Isothermal Stress Research Articles

Thermo-Mechanical Analysis and Lifing of Ni-Cr SPF Tool

The paper describes the aniso-thermo-mechanical analysis of a representative large industrial SPF (XN40F, high nickel-chromium) tool. Sequentially coupled thermo-mechanical analyses under realistic loading conditions are developed within a general purpose non-linear FE code, ABAQUS to predict and analyse the complex temperature-stress-strain cycles of the SPF tool. A temperature-dependent, two-layer visco-plasticity model which combines both creep and combined isotropic-kinematic plasticity is chosen to represent the material behaviour. The material constants are identified from multiple strain-range isothermal cyclic tests and stress relaxation tests, over a range of temperatures between 20°C and 900°C. The FE predicted stress-strain data is used in stress-strain-life equations obtained from isothermal fatigue lifing tests at 900°C and the identified constants are validated against the TMF tests and simulative SPF tool test, designed to represent the temperature and stress-strain cycling associated with the most damaging phase of the tool cycle. The predicted SPF tool life is consistent with the simulative SPF tool test life and industrial observations.

Stress- and magnetic field-induced entropy changes in Fe-doped Ni–Mn–Ga shape-memory alloys

Isothermal stress- and magnetic field-induced entropy changes in a Fe-doped Ni–Mn–Ga alloy have been measured in the limits of low applied stress and magnetic field. We have obtained that in this limit while elastocaloric is conventional, giving rise to an increase of entropy when a stress is applied, magnetocaloric effect is inverse, which means that entropy decreases by application of an applied magnetic field. This inverse effect is a consequence of the magnetostructural coupling driven by the martensitic transition.

Predicting the lifetime of fluorosilicone o-rings

Long-term (up to 1000 days) accelerated oven-aging studies on a commercial fluorosilicone o-ring seal are used to predict the sealing lifetime at room temperature (23 °C). The study follows force decay (relaxation) on squeezed o-ring material using isothermal compression stress relaxation (CSR) techniques. The relaxation is normally a complex mix of reversible physical effects and non-reversible chemical effects but we utilize an over-strain approach to quickly achieve physical equilibrium. This allows us to concentrate the measurements on the chemical relaxation effects of primary interest to lifetime assessment. The long-term studies allow us to access a fairly broad temperature range (80–138 °C) which results in improved modeling of the temperature dependence of the accelerated data. Non-Arrhenius behavior is observed with evidence of a significant lowering of the activation energy at the lowest accelerated aging temperature (80 °C). This observation is consistent with numerous recent accelerated aging studies that probed temperature ranges large enough to observe similar non-Arrhenius behavior. The extrapolated predictions imply that significant loss of sealing force requires on the order of 50–100 years at 23 °C. Field aging results out to ∼25 years at 23 °C are shown to be in reasonable accord with the significant change in Arrhenius slope observed from the accelerated aging study.

Silicon Steel: Hot Working Constitutive Behaviour and Multistage Rolling Simulation

From torsion testing of silicon steel over the range 600 to 900 °C and 0.1-5 s-1, the constitutive analysis was performed by the sin h law to establish activation energies. As the temperature rose and strain rate declined, the fracture strain increased to as much as 14.3 and the subgrains in the elongated grains became larger. These results are shown graphically to agree with data from a wide collection of published reports on Si steels. The strengths were intermediate between those of 409 (11Cr) and 434 (16Cr-1Mo) that indicated Si has a strengthening effect almost 4 times that of Cr per unit weight; however, the activation energies of 409 and 434 were higher. Simulation of multistage rolling was done with T declining from 900 to 600 °C, pass strains of 0.2 or 0.3, strain rate of 1 s-1 and intervals of 20 or 40 seconds. Because of the short intervals, the pass flow curves did not exhibit the yield points of isothermal tests so that it was not possible to calculate fractional softening in the intervals at a declining temperature. Two new measures were suggested: a) the relative softening during an interval that decreased as temperature fell and b) relative reduction of the maximum pass stress relative to isothermal peak stress that increased at a decreasing rate as temperature declined.

Usual stress relaxation in an ‘unusual’ Pd 40Cu 40P 20 metallic glass

This work presents isochronal and isothermal shear stress relaxation measurements in glassy Pd 40Cu 40P 20, which is uniquely denser as compared with the reference tetragonal Pd 2Cu 2P crystal. However, the relaxation kinetics of this glass in different structural states is quite similar to that in ‘usual’ metallic glasses confirming a minor role of the free volume in the flow behavior below T g .

Development of Localized Deformation in AA 2024-O

An experimental study is presented to (1) quantify the rate-sensitive mechanical response and (2) examine the localized deformation behavior under an applied temperature gradient in the alloy AA 2024. Isothermal flow stresses are obtained at temperatures from −100°C to 495°C and strain rates from 10−2/s to 10−5/s using routine compression tests and a novel cyclic test, which expedites the characterization. The material displays two distinct kinetic responses with both being amenable to localization phenomena. The lower temperature/high strain rate regime displays a rate-insensitive yield with Stage III/IV work hardening. At higher temperature/low strain rates, a rate-sensitive response with little work hardening is observed. In order to relate the material constitutive behavior to the development of localized deformation, a temperature gradient test is performed wherein temperature differences of approximately 30°C are enforced between the top and bottom surfaces of a cylindrical compression test specimen. Deformation heterogeneity developed in the two distinct regimes of material response is illustrative of warm and hot working conditions typical of industrial processes, such as rolling.

Forgeability and Flow Stress of Mg-Zn-Y Alloys with Long Period Stacking Ordered Structure at Elevated Temperatures

To develop a forging process for high strength Mg-Zn-Y alloys with a long period stacking ordered (LPSO) structure, the forgeability and flow stress of Mg97Zn1Y2 (at%) alloys were investigated with the upsettability test at an initial strain rate of 0.55 s � 1 . As-cast and extruded Mg97Zn1Y2 alloys can be successfully forged without fracture up to 0.5 and 1.0 respectively of average equivalent strain at a forging temperature of 773 K. To remove the influence of the temperature change during upsetting from the experimentally obtained flow stress curve, the isothermal flow stress curve was estimated by combining experimental results with finite element analysis. Furthermore, the mechanical properties of the forged Mg-Zn-Y alloys were measured, and the influence of microstructure on the forging properties was discussed.

Compatibility Study of Lamivudine with Various Cellulose Polymers

For the development of any formulation, techniques such as thermal and isothermal stress testing were used to assess the compatibility of drug with excepients. Differential scanning calorimetry (DSC) and FTIR were the common methods for the study of compatibility. Isothermal stress testing (IST) is also a method for the compatibility study during proto type formulation. In the present study drug excepient compatibility study of lamivudine was conducted with different controlled release polymers. The drug and polymer mixtures were stored at 50 °C for 2 weeks. The samples were then characterized using DSC, FTIR and UV spectrophotometric methods. The results show that lamivudine was compatible with the all the polymers used in the study. The polymers used in the present study were definitely incorporated in the extended release lamivudine formulation.

Pseudoelastic Behavior of Cu–Al–Be Shape Memory Wires Subjected to Varied Loading and Unloading Cycles

Cyclic loading and unloading tensile tests were carried out for Cu-11.30wt. %Al-0.32wt. % Be Shape Memory Alloys (SMA) wires to investigate the effect of temperature and cyclic condition on the pseudoelastic behavior. It is shown that σc increases with the isothermal external temperature in single cycle tests and loading stress in constant stress increment tests while tends to a stable level after the 1st cycle in constant maximum strain tests. Both modes of loading and unloading improve the pseudoelasticity obviously.

Dynamic Response of Rapidly Heated Cylindrical Rods: Longitudinal and Flexural Behavior

A very fast temperature increase, produced by a nonuniform heat generation, induces in a simply supported, isotropic, cylindrical rod both longitudinal and flexural vibrations. This paper presents an analytical method to study these vibrations and determine the stresses they provoke. The proposed procedure relies on three main steps: an exact solution for the temperature field is first obtained, by means of Fourier–Bessel expansions; quasistatic thermal stresses are then computed as a function of the calculated temperature distribution, making use of the thermoelastic displacement potential and of the solution to the equivalent isothermal two-dimensional stress problem; finally, longitudinal and flexural vibrations excited by an equivalent thermal force and thermal bending moment are determined using the mode-summation method. The influence of thermal shock duration on the maximum value of the longitudinal dynamic stress and of the ratio between the characteristic thermal time and structural response time on the dynamic bending deflection is analyzed and discussed. Finally, a comparison between the analytical model and experimental measurements is presented. The analytical model described in this paper allows the complete evaluation, within the linear elastic domain, of quasistatic and dynamic thermal stresses induced in an isotropic cylindrical rod by rapid internal heating.

Preformulation Considerations for Controlled Release Dosage Forms: Part II—Selected Candidate Support

Practical examples of preformulation support of the form selected for formulation development are provided using several drug substances (DSs). The examples include determination of the solubilities vs. pH particularly for the range pH 1 to 8 because of its relationship to gastrointestinal (GI) conditions and dissolution method development. The advantages of equilibrium solubility and trial solubility methods are described. The equilibrium method is related to detecting polymorphism and the trial solubility method, to simplifying difficult solubility problems. An example of two polymorphs existing in mixtures of DS is presented in which one of the forms is very unstable. Accelerating stability studies are used in conjunction with HPLC and quantitative X-ray powder diffraction (QXRD) to demonstrate the differences in chemical and polymorphic stabilities. The results from two model excipient compatibility methods are compared to determine which has better predictive accuracy for room temperature stability. A DSC (calorimetric) method and an isothermal stress with quantitative analysis (ISQA) method that simulates wet granulation conditions were compared using a 2 year room temperature sample set as reference. An example of a pH stability profile for understanding stability and extrapolating stability to other environments is provided. The pH-stability of omeprazole and lansoprazole, which are extremely unstable in acidic and even mildly acidic conditions, are related to the formulation of delayed release dosage forms and the resolution of the problem associated with free carboxyl groups from the enteric coating polymers reacting with the DSs. Dissolution method requirements for CR dosage forms are discussed. The applicability of a modified disintegration time (DT) apparatus for supporting CR dosage form development of a pH sensitive DS at a specific pH such as duodenal pH 5.6 is related. This method is applicable for DSs such as peptides, proteins, enzymes and natural products where physical observation can be used in place of a difficult to perform analytical method, saving resources and providing rapid preformulation support.

Size effects on yield strength and strain hardening for ultra-thin Cu films with and without passivation: A study by synchrotron and bulge test techniques

We present a systematic study of the mechanical properties of different Cu, Ta/Cu and Ta/Cu/Ta films systems. By using a novel synchrotron-based tensile testing technique isothermal stress–strain curves for films as thin as 20 nm were obtained for the first time. In addition, freestanding Cu films with a minimum thickness of 80 nm were tested by a bulge testing technique. The effects of different surface and interface conditions, film thickness and grain size were investigated over a range of film thickness up to 1 μm. It is found that the plastic response scales strongly with film thickness but the effect of the interfacial structure is smaller than expected. By considering the complete grain size distribution and a change in deformation mechanism from full to partial dislocations in the smallest grains, the scaling behavior of all film systems can be described correctly by a modified dislocation source model. The nucleation of dissociated dislocations at the grain boundaries also explains the strongly reduced strain hardening for these films.

Strong single-crystalline Au films tested by a new synchrotron technique

Although it is well known that thin films exhibit mechanical properties very different from those of their bulk counterparts, knowledge of the underlying mechanisms is incomplete. Single-crystalline films have a favorable microstructure for investigating the scaling behavior of mechanical properties. We present a novel experimental route for preparing single-crystalline Au films on a compliant polyimide substrate. For such single-crystals, we have developed a synchrotron-based tensile testing technique to measure the isothermal stress–strain curves and average peak widths. The analysis of Laue diffraction patterns as well as a parallel transmission electron microscopy study give new insight in the initial and evolving microstructure of the films. Complex novel deformation mechanisms are found, including a transition of the dominant deformation mechanism from full to partial dislocations in films thinner than 160 nm. The scaling behavior is described in view of the coexistence of different deformation mechanisms where the nucleation stress for single dislocations very likely governs the behavior.

A model based creep equation for 9Cr–1Mo–0·2V (P91 type) steel

The isothermal constant stress creep tests data for a 9Cr–1Mo–0·2V (P91 type) steel were submitted for a phenomenological analysis in order to obtain the relevant creep equation for such steel. Namely, the minimum creep strain rate of P91 type steel cannot be described by the simple Arrhenius type power law constitutive model. The incorporation of the threshold stress concept in the analysis of creep data leads to a modified power law, which satisfactorily describes the creep behaviour of the examined P91 steel. However, the threshold stress is not a good material parameter, as it often varies with temperature and/or applied stress. This adds uncertainty to the extrapolation of the creep rates into ranges where experimental data are not available. Besides the fact that the physical foundation for a threshold stress is questionable from a scientific point of view, this is a serious practical limitation of the modified power law creep equation. The second creep equation proposed in the present paper is the improved stress dependent energy barrier model. The improvement of the standard model is based on two assumptions: first, on the hypothesis that the application of a stress also affects the energy barrier to be overcome when a local region transitions from the initial to the final state, and second, by applying a simple power function of stress instead of a hyperbolic sin function in the model based equation. The obtained value of stress exponent, n=5·5, is too high for entirely climb controlled creep. The apparent activation energy of approximately 510 to 545 kJ mol−1, which is considerably higher than the activation energy for lattice diffusion, is the stress dependent activation energy of the slowest, dominant rate controlling process of the supposed multiple creep mechanisms.

Characterization and relevance of physicochemical interactions among components of a novel multiparticulate formulation for colonic delivery

The primary objective of this study was to investigate potential interactions among a model drug (azathioprine; AZA), polymers and a divalent metal ion, which were utilized in the development of a novel multiparticulate formulation for colonic delivery. The approach involved preparation of beads by ionotropic gelation of deacylated gellan gum (DGG) in the presence of Ca 2+ ions, followed by coating with Eudragit ®S-100. Various possible physicochemical interactions among formulation components were characterized by DSC, FT-IR, XRPD, 1H-NMR, and an isothermal stress test. Results of isothermal stress testing indicated that there was no significant interaction of AZA with DGG and Eudragit ® S-100. However, results of DSC and XRPD studies suggested that there could be interactions between AZA and DGG, and ionotropic gelation can affect the physical state of AZA, which may have implications for drug release characteristics and, physical and chemical stability. The results of FT-IR studies were suggestive of interactions of DGG with AZA and Eudragit ® S-100, and provided evidence for interactions of AZA and DGG with Ca 2+ ions. The electrostatic interaction of DGG with Ca 2+ was also supported by results of DSC studies while that between AZA and Ca 2+ was confirmed by 1H-NMR studies. This study, to our knowledge, is first reported investigation in which the unique thermal property of gellan gum gels, and possible interactions between a drug and counter ions of an ionotropic agent have been demonstrated through bead characterization studies. The formation of AZA–Ca 2+ complex could have an impact on drug release kinetics, product stability and clinical efficacy for treatment of inflammatory bowel diseases or other diseases, which merit further investigation.

Synchrotron Characterization of Texture and Stress Evolution in Ag Films

AbstractReal-time in-situ synchrotron x-ray diffraction measurements were performed at the Cornell High Energy Synchrotron Source to characterize both the texture evolution and stresses within the individual texture components of Ag films during recrystallization. As deposited films had a nearly perfect (111) fiber texture. During isothermal anneals, stress and texture were characterized in real-time as the texture evolved into a strong (001) fiber. An Avrami analysis of the evolving texture fractions yielded very different activation energies for films on different barrier layers, suggesting different governing mechanisms were responsible for secondary grain growth. The strains were used to test a common model for texture prediction that assumes the same strain within each texture component. It was found that secondary (001) grains were able to grow primarily strain free. Selection for this strain energy minimizing orientation occurred during the nucleation process during which texture interactions play an important role. By using these real time measurements, we are able to show that driving forces for texture transformations in metal films may not be as simple previously described.

Modeling and numerical simulation of the pseudoelastic behavior of shape memory alloycircular rods under tension–torsion combined loading

Most research on the behavior of shape memory alloys (SMAs) under tension–torsioncombined loading has focused on tubular materials. In contrast to tubular SMAs, SMArods have unique characteristics. When an SMA rod is twisted, the central regionremains elastic while the outer layer undergoes a martensite transformation. Thenonlinear stress distribution of an SMA rod through the radial direction givesthe SMA rod peculiarities, and this is the primary factor that makes analysis ofthe stress–strain behavior difficult. The authors suggest a material model thatcan represent the stress–strain behavior of an SMA rod under tension–torsioncombined loading. The proposed model is based on Brinson’s phase transformationkinetics and the plastic flow rules. Furthermore, the yield is assumed to be inaccordance with the Von Mises criterion and the effect of thermal expansion is notconsidered. The authors also present some numerical solutions of the proposedmodel in the context of isothermal strain and stress processes (proportional andnonproportional loading) in the pseudoelastic region. According to simulationresults, the stress–strain behavior of an SMA solid rod under biaxial loadingdiffers from that of a thin-walled tube or a rod under one-dimensional loading.

Isothermal Stress Relaxation of Bulk and Ribbon Pd<sub>40</sub>Cu<sub>30</sub>Ni<sub>10</sub>P<sub>20</sub> Metallic Glass

The paper presents the results of detailed measurements of the isothermal tensile stress relaxation in Pd40Cu30Ni10P20 bulk and ribbon samples, which differ by ∼ 104 times in their production quenching rates. The relaxation law is derived. It is shown that the relaxation kinetics is not much affected by the quenching rate.

Constitutive equations for rate-dependent pseudoelastic behaviour of shape memoryalloys

In this paper we study the isothermal stress–elongation rate response of a bar capable ofundergoing austenite–martensite phase transformations. Our approach is based on athermodynamically consistent three-dimensional phase field theory of Fried and Gurtin(1994 Physica D 72 287–308) capable of very general kinetics. We propose specificconstitutive equations and develop an algorithm for analysing displacement controlleduniaxial extension. A finite difference solution of the governing equations is carriedout using material parameters for Ni–Ti. Most of the significant features of theisothermal behaviour for this alloy are quantitatively predicted very well whencompared with experimental data from the literature. The proposed constitutiveequations model the dependence of hysteresis on the elongation rate particularlywell.

THERMAL STRESS OF HIGHLY ORIENTED POLY(ETHYLENE TEREPHTHALATE)

The isothermal shrinkage stress of highly oriented semi-crystalline poly(ethylene terephthalate) (PET) fiber was studied in the temperature range from 102℃ to 221℃.The data of the isothermal shrinkage stress on logarithmic time scales could be simply shifted to obtain a superimposed curve when the temperature was not above 186℃.The relationship between the shift factors and the experiment temperatures could be described satisfactorily by the Williams-Landel-Ferry equation with C_1=71.9 and C_2=511.7.Therefore,the increasing of shrinkage stress with treated time,resulted from the release of “frozen" stress in the oriented polymer materials,was resulted from the activation of the segments due to the glass transition.The much higher values of C_1 and C_2 than those of the non-crystalline materials were due to the higher stiffness of the molecules contributed to the stress increasing,which were constrained by the crystallization and the chain entanglement in a highly oriented semi-crystalline polymer.It was also proven by a wide α-relaxation peak up to 170℃ indicated by the tanδ-temperature curve.When the temperature was equal to or higher than 186℃,the decreasing of the isothermal shrinkage stress could be observed after it reached the maximum stress.According to the three-phase model of PET fiber provided by Prevorsek,such high temperature stress relaxation was due to the viscose flow of the extended non-crystalline molecules located between micro-fibrils,which was also indicated by the α′-relaxation from 180℃ to 250℃.