependency is underlined, both by considering numerical solutions and in analysing unstable natural and experimental phenomena.

Mechanisms-based constitutive equations for the superplastic behaviour of a titanium alloy:

Abstract: Mechanisms-based constitutive equations are proposed for the high-temperature behaviour of a class of titanium alloys, for which the deformation mechanisms include diffusional creep, grain boundary sliding, dislocation creep and grain growth A computational procedure has been developed for the determination of the constitutive equations from a material database The constitutive equations and the procedure for their determination have been validated by modelling the behaviour of the titanium alloy Ti-6Al-4V at 927°CIt is shown that the procedure developed for the determination of the mechanisms-based constitutive equations can be used to identify the important deformation mechanisms in operation for particular stress, temperature and strain rate conditions For the case of the Ti-6Al-4V material, the procedure developed correctly predicts the material hardening due to grain growth and indicates that an additional hardening mechanism operates In addition, the procedure is able to identify grain

Enhanced ductility in coarse-grained Al-Mg alloys

Abstract: Enhanced ductilities, i.e., values of tensile ductility exceeding those normally expected in metallic alloys, have been observed at warm temperatures in coarse-grained Al-Mg alloys which exhibit viscous-glide controlled creep. Numerous tests have been conducted in order to quantify this phenomenon over wide ranges of temperature and magnesium concentration. The contributions of strain-rate sensitivity and strain hardening have been analyzed in relation to the observed tensile ductilities. It is shown that an analysis based only on flow instability in tension cannot be used to predict failure in a unique manner.

High-temperature measurements of lattice parameters and internal stresses of a creep-deformed monocrystalline nickel-base superalloy

Abstract: High-temperature X-ray line profile measurements were performed to maximal temperatures of 1050 °C on samples of the nickel-base superalloy SRR 99. The samples with rod axes near the [001] direction were investigated in the initially undeformed state and after creep deformation at different temperatures and stresses. For the measurements of the (002) and (020) line profiles, a special X-ray double crystal diffractometer with negligible line broadening was used which was equipped with a high-temperature vacuum chamber. The line profiles were evaluated for the lattice parameters of the matrix phase γ and the precipitated γ′ phase and for values of the lattice mismatch parallel and perpendicular to the stress axis, respectively, which were found to be different. Elastic, tetragonal distortions of the phases γ and γ′ could be determined between room temperature and about 900 °C. These distortions are thermally induced due to the different thermal expansion coefficients of the two phases and deformation induced due to interfacial dislocation networks which were built up during deformation. At the high temperatures of the X-ray measurements, at least partial recovery of the deformation-induced internal stresses occurred, depending on the temperature of the X-ray measurements. The results are discussed and compared with data obtained by complementary techniques.