Characterizing the mechanical response of metallic glasses to uniaxial tension using a spring network model

Metallic glasses are frequently used as structural materials. Therefore, it is important to develop methods to predict their mechanical response as a function of the microstructure prior to loading. We develop a coarse-grained spring network model, which describes the mechanical response of metallic glasses using an equivalent series network of springs, which can break and re-form to mimic atomic rearrangements during deformation. To validate the model, we perform simulations of quasistatic, uniaxial tension of Lennard-Jones and embedded atom method (EAM) potentials for Cu$_{50}$Zr$_{50}$ metallic glasses. We consider samples prepared using a wide range of cooling rates and with different amounts of crystalline order. We show that both the Lennard-Jones and EAM models possess qualitatively similar stress $\sigma$ versus strain $\gamma$ curves. By specifying five parameters in the spring network model (ultimate strength, strain at ultimate strength, slopes of $\sigma(\gamma)$ at $\gamma=0$ and at large strain, and strain at fracture where $\sigma=0$), we can accurately describe the form of the stress-strain curves during uniaxial tension for the computational studies of Cu$_{50}$Zr$_{50}$, as well as recent experimental studies of several Zr-based metallic glasses. For the computational studies of Cu$_{50}$Zr$_{50}$, we find that the yield strain distribution is shifted to larger strains for slowly cooled glasses compared to rapidly cooled glasses. In addition, the average number of new springs and their rate of formation decreases with decreasing cooling rate. These effects offset each other at large strains, causing the stress-strain curve to become independent of the sample preparation protocol in this regime. In future studies, we will extract the parameters that define the spring network model directly from atomic rearrangements that occur during uniaxial deformation.Comment: 16 pages, 13 figure

Avalanche Statistics from Data with Low Time Resolution

Extracting avalanche distributions from experimental microplasticity data can be hampered by limited time resolution. We compute the effects of low time resolution on avalanche size distributions and give quantitative criteria for diagnosing and circumventing problems associated with low time resolution. We show that traditional analysis of data obtained at low acquisition rates can lead to avalanche size distributions with incorrect power-law exponents or no power-law scaling at all. Furthermore, we demonstrate that it can lead to apparent data collapses with incorrect power-law and cutoff exponents. We propose new methods to analyze low-resolution stress-time series that can recover the size distribution of the underlying avalanches even when the resolution is so low that naive analysis methods give incorrect results. We test these methods on both downsampled simulation data from a simple model and downsampled bulk metallic glass compression data and find that the methods recover the correct critical exponents

Evaluation of Temporal Variability in Surface Chlorophyll-a Concentration Estimated by GCOM-C/SGLI in Onagawa Bay

Young Researchers Sessio

Universal Slip Dynamics in Metallic Glasses and Granular Matter – Linking Frictional Weakening with Inertial Effects

Slowly strained solids deform via intermittent slips that exhibit a material-independent critical size distribution. Here, by comparing two disparate systems - granular materials and bulk metallic glasses - we show evidence that not only the statistics of slips but also their dynamics are remarkably similar, i.e. independent of the microscopic details of the material. By resolving and comparing the full time evolution of avalanches in bulk metallic glasses and granular materials, we uncover a regime of universal deformation dynamics. We experimentally verify the predicted universal scaling functions for the dynamics of individual avalanches in both systems, and show that both the slip statistics and dynamics are independent of the scale and details of the material structure and interactions, thus settling a long-standing debate as to whether or not the claim of universality includes only the slip statistics or also the slip dynamics. The results imply that the frictional weakening in granular materials and the interplay of damping, weakening and inertial effects in bulk metallic glasses have strikingly similar effects on the slip dynamics. These results are important for transferring experimental results across scales and material structures in a single theory of deformation dynamics

Role of PKCΖ translocation in the development of type 2 diabetes in rats following continuous glucose infusion

Aim We investigated the molecular mechanisms of hyperglycaemia-induced insulin resistance and type 2 diabetes in rats receiving a continuous glucose infusion (GI). Methods Female Wistar rats were infused with either 2.8 mol/L glucose or saline (2 mL/h) for durations varying from 0 to 15 days. Blood samples were analysed daily to determine glucose and insulin dynamics. Subsets of animals were sacrificed and solues muscles were extracted for determination of protein expression, subcellular location, and activities of insulin-signalling proteins. Results Rats accommodated this systemic glucose oversupply and developed insulin resistance on day 5 (normoglycaemia/hyperinsulinaemia) and type 2 diabetes on day 15 (hyperglycaemia/normoinsulinaemia). The effect of GI on protein kinase CΖ (PKCΖ) activity was independent of changes in phosphatidylinositol 3-kinase activity, and occurred in parallel with an increase in PDK1 activity. Activated PKCΖ was mainly located in the cytosol after 5 days of GI that was coincident with the translocation of GLUT4 to the plasma membrane, and normoglycaemia. After 15 days of GI, PKCΖ translocated from the cytosol to the plasma membrane with a concomitant decrease in PDK1 activity. This caused an increase in the association between PKCΖ and PKB and a decrease in PDK1–PKB reactions at the plasma membrane, leading to reduced PKB activity. The activity of PKCΖ per se was also compromised. The PKCΖ and PKB activity reduction and the blunted insulin-stimulated GLUT4 translocation eventually led to hyperglycaemia and diabetes. Conclusion Translocation of PKCΖ may play a central role in the development of type 2 diabetes. Copyright © 2009 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/64921/1/1056_ftp.pd

Terrace Standard, July, 09, 1997

Slowly strained solids deform via intermittent slips that exhibit a material-independent critical size distribution. Here, by comparing two disparate systems - granular materials and bulk metallic glasses - we show evidence that not only the statistics of slips but also their dynamics are remarkably similar, i.e. independent of the microscopic details of the material. By resolving and comparing the full time evolution of avalanches in bulk metallic glasses and granular materials, we uncover a regime of universal deformation dynamics. We experimentally verify the predicted universal scaling functions for the dynamics of individual avalanches in both systems, and show that both the slip statistics and dynamics are independent of the scale and details of the material structure and interactions, thus settling a long-standing debate as to whether or not the claim of universality includes only the slip statistics or also the slip dynamics. The results imply that the frictional weakening in granular materials and the interplay of damping, weakening and inertial effects in bulk metallic glasses have strikingly similar effects on the slip dynamics. These results are important for transferring experimental results across scales and material structures in a single theory of deformation dynamics

Restoration of Hepatic Glucokinase Expression Corrects Hepatic Glucose Flux and Normalizes Plasma Glucose in Zucker Diabetic Fatty Rats

OBJECTIVE—We examined in 20-week-old Zucker diabetic fatty (ZDF) rats whether restoration of hepatic glucokinase (GK) expression would alter hepatic glucose flux and improve hyperglycemia

Development and potential role of type-2 sodium-glucose transporter inhibitors for management of type 2 diabetes

There is a recognized need for new treatment options for type 2 diabetes mellitus (T2DM). Recovery of glucose from the glomerular filtrate represents an important mechanism in maintaining glucose homeostasis and represents a novel target for the management of T2DM. Recovery of glucose from the glomerular filtrate is executed principally by the type 2 sodium-glucose cotransporter (SGLT2). Inhibition of SGLT2 promotes glucose excretion and normalizes glycemia in animal models. First reports of specifically designed SGLT2 inhibitors began to appear in the second half of the 1990s. Several candidate SGLT2 inhibitors are currently under development, with four in the later stages of clinical testing. The safety profile of SGLT2 inhibitors is expected to be good, as their target is a highly specific membrane transporter expressed almost exclusively within the renal tubules. One safety concern is that of glycosuria, which could predispose patients to increased urinary tract infections. So far the reported safety profile of SGLT2 inhibitors in clinical studies appears to confirm that the class is well tolerated. Where SGLT2 inhibitors will fit in the current cascade of treatments for T2DM has yet to be established. The expected favorable safety profile and insulin-independent mechanism of action appear to support their use in combination with other antidiabetic drugs. Promotion of glucose excretion introduces the opportunity to clear calories (80–90 g [300–400 calories] of glucose per day) in patients that are generally overweight, and is expected to work synergistically with weight reduction programs. Experience will most likely lead to better understanding of which patients are likely to respond best to SGLT2 inhibitors, and under what circumstances