Creep analysis of directionally solidified GTD111 based on stress relaxation testing

A new approach to creep analysis based on short time high precision stress relaxation testing is applied to a directionally solidified nickel based superalloy, GTD111. For specimens taken in the longitudinal, transverse and diagonal orientations, it is shown that extensive data may be generated on a single specimen. The analysis, either in terms of stress vs creep rate, or in terms of stress vs predicted times for a specific creep strain, may be used directly in design. At low stresses and high temperatures the longitudinal orientation may offer the highest creep strength, but at high stresses and lower temperatures the diagonal and transverse orientations are stronger. This new methodology allows rapid evaluation of the consequences of microstructural evolution, rather than attempting to incorporate such changes in the actual test as in the conventional long time creep-rupture testing approach. For high strength cast superalloys, such as GTD111, the creep strength is not strongly affected by thermal exposures or moderate deformation at high temperatures. However, such exposures may lead to embrittlement which requires a separate test for evaluation. This decoupling of creep strength and fracture resistance allows enhanced efficiency in both alloy optimization and component life assessment.     

Interaction between thermal dilation and autogenous deformation in high performance concrete

The paper concerns the driving forces to stress generation and cracking in high performance concrete,i.e. thermal dilation and autogenous deformation and presents experimental results from a recently finalized Dr. program. The paper focuses on the effect of temperature, and discusses how to separate thermal dilation and autogenous deformation with the view to describe each with a model for use in stress calculations. The results show clearly that especially autogenous deformation is very complicated since autogenous deformation from isothermal tests appears to be fundamentally different from autogenous deformation in tests using realistic temperature development. The implication is that autogenous deformation under realistic temperature development cannot be predicted from isothermal test results and that the simple maturity concept cannot be used. The paper gives a new experimental approach to separate thermal dilation and autogenous deformation. The results from such tests show, that the thermal dilation coefficient varies more systematically with temperature. Consequently, the maturity concept appears to be more usable for the thermal dilation coefficient.

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Résumé Cet article concerne les mécanismes physiques qui sont à la base de la génération de contrainte et de la fissuration des bétons hautes performances: la dilatation thermique et le retrait endogène. Il présente les résultats expérimentaux acquis dans le cadre d'une thèse de doctorat récement soutenue. Il traite des effets de la température et des méthodes de séparation des composantes thermiques et endogènes du retrait. L'objectif final est de modéliser chacune des ces contributions, pour les utiliser dans des calculs de développement de contraintes. Les résultats présentés montrent clairement que la caractérisation du retrait endogène est difficile puisque celui qui est mesuré dans des conditions isothermes est fondamentalement différent de celui mesuré dans des conditions d'évolution, réaliste de la température. L'implication de ces travaux est que le retrait endogène se produisant dans des conditions non isothermes ne peut pas être prédit au travers d'essais isothermes et que le concept de maturité ne peut pas être utilisé. Cet article décrit enfin une nouvelle approche expérimentale pour séparer la dilatation thermique et le retrait endogène. Son application montre que le coefficient de dilatation thermique varie assez systématiquement avec la température. En conséquence, le concept de maturité semble plus adéquat pour la détermination de coefficient de dilatation thermique.

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Editorial Note This paper was presented at the International RILEM Workshop Shrinkage of Concrete (‘Shirnkage 2000’) held on 16–17 October 2000 in Paris, France. Prof. E. J. Sellevold is a RILEM Series Member     

Hot Electrons Effect in a #23 NTD Ge Sample

We have measured thermal properties of Neutron Transmutation Doped Ge (type #23 from E. E. Haller) at very low temperatures (down to 20 mK). Both R versus T characteristics and thermal decoupling were considered. We show that, for our sample, the thermal decoupling can be well described with the hot electron model. Our results are consistent with those obtained for other doping concentrations NTD Ge. Finally, we compare the two decoupling mechanisms (electron-phonon and phonon-bath) and show that they are comparable in magnitude for standard running conditions of bolometers.     

A thermal comparator sensor for measuring autogenous deformation in hardening Portland cement paste

This paper describes a simple and accurate experimental device specially developed to measure autogenous deformation in hardening cement-based materials. The measuring system consists of a so-called thermal coparator sensor and a modular thermostatically controlled system. The operating principle of the thermal comparator is based on thermal expansion of aluminium. A particular characteristic of the measuring system is the fixation of the thermal comparator sensor to the deforming speciment. The modular system ensures effective thermostatic control of the hydrating cement paste samples. The technique allows continuous measurement with high accuracy of the linear deformation as well as determination of the activation energy of autogenous deformation.     

Effect of drying conditions on autogenous shrinkage in ultra-high performance concrete at early-age

This experimental study investigated the effects of drying conditions on the autogenous shrinkage of ultra-high performance concrete (UHPC) at early-ages. UHPC specimens were exposed to different temperatures, namely, 10, 20 and 40°C under a relative humidity (RH) ranging from 40 to 80%. The effects of using a shrinkage-reducing admixture (SRA) and a superabsorbent polymer (SAP) as shrinkage mitigation methods were also investigated. The results show that autogenous and drying shrinkage are dependent phenomena. Assuming the validity of the conventional superposition principle between drying and autogenous shrinkage led to overestimating the actual autogenous shrinkage under drying conditions; the level of overestimation increased with decreasing RH. Both SRA and SAP were very effective in reducing autogenous shrinkage under sealed conditions. However, SRA was efficient in reducing drying shrinkage under drying conditions, while SAP was found to increase drying shrinkage. Generally, results indicate that adequate curing is essential for reducing shrinkage in UHPC even when different shrinkage mitigation methods are applied.     

The influence of temperature on autogenous volume changes in cementitious materials containing shrinkage reducing admixtures

This research examines the influence of temperature on unrestrained and restrained autogenous volume changes in cementitious systems containing shrinkage reducing admixtures (SRAs). The apparent activation energy of cement hydration is determined using measurements of isothermal conduction calorimetry. Time-temperature (equivalent-age based) transformations are applied to extract the apparent activation energy of cement hydration (reactions). The results indicate that while equivalent-age transformations are a suitable procedure for describing the influence of temperature on chemical reactions, they are an inappropriate approach to describe the evolution of volume changes in cementitious materials cured at different temperatures. It is noted that while SRAs do not substantially alter the temperature sensitivity of hydration reactions, their ability to induce early-age expansions negates the use of maturity (equivalent age) approaches in describing autogenous deformations in these materials. Efforts are made to better describe the thermodynamic-limitations of autogenous RH change (self-desiccation) and the need to account for viscoelastic (i.e., creep) and damage (i.e., microcracking) considerations in interpreting the residual stress development response of cement-based materials cured at different temperatures.     

Effects of Surface Morphology and Composition Modification on Spectral Emissivity and Thermal Diffusivity Profile at High Temperatures

A new all-spectroscopic method for depth-resolved thermal diffusivity measurement of metallic specimens has been demonstrated. The method entails measurement of the mass entrained into a laser-produced plasma (LPP) plume in such a manner that the plume is representative of the specimen in elemental composition. Both the abundance of matter and its elemental composition are measured by time-resolved spectroscopy for each LPP plume. In order to delineate the morphology versus composition basis of the depth dependence, a new study on a Nichrome ribbon specimen heated by ohmic heating in a vacuum is presented. A set of depth-resolved thermal diffusivity measurements is carried out, while noting the attendant changes in the spectral emissivity and elemental composition at succeeding ablation layers. Additional measurements are carried out after the specimen has been treated under varying heating conditions. Preferential diffusion of chromium at high temperatures has been found to contribute to the dynamics of surface thermophysical properties at high temperatures. Representative LPP ablation is well suited for removal of surface impurities prior to thermophysical property measurements by the pulse heating technique.     

Theoretical and experimental validation of a simple method to reproduce representative DEF-prone conditions in laboratory

Delayed Ettringite Formation (DEF) is a possible autogenous expansive reaction of concrete. It can affect materials that have experienced temperatures higher than about 65 °C at early age. This temperature increase can be the result of the cement heat of hydration in massive structures where the heat transfers are particularly low. To understand the effects of DEF, it is necessary to be able to reproduce thermal DEF-prone conditions in laboratory. This paper proposes a method developed during an extensive experimental programme that aimed at studying the mechanical effects of DEF on structures built with different concretes. The objective was to design a single heat treatment profile to generate homogeneous and similar temperature fields in specimens of various geometries, concrete constituents and mix proportions. It has been demonstrated that quasi-adiabatic conditions were to be realized at early age, whatever the samples size, to simulate the curing conditions in massive structures. The experimental method developed is described in details: the design of the temperature profile, the experimental device and its operation are presented. Both a numeric and an experimental validation are proposed. They emphasize the good accuracy of the process and demonstrate the possibility to trigger similar DEF potential expansions due to the thermal history for all the specimens of the programme.     

Coefficient of thermal expansion of cement paste and concrete: Mechanisms of moisture interaction

Crack sensitivity calculations for young concrete are strongly influenced by the coefficient of thermal expansion (CTE) values for the concrete. This paper demonstrates the strong effect of moisture content on CTE, and discusses the mechanism(s) based on experimental results on mature cement paste. The temperature variation of the relative humidity (RH) exerted by the pore water is quantified and used to explain the high CTE of partly dried specimens. The relevance for early age crack sensitivity is that the concrete should be kept as wet as possible. This minimizes CTE and will largely eliminate autogenous shrinkage. However, trustworthy procedures to separate autogenous- and thermal deformations require a better fundamental understanding of moisture effects and the nature of delayed deformations.     

Using a modified Knoop Indentation Technique to estimate the cavitation erosion resistance of NiTi

An Ni-rich (50.8 at.% Ni) NiTi alloy was heat treated with different aging temperatures to obtain specimens possessing different cavitation erosion resistance. A modified Knoop Indentation Technique that combines indentation measurements and thermal recovery has been used to determine the total-recovery/deformation ratio for the heat-treated NiTi specimens. This ratio, which reflects both superelasticity and pseudoplasticity, has been found to correlate well with the cavitation erosion resistance for NiTi. The technique thus serves as a simple method for estimating the performance of NiTi against cavitation erosion.     

Chemical and thermomechanical tailoring of the shape memory effect in poly(ε-caprolactone)-based systems

The thermally activated shape memory response of polymeric materials results from a combination of the material molecular architecture with the thermal/deformational history, or ‘programming’. In this work, we investigate the shape memory response of systems based on poly(ε-caprolactone) (PCL) so as to explore the adoption of proper chemical and thermomechanical tailoring routes. Cross-linked semicrystalline PCL-based materials are prepared by different molecular architectures starting from linear, three- and four-arms star PCL functionalized with methacrylate end groups, allowing to tune the melting temperature, T _m, ranging between 36 and 55 °C. The materials’ ability to display the shape memory is investigated by the application of proper thermomechanical cycles on specimens deformed at two different temperatures (23 and 65 °C, i.e. below and above the T _m, respectively). The shape memory response is studied under dynamic thermal conditions in thermally activated recovery tests, to identify the typical transformation temperatures, and under isothermal conditions at given recovery temperatures, to monitor shape recovery as a function of time. All the specimens are capable of full recovery on specific thermal ranges influenced by both melting and deformation temperatures. Specimens deformed above T _m are able to recover the whole deformation in a very narrow temperature region close to T _m, while those deformed at room temperature display broader recovery processes, those onset at about 30 °C. Isothermal tests reveal that when the deformed material is subjected to a constant recovery temperature, the amount of recovered strain and the time required strongly depend on the particular combination of melting temperature, deformation temperature and recovery temperature.     

Measurements of transient high temperatures during the deformation of polymers

A novel and simple technique for the measurement of the magnitude and spatial distribution of transient high temperatures is described. The technique is applied to the measurement of the temperatures produced during the deformation under impact of a range of polymers. It is found that very high temperatures (up to 700‡ C) can be obtained in materials which undergo catastrophic failure, and that the temperatures obtained are related to the material's thermal and mechanical properties.     

Expansive concrete cured in pressurized water at high temperature

Behaviours of expansive concretes cured in pressurized water at high temperature (TPC curing) are investigated with regard to their variations in strain, weight and strength. These behaviours are compared with those cured by autoclave curing (AC curing) at the same pressure and temperature. The experimental data show that both of these curing methods are effective in reducing autogenous shrinkage and drying shrinkage at longer age after the curing. Particularly when TPC curing is adopted, the required expansion of concrete can be attained at smaller dosage of expansive agents, and after the curing length changes due to hygrometric or hygroscopic conditions are almost eliminated for later age. Therefore, this curing method is anticipated to be effective in introducing chemical prestress in a reinforced concrete member, which is proved by loading tests of spirally reinforced pipe specimens [Ei.-i. Tazawa, K. Miyaguchi, in: Proceedings of the 25th JUCC Congress on Cement and Concrete, 1998, pp. 71–76].     

A new application of fiber-optic reflection spectroscopy (FORS): characterization of low-temperature alteration of chlorite schist and implications for understanding ancient stone cooking vessels

Bulk thermal alterations to chlorite schist occurring at temperatures above 450 °C are traditionally studied using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, or thermogravimetric analysis (TGA), but lower temperature changes that lead to structural and chemical alteration, including changes in oxidation state that are not followed by a crystalline modification, are not detectable using these techniques. In this paper we present the results of the first study to examine the application of fiber-optic reflection spectroscopy (FORS) for the detection and quantification of low-temperature thermal alterations in chlorite schist. Such changes have been observed during research on the thermal behavior of medieval (12th-13th centuries AD) chlorite cooking pots from the archaeological site of Merv, Turkmenistan. FORS was used to investigate these changes, testing the archaeological samples against a model data set of experimental reference specimens. The results demonstrate the potential of FORS for tracking low-temperature thermal alterations and offer the opportunity to examine temperatures attained by ancient chlorite vessels during their past use in cooking activities.     

Stochastic nonlinear bending response of functionally graded material plate with random system properties in thermal environment

This study deals with the stochastic nonlinear bending response of functionally graded materials (FGMs) plate with uncertain system properties subjected to transverse uniformly distributed load in thermal environments. The system properties such as material properties of each constituent’s material, volume fraction index and transverse load are taken as independent random input variables. The material properties are assumed to be temperature independent (TID) and temperature dependent (TD). The basic formulation is based on higher order shear deformation theory with von-Karman nonlinear strain kinematics using modified C ⁰ continuity. A direct iterative based nonlinear finite element method in conjunction with first-order perturbation technique developed by last two authors for the composite plate is extended for the FGM plate to compute the second order statistics (mean and standard deviation) of the nonlinear bending response of the FGM plates. Effects of TD, TID material properties, aspect ratios, volume fraction index and boundary conditions, uniform temperature and non-uniform temperature distribution on the nonlinear bending are presented in detail through parametric studies. The present outlined approach has been validated with the results available in the literature and independent Monte Carlo simulation.     

Determination of the coefficient of thermal expansion of high performance concrete from initial setting

Autogenous shrinkage, which is a consequence of the absolute volume contraction resulting from cement hydration, occurs in any concrete but its effect is particularly amplified in high performance concrete in which it can be as large as drying shrinkage. Autogenous shrinkage can be directly measured in concrete samples under isothermal conditions but from a practical standpoint the experimental procedure is not always possible. On the other hand, it can be evaluated after having taken into account volumetric variations due to the release in heat during cement hydration. To separate the thermal effect from autogenous shrinkage, it is necessary to know at any moment the evolution of the coefficient of thermal expansion of the concrete from initial setting. A simple method to determine the coefficient of thermal expansion at early ages is proposed in this paper. It consists in submitting concrete samples instrumented with vibrating wire extensometers to thermal shocks. The response of the concrete sample to this shock results in a nearly instantaneous deformation, which is measured by the sensor. These deformations, as well as the temperature signal, are used to calculate the coefficient of thermal expansion. By repeating this experiment at various ages, it is possible to follow the variation in the coefficient of thermal expansion of the concrete over time.     

Effect of thermal cycling process parameters on recrystallization kinetics for processing of fine-grained pure copper

The present work developed a two-step thermal cycling technique for processing of fine-grained pure copper for improved properties. This included initial annealing of specimens followed by heavy cold rolling and a series of heating and cooling cycles. The study investigated the important microstructural changes occurring in the cold deformed grains in the absence of phase transformations. A major interest of the present research was to closely describe the effect of thermal cycling parameters on the recrystallization and grain growth kinetics for processing of fine-grained structure. The study determined the optimum values of process parameters for the developed thermal cycling route including the extent of cold deformation, annealing temperatures, holding periods, and the number of thermal cycles. The thermal cycling process produced closely distributed fine grains with bi-modal microstructure leading to increased hardness and strength without any significant loss in electrical conductivity.     

Thermal postbuckling analysis of laminated composite plates

The thermal postbuckling behavior of graphite/epoxy multi-layered rectangular plates of various boundary conditions is studied using the finite element method. Temperature dependent thermal and elastic properties of the material are used in the analysis. The nonlinear finite element equations are solved as a sequence of linear eigenvalue problems to trace the thermal postbuckling paths of 15-layered symmetric angle-ply plates. The presence of secondary instability with an unsymmetric deformation mode has been identified for symmetric laminates under uniform temperature rise. In the case of linearly varying temperature rise through the thickness of the plate, the nonlinear equilibrium equations are solved by the modified Newton–Raphson technique to get the temperature-displacement curves.     

Transient strain of self-compacting concrete loaded in compression heated to 700 °C

Self-compacting concrete (SCC) tends to spall when subject to high temperature in case of an accidental fire. Proper understanding its deformation properties at elevated temperatures is necessary in avoiding detrimental effects. This paper presents the experimental results carried out on SCC specimens subjected to high temperatures when they are loaded. The transient strain and load induced thermal strain of SCC are measured under the variation of temperature level, heating rate, stress level, strength grade of SCC, and content of polypropylene fibres. Furthermore, measurements with differential thermal analysis, SEM and mercury intrusion porosimetry had been conducted on samples taken from thermal-loading tests to investigate the mechanism that induces transient strain.     

Low-Temperature Internal Friction and Thermal Conductivity of Plastically Deformed, High-Purity Monocrystalline Niobium

The low-temperature internal friction Q ^–1 and thermal conductivity of plastically deformed, high-purity niobium monocrystals have been investigated and compared with measurements on an amorphous SiO₂ (a-SiO₂) specimen. After plastic deformation at intermediate temperatures, an approximately temperature independent internal friction Q ^–1 was observed with a magnitude comparable to that of the a-SiO₂ specimen. Plastic deformation at low temperatures leads to an internal friction Q ^–1 with a considerably smaller magnitude. In the temperature range between about 0.3 and 1.5K, the lattice thermal conductivity k of the deformed specimens decreases with increasing deformation. It is, however, nearly independent of the amount of deformation at the lowest temperatures investigated. In this temperature regime, the lattice thermal conductivity of the specimens varies proportional to T ³ and has a magnitude as would be expected for an undeformed sample. Additional heat release experiments on an undeformed sample clearly show no long-time energy relaxation effects. We conclude that the defects introduced by plastic deformation cannot be described with the tunneling model which had been proposed to describe the low temperature elastic and thermal properties of amorphous solids. The phonon scattering mechanisms observed in deformed niobium are tentatively related to the dynamic interaction of phonons with geometrical kinks in dislocations.