Comparisons of elastic and creep deformation linearly dependent upon stress

The theory of linear elasticity provides a complete description of reversible deformation under small stresses for both isotropic and anisotropic solids. At elevated temperatures, creep deformation sometimes occurs at a rate that is linearly dependent upon stress. When this form of creep arises from vacancy movement, there is possibility of anisotropic behaviour through the orientational dependence of average grain dimensions. This indicates that the elasticity theory may be utilised to provide comparable descriptions of such creep deformation, with creep strain built up of equal increments of strain occurring in equal intervals of time. The extent of this analogy is explored with the conclusion that its usefulness is substantial when grains are small in relation to geometrical features of the component but it is no longer applicable when the grains approach the size of these features and where there is a high gradient of stress

Constraints Imposed by the Wilshire Methodology on Creep Rupture Data and Procedures for Testing the Validity of Such Constraints: Illustration Using 1Cr-1Mo-0.25V Steel

A new parametric approach, termed the Wilshire equations, offers the realistic potential of being able to accurately life materials operating at in service conditions from accelerated test results lasting no more than 5000 hours. These Wilshire equations contain discontinuities that have in the literature been interpreted either in terms of changing deformation mechanisms or changes in where deformation occurs within a material (i.e., within boundaries or crystals). This paper demonstrates that the rather restrictive nature of these discontinuities within the Wilshire equations can lead to problems in identifying an appropriate model for long-term life prediction. An alternative framework is developed that removes these restrictions but still maintains the fundamental nature and characteristics of the Wilshire methodology. Further, when this alternative structure is applied to 1Cr-1Mo-0.25V steel, it produces more accurate and realistic looking long-term predictions of the time to failure

The importance of creep strain in linking together the Wilshire equations for minimum creep rates and times to various strains (including the rupture strain): An illustration using 1CrMoV rotor steel

This paper highlights the observation that the Wilshire equations for failure times and times to various strains, as reported in the original literature, may not be the most appropriate ones for all materials—including the one selected in this study. Further, such appropriateness can be determined by looking at the consistencies between the parameter estimates obtained using minimum creep rates in comparison to using failure times. It is shown, using 1CrMoV steel as an illustration, that the parameter consistency can be achieved by generalising the Monkman–Grant relation so that it contains a temperature correction. Indeed, the ability of the Wilshire equations to produce meaningful physical parameters, such as the activation energy, is shown to be highly dependent upon a valid specification for the Monkman–Grant relation. It is shown that variations in the measured values for some of the Wilshire parameters (w and k 3) with strain indicate that the causes of deformation are different at different strains and different stresses. Finally, the measured variations in the parameters of the Monkman–Grant relation with strain enable accurate interpolated and extrapolated creep curves to be calculated for any test condition

The effect of endurance exercise on bone dimensions, collagen, and calcium in the aged male rat

Sixteen weeks of a relatively mild running program, started at 22 months of age, lowered the body weights of 26-month-old male rats to the level of 9-month-old rats and increased the weights and the collagen densities of hind limb bones to levels greater than those of 9-, 22-, and 26-month-old sedentary rats. The densities (g/cm3) and the calcium densities (mg/cm3) of the hind limb bones decreased with age and were restored to the 9-month level by training the elderly rats to run. These data suggest that exercise is capable of inducing a compensation for, or a reversal of, age-associated bone loss (osteoporosis) and restoring the bone mineral content in aged rats to the level of those of mature young adult animals.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25887/1/0000450.pd

Presence of Germline and Full-Length IgA RNA Transcripts Among Peritoneal B-1 Cells

Next to conventional B cells (or B-2 cells), peritoneal B-1 cells have been shown to contribute significantly to the production of IgA-secreting plasma cells in the gut. Evidence for this was mainly based on studies comprising manipulated animals, including lethally X-irradiated and transgenic mice. To examine the ability of peritoneal B-1 cells from untreated mice to switch actively to IgA in vivo, we performed RT-PCR analysis on FACS-sorted peritoneal B-cell subsets from untreated BALB/c mice in order to examine the presence of germline Cα mRNA and mature Cα mRNA transcripts. Germline Cα and mature Cα transcripts were readily detectable in peritoneal B-1 cells (defined as IgMbright/IgDdull), but not, or very little, in peritoneal B-2 cells (defined as IgMdull/IgDbright). Moreover, by subdividing the B-l-cell population in CD5+ B-1a cells and CD5- B-1b cells, it was shown that in vivo expression of germline Cα and mature Cα transcripts was largely restricted to the B-1b-cell lineage. These results indicate that peritoneal B-1 cells indeed are capable to switch to IgA under normal physiological conditions and hereby further support the view that B-1 cells contribute significantly to the mucosal IgA response, albeit this function appears to be restricted to the B-1b-cell subset

Experimental and numerical analysis of initial plasticity in P91 steel small punch creep samples

To date, the complex behaviour of small punch creep test (SPCT) specimens has not been completely understood, making the test hard to numerically model and the data difficult to interpret. This paper presents a novel numerical model able to generate results that match the experimental findings. For the first time, pre-strained uniaxial creep test data of a P91 steel at 600 °C have been implemented in a conveniently modified Liu and Murakami creep damage model in order to simulate the effects of the initial localised plasticity on the subsequent creep response of a small punch creep test specimen. Finite element (FE) results, in terms of creep displacement rate and time to failure, obtained by the modified Liu and Murakami model are in good agreement with experimental small punch creep test data. The rupture times obtained by the FE calculations which make use of the non-modified creep damage model are one order of magnitude shorter than those obtained by using the modified constitutive model. Although further investigation is needed, this novel approach has confirmed that the effects of initial localised plasticity, taking place in the early stages of small punch creep test, cannot be neglected. The new results, obtained by using the modified constitutive model, show a significant improvement with respect to those obtained by a state of the art creep damage constitutive model (the Liu and Murakami constitutive model) both in terms of minimum load-line displacement rate and time to rupture. The new modelling method will potentially lead to improved capability for SPCT data interpretatio

A study on the evolution of the contact angle of small punch creep test of Ductile materials

The work discussed in the present paper reports a novel investigation of the applicability of Chakrabarty's theory, for membrane stretching of a circular blank over a rigid punch, to small punch creep test (SPCT). The Chakrabarty solution was compared with corresponding results obtained by numerical finite element (FE) analyses and experimental tests. The Liu and Murakami creep damage model was used in the FE analyses. The aim of the work is also to improve the understanding of the mechanism governing the deformation and the failure of the specimen and to verify the range of applicability of the CEN Code of Practice CWA 15627, which is based on Chakrabarty's theory. The effects of various parameters, such as the initial thickness of the specimen, the radius of the punch, the load magnitude, the friction coefficient and different plasticity constitutive models, on the variation of the contact angle, θ0, and the central displacement of the punch, Δ, were identified and correlated by fitting equations. The variation of θ0 with Δ, obtained from Chakrabarty's solution was compared with that obtained by FE analyses of the SPCT. When the initial thickness of the specimen increased and the radius of the punch decreased, the FE results, in terms of the variation of θ0 versus Δ, showed to differ from Chakrabarty's solution, therefore new ranges of applicability of the CEN Code of Practice CWA 15627 were determined

Influence of Grain Boundary Character on Creep Void Formation in Alloy 617

Alloy 617, a high temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP) which will operate at temperatures exceeding 760oC. Orientation imaging microscopy (OIM) is used to characterize the grain boundaries in the vicinity of creep voids that develop during high temperature creep tests (800-1000oC at creep stresses ranging from 20-85 MPa) terminated at creep strains ranging from 5-40%. Observations using optical microscopy indicate creep rate does not significantly influence the creep void fraction at a given creep strain. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular or 45o to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids while CSL-related grain boundaries did not appear to be consistently associated with void development

Dietary carbohydrate restriction as the first approach in diabetes management:Critical review and evidence base

AbstractThe inability of current recommendations to control the epidemic of diabetes, the specific failure of the prevailing low-fat diets to improve obesity, cardiovascular risk, or general health and the persistent reports of some serious side effects of commonly prescribed diabetic medications, in combination with the continued success of low-carbohydrate diets in the treatment of diabetes and metabolic syndrome without significant side effects, point to the need for a reappraisal of dietary guidelines. The benefits of carbohydrate restriction in diabetes are immediate and well documented. Concerns about the efficacy and safety are long term and conjectural rather than data driven. Dietary carbohydrate restriction reliably reduces high blood glucose, does not require weight loss (although is still best for weight loss), and leads to the reduction or elimination of medication. It has never shown side effects comparable with those seen in many drugs. Here we present 12 points of evidence supporting the use of low-carbohydrate diets as the first approach to treating type 2 diabetes and as the most effective adjunct to pharmacology in type 1. They represent the best-documented, least controversial results. The insistence on long-term randomized controlled trials as the only kind of data that will be accepted is without precedent in science. The seriousness of diabetes requires that we evaluate all of the evidence that is available. The 12 points are sufficiently compelling that we feel that the burden of proof rests with those who are opposed