Validation of the time–temperature superposition principle for crack propagation in bituminous mixtures

The time–temperature superposition principle (TTSP) is known to be valid in the small strain domain where the behaviour of bituminous mixtures is linear viscoelastic (LVE). The behaviour is then called thermorheologically simple. In this work, an experimental campaign was performed at University of Lyon/ENTPE (France) to check the validity of the TTSP in the linear domain in the tridimensional case and also when cracks occur and propagate in bituminous mixture. A four-point bending test, which has been designed at University of Lyon/ENTPE, was used as crack propagation test. First, a complex modulus test is performed on cylindrical specimen in the LVE domain. Then, a series of crack propagation tests are carried out at different temperatures and different imposed displacement rates. The same shift factors obtained for master curve of complex modulus is also applied for the crack propagation tests analysis. The results allow obtaining a unique curve, for identical loadings when plotting as a function of reduced time. This result confirms that the TTSP is also valid for crack propagation in bituminous mixtures.     

Application of time–temperature–stress superposition on creep of wood–plastic composites

Time–temperature–stress superposition principle (TTSSP) was widely applied in studies of viscoelastic properties of materials. It involves shifting curves at various conditions to construct master curves. To extend the application of this principle, a temperature–stress hybrid shift factor and a modified Williams–Landel–Ferry (WLF) equation that incorporated variables of stress and temperature for the shift factor fitting were studied. A wood–plastic composite (WPC) was selected as the test subject to conduct a series of short-term creep tests. The results indicate that the WPC were rheologically simple materials and merely a horizontal shift was needed for the time–temperature superposition, whereas vertical shifting would be needed for time–stress superposition. The shift factor was independent of the stress for horizontal shifts in time–temperature superposition. In addition, the temperature- and stress-shift factors used to construct master curves were well fitted with the WLF equation. Furthermore, the parameters of the modified WLF equation were also successfully calibrated. The application of this method and equation can be extended to curve shifting that involves the effects of both temperature and stress simultaneously.     

Flexural creep behavior of discontinuous thermoplastic composites: Non-linear viscoelastic modeling and time–temperature–stress superposition

Flexural creep behavior of nylon 6/6, polypropylene and high-density polyethylene long fiber thermoplastic (LFT) composites was studied according to ASTM D-2990. Neat polymers were tested for baseline data and compared with the 40 wt.% E-glass reinforced LFTs, all processed by compression molding. All materials exhibited non-linear viscoelasticity and showed a succession in creep resistance consistent with static flexural yield strength. A four parameter empirical model used for short fiber thermoplastics (SFT), proposed by Hadid et al., was found to provide an excellent fit to the experimental data. Time-compliance data from flexural creep and dynamic mechanical analysis (DMA) were combined to utilize short-term flexural creep tests to predict lifetime of the composites. A time–temperature–stress superposition (TTSSP) procedure was used, where stress-based vertical shifts were applied in addition to horizontal shifts used in a traditional time–temperature superposition (TTSP). Master curves obtained by this method projected the long-term creep properties, the order of creep resistance being consistent with the flexural creep data.     

Prediction of the temperature dependence of fracture toughness of 12Cr–2Ni–Mo refractory steel

We study the influence of temperature and the size of the specimens on the characteristics of static crack resistance of 12Cr–2Ni–Mo refractory steel. It is shown that, in the temperature range 20–450°C, the increase in the thickness of specimens leads to an insignificant increase in fracture toughness obtained along a 5% secant line according to the standards of evaluation of the characteristics of crack resistance. The evaluation of the characteristics of crack resistance of 12Cr–2Ni–Mo steel with regard for the scale effect according to an earlier developed numerical-experimental model reveals the existence of satisfactory agreement with the experimental data in the entire investigated temperature range. Translated from Problemy Prochnosti, No. 4, pp. 78–88, July–August, 2009.     

Shear failure characterization of time–temperature sensitive interfaces

Poor interlayer bonding can lead to early failures and thus to a reduction in service life of bituminous pavements. For this reason, it is important to identify the parameters influencing the interlayer shear failure and to characterize their effect by means of laboratory test. In particular, this study is focussed on the effects of test temperature and deformation rate on the interlayer shear strength (ISS) of double-layered asphalt concrete specimens. First, the ISS was measured at temperatures ranging from 0 °C to 30 °C and deformation rates ranging from 0.5 mm/min to 9 mm/min using the Ancona Shear Testing Research and Analysis (ASTRA) device. Then the experimental data were analyzed using a two-stage statistical modelling approach. In the first stage, the variation of ISS versus deformation rate, at each testing temperature, was modelled using both a power-law and a logarithmic function. In the investigated range of deformation rate, the models allowed to estimate the mean ISS with residual standard error varying from 0.062 MPa to 0.128 MPa. Moreover, the linear regression coefficients, which measure the influence of the deformation rate on ISS, changed with temperature. In the second stage, both temperature and deformation rate were used as joint predictors of ISS by using an approach based on the superposition of their effects. Results showed that the time–temperature superposition approach is applicable and a sigmoid-shaped master curve for ISS was obtained. The proposed approach was validated by using ISS measurements obtained on the same materials with different test devices.     

Prediction of the mechanical properties of the post-forged Ti–6Al–4V alloy using fuzzy neural network

Isothermal compression of the Ti–6Al–4V alloy was conducted at a 2500 ton isothermal hydrostatic press, and the mechanical properties including ultimate tensile strength, yield strength, elongation and area reduction of the post-forged Ti–6Al–4V alloy were measured at a ZWICK/Z150 testing machine. A fuzzy neural network (FNN) was applied to acquire the relationships between the mechanical properties and the processing parameters of post-forged Ti–6Al–4V alloy. In establishing those relationships, the forging temperature, strain and strain rate were taken as the inputs, whilst the ultimate tensile strength, yield strength, elongation and area reduction were taken as the output respectively. The predicted results using the present FNN model is in a good agreement with the experimental data of the post-forged Ti–6Al–4V alloy, and the optimum processing parameters can be quickly and conveniently selected to achieve the desired mechanical properties by means of the prediction based on the fuzzy neural network model.     

Growth of phases in the solid-state from room temperature to an elevated temperature in the Pd–Sn and the Pt–Sn systems

The solid-state growth of the product phases in bulk and electroplated diffusion couples of the Pd–Sn and the Pt–Sn systems is reported at various temperatures, ranging from room temperature to 215 °C. The growth rate of the product phase in the Pt–Sn system is found to be much lower compared to the Pd–Sn system and the Au–Sn system also, which is currently used in the microelectronics industry. The time dependent experiments indicate that the growth rate in the Pd–Sn system is parabolic in nature, i.e., it is controlled by the diffusion rates of components through the product phases. However, the growth rate is linear and hence reaction-controlled in the Pt–Sn system, which indicates that the formation of the compound is the rate-limiting step rather than the diffusion rates of components. The PdSn₄ phase covers almost whole interdiffusion zone in the Pd/Sn couple, while PtSn₄ is the only phase found in the Pt/Sn couple. The marker experiments indicate that both PdSn₄ and PtSn₄ grow mainly by the diffusion of Sn, with negligible diffusion of Pd and Pt, respectively. Furthermore, the analysis considering the same crystal structure (i.e., oC20) of these phases along with the concept of sublattice diffusion mechanism indicates that the diffusion rates of both Pd and Pt are negligible via both the lattice and the grain boundaries.     

Prediction of the mechanical properties of forged Ti–10V–2Fe–3Al titanium alloy using FNN

In this paper, a fuzzy neural network (FNN) prediction model has been employed to establish the relationship between processing parameters and mechanical properties of Ti–10V–2Fe–3Al titanium alloy. In establishing these relationships, deformation temperature, degree of deformation, solution temperature and aging temperature are entered as input variables while the ultimate tensile strength, yield strength, elongation and area reduction are used as outputs, respectively. After the training process of the network, the accuracy of fuzzy model was tested by the test samples and compared with regression method. The obtained results with fuzzy neural network show that the predicted results are much better agreement with the experimental results than regression method and the maximum relative error is less than 7%. And the optimum matching processing parameters can be quickly selected to achieve the desired mechanical property based on the fuzzy model. It proved that the model has a good precision and excellent ability of predicting.     

Investigation of the heat conductivity of niobium in the temperature range 0.05–23 K

We report thermal conductivity measurements on a single-crystal niobium specimen of resistivity ratio 33,000 over the temperature range 0.05–23 K in the superconducting state and above 9.1 K in the normal state. The axis of the niobium rod was [110] oriented. The surface roughness was varied by sandblasting of the sample. The values of the thermal conductivity in the range from the lowest temperatures up to the maximal value covered a range of six orders of magnitude (=2×10^–5 W cm^–1 K^–1 at 50 mK to =22 W cm^–1 K^–1 at 9 K). Above 2 K the results for the untreated and the sandblasted sample are in accord, whereas below 2 K the influence of the sample surface is discernible. The various conduction and scattering mechanisms are discussed.     

Thermal diffusivity of gadolinium in the temperature range of 287–1277 K

New experimental data on the thermal diffusivity of gadolinium in the temperature interval from 287 to 1277 K obtained by the laser flash method with an error of 3–4% are presented. Results are compared with the available literature data. Reference tables on the heat transfer coefficients of gadolinium for scientific and practical use are developed. Critical indices for the thermal diffusivity of gadolinium above the Curie point are determined. The limitations of the laser flash method during measurement in the region of phase transformations are briefly discussed.     

Alumina formation and microstructural changes of aluminized CoNiCrAlY coating during high temperature exposure in the temperature range 925°C–1075°C

Abstract

MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. In the present work the microstructural features and oxidation behavior of an aluminized Co-base MCrAlY-coating on a Ni-based superalloy have been investigated in the temperature range 925–1075 °C. Microstructural studies of the oxidized coatings by SEM/EBSD were complemented with numerical thermodynamic calculations using the software package ThermoCalc. In the as-received condition the outer part of the coating consisted mostly of β-(Ni,Co)Al. Formation of σ-CoCr was observed at the interface between the β-layer and the inner initial CoNiCrAlY. During high-temperature air exposure alumina based surface scales were formed but the oxidation induced Al depletion of the aluminized coating did not result in formation of the γ’-(Ni₃Al) phase. Rather, the subscale formation of Co/Cr-rich phases was observed and a direct transformation of β- into γ-Ni phase after longer times. It is expected that these subscale microstructural changes thus affect the alumina formation and growth as well as the critical aluminum depletion in a different manner as in the case of corresponding β-NiAl coatings, although a direct comparison between various coating systems was not possible on the basis of the present results.     

Prediction of flow stress in isothermal compression of Ti–6Al–4V alloy using fuzzy neural network

Isothermal compression of Ti–6Al–4V alloy at the deformation temperatures ranging from 1093 K to 1303 K with an interval 20 K, the strain rates ranging from 0.001 s⁻¹ to 10.0 s⁻¹ and the height reductions ranging from 20% to 60% with an interval 10% were carried out on a Thermecmaster-Z simulator. Based on the experimental results, a model for the flow stress in isothermal compression of Ti–6Al–4V alloy was established in terms of the fuzzy neural network (FNN) with a back-propagation learning algorithm using strain, strain rate and deformation temperature as inputs. The maximum difference and the average difference between the predicted and the experimental flow stress are 18.7% and 4.76%, respectively. The comparison between the predicted results based on the FNN model for flow stress and those using the regression method has illustrated that the FNN model is more efficient in predicting the flow stress of Ti–6Al–4V alloy.     

Effect of concentration and temperature on the properties of the microspheres prepared using an emulsion–solvent extraction process

Methylaluminoaxane (MAO) microspheres of average size smaller than 100 μm were prepared using a solvent extraction process; excess of perfluorocarbon (PFC) was added to the hydrocarbon-in-perfluorocarbon emulsion to solidify the MAO droplets. The effect of the MAO concentration in the dispersed phase, the temperature of the PFC added to the emulsion and the preparation temperature on the size, the size distribution and the morphology of the MAO microspheres was studied. MAO droplets broke up more easily with decreasing dispersed phase viscosity and hence the size of the MAO microspheres decreased. The temperature of the PFC added to the emulsion did not significantly affect the size and the morphology of the MAO microspheres. The size of the MAO microspheres increased with increasing preparation temperature. The main reason for the increased size of the MAO microspheres was the increased coalescence with increasing preparation temperature. In addition the porosity of the MAO microspheres increased with increasing preparation temperature.     

Influence of solution treatment temperature on mechanical properties of a Fe–Ni–Cr alloy

Influence of solution treatment temperature on mechanical properties of a Fe–Ni–Cr alloy was studied in this work. The results indicate that the strength and the ductile properties are optimum after solution treatment at 1000 °C followed by conventional two-step aging, but decrease markedly with the increase of solution temperature. Microstructure analyses show that TiC phase particles in the microstructure partly dissolves into the matrix when the solution treatment temperature is higher than 1100 °C, resulting in the coarsening of austenitic grain. Flake-like M₃B₂ phase precipitates at the grain boundary in the specimens solution-treated at temperatures higher than 1050 °C and its formation induces the mechanical properties to be worse.     

Application of Liquid Solution Models for the Prediction of Corrosion Phenomena in the Na–K Melt

With the mathematical tool of pseudo-regular solutions, we calculated the solubility of Fe, Cr, Ni, V, Mn, and Mo in eutectic Na–K melt and compared the calculation results with the available experimental data in the literature on the mass transfer of austenitic chrome-nickel steel components in a sodium-potassium loop in nonisothermal conditions. We show that the Wagner parameters of the interaction between oxygen and transition metal might be applied to predict the corrosion behavior of the construction materials in sodium-potassium melt under the presence of oxygen impurity. We present the calculation results of the threshold oxygen concentration required to form ternary sodium oxides with transition metals (Fe, Cr, Ni, V, Mn, Mo) in conditions in which the pure metal is in contact with eutectic Na–K melt.     

Prediction of iron ore mineral liberation behavior using the Andrews–Mika diagram and beta distribution

A combined grinding and liberation model was studied to predict the liberation characteristics of iron ore comminuted by a ball mill. Comminution characteristics were obtained using the one-size-fraction method; then, the iron ore samples were crushed and separated into several narrow-size fractions by screening. The size fractions were further divided into three classes by magnetic separation, namely concentrate, middling, and tailings. Subsequently, various size classes were subjected to ball milling, and the results were analyzed in the kinetic grinding model. The mineralogical textures of the ground products were analyzed using mineral liberation analysis. The beta distribution and Andrews–Mika diagram characterized the distribution of the iron content within the size fractions. The breakage characteristics of the iron ore samples varied slightly with the iron content. However, the liberation characteristics were well-described by adjusting the four parameters of the beta distribution. The variation in grinding kinetics with composition and liberation model parameters was used in the combined liberation–comminution model to predict the evolution of the size–composition matrix as grinding proceeded. The predicted results align with the experimental results. The developed model can construct a yield–recovery graph and calculate the liberation efficiency to determine the size reduction threshold for efficient separation.     

The decomposition of the solid solution state in the temperature range 20 to 200° C in an Al-Zn-Mg alloy

The decomposition of the supersaturated solid solution of an Al-3.2wt% Zn-2.2 wt% Mg alloy has been investigated in the temperature of 20 to 200° C, by small-angle X-ray scattering, electrical resistivity and mechanical measurements. On the basis of the results obtained, three subsequent stages of the decomposition process can be distinguished. Between 20 and 70° the basic process is the nucleation and growth of G.P. zones, the volume fraction of which increases logarithmically with time. A transition stage is observed between 80 and 100° in which the volume fraction increases linearly with time. Above 90° C, the growth kinetics of the volume fraction shows a definite incubation period at the beginning of ageing, while the yield stress increases monotonically. In the temperature range 100 to 160° C, the formation of the phase takes place. Below 100° a linear connection between the yield stress and (fR)^1/2 is found from which the specific surface energy to cut a G.P. zone is calculated to be ₀= 0.21 Nm m^–2.     

Electroelastic study of the relaxor–normal phase transition in PFW–PT solid solution

Relaxor–normal transition in ferroelectric materials have been extensively studied through their optical and dielectric properties. However, only few reports concerning simultaneously elastic and dielectric characterization were presented. In this work, multiferroic ceramic solid solutions between the ferroelectric relaxor Pb(Fe_2/3W_1/3)O₃ (PFW) and “normal” ferroelectric PbTiO₃ (PT) [PFW–PT] have been synthesized by a modified B-site precursor method and investigated by the ultrasonic pulse echo technique and dielectric measurements, as a function of the frequency and temperature. The nature of the phase transition was characterized through the diffusivity exponent obtained from both measurements. Additionally, an interplay between ferroelectric and ferromagnetic properties of the samples, as well as the influence of the proximity of the temperatures of the establishment of both orderings with electroelastic coupling coefficient was represented.     

An analytic solution to the coupled pressure–temperature equations for modeling of photoacoustic trace gas sensors

Trace gas sensors have a wide range of applications including air quality monitoring, industrial process control, and medical diagnosis via breath biomarkers. Quartz-enhanced photoacoustic spectroscopy and resonant optothermoacoustic detection are two techniques with several promising advantages. Both methods use a quartz tuning fork and modulated laser source to detect trace gases. To date, these complementary methods have been modeled independently and have not accounted for the damping of the tuning fork in a principled manner. In this paper, we discuss a coupled system of equations derived by Morse and Ingard for the pressure, temperature, and velocity of a fluid, which accounts for both thermal effects and viscous damping, and which can be used to model both types of trace gas sensors simultaneously. As a first step toward the development of a more realistic model of these trace gas sensors, we derive an analytic solution to a pressure–temperature subsystem of the Morse–Ingard equations in the special case of cylindrical symmetry. We solve for the pressure and temperature in an infinitely long cylindrical fluid domain with a source function given by a constant-width Gaussian beam that is aligned with the axis of the cylinder. In addition, we surround this cylinder with an infinitely long annular solid domain, and we couple the pressure and temperature in the fluid domain to the temperature in the solid. We show that the temperature in the solid near the fluid–solid interface can be at least an order of magnitude larger than that computed using a simpler model in which the temperature in the fluid is governed by the heat equation rather than by the Morse–Ingard equations. In addition, we verify that the temperature solution of the coupled system exhibits a thermal boundary layer. These results strongly suggest that for computational modeling of resonant optothermoacoustic detection sensors, the temperature in the fluid should be computed by solving the Morse–Ingard equations rather than the heat equation.     

Oxidation of ion-implanted niobium in the 300–700°C temperature range

Ion implantation of different species was shown to have a beneficial influence on the thermal oxidation kinetics of niobium in pure oxygen at temperatures below 500°C. The implants were chosen with regard to their affinity for oxygen compared to that of niobium and their solubility in niobium. The effects of the treatment was to delay the appearence of the linear catastrophic kinetics. The mechanisms involved are discussed.