Aging Regime Research Articles

Effect of temperature on the mode I and mixed mode I/III fracture toughness of SA333 steel

The effect of temperature on tensile properties, mode I and mixed mode I/III fracture toughness of SA333 Grade 6 steel was investigated. The variation of ultimate tensile strength and strain hardening exponent with temperature as well as the appearance of serrations in the stress–strain plots indicated that dynamic strain aging regime in this steel is in the temperature range 175–300 °C at a nominal strain rate of 3 × 10 −3 s −1. Both mode I and mixed mode I/III fracture toughness values were found to exhibit a significant reduction in the DSA regime. The mixed mode I/III fracture toughness was found to be significantly lower than the mode I fracture toughness at all temperatures. However, the difference between the two toughness values was much higher prior to the onset of DSA. The results are explained on the basis of the nature of deformation fields under mode I and mixed mode I/III loading as well as the fracture mechanism prevalent in these steels at different temperatures.

Exact two-time correlation and response functions in the one-dimensionalcoagulation–diffusion process by the empty-interval method

The one-dimensional coagulation–diffusion process describes the strongly fluctuatingdynamics of particles, freely hopping between the nearest-neighbour sites of achain, such that one of them disappears with probability one if two particles meet.The exact two-time correlation and response functions in the one-dimensionalcoagulation–diffusion process are derived from a generalization of the empty-intervalmethod. The main quantity is the conditional probability of finding an empty interval ofn consecutive sites,if at distance d a site is occupied by a particle. Closed equations of motion are derived such that theprobabilities needed for the calculation of correlators and responses, respectively, aredistinguished by different initial and boundary conditions. In this way, the dynamicalscaling of these two-time observables is analysed in the long-time ageing regime. Anew generalized fluctuation-dissipation ratio with a universal and finite limit isproposed.

Frequency of SMBG correlates with HbA1c and acute complications in children and adolescents with type 1 diabetes

The aim of this study was to correlate the frequency of self-monitoring of blood glucose (SMBG) to the quality of metabolic control as measured by hemoglobin A1c (HbA1c), the frequency of hypoglycemia and ketoacidosis, and to see whether the associations between SMBG and these outcomes are influenced by the patient's age or treatment regime. We analyzed data from the DPV-Wiss-database of 26 723 children and adolescents aged 0-18 yr with type 1 diabetes recorded during 1995-2006. Variables evaluated were gender, age at visit, diabetes duration, therapy regime, insulin dose, body mass index-standard deviation scores (BMI-SDS), HbA1c, rate of hypoglycemia, and ketoacidosis. In the youngest age group of children under the age of 6 yr, the frequency of SMBG was the highest compared with that in children aged 6-12 yr or children aged > 12 yr: 6.0/d vs. 5.3/d vs. 4.4/d (p < 0.001). Frequency of SMBG differed significantly also in the different groups of treatment (p < 0.001), but only for the continuous subcutaneous insulin infusion (CSII) group the frequency was considerably higher: 5.3/d (CSII) vs. 4.7/d (multiple daily injections) vs. 4.6/d (conventional therapy). Adjusted for age, gender, diabetes duration, year of treatment, insulin regimen, insulin dose, BMI-SDS, and center difference, SMBG frequency was significantly associated with better metabolic control with a drop of HbA1c of 0.20% for one additional SMBG per day (p < 0.001). Increasing the SMBG frequency above 5/d did not result in further improvement of metabolic control. A higher frequency of SMBG measurements was related to better metabolic control. But only among adolescents aged > 12 yr, metabolic control (HbA1c) improved distinctively with two or more blood glucose measurements.

Prediction of Long and Short Time Rheological Behavior in Soft Glassy Materials

We present an effective time approach to predict long and short time rheological behavior of soft glassy materials from experiments carried out over practical time scales. Effective time approach takes advantage of relaxation time dependence on aging time that allows time-aging time superposition even when aging occurs over the experimental time scales. Interestingly, experiments on a variety of soft materials demonstrate that the effective time approach successfully predicts superposition for diverse aging regimes ranging from subaging to hyperaging behaviors. This approach can also be used to predict behavior of any response function in molecular as well as spin glasses.

Spatially heterogeneous dynamics in a thermosensitive soft suspension before and after the glass transition

The microscopic dynamics and aging of a soft thermosensitive suspension were investigated by looking at the thermal fluctuations of tracers in the suspension. Below and above the glass transition the dense microgel particle suspension was found to develop heterogeneous dynamics, characterized by a non Gaussian Probability Distribution Function (PDF) of the probes' displacements, with an exponential tail. We show that non Gaussian shapes are a characteristic of the ensemble-averaged PDF, while local PDFs remain Gaussian. This shows that the scenario behind the non Gaussian van Hove functions is a spatially heterogeneous dynamics, characterized by a spatial distribution of locally homogeneous dynamical environments through the sample, on the considered time scales. We characterize these statistical distributions of dynamical environments, in the liquid, supercooled, and glass states, and show that it can explain the exponential tail of the van Hove functions observed in the concentrated states. The intensity of spatial heterogeneities was found to amplify with increasing volume fraction. In the aging regime, it tends to increase as the glass gets more arrested.

Human glial-restricted progenitors survive, proliferate, and preserve electrophysiological function in rats with focal inflammatory spinal cord demyelination

Transplantation of glial progenitor cells results in transplant-derived myelination and improved function in rodents with genetic dysmyelination or chemical demyelination. However, glial cell transplantation in adult CNS inflammatory demyelinating models has not been well studied. Here we transplanted human glial-restricted progenitor (hGRP) cells into the spinal cord of adult rats with inflammatory demyelination, and monitored cell fate in chemically immunosuppressed animals. We found that hGRPs migrate extensively, expand within inflammatory spinal cord lesions, do not form tumors, and adopt a mature glial phenotype, albeit at a low rate. Human GRP-transplanted rats, but not controls, exhibited preserved electrophysiological conduction across the spinal cord, though no differences in behavioral improvement were noted between the two groups. Although these hGRPs myelinated extensively after implantation into neonatal shiverer mouse brain, only marginal remyelination was observed in the inflammatory spinal cord demyelination model. The low rate of transplant-derived myelination in adult rat spinal cord may reflect host age, species, transplant environment/location, and/or immune suppression regime differences. We conclude that hGRPs have the capacity to myelinate dysmyelinated neonatal rodent brain and preserve conduction in the inflammatory demyelinated adult rodent spinal cord. The latter benefit is likely dependent on trophic support and suggests further exploration of potential of glial progenitors in animal models of chronic inflammatory demyelination.

Effects of ageing conditions and block copolymer concentration on the stability and micellization of P123-Ti4+ sols prepared by the templating method

In the work reported here we studied, for the first time, the effects of ageing conditions (temperature and time) on the stability and micellization of two sets of sols with different P123 block copolymer (BC) concentrations by use of dynamic light scattering (DLS). Further, for a comparative study the aggregation/clustering behavior of pure P123 in IPA solution was also examined by DLS, and 1H NMR, and Raman spectroscopy at room temperature. Different ageing regimes applied to the samples prepared included: (i) in-situ ageing of the sols in the sealed capillary cell of the DLS system at room temperature for 0, 1, 2, 3, 4, 5, 6, and 19 h, (ii) ageing of the sols in a Teflon-lined sealed vessel at 50, 55, 60, and 65 °C for 30 min, and (iii) isothermal ageing of the synthesized sols at 50 °C for 1, 4, 8, and 14 days. On the basis of the results obtained it is shown that both ageing time and temperature have remarkable effects on the clustering and aggregation of unimer/micelles formed in the sol system studied. Further, quantitative analysis of interparticle potential energy carried out for the prepared sol at low concentration confirmed that steric interactions play the major role among the other contributing sources of energy. This is mainly related to the presence of BC and the complex polymer structure formed by acetyl acetone (ACAC). Moreover, on the basis of the calculated stability ratio for this sol, we believe that reaction-limited cluster aggregation (RLCA) is possibly the major governing mechanism. Finally, kinetic and dynamic aspects of the micellar aggregates formed are also discussed in this paper.

Subdiffusion and intermittent dynamic fluctuations in the aging regime of concentrated hard spheres

We study the nonequilibrium aging dynamics in a system of quasihard spheres at large density by means of computer simulations. We find that, after a sudden quench to large density, the relaxation time initially increases exponentially with the age of the system. After a surprisingly large crossover time, the system enters the asymptotic aging regime characterized by a nearly linear increase in the relaxation time with age. In this aging regime, single-particle motion is strongly non-fickian, with a mean-squared displacement increasing subdiffusively, associated with broad non-gaussian tails in the distribution of particle displacements. We find that the system ages through temporally intermittent relaxation events, and a detailed finite-size analysis of these collective dynamic fluctuations reveals that these events are not spanning the entire system, but remain spatially localized.

Influence of dynamic strain aging on plastic instability behaviour of near alpha titanium alloy Timetal 834

The deformation behaviour of near alpha titanium alloy Timetal 834 was investigated by analysing Considére criterion in temperature range between 300 and 500°C at an interval of 25°C. In the dynamic strain aging (DSA) regime (400–475°C) studied in this material, Considére criterion was satisfied partially at peak DSA temperature (450°C). This was attributed to maximum DSA effect at 450°C which eventually resulted in specimen fracture at low ductility levels. Strain hardening exponent (n) values was determined using Ludwik plot, Considére criterion and obtained experimentally from uniform strain. The n values obtained from Ludwik plot and Considére criterion were greater than the uniform strain in DSA regime studied.

Predictors of responders to insulin therapy at 1 year among adults with type 2 diabetes

The effectiveness of insulin therapy to lower blood glucose levels in patients with type 2 diabetes (T2D) may depend on a variety of factors. This study aims to determine baseline parameters including body mass index (BMI) threshold that might predict responders to insulin therapy. This was a retrospective UK population-based study derived from 358 general practices electronic dataset. We included all patients with T2D, diagnosed at the age of >18 years old and who were initiated on insulin from January 2000 to December 2006. Insulin responders were defined as HbA1c <7.5% and/or HbA1c reduction by >1% at 12 months postinsulin initiation. Results are expressed in mean (s.d.). A total of 6032 patients were identified. Baseline age was 63 years (11.7). In all, 61% of patients (3696) responded to insulin. At 1-year postinsulin initiation, HbA1c was significantly reduced (9.8 vs. 8.4%, p < 0.001) and weight increased (85.7 vs. 87.9 kg, p < 0.001). Using logistic regression model, older age (p < 0.001), lower BMI (p = 0.046), higher HbA1c (p < 0.001), basal-bolus insulin therapy and premixed insulin compared to basal insulin alone at baseline were independent predictors of responders to insulin. Gender and social class were not significant predictors of insulin responders. A BMI of <35.3 was derived as a cut-point for response to insulin (p = 0.038). Overall, insulin therapy confers significant HbA1c reduction and weight increase in patients with T2D. The responsiveness to insulin therapy however appears to depend on baseline age, BMI, HbA1c and insulin regime. Clinicians should take these factors into consideration when making a decision to initiate insulin therapy in patients with T2D.

Laboratory evaluation of antioxidants for asphalt binders

A laboratory study was carried to investigate whether certain antioxidants could be used to reduce age-hardening in asphalt binders. Several additives including: Vitamin E, Irganox 1010, Irgafos P-EPQ, carbon black, hydrated lime, and DLTDP/furfural, were evaluated. The modified asphalt binders were subjected to two accelerated oxidative aging regimes in the laboratory. Short-term oxidative aging of the binders was simulated using a rolling thin film oven while long-term aging was performed using a pressure aging vessel. The effects of aging on the binders were evaluated based on changes in rheological properties of asphalt binders undergoing oxidative aging. Binders modified with a combination of furfural and DLTDP was found to posses the lowest aging index. The antioxidant-treated binders exhibited improved performance at both high and low pavement temperatures.

A study of retrogression in aging Invars

N30K10T3 Invar with ferromagnetic austenite represented by a supersaturated solid solution capable of aging in subsequent heating is studied. The possibility of the use of the “method of higher harmonics” for studying the decomposition of supersaturated solid solutions in Invars is considered. This method is used for studying modes of double aging including low-temperature and high-temperature aging stages. The temperature regimes of double aging in which the structure of the Invar undergoes “retrogression” are determine.

Exploring basic biochemical constituents in the body tissues of South African abalone Haliotis midae reared in shore-based mariculture systems

The study quantifies the effects of season, age and feeding regime on glucose, glycogen, proteins and lipids in abalone Haliotis midae tissues, and to assess water loss during simulated live export. Abalone of two age classes, younger (∼30 months old) and older (∼38 months old), were subjected to four dietary regimes: two single feeds, kelp and an artificial feed, and two rotational feeding regimes utilising kelp and the artificial feed in varying ratios. Total protein, free glucose, glycogen and total lipid content were determined in the digestive gland and foot/adductor muscle. One subset of older animals, from each feeding treatment, was subjected to water content analysis by drying. Subsets of older and younger animals were subjected to a simulated live export trial to determine mass loss. Digestive gland tissue proved to be of little value for routine monitoring. Muscle glucose, muscle lipid and muscle protein concentrations were affected by month and age. Muscle glycogen concentration was affected by month, age and feeding regime. During simulated export trials, animals lost on average 15% body mass, being significantly higher (p < 0.01) in younger compared to older animals. Feeding regime did not influence susceptibility to mass loss during export. Animal age influenced the biochemical composition and export performance of abalone.

Influence of nitrogen alloying on dynamic strain ageing regimes in low cycle fatigue of AISI 316LN stainless steel

The effect of nitrogen alloying (0.078% to 0.22%) in 316 LN stainless steel has been investigated for its Low Cycle Fatigue properties in the temperature range 300 to 873 K. The effect of dynamic strain ageing has been evaluated as a function of nitrogen content, temperature, strain amplitude and strain rate. The temperature range of operation of DSA has been found to be a strong function of the above parameters. Increase in nitrogen content shifted the range of operation of DSA to higher temperatures. For temperatures ≥673 K, fatigue life is found to be maximum for a nitrogen content of 0.14 wt%, while it is found to saturate or increase with increasing nitrogen content at temperatures <673 K.

A viscoplastic constitutive model for cyclic hardening in the dynamic strain aging regime

Negative strain rate sensitivity in certain temperature regimes is considered to be a manifestation of dynamic strain aging. Within the framework of unified state variable theory, viscoplastic constitutive equations incorporating the Mises yield criterion are presented to model negative rate sensitivity of the flow stress and its gradual variation with cyclic loading. The modeling capabilities are essentially accomplished by introducing a rate sensitivity parameter into the evolution law of the back stress. The influence of the drag stress and the isotropic stress as well as the rate sensitivity parameter both on the cyclic hardening and the variation of strain rate sensitivity is systemically investigated. Excellent agreement between experimental results reported in the literature and model simulations is obtained, confirming the efficacy of the proposed constitutive model.

Evolutionary stellar population synthesis with MILES - I. The base models and a new line index system

[Abridged]. We present SEDs for single-age, single-metallicity stellar populations (SSPs) covering the optical range at resolution 2.3A (FWHM). These SEDs constitute our base models, as they combine scaled-solar isochrones with MILES empirical stellar library, which follows the chemical evolution pattern of the solar neighbourhood. The models rely as much as possible on empirical ingredients, not just on the stellar spectra, but also on extensive photometric libraries. The unprecedented stellar parameter coverage of MILES allowed us to safely extend our optical SSP SED predictions from intermediate- to very-old age regimes, and the metallicity coverage of the SSPs from super-solar to [M/H]=-2.3. SSPs with such low metallicities are particularly useful for globular cluster studies. Observed spectra can be studied by means of full spectrum fitting or line-strengths. For the latter we propose a new Line Index System (LIS) to avoid the intrinsic uncertainties associated with the popular Lick/IDS system and provide more appropriate, uniform, spectral resolution. Apart from constant resolution as function of wavelength the system is also based on flux-calibrated spectra. Data can be analyzed at three different resolutions: 5A, 8.4A and 14A (FWHM), which are appropriate for studying globular cluster, low and intermediate-mass galaxies, and massive galaxies, respectively. Polynomials to transform current Lick/IDS line index measurements to the new system are provided. A web-page with a suite of on-line tools to facilitate the handling and transformation of the spectra is available at http://miles.iac.es.

Influence of Composition, Humidity, and Temperature on Chemical Aging in Epoxies: A Local Study of the Interphase with Air

The developing chemical depth profile in an epoxy adhesive bulk (with varying amine content) is monitored during aging by FTIR microspectroscopy on sample cuts prepared with low angle microtomy. Three aging regimes are applied in order to separate the role of temperature and water: dried or moist air (90% rel. humidity) at 60°C and dried air at 120°C for up to 300 days. Quantitative evaluation of the IR spectra shows: thermo-oxidative aging (= dried air) is controlled by the diffusion of atmospheric oxygen. It affects a gradient region of more than 200 μm in depth. At given aging time, the depth profiles depend on temperature, humidity, and on the epoxy-amine ratio. Humidity mainly affects the IR band intensities. The plasticizing effect of water promotes the loss of small network fragments. At 120°C, autoxidation of α-CH2 at ether and amine groups and the oxidative attack on tertiary amines dominate aging. At 60°C in dried air, these processes proceed only very slowly. In the case of amine excess, aging is extended by the additional oxidation of primary and secondary amines to carbonyls. Carbonyls undergo consecutive reactions, especially in the presence of water. Hence, increasing temperature does not simply accelerate the aging mechanisms but it reduces their selectivity and changes their hierarchy. Thus, the long-term aging behavior at moderate temperatures cannot be predicted safely from accelerated aging tests.

Glassy dynamics of gelatin gels

We measure the elasticity of thermoreversible gelatin gels during slow cool and heat cycles, with and without stops. The responses are measures of the system energy, since elasticity is a function of the helix fraction, which quantifies the system's order. They show the same memory and rejuvenation effects as magnetic spin glasses. At constant temperature, all gels reach an aging regime, where their responses decrease linearly in log(time), like many other glassy systems. Complex memory effects occur after temperature jumps. We suggest that the parallels between gelatin gels and other glassy systems are due to their temperature-dependent, rugged, hierarchical energy landscapes. Gelatin gels act rather like “colloidal spin glasses”. However, unlike magnetic spin glasses, their size and time scales should allow experimental measurement of the relation between mesoscopic dynamics and macroscopic behaviour.

Nanoscale non-equilibrium dynamics and the fluctuation–dissipation relation in an ageing polymer glass

Response functions and fluctuations measured locally in complex materials should equally well characterize mesoscopic-scale dynamics. The fluctuation-dissipation-relation (FDR), relates the two in equilibrium, a fact used regularly, for example, to infer mechanical properties of soft matter from the fluctuations in light scattering. In slowly-evolving non-equilibrium systems, such as aging spin and structural glasses, sheared soft matter, and active matter, a form of FDR has been proposed in which an effective temperature, Teff, replaces the usual temperature, and universal behavior is found in mean-field models and simulations. Thus far, only experiments on spin-glasses and liquid crystals have succeeded in accessing the strong aging regime, where Teff >T and possible scaling behavior are expected. Here we test these ideas through measurements of local dielectric response and polarization noise in an aging structural glass, poly-vinyl-acetate . The relaxation-time spectrum, as measured by noise, is compressed, and by response, is stretched, relative to equilibrium, requiring an effective temperature with a scaling behavior similar to that of certain mean-field spin-glass models.

Prediction of mechanical properties of extra deep drawn steel in blue brittle region using Artificial Neural Network

Blue brittle region also known as dynamic strain ageing (DSA) regime is very important in the materials because in this region material properties behave in very unpredictable ways. In this work, Artificial Neural Network (ANN) models are developed for the prediction of mechanical properties such as yield strength (YS), ultimate tensile strength (UTS), % elongation, strength coefficient ( K) and strain hardening exponent ( n) for the extra deep drawn (EDD) quality steel in blue brittle region. To calculate the mechanical properties at elevated temperatures, experiments were conducted at the interval of 25 °C from room temperature till 700 °C in three rolling directions. Based on the experimental results, the blue brittle region for EDD steel is identified between 350 °C and 450 °C and ANN model is trained in all the three rolling directions. Trained up ANN model is tested with the experimental results at two different temperatures with in blue brittle region. Experimental and modeling errors in the prediction of mechanical properties are found within the permissible range.