Early Stage Of Aging Research Articles

Mechanical, Thermal, and Morphological Behavior of Silicone Rubber during Accelerated Aging

ABSTRACTAn accelerated aging study on silicone rubber exploring the effects of exposure to a functional oil (polyalkylene glycol) at elevated temperature (195°C) is reported in this paper. Variations in mechanical (tensile, tear, hardness) and thermal (conductivity, specific heat capacity) properties were monitored versus aging time while permanent deformation of the rubber was evaluated through creep and recovery measurements. Morphology and surface chemistry of the aged rubber were also investigated through scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy, respectively. Aging had a significant impact on the mechanical properties with the ultimate tensile strength and elongation at break decreasing from 7.4 MPa and 2250% in unaged samples to 1.5 MPa and 760% in 6-week aged samples, respectively. The tear strength and hardness exhibited an initial increase during the early stages of aging, followed by a decreasing trend. In contrast, the thermal properties did not change significantly and FTIR did not detect any changes in the surface chemistry of the rubber with aging. SEM however, provided evidence of an increase in brittle behavior from the morphology of the fractured surfaces.

Effects of cluster characteristics on two-step aging behavior in Al-Mg-Si alloys with different Mg/Si ratios and natural aging periods

Age-hardening behavior during the early stages of artificial aging (AA) is associated with the clustering behavior of high-Mg and high-Si Al-Mg-Si alloys. Herein, the cluster characteristics during natural aging (NA) and the early stages of subsequent AA are obtained using atom probe tomography. Alloy composition exerts a significant impact on the internal composition of clusters after short-term NA, and clusters with a similar number of Mg and Si atoms increase after long-term NA. The cluster behavior during the early stages of AA is highly dependent on the internal chemical composition of clusters, which suggests that Mg-Si clusters with Mg/(Mg+Si) values of over 0.4 decrease, independent of cluster size. In addition, the results also suggest that Si-rich clusters with Mg/(Mg+Si) values of below 0.4 retard the formation of the β″ phase during AA. These findings are discussed in relation to the hardness change. The increase in the hardness during the early stage of AA is associated with the proportion of Si-rich and Mg-Si clusters in both high-Mg and high-Si alloys.

Influence of stress on γ″ precipitation behavior in Inconel 718 during aging

The precipitation behavior of the γ″ phase and mechanical properties in Inconel 718 alloy during aging under an external applied stress or internal quench-in residual stress were studied. The results show that nucleation and precipitation of the γ″ phase are strongly promoted by external stress in the early stages of aging. The γ″ precipitates exhibit homogeneous nucleation and always maintain a coherent relationship with the matrix. Further, the competitive growth of γ″ particles, namely coarsening, is the dominant process in the intermediate and late stages of aging. Aging under external stress may promote the nucleation rate, and therefore produce a higher number density of nuclei and, consequently, smaller precipitates after aging for 8 h. Moreover, the strength increment is sensitive to the mean size of γ″ precipitates. The decrease in strength is mainly due to the size reduction of precipitates in samples aged under external stress for 8 h. In the second experiment, the internal residual stress is distributed unevenly, resulting in inhomogeneous microstructure and mechanical properties in the workpiece.

Exercise Intervention Associated with Cognitive Improvement in Alzheimer's Disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disease with the syndrome of cognitive and functional decline. Pharmacotherapy has always been in a dominant position for the treatment of AD. However, in most cases, drug therapy is accompanied with clinical delays when older adults have suffered from cognitive decline in episodic memory, working memory, and executive function. On the other hand, accumulating evidence suggests that exercise intervention may ameliorate the progression of cognitive impairment in aging ones while the standard strategy is lacking based on different levels of cognitive decline especially in mild cognitive impairment (MCI) and AD. MCI is the preclinical stage of AD in which neurodegeneration may be reversed via neuroplasticity. Therefore, taking exercise intervention in the early stage of MCI and healthy aging at the risk of AD could slow down the process of cognitive impairment and provide a promising cost-effective nonpharmacological therapy to dementia.

Formation of equiaxed α phase in Ti-5Al-5Mo-5V-3Cr alloy deformed by high-pressure torsion

Formation of ultrafine equiaxed α phase particles was observed in Ti-5Al-5Mo-5V-3Cr (mass%) alloy after aging of samples deformed by high-pressure torsion (HPT) while acicular α phase was observed in undeformed samples. Burgers orientation relationship was obeyed between equiaxed α and β phases. The numerous nucleation sites and enhanced diffusivity in HPT processed samples due to high density of dislocations promoted the nucleation of and growth of α phase at the early aging stage. The segregation of β stabilizers near the α/β interfaces retarded the growth of α phase during aging, leading to equiaxed α phase with ultrafine particle size.

Effect of ZrO2 contents and ageing times on mechanical and electrical properties of Al–4.5 wt.% Cu nanocomposites prepared by mechanical alloying

Al–4.5 wt.% Cu-based nanocomposites, reinforced with various weight percentages of ZrO2, have been synthesized via mechanical alloying method. The mixture powders were cold pressed and sintered at various firing temperatures up to 500 °C for 1 h in argon atmosphere. X-ray diffraction technique is utilized to investigate structural phases and elemental diffusion of milled powders. Transmission and scanning electron microscopies coupled with energy dispersive spectroscopy were employed to examine the morphology of milled powders and the microstructure of sintered specimens, respectively. Physical, mechanical and electrical properties of sintered samples were measured. Moreover, the sintered samples followed by heat treatments at 460 °C for 3 h, quenched and then, artificial aging was performed. The results revealed that Al2Cu precipitate phase was clearly observed after milling reflecting the formation of supersaturated solution of Cu in Al. Increase of ZrO2 contents caused weakness of Al2Cu phase, decrease in the crystal and particle sizes. Furthermore, successive increase of ZrO2 contents led to, in contrast to relative density and electrical conductivity, remarkable enhancement of mechanical properties of the nanocomposites. The results pointed out that microhardness, compressive strength and conductivity, for specific ZrO2 content, obviously increased at the early stage of aging and then, decreased.

Impact of frailty and metabolic syndrome on the incidence of loss of independence in community-dwelling older Japanese: the Kusatsu-town study

Objectives To investigate the relationship between frailty, metabolic syndrome (MetS), and loss of independence leading to the onset of disability or death among community-dwelling older Japanese.Methods Between 2002 and 2011, we conducted a 7-year prospective study involving 1,453 residents aged 65 years and over, initially free of disability, who underwent a comprehensive geriatric assessment in Kusatsu town. Statistical analysis was conducted for those with complete values, resulting in a variable number of subjects for each data point. Frailty was defined for 1,335 subjects as three or more of the following criteria: weight loss (>2-3 kg in the past six months), weakness (grip strength was <26 kg for men and <18 kg for women), exhaustion (answer of "no" to the question "Do you feel full of energy?" on the Geriatric Depression Scale), slowness (usual gait speed<1.0 m/s), and low physical activity (answer of "less than once a week" to the question "How often do you usually go outdoors?"). MetS was defined for 1,450 subjects from the Japanese definition. The Cox proportional-hazard regression model was used to estimate hazard ratio (HR) of loss of independence for 1,217 subjects.Results During the follow-up, 494 cases of loss of independence, including 376 disabilities and 118 deaths were identified. In men and women, the incidence of the loss of independence showed a significant linear trend according to the severity classification of frailty. By contrast, there was no relationship between MetS and the loss of independence. Subjects classified as prefrail and frail had an increased risk of loss of independence compared with robust participants, with sex- and age-adjusted HRs (95% CIs) of 1.5 (1.2-1.9) and 2.4 (1.8-3.3), respectively. After adjusting for sex, age, the presence of MetS, low serum total cholesterol, chronic kidney disease, anemia, low serum albumin, cognitive impairment, and past history of stroke, the respective multivariable HRs (95% CIs) of loss of independence were 1.5 (1.2-1.9) and 2.1 (1.5-2.9). The magnitude of the risk of loss of independence incidence for frailty was greater among the subjects aged 65-74 years than those aged ≥75 years.Conclusions Frailty was significantly associated with incident loss of independence among elderly Japanese. The presence of MetS had no effect on subsequent onset of disability and mortality in elderly Japanese. Our findings suggest screening and intervention for frailty in the early stage of aging are beneficial to prolong healthy life expectancy of elderly Japanese.

Investigation of discontinuous precipitation upon age-hardening of invar-based Sn alloy

Abstract

Hydration at early ages of styrene-butadiene copolymers cementitious systems

Oil well cementing is a vital operation to assure casing stability and zonal isolation for oil and gas exploration. However, some scenarios demand the cemented region to withstand high thermal gradients and imposed deformations, as occurs in the case of oil wells subjected to cyclic steam injection at temperatures up to 250 °C, to reduce oil viscosity and to increase well pressure to facilitate heavy oil recovery. In this paper, the hydration of ductile special cement systems using styrene-butadiene latex (SBR) and carboxylated styrene-butadiene latex (XSBR) addition was studied by conduction calorimetry. The resulting heat flow curves, presented in log–log plots, were used to analyze the influence of those copolymers on the hydration stages of three families of cement pastes of different complexity. The simpler cement systems (SCCS) contained water, oil well Portland cement class G and SBR or XSBR in its composition. In medium complexity systems silica fume was added and in the higher complexity ones (HCCS), superplasticizer as well. The primary objective of adding those copolymers into the Portland cement paste is to obtain higher ductility properties after setting, silica fume to have good thermal stability up to 300 °C, while superplasticizer was added to guarantee good workability. Rheological tests were carried out to evaluate the effect of the copolymers on the composite viscosity. Thermogravimetric analysis of selected SCCS and HCCS samples was performed to quantify the main formed phases up to 24 h of cement hydration. From the obtained results, it was noticed that SBR and XSBR addition substantially affects hydration kinetics at all early age stages. Starting from pre-induction and induction periods, the main observed effect during these stages, was related to the increased viscosity of the pastes, which was higher in XSBR containing pastes, retarding the hydration reactions of respective following stages, when compared to pastes with the same cementitious matrix without copolymer addition.

Improvement of Early Strength of Cement Mortar Containing Granulated Blast Furnace Slag Using Industrial Byproducts.

In the field of construction, securing the early strength of concrete (on the first and third days of aging) has been an important problem in deciding the mold release time (i.e., shortening the construction time period). Therefore, the problem of reduced compressive strength in the early aging stage caused by mixing granulated blast furnace slag (GBFS) with concrete must certainly be resolved. In this study, we conduct experiments to explore methods for generating a concrete that develops an early strength equivalent to that of 100% OPC. The objective of this study is the development of an early-strength accelerator (ESA) made from an industrial by-product, for a GBFS-mixed cement mortar. This study also analyzes the mechanism of the early-strength generation in the concrete to evaluate the influence of the burning temperature of ESA on the optimal compressive strength of the concrete. According to the results of the experiment, GBFS, whose ESA is burnt at 800 °C, shows an activation factor of 102.6–104.7% in comparison with 100% OPC on the first and third days during early aging, thereby meeting the target compressive strength. The results of the micro-analytic experiment are as follows: ESA showed a pH of strongly alkaline. In addition, it was found that the content of SO3 was high in the chemical components, thus activating the hydration reaction of GBFS in the early age. This initial hydration reaction was thought to be due to the increase in the filling effect of the hydrate and the generation of C-S-H of the early age by the mass production of Ettringite.

The effects of early stages of aging on postural sway: A multiple domain balance assessment using a force platform.

Technical advancements in instrumentation and analytical methods have improved the ability of assessing balance control. This study investigated the effects of early stages of aging on postural sway using traditional and contemporary postural indices from different domains. Eleven healthy young adults and fourteen healthy non-faller older adults performed two postural tasks: (a) functional limits of stability and (b) unperturbed bipedal stance for 120s. Postural indices from spatial, temporal, frequency, and structural domains were extracted from the body's center of pressure (COP) signals and its Rambling and Trembling components. Results revealed a preservation of functional limits of upright stability in older adults accompanied by larger, faster, and shakier body sway in both anterior-posterior and medio-lateral directions; increased medio-lateral sway frequency; increased irregularity of body sway pattern in time in both directions; and increased area, variability, velocity, and jerkiness of both rambling and trembling components of the COP displacement in the anterior-posterior direction (p<0.02). Such changes might be interpreted as compensatory adjustments to the age-related decline of sensory, neural, and motor functions. In conclusion, balance assessment using postural indices from different domains extracted from the COP displacement was able to capture subtle effects of the natural process of aging on the mechanisms of postural control. Our findings suggest the use of such indices as potential markers for postural instability and fall risk in older adults.

Age-hardening behavior and microstructure of Cu-15Ni-8Sn-0.3Nb alloy prepared by powder metallurgy and hot extrusion

Cu-15Ni-8Sn-0.3Nb alloy rods were prepared by means of powder metallurgy followed by hot extrusion. Element maps obtained by electron probe micro analyzer (EPMA) showed that Nb-rich phases were formed and distributed within grains and at grain boundaries of the Cu-15Ni-8Sn-0.3Nb alloy. Transmission electron microscope (TEM) results indicated that there was no obvious orientation relationship between these phases and the matrix. Spinodal decomposition and ordering transformation appeared at early stages of aging at 400 °C and caused significant strengthening. Cu-15Ni-8Sn-0.3Nb alloy exhibited both higher strength (ultimate tensile strength >1030 MPa) and higher tensile ductility (elongation>9.1%) than Cu-15Ni-8Sn alloy after aging treatment. The improvement was caused by Nb-rich phases at grain boundaries which led o the refinement of grain size and postponed the growth of discontinuous precipitates during aging.

Aging Strengthening Mechanism of the Cu-1.0Zr Alloy

The effects of different aging processes on electrical conductivity and microhardness of a Cu-1.0Zr alloy were investigated. Microstructure and precipitates of the aged alloy were analyzed by transmission electron microscopy, and the aging strengthening mechanism of the Cu-1.0Zr alloy is discussed. Good comprehensive performance of the Cu-1.0Zr alloy can be obtained by aging at 773K (500 °C) for 1 hour, for which electrical conductivity reached 80.2 pct IACS, while microhardness reached 155.6 HV. Small amounts of a Zr-rich phase and annealing twins were present in the solid solution. A large amount of 3-18 nm Cu10Zr7 precipitates were present in the copper matrix. At the early stage of aging, the precipitates were small and their density was relatively low. As aging time progressed, the precipitates gradually increased in size. After overaging, the precipitated phase was dissolved, resulting in reduced microhardness. After aging at 723 K (450 °C) for 6 hours, the precipitates were Cu10Zr7. The peak Cu-1.0Zr alloy performance was achieved after aging at 773 K (500 °C) for 1 hour, and the main reason for the performance increase is coherent strain hardening.

Microstructure Evolution and Age-Hardening Behavior of Microalloyed Austenitic Fe-30Mn-9Al-0.9C Light-Weight Steels

The aging behavior and mechanical properties of microalloyed austenitic Fe-30Mn-9Al-0.9C light-weight steels were investigated through transmission electron microscopy analysis, electron backscatter diffraction analysis, tensile tests, and Vickers hardness tests. The base steels were aged at 823 K (550 °C) for up to 10,000 minutes. The true stress–strain responses of solution-treated samples before aging showed that the addition of Nb and/or V improved the strength by grain refinement and precipitation hardening. During the process of tensile deformation, the strain-hardening rate of Fe-30Mn-9Al-0.9C steel steadily increased due to the microband-induced plasticity (MBIP) from the onset of plastic deformation to e = 25 pct, while such behavior was weakened and not observed in Nb- and/or V-added steels despite MBIP. In the early stage of aging, the Vickers hardness gradually increased with an increase in the aging time due to the precipitation of κ-carbide of (Fe,Mn)3AlC and remained stagnant between the aging times of 1000 and 3000 minutes. The hardness increased again after 3000 minutes due to the formation of ferrite and brittle β-Mn.

Long term ageing of polyamide 6 and polyamide 6 reinforced with 30% of glass fibers: physicochemical, mechanical and morphological characterization

Hygrothermal ageing of polyamide 6 (PA6) and polyamide 6 reinforced with 30 wt% of glass fibers (PA6GF30) was undertaken. Immersion was conducted in distilled water at 90 degrees C and 100% relative humidity (RH) for up to 80 days (1920 h). Results revealed a noteworthy decrease either in glass transition temperature T-g or in tensile properties, at early stage of ageing, for both studied materials. This decline was mainly caused by the plasticization effect of water and the weakness of the interfacial interactions leading as a consequence to a loss of adhesion between fiber and matrix. Afterwards, physical and mechanical properties decrease monotonically testifying the occurrence of exhaustive damages and chemical reaction phenomena. Such phenomena were yellowing and crazing formation which were observed for both materials after 1920 h of conditioning. The former is caused by the thermo-oxidation whereas the latter results from the release of internal stresses induced by water sorption. These chemical reactions were monitored by infrared spectroscopy. Thus, an increase of the free N-H stretch and the carbonyl groups (imides) was noted. Accordingly, it seems that long term immersion in distilled water at high temperature induces chemical reactions which indicate the severity of the damage.

Attentional Orienting and Dorsal Visual Stream Decline: Review of Behavioral and EEG Studies

Every day we are faced with an overwhelming influx of visual information. Visual attention acts as the filtering mechanism that enables us to focus our limited neural resources, by selectively processing only the most relevant and/or salient aspects of our visual environment. The ability to shift attention to the most behaviorally relevant items enables us to successfully navigate and interact with our surroundings. The dorsal visual stream is important for the rapid and efficient visuospatial orienting of attention. Unfortunately, recent evidence suggests that the dorsal visual stream may be especially vulnerable to age-related decline, with significant deterioration becoming evident quite early in the aging process. Yet, despite the significant age-related declines to the dorsal visual stream, the visuospatial orienting of attention appears relatively well preserved in older adults, at least in the early stages of aging. The maintenance of visuospatial orienting of attention in older adults appears to be facilitated by the engagement of compensatory neural mechanisms. In particular, older adults demonstrate heightened activity in the frontal regions to compensate for the reduced activity in the posterior sensory regions. These findings suggest that older adults are more reliant on control processes mediated by the anterior regions of the frontoparietal attention network to compensate for less efficient sensory processing within the posterior sensory cortices.

A SANS and APT study of precipitate evolution and strengthening in a maraging steel

In this work a combination of the characterisation techniques small angle neutron scattering (SANS) and atom probe tomography (APT) are used to study the precipitation in a maraging steel. Three similar maraging steel alloys were aged at different temperatures and ageing times, and then characterised using SANS, APT and microhardness. The alloys consist of two types of precipitates, namely Laves phase and β-NiAl, the precipitates have different composition and hence precipitate ageing, which makes it complicated to model. The SANS experimental set-up was relatively simple and allowed the precipitate size and fraction of a large number of samples to be measured in a single experiment. The APT results were used for constraining the SANS modelling, particularly the composition, shape and distribution of phases. The characterisation led to the following description of precipitation: NiAl phase reaches coarsening at early stages of ageing and shifts its strength mechanisms from shearing to Orowan looping, which cause the characteristic peak strength; the Laves phase is in growth throughout and its strength contribution increases with ageing time. These observations were shown to be consistent with precipitate evolution and strengthening models, and the work of others. Although, there are some issues with the combination of SANS and APT approach, which are discussed, the methodology provides a valuable tool to understand complex precipitation behaviours.

Thermal Aging Effects on Fire Performance of the Cross-Linked Polyethylene Insulated Cable

To evaluate the fire performance of the cable in service, the ignition time, heat release rate and insulation failure parameters of the accelerated thermal aging cable on stimulated fire condition were studied. The results show that, the extended ignition time and decreasing peak of heat release rate of the cable in the early stage of aging are the addictive including the lubricant, plasticizer and anti-oxygen. In the aggravating of thermal aging, labile materials with low heating value will be exhausted, while the thermal stability of insulation sheath material decreases, its corresponding ignition time will start to shorten and initial peak of heat release rate will start to rise. The insulation failure temperature of the cable is related with the decomposition temperature of insulation materials of the cable by heating. In each aging stage, the decomposition temperatures of insulation materials by heating are all lower than the insulation failure temperature of the cable for about 10°C. The reason for the insulation failure of the cable is that the ambient heat , gives rise to the decomposition by heating of internal insulation materials of the cable through sheath materials finally under the thermal transmission effect.

Precipitation in simultaneously nitrided and aged Mo-containing maraging steel

The excellent mechanical properties of maraging steels are ascribed to nanometer-sized intermetallics which precipitate during aging in the ductile very low carbon Ni-martensite. Their wear and fatigue properties can be improved by nitriding. The non-equilibrium precipitation reactions in Fe-Ni-Co-Mo maraging steels are studied during an aging heat treatment executed in a nitriding atmosphere. The precipitates formed during the initial stages of precipitation are characterized with transmission electron microscopy and atom probe tomography. Spherical intermetallic precipitates having a diameter of around 3nm were detected in the aged, bulk material. These ω-type precipitates formed during the early stages of aging, have a trigonal crystal lattice and their chemical composition is close to (Fe,Ni)7Mo2. In the nitrided layer, Mo-N disc-shaped nitrides on the {100} martensitic lattice having a diameter of 3 to 4nm were found but their exact crystal structure could not be determined with microstructural characterization techniques. Density functional theory calculations confirmed that a single layer of Mo atoms, substituting Fe on the {100} plane of the Fe-matrix, is stable and showed that the N atoms prefer to be in the Mo-layer, on the octahedral sites with Fe as nearest neighbors.

Artificial Neural Network-Based Early-Age Concrete Strength Monitoring Using Dynamic Response Signals.

Concrete is one of the most common materials used to construct a variety of civil infrastructures. However, since concrete might be susceptible to brittle fracture, it is essential to confirm the strength of concrete at the early-age stage of the curing process to prevent unexpected collapse. To address this issue, this study proposes a novel method to estimate the early-age strength of concrete, by integrating an artificial neural network algorithm with a dynamic response measurement of the concrete material. The dynamic response signals of the concrete, including both electromechanical impedances and guided ultrasonic waves, are obtained from an embedded piezoelectric sensor module. The cross-correlation coefficient of the electromechanical impedance signals and the amplitude of the guided ultrasonic wave signals are selected to quantify the variation in dynamic responses according to the strength of the concrete. Furthermore, an artificial neural network algorithm is used to verify a relationship between the variation in dynamic response signals and concrete strength. The results of an experimental study confirm that the proposed approach can be effectively applied to estimate the strength of concrete material from the early-age stage of the curing process.