Experimental Characterization of Dynamic Property Changes in Aged Sands

This study investigates the aging effects on the small-strain shear modulus and damping ratio of sands and offers explanations for the measured results based on the concept of contact-force homogenization. Resonant column tests of aged sands under various aging conditions were conducted. The results show that loose sands exhibit greater aging effects than dense sands do at a confining pressure of 35?kPa and the effects are completely opposite when the aging pressure is increased to 100?kPa. The aging effects can be partially erased by unloading-reloading; the remaining effects can be restored when the applied pressure is the same as the original pressure used during aging and cannot be further erased by additional unloading-reloading cycles. The stress history is also a factor that affects aging behavior: unloading reloading and overconsolidation can reduce the aging rate in terms of the shear-modulus increase. The aging effects, however, can be wiped out by large strain shearing. An addition of fines (dry kaolinite powder) in the sand samples can increase the aging rate because of higher creep made by the kaolinite.     

The burden of aging: a theoretical framework for understanding the shifting balance of caregiving and care receiving as cohorts age

Unless changes occur in the way cohorts age, the future aging of the population will make the burden of caring for older persons an increasingly salient political issue in American society. There is no reason, however, why aging in the future should replicate the pattern of aging that currently exists. A helpful step toward understanding what social changes would reduce the burden that aging cohorts place on society is the development of aging theory. This article develops a theoretical framework that explicates factors determining the level of care given and care received by cohorts moving through different stages of later life. Four proximate determinants of caregiving and three proximate determinants of care receiving are specified. Once the proximate determinants are identified, attention is focused on social changes that could reduce the burden of aging produced by cohorts aging through later life in the future.     

Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K

The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature (<600 K) rarely used for practical applications and at a high temperature (>600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.     

Static strain aging of α-Fe-P alloys

Interstitial-free Fe-P alloys (0.083, 0.23 and 0.40 at. pct P) and a rephosphorized low-carbon steel were strained 4 pct in tension, aged isothermally at temperatures between 373 and 723 K, then strained to fracture at room temperature. The yield strength increment of the Fe-P alloys increases with aging time, reaches a maximum, then decreases. As with other substitutional solutes, only the first stage of strain aging was seen; the UTS and elongation remained unchanged. Phosphorus is more effective than other substitutional solutes in causing strain aging. Because of the limited solubility of P in α-Fe, increasing creasing the P content from 0.083 to 0.40 at. pct had only a slight effect on strain aging. The activation energy for strain aging is about 220 kJ/mole, indicating that aging is controlled by lattice diffusion of P. When both interstitial and substitutional solutes are present, as in the rephosphorized steel, aging by interstitials masks any aging by P. E. J. DZIURA, formerly Graduate Student, Department of Materials and Metallurgical Engineering, University of Michigan, is now with the     

The anisotropic kinetics of strain aging in a martensitic steel

An experimental study has been conducted to evaluate the effect of static strain aging (bake hardening) on the yield strength and yield strength asymmetry in torsion and torsion/reverse torsion tests on a low-carbon martensitic steel. It is shown that vastly different strain aging responses are observed in the forward and reverse portions of the torsion test following strain aging. While the yield stresses can be described phenomenologically by classic segregation kinetics, the observed asymmetry in the kinetics and magnitude of strain aging call into question the simple interpretation of dislocation de-pinning. This is further emphasised by the fact that strain aging modifies the apparent work hardening rate in the reverse loading tests.     

Yield behavior of a mild steel after prestraining and aging under reversed stress

Tubular specimens of mild steel are subjected to tensile prestraining and then aged under compressive stresses along the Bauschinger curve, at various temperatures below 175 °C. The compressive (reversed) stress imposed on the specimen during aging, called here the aging stress, is employed up to a maximum level of 90 pct of the prestress. Reloading tests in tension and in compression are carried out at room temperature, and the critical aging stress at which the yield stresses in the two reloading directions become equal is determined as a function of aging temperature. The critical aging stress decreases with increasing aging temperature and becomes constant at aging temperatures above 100 °C. Under such critical aging conditions, the aging index, i.e., the ratio of yield stress to prestress, rises with increasing aging temperature and approaches a saturation value of 0.86 at 150 °C. It is shown that stress aging is an effective means of reducing the Bauschinger effect while preserving the work hardening induced by the prestrain.     

Oxidative damage during aging targets mitochondrial aconitase

The mechanisms that cause aging are not well understood. The oxidative stress hypothesis proposes that the changes associated with aging are a consequence of random oxidative damage to biomolecules. We hypothesized that oxidation of specific proteins is critical in controlling the rate of the aging process. Utilizing an immunochemical probe for oxidatively modified proteins, we show that mitochondrial aconitase, an enzyme in the citric acid cycle, is a specific target during aging of the housefly. The oxidative damage detected immunochemically was paralleled by a loss of catalytic activity of aconitase, an enzyme activity that is critical in energy metabolism. Experimental manipulations which decrease aconitase activity should therefore cause a decrease in life-span. This expected decrease was observed when flies were exposed to hyperoxia, which oxidizes aconitase, and when they were given fluoroacetate, an inhibitor of aconitase. The identification of a specific target of oxidative damage during aging allows for the assessment of the physiological age of a specific individual and provides a method for the evaluation of treatments designed to affect the aging process.     

The dynamics of aging and rehabilitation following coronary bypass operation]

The dynamics of aging encompass the topics "aging of society" and "coping with chronic illness". Both are discussed using data from a qualitative study on return to work after coronary artery bypass surgery. Data from the retrospective substudy reveal four forms of aging as biographical development toward early retirement, old-age retirement, or related stages. It emerges that the manner of aging is less influenced by medical rehabilitation or medical history, and more by the patient's occupation and socio-economic status. Case reports are used to show that social class is a facilitation context, but not a determining force, regarding the choice between postoperative retirement or return to work. The final section briefly discusses whether the findings concerning the dynamics of aging of the cohort operated on in the late 1970s apply also to that of the cohort operated on in the late 1980s; this is addressed using some data from the prospective sub-study.     

高强韧铝合金非等温时效工艺的研究进展

非等温时效工艺作为一种新兴的时效处理方法,能够有效地提高高强韧铝合金的综合性能。通过简要归纳近些年来应用于高强韧铝合金的非等温时效工艺,总结出经不同非等温时效处理后高强韧铝合金析出相的特征、合金力学性能和腐蚀性能的变化情况。非等温时效工艺的效率相较于传统时效工艺有很大提高,并且能够同时调控高强韧铝合金内基体析出相和晶界析出相的种类、尺寸和分布情况,使高强韧铝合金兼具与T6峰值时效态相差不多的力学性能和近T7x过时效态的腐蚀性能。最后,对未来高强韧铝合金非等温时效工艺的研究和应用进行了展望。      

The effect of aging on hydrogen embrittlement of a nickel alloy

The effect of aging at 500° C on the hydrogen embrittlement tendency of a cold worked Ni-base superalloy was investigated in a series of experiments which included hydrogen charging studies, mechanical tests in hydrogen and in air, and fractographic and slip line investigations. Embrittlement tendency increased (time-to-failure decreased) markedly during the first hour of aging and then remained constant until about 1000 h aging time, whereupon it increased rapidly again. The short-time embrittlement could be accounted for either by a mechanism involving segregation of P to grain boundaries or by one involving planar slip induced by short-range order. The hydrogen charging studies indicated that hydrogen uptake decreases during aging, a result which is not consistent with the P segregation hypothesis. The increase in embrittlement at long aging times is most readily explained in terms of planar slip induced by long range order. Tensile tests over a range of strain rates suggested that accelerated transport of hydrogen by dislocation dragging of hydrogen atmospheres is involved in embrittlement. Formerly Graduate Assistant, Dept. of Met. Eng. & Mat. Sci., University of Notre Dame     

Strain aging and load relaxation behavior of type 316 stainless steel at room temperature

The strain aging and load relaxation behavior of type 316 stainless steel (SS) at room temperature were studied. It is shown that rapid aging occurs in 316 SS at room temperature to an extent that affects the load relaxation behavior of the material. Qualitatively, the aging behavior was found to agree with those reported earlier for Fe-Ni-C-alloys, and the observed aging characteristics could be explained by using an earlier proposed vacancy-interstitial mechanism. The load relaxation behavior is analyzed in terms of Hart’s state variable model. Effects of strain aging and strain hardening on the load relaxation behavior and the scaling of the relaxation curves are determined. It is shown that aging can be accounted for by a time-dependent change in a model parameter, which is dependent on the mobile dislocation density and the dislocation mobility. In addition, a dependency on plastic state of the same parameter previously held constant was found. It is concluded that this phenomenon, which in 316 SS could be rationalized in terms of increasing forest dislocation density, is likely to be more general, and a provision for it should be made in the state variable theory. S. P. Hannula formerly Research Associate in the Department of Materials Science and Engineering, Cornell University, Ithaca, NY M. A. Korhonen, formerly Visiting Assistant Professor in the Department of Materials Science and Engineering, Cornell University, Ithaca, NY     

Primary aging, secondary aging, and intelligence.

The distinction between primary aging, representing innate maturational processes, and secondary aging, representing the effects of environment and disease (E. W. Busse, 1969), was used to develop a model for the assessment of factors that are associated with age-related individual differences in intelligence. Intelligence was measured by performance on a number of tests that measure cognitive abilities known to decline with age. In a hierarchical multiple regression analysis, primary aging and education but not health explained a significant portion of the variance in fluid intelligence. Chronological age had a residual effect over and above that of primary and secondary aging, indicating that there was remaining age-associated variance unaccounted for in the proposed model. The results suggest that the model of primary and secondary aging is a valid means of operationalizing chronological age. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Modeling of the Precipitation Kinetics During Aging a Predeformed Fe-Cu Alloy

A model is developed to describe the microstructure evolution during aging a Fe-Cu alloy. The precipitation process during aging a predeformed Fe-1.5 wt pct Cu is calculated and the kinetic details of aging are discussed. The model is satisfactorily tested by comparing with the reported experimental results. The numerical results demonstrate that the nucleation of the precipitates occurs on dislocations first. However, the maximum nucleation rate of the precipitates in the matrix is much higher than that on dislocations and it is not affected by the appearance of dislocations. Only a small fraction of precipitates locate on dislocations immediately after nucleation. The particles in the matrix dissolve preferentially during coarsening, and most of the remained precipitates locate on dislocations in an overaged sample. Dislocations have little effect on the number density and average radius of the precipitates in the late stage of aging, although they promote the beginning of precipitation in the early stage of aging. The heating rate of the specimen almost does not affect the summit of the particle number density as well as its corresponding mean particle radius.     

Effect of aging and deformation on the microstructure and properties of Fe-Ni-Ti maraging steel

The age-hardening behavior of Fe-25.3Ni-1.7 Ti (wt pct) alloy both in undeformed specimens and in specimens cold deformed by 10 or 20 pct prior to aging was studied. The microstructural changes during aging were observed using transmission electron microscopy (TEM) and atom probe analysis and there were related to the mechanical properties as measured by microhardness and shear punch testing. An excellent combination of hardness, strength, and ductility was achieved after only 5 seconds aging at 550 °C. We propose that this rapid strengthening is due to a dislocation friction effect arising from the formation of a fine dispersion of Ni-Ti atomic co-clusters during this short aging time. The concomitant effects of a reverse transformation of martensite to austenite during aging and a gradual increase in both size of the clusters and distance between them contributed to a decrease in strength after aging for 15 seconds. This decline proceeded until aging for 300 seconds and was followed by a secondary hardening reaction toward peak hardness (at 10,800 seconds) and subsequent overaging. This secondary hardening was associated with fine-scale precipitation of Ni₃Ti and this process was accelerated by deformation prior to aging, leading to a reduction or elimination of hardness decline after the initial cluster hardening.     

Psychology's response to the graying of America.

Asserts that changes set in motion by major public health advances early in the century have increased the demand for knowledge about the aging process, especially the psychological aspects of aging. There is need for basic and applied research on aging in areas such as memory; federal support should be provided for training at the predoctoral level; and information on the psychological aspects of aging should be transmitted to society, policy makers, and members of other professions who serve the elderly. Knowledge of aging as a biological, psychological, and social process is applicable in fields such as industry, education, and engineering. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

改型GH4133A合金长期时效的性能稳定性研究

对经过标准热处理改型GH4133A合金,在650、700和750℃进行长时间的时效处理,测量其室温拉伸和冲击性能、硬度、400℃拉伸性能和在750℃/343 MPa条件下持久性能.结果表明:在650℃和700℃下时效时,合金的性能变化规律一致,性能具有良好的稳定性.而在750℃时效时,合金的室温拉伸和400℃拉伸性能在时效500 h以前保持稳定,500 h以后随时效时间的增长而下降;合金的室温冲击性能在时效300 h以前保持稳定,300 h后下降;合金在750℃/343 MPa条件下,在时效600 h以前,持久性能寿命很稳定,基本保持在100 h左右,时效600 h以后的持久寿命迅速下降.     

Natural killing and antibody-dependent cytotoxicity by lymphocyte subpopulations in young and aging humans

Natural killer (NK) and antibody-dependent cellular cytotoxicity (ADCC) were examined in the peripheral blood lymphocytes and their major subpopulations from young and aging subjects. Monocyte-depleted unseparated lymphocyte-mediated NK activity (against cells of K-562) and ADCC (against IgG-coated chicken erythrocytes) were comparable between young and aging subjects. Similarly no significant difference was observed in T cell-mediated NK and ADCC and non-T cell-mediated ADCC between young and aging subjects. Non-T cell-mediated NK activity, however, was significantly (P less than 0.025) greater in aging humans compared to that of young subjects. When the data were analyzed according to gender, T cell-mediated ADCC in aging males was significantly (P less than 0.05) greater than that found in young males. No significant difference was observed between T-cell ADCC among young and aging females. T cell-mediated NK was comparable among young and aging males and young and aging females. Non-T cell-mediated NK as well as ADCC activity was significantly (P[ less than 0.025 or less than 0.05) greater in aging males compared to that in young males. Both non-T-cell NK and ADCC were comparable among young and aging females. This study demonstrates an increase in NK and ADCC activity in aging subjects that is primarily shared by males and not by females. No correlation was observed between the proportion of T gamma cells and T-cell NK or ADCC activity.     

Ti-Al-Mo合金的时效析出行为

研究了一种新型Ti-Al-Mo近β型钛合金在900℃×30 min/WQ固溶处理后,在不同条件时效处理后的析出相、显微硬度及力学性能。研究结果表明,在500℃时效4 h后的合金显微维氏硬度最高,为4 273 MPa;时效温度在400~700℃范围内时,随着时效温度的升高,析出的片层状α相尺寸逐渐增大,体积分数先增加后降低。由于加入了β稳定元素Mo,能提高强度但也会降低塑性,为了获得较好的强塑性匹配,在时效时间一定的前提下,时效温度应选取500℃左右;而为了得到较高的塑性和断裂韧性,在600~700℃之间时效较为适宜。     

Postirradiation aging affects stress and strain in polyethylene components

Ultrahigh molecular weight polyethylene components oxidatively degrade because of gamma radiation sterilization and subsequent shelf aging in air. The effects of shelf aging on the stresses and strains associated with surface damage in tibial and acetabular components were examined. A material model was developed to predict the stress and strain relationship of oxidatively degraded polyethylene as a function of density using samples of polyethylene that were gamma radiation sterilized and evaluated immediately after irradiation and after 42 months of shelf aging. The finite element method was used to determine the stresses and strains before and after shelf aging for two tibial components with different conformities between the articulating surfaces and for an acetabular component. The stresses increased by 10% to 14% in the conforming tibial model after 42 months of aging, whereas the stresses in the nonconforming tibial model and in the acetabular model increased by only 4% to 8%. Aging decreased the principal strains by 5% to 10% in both tibial models and by 15% to 17% in the acetabular model. Postirradiation aging during shelf storage of polyethylene joint components is likely to worsen long term wear, based on the increased stresses and decreased strains predicted to occur as a result of aging.