Isothermal Stress Research Articles

Adapting a multi-component model to cyclic stress–strain behaviour under thermomechanical fatigue conditions

The cyclic stress–strain behaviour of metals and alloys in cyclic saturation can reasonably be described by means of simple multi-component models. In this study, this model concept was applied to thermomechanical fatigue loading. Two metallic engineering materials, which were found to be oppositional with respect to cyclic plastic deformation, were dealt with. One material is an austenitic stainless steel of type AISI304L which can be considered as an example for a rather ductile alloy. The second alloy is a third-generation near-gamma TiAl alloy which is characterized by a very pronounced ductile-to-brittle transition (DBT) within the temperature range of TMF cycling. The experimental observations regarding isothermal and non-isothermal stress–strain behaviour and the correlation to the underlying microstructural processes were used to further develop the TMF multi-composite model in order to accurately predict the TMF stress–strain response by taking the alloy-specific features into account.

Stress Relaxation Continuum Damage Constitutive Equations for Relaxation Performance Prediction

Stress relaxation constitutive equations based on Continuum Damage Mechanics, Kachanov-Robatnov creep model, and stress relaxation equation has been developed by analyzing stress relaxation damage mechanisms and considering the relationship that stress relaxation is creep at various stresses. And, the constitutive differential equations were integrated to predict stress relaxation performance by using numerical analysis technique. In order to validate the approach, the predicted results are compared to the experimental results of uni-axial isothermal stress relaxation tests conducted on 1Cr10NiMoW2VNbN steel with the same temperature of creep tests. Good agreement between results of relaxation tests and the predicted results indicates that the developed constitutive models can be used in the relaxation behavior evaluation of high temperature materials.

Compatibility studies between propafenone and selected excipients used in the development of controlled release formulations

The objective of the present study was to evaluate the compatibility of propafenone HCl (PFH) with the selected excipients used in a controlled drug delivery system. The studies were conducted by an isothermal stress test method. The differential scanning calorimetry (DSC) and high-performance liquid chromatography techniques were used as tools to assess the compatibility of the drug with the selected excipients. Complementary techniques such as powder X-ray powder diffractometry (pXRD) and fourier transform infrared (FTIR) spectroscopy were used to assist in the interpretation of the DSC results. On the basis of the DSC results, the drug was found to be compatible with gum kondagogu (GKG), chitosan, polyelectrolyte complex of GKG and chitosan, HPMC K100M, carbopol 934P, Benecel® and A-tab®. Some degree of interaction was observed with lactose monohydrate; however, the additional studies using FTIR spectroscopy and pXRD confirmed that PFH is compatible with lactose monohydrate.

Measurement of Flow Stress in Supercooled Austenite for High Hardenability Steel

In this article, the isothermal flow stress in supercooled austenite was measured for a high hardenability steel. Supercooled austenite forms at the nonequilibrium phase and changes into other phases within a short time. It was confirmed that conventional tensile tests, which require maintaining a constant temperature before stretching, cannot accurately measure the flow stress in supercooled austenite. Therefore, a new tensile test named “the continuous cooling tensile test” was developed. In this test, stretching is conducted during continuous cooling. In the continuous cooling tensile test, the flow stress between 673 K and 973 K (400 °C and 700 °C) was measured. Microscopic observations of the continuous cooling test results verified that the microstructures were supercooled austenite.

Evaluation of Trapa bispinosa Roxb. starch as pharmaceutical binder in solid dosage form

Objective To evaluate binding efficiency of Trapa bispinosa Roxb. starch (TBS) in diclofenac sodium tablets. Methods Diclofenac sodium tablets were prepared using wet granulation method. The starch paste in different concentrations (5%-15% w/w) was evaluated for optimized binder concentration. Preformulation study of the drug with TBS and different excipients were also analyzed using fourier transform infrared spectroscopy (FTIR) and isothermal stress testing (IST). Results Preformulation study of the drug, water chestnut starch and different excipients showed no interaction or, drug degradation in FTIR and IST, respectively. The tablets were evaluated for hardness, friability, drug content, disintegration and dissolution studies, and all the parameters were found within the official specifications. Conclusions The results reveal that this starch has potential to be used as binder at industrial scale in pharmaceutical solid dosage form development.

Chip and package-related degradation of high power white LEDs

With this paper we present an analysis of the degradation of state-of-the-art high power LEDs. Three different kinds of commercially available samples, from leading manufacturers, were submitted to stress under various current and temperature levels. Based on an accurate estimation of the thermal resistance of the devices, iso-thermal and iso-current stress tests have been carried out, with the aim of separately evaluating the role of current and temperature in determining the degradation of the LEDs. Results indicate that state-of-the-art LEDs can show a significant degradation of their electrical and optical characteristics, when they are operated close to their current/temperature limits. In particular, data reveal the presence of two different degradation mechanisms: (i) the degradation of the blue semiconductor chip, due to the increase in non-radiative recombination, or to the decrease in the acceptor dopant concentration at the p-side of the diodes; (ii) the chemical degradation of the package, with subsequent worsening of its optical properties. Results suggest that even high-performance LEDs can suffer from limited lifetime: thermal management and driving conditions must be carefully optimized with the aim of achieving high reliability for LEDs to be adopted in high efficiency lighting systems.

Degradation mechanisms of high-power white LEDs activated by current and temperature

This paper presents a study of the degradation mechanisms that limit the reliability of commercially-available white LEDs. Purely thermal stress and biased iso-thermal stress were carried out for several thousands hours on 1W-power LEDs, produced by a leading manufacturer. Results reveal that temperature and operating current have different roles in determining the optical degradation of these devices: (i) pure thermal stress induces a short-term optical power decay, strictly correlated to the decrease in the reflectivity of the package/reflector system and with no effects on the electrical characteristics of the blue chip; the activation energy of thermally-induced degradation is equal to 1.8eV; (ii) constant current stress induces a long-term degradation process, with a degradation rate which is strongly dependent on the stress current level. In this latter case, optical degradation is ascribed to the degradation of the blue semiconductor chip: details are provided through the analysis of forward voltage and wavelength shift during stress time.

Synthesis, characterization, and compatibility study of acetylated starch with lamivudine

In this study, highly substituted starch acetate was prepared by reaction with native moth bean starch and acetic anhydride. Physicochemical characterization of this modified starch was done using scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis. Their formation was confirmed by titrimetric analysis and highest degree of substitution was observed with a value of 2.35. The synthesized modified starch was further studied for compatibility with model drug lamivudine using differential scanning calorimetry and isothermal stress testing for its controlled release tablet formulation.

Influence of heat treatment on stress–strain behaviour and lattice parameters of Sn–In alloy

Sn based alloys have important industrial applications specially as pewters and soldering materials. One of these alloys, Sn–5·2 wt-%In alloy, is designed to be examined in the present work. The differential thermal analysis of this alloy gives a melting temperature value of 493 K. An empirical equation that can be used to determine the melting temperature of some Sn–In alloys is derived. Two different heat treated groups of samples, slowly cooled and quenched, are prepared. The phases present in these two groups of samples are determined from their X-ray diffraction patterns. The isothermal tensile stress–strain tests of all samples are reported at temperatures between 343 and 403 K. The changes of the work hardening parameters and also of the lattice parameters of the β-Sn phase with the deformation temperature are discussed. The values of the activation energy characterise a dislocation fracture mechanism.

Compatibility studies of nateglinide with excipients in immediate release tablets

Experiments were done to assess the compatibility of nateglinide with selected excipients in the development of immediate release tablets of nateglinide by thermal and isothermal stress testing (IST) techniques. To evaluate the drug-excipient compatibility, different techniques such as differential scanning calorimetric (DSC) study, infra-red (IR) spectrophotometric study and isothermal stress testing were adopted. The results of DSC study showed that magnesium stearate exhibited some interaction with nateglinide. However, the results of IR, and IST studies showed that all the excipients used in the formula were compatible with nateglinide. Optimized formulations developed using the compatible excipients were found to be stable over 3 months of accelerated stability studies (40 ± 2°C and 75 ± 5% RH). Overall, compatibility of excipients with nateglinide was successfully evaluated using a combination of thermal and IST methods and the formulations developed using the compatible excipients were found to be stable.

Solid state compatibility studies with tablet excipients using non thermal methods

Compatibility between two new active pharmaceutical ingredients (API) and several pharmaceutical excipients used in solid formulations has been investigated by FT-IR and HPLC following storage under two different conditions. Compatibility was investigated by storage at isothermal stress conditions for (i) 3 days and subsequently analysed by FT-IR and (ii) 12 weeks of storage and analysis by HPLC. For the majority of the examined excipients a large degradation measured by HPLC after 12 weeks storage was also detected by FT-IR following storage at isothermal stress conditions for 3 days, i.e. there was a general agreement between the results obtained by the two protocols. Further, the FT-IR method showed clear incompatibility with three excipients where no degradation products were detected by HPLC, but where a significant decrease in the API quantified by the HPLC assay, was observed. The accelerated method thus showed a clear advantage: incompatibility found after 12 weeks using HPLC was seen after 3 days with FT-IR. Furthermore, FT-IR provides an insight into structural changes not seen with HPLC. This is exemplified by the desalting of a hydrogen bromide salt of one of the two compounds, which might lead to changes of the intrinsic dissolution rate and potentially affect the bioavailability of the API.

Application of DSC, IST, and FTIR study in the compatibility testing of nateglinide with different pharmaceutical excipients

Experiments were done to assess the compati- bility of nateglinide (NTG) with selected excipients in the development of immediate release tablets of NTG by thermal and isothermal stress testing (IST) techniques. To evaluate the drug excipient compatibility, different tech- niques such as differential scanning calorimetric (DSC) study, infrared (IR) spectrophotometric study, and IST were adopted. The results of DSC study showed that magnesium stearate exhibited some interaction with NTG. However, the results of IR and IST studies showed that all the excipients used in the formula were compatible with NTG. The optimized formulation developed using the compatible excipients were found to be stable after 3 months of accelerated stability studies (40 ± 2 C and 75 ± 5% RH). Overall, compatibility of excipients with NTG was successfully evaluated using the combination of thermal and IST methods and the formulations developed using the compatible excipients was found to be stable.

Application of stress relaxation testing in evaluation of creep strength of a tungsten-alloyed 10% Cr cast steel

Uniaxial isothermal stress relaxation tests (SRT) were performed on a tungsten-alloyed 10% Cr cast steel (G-X12Cr Mo W V Nb N 10 1 1) at temperatures of 580, 600 and 620 °C and initial strain levels of 0.2, 0.5 and 0.8%. Inelastic strain rates for different stresses were estimated from the stress versus time data generated from the tests. Conventional creep tests were also conducted on the same material at 580, 600 and 620 °C and at different stress levels. The strain rate data estimated from SRT were compared with minimum creep rates derived from the creep tests; the strain rates estimated from SRT with 0.8% initial strain level are in better agreement than those estimated from SRT with 0.2 and 0.5% initial strain levels. In order to ascertain the technique, stress relaxation behaviour was estimated from creep test data and compared with the stress relaxation curves obtained from SRT at corresponding temperatures. The stress relaxation curves obtained from SRT with 0.8% initial strain level are in good agreement with the stress relaxation curves estimated from the creep tests. It is concluded that the stress relaxation test with initial strain level of 0.8% could be considered as an appropriate short-term test technique for estimation of creep strength of newly developed materials.

Effects of Physical Hardening on Stress Relaxation in Asphalt Cements

This paper documents and discusses an investigation of how physical hardening phenomena during isothermal conditioning affect stress relaxation in asphalt cement. The importance of physical hardening is reviewed with data from trial sections and regular contracts. Seven asphalts were recovered from an Ontario, Canada, pavement trial. Tensile specimens were poured and conditioned for either 20 min or 72 h at low temperatures before being subjected to a stress relaxation test at −10°C. The extended conditioning period could more than double the residual thermal stress at the end of the test. The one asphalt to show no physical hardening came from the section that remained largely free of cracking. A second material that showed a moderate degree of physical hardening only recently started to crack by an appreciable amount. In contrast, the remaining five materials significantly hardened during the extended conditioning period and cracked prematurely and excessively in service. These findings clearly refute the hypotheses that physical hardening is not important and that stress relaxation can reverse the effect or reduce it to insignificance. Materials with an unstable colloidal structure were found most sensitive to physical hardening. The results of this study agree with earlier creep data obtained according to an extended bending beam rheometer test method. Hence, it is imperative that pavements be designed with criteria that take physical hardening effects into account to limit premature and excessive performance failures.

Load-biased shape-memory and superelastic properties of a precipitation strengthened high-temperature Ni 50.3Ti 29.7Hf 20 alloy

A slightly Ni-rich NiTi–20Hf (at.%) alloy was aged for 3 h at 550 °C to form a homogeneous distribution of 10–20 nm precipitates. To determine the effect of such structures on the martensite–austenite transformation, preliminary load-biased shape-memory and superelastic properties were measured. The alloy exhibited reasonably high transformation temperatures, near-perfect dimensional stability and a work output as high as 18.7 J cm −3 during load-biased thermal cycling. Isothermal stress cycling of the austenite between 180 and 220 °C resulted in near-perfect superelastic behavior up to 3% applied strain.

Response of the cell membrane–cytoskeleton complex to osmotic and freeze/thaw stresses

In order to develop successful cryopreservation protocols a better understanding of the freeze- and dehydration-induced changes occurring in the cell membrane and its underlying support, the actin cytoskeleton, is required. In this study, we compared the biophysical response of model mammalian cells (human foreskin fibroblasts) to hyperosmotic stress and freeze/thaw. Transmitted light, infrared spectroscopy, fluorescence- and cryo-microscopy were used to investigate the changes in the cell membrane and the actin cytoskeleton. We observed that a purely hyperosmotic challenge at room temperature resulted in bleb formation. A decrease in temperature abrogated the blebbing behavior, but was accompanied by a decrease in viability. These results suggested that cell survival depended on the availability of the membrane material to accommodate the volumetric expansion back to the original cell volume at isotonic conditions. Our data also showed that freeze/thaw stresses altered the cell membrane morphology resulting in a loss of membrane material. There was also a significantly lower incidence of blebbing after freeze/thaw as compared to isothermal osmotic stress experiments at room temperature. Significant depolymerization of the actin cytoskeleton was seen in cells whose membranes had been compromised by freeze/thaw stresses. Actin depolymerization using cytochalasin D affected the stability of the membrane against mechanical stress at isothermal conditions. This study shows that both the membrane and cytoskeleton, as a system, are involved in the osmotic and freeze/thaw-induced responses of the mammalian cells.

A modified Johnson–Cook constitutive model for Mg–Gd–Y alloy extended to a wide range of temperatures

In this paper, a new phenomenological and empirically based constitutive model was proposed to change the temperature term in the original Johnson–Cook constitutive model. The new model can be used to describe or predict the stress–strain relation of the metals deformed over a wide range of temperatures even though the current temperatures were lower than the reference temperature. Based on the impact compression data obtained by split Hopkins pressure bar technique, the material constants in the new model can be experimentally determined using isothermal and adiabatic stress–strain curves at different strain rates and temperatures. Good agreement is obtained between the predicted and the experimental stress–strain curves for a hot-extruded Mg–10Gd–2Y–0.5Zr alloy at both quasi-static and dynamic loadings under a wide range of temperatures ever though the current temperatures were lower than the reference temperature.

Kinematics and kinetics description of thermoelastic finite deformation from multiplicative decomposition of deformation gradient viewpoint

In this paper, using the multiplicative decomposition of the deformation gradient into mechanical and thermal parts, both kinematic and kinetic aspects of finite deformation thermoelasticity are considered. At first, the kinematics of the thermoelastic continua in the purely thermal process of nonisothermal deformation is investigated for finite deformation thermoelasticity. Also, a linear relation between the thermal expansion tensor and the rate of the thermal deformation tensor is presented. In order to model the mechanical behavior of thermoelastic continua in the stress-producing process of nonisothermal deformation, an isothermal effective stress–strain equation based on the logarithmic strain measure is considered for implementation in the governing equations of finite deformation thermoelasticity.

Analysis of Garofalo equation parameters for an ultrahigh carbon steel

Isothermal stress–strain curves data from torsion tests conducted at high temperature (950–1200 °C) and strain rates (2–26 s−1) were analyzed in an ultrahigh carbon steel (UHCS) containing 1.3%C. The sine hyperbolic Garofalo equation was selected as an adequate constitutive equation for the entire range of the forming variables considered. The Garofalo parameters were assumed strain dependent allowing the prediction of stress–strain curves under transient and steady-state conditions. The average relative errors obtained were below 3% in stress. In addition, the creep deformation mechanisms in the UHCS were analyzed from the Garofalo equation parameters. For this aim, the stress exponent of the Garofalo equation was, for the first time, related to that of the power law equation. The results show that the controlled deformation mechanism at steady state is lattice diffusion-controlled slip creep.

Microstructural and alloy influence on the low-temperature strengthening behavior of commercial steels used as plates

Steel plates with a variety of microstructures including ferrite–pearlite, tempered martensite, and acicular ferrite are currently used for low-temperature applications. While ductility is a primary concern for steels at low temperatures (a concern which has led to extensive research in this area), a comparison of the strengthening mechanisms between microstructures gives insight into the strengthening behavior of these steels as a function of temperature. The focus of the present work is to compare both thermally activated and athermal strengthening mechanisms between six steel alloys to provide insight into steel design for low-temperature plate applications. The alloys selected for examination were A516 normalized; A514 in the as rolled, quenched and tempered, and as-quenched conditions; HSLA100 quenched and tempered; and A553 quenched and tempered. These steels gave a wide range of industrially relevant steel microstructures. The A514 conditions also provide the opportunity to compare strengthening mechanisms as a function of microstructure for a constant composition. A combination of isothermal tensile and stress relaxation tests were performed at temperatures between 79 and 295 K (−194 and 22 °C) to determine flow stress and strengthening components for each of the alloys as functions of temperature. The relative flow stresses of all of the steels showed that even though the strength of all the steels did increase with decreasing temperature, the strength did not increase to the same degree. The two alloys with significantly different compositions, HSLA100 and A553, displayed very different temperature-dependent effective stress and dislocation-velocity stress exponent exponent as compared to the other steels. These alloys have a lower overall effect from solid-solution alloying. The temperature-independent strength component, internal stress, was successfully related to effective grain size using a Hall–Petch type plot. Internal stress is the dominant strengthening mechanism for all of the alloys over the range of conditions tested.