Creep ruptures in heterogeneous materials.

We present creep experiments on fiber composite materials with different controlled heterogeneity. All samples exhibit a power-law relaxation of the strain rate in the primary creep regime (Andrade's law) followed by a power-law acceleration up to rupture. We discover that the rupture time is proportional to the duration of the primary creep regime, showing the interplay between the two regimes and offering a method of rupture prediction. These experimental results are rationalized by a mean-field model of representative elements with nonlinear viscoelastic rheology and with a large heterogeneity of strengths.

Measurement of hydrostatic intraperitoneal pressure: a useful tool for the improvement of dialysis dose prescription.

The prescription of peritoneal dialysis should be individualized based on parameters of tolerance and adequacy. Determination of the intraperitoneal fill volume is essential for optimal patient care. Fill volume enhancement is a factor of exchange surface area recruitment: the wetted, contact peritoneal dialysis membrane. Nevertheless, fill volume enhancement can also lead to patient discomfort, with the potential risk of too high an intraperitoneal pressure (hernia, gastro-esophageal reflux). The perception of the individual patient is also a subjective parameter of fill volume tolerance assessment. In contrast, measurement of the hydrostatic intraperitoneal pressure (IPP, cmH(2)O) allows an objective approach to fill volume tolerance. From our clinical experience of more than 10 years of IPP measurements in child care, we can give a recommendation for normal values in children: less than 18 cm of water, usually between 5 and 15 cm, correlated to the intraperitoneal fill volume (naturally), but individually taking into account age, gender, "accustomization" and overall body mass index.

Occurrence of finite-time singularities in epidemic models of rupture, earthquakes, and starquakes.

We present a new kind of critical stochastic finite-time singularity, relying on the interplay between long-memory and extreme fluctuations. We illustrate it on the well-established epidemic-type aftershock model for aftershocks, based solely on the most solidly documented stylized facts of seismicity (clustering in space and in time and power law Gutenberg-Richter distribution of earthquake energies). This theory accounts for the main observations (power law acceleration and discrete scale invariant structure) of critical rupture of heterogeneous materials, of the largest sequence of starquakes ever attributed to a neutron star, as well as of earthquake sequences.

Diffusion of epicenters of earthquake aftershocks, Omori's law, and generalized continuous-time random walk models.

The epidemic-type aftershock sequence (ETAS) model is a simple stochastic process modeling seismicity, based on the two best-established empirical laws, the Omori law (power-law decay approximately 1/t(1+theta) of seismicity after an earthquake) and Gutenberg-Richter law (power-law distribution of earthquake energies). In order to describe also the space distribution of seismicity, we use in addition a power-law distribution approximately 1/r(1+mu) of distances between triggered and triggering earthquakes. The ETAS model has been studied for the last two decades to model real seismicity catalogs and to obtain short-term probabilistic forecasts. Here, we present a mapping between the ETAS model and a class of CTRW (continuous time random walk) models, based on the identification of their corresponding master equations. This mapping allows us to use the wealth of results previously obtained on anomalous diffusion of CTRW. After translating into the relevant variable for the ETAS model, we provide a classification of the different regimes of diffusion of seismic activity triggered by a mainshock. Specifically, we derive the relation between the average distance between aftershocks and the mainshock as a function of the time from the mainshock and of the joint probability distribution of the times and locations of the aftershocks. The different regimes are fully characterized by the two exponents theta and mu. Our predictions are checked by careful numerical simulations. We stress the distinction between the "bare" Omori law describing the seismic rate activated directly by a mainshock and the "renormalized" Omori law taking into account all possible cascades from mainshocks to aftershocks of aftershock of aftershock, and so on. In particular, we predict that seismic diffusion or subdiffusion occurs and should be observable only when the observed Omori exponent is less than 1, because this signals the operation of the renormalization of the bare Omori law, also at the origin of seismic diffusion in the ETAS model. We present predictions and insights provided by the ETAS to CTRW mapping which suggest different ways for studying seismic catalogs. Finally, we discuss the present evidence for our predicted subdiffusion of seismicity triggered by a main shock, stressing the caveats and limitations of previous empirical works.

Effects of automated peritoneal dialysis on residual daily urinary volume in children.

Preservation of residual renal function (RRF) is an important goal. In children, a more rapid decline in RRF has been observed under hemodialysis (HD) therapy as compared with peritoneal dialysis (PD) therapy. In adults, however, automated peritoneal dialysis (APD) may cause a more rapid decline of RRF than continuous ambulatory peritoneal dialysis (CAPD) does. The objective of the present study, a survey in a single center over the last 15 years, was to assess the impact of APD versus hemodiafiltration (HDF) on daily urinary volume (dUV) outcome. We included 97 children who were dialyzed for at least a 12-month period between January 1985 and December 1999, using either HDF (n = 60; 62%) or PD [n = 37; 38% (86% of those on APD)]. The endpoint was anuria occurrence, defined as a dUV below 50 mL/m2 body surface area (BSA) at three consecutive monthly determinations. Despite the use of HDF as hemodialysis therapy (that is, biocompatible membranes and a very low incidence of vascular instability during ultrafiltration), PD--even predominantly prescribed as APD--was associated with better preservation of residual dUV. At dialysis end, anuria occurred in 65% of the children undergoing HD as compared with 23% of those undergoing PD. The mean age of the children at dialysis start was lower in the PD group. No other significant differences were noted between the groups, either for the rate of uropathies or for the RRF at initiation of dialysis.

Biological response to variation of acid-volatile sulfides and metals in field-exposed spiked sediments.

Vertical and temporal variations of acid-volatile sulfides (AVS) and simultaneously extracted metals (SEM) in sediment can control biological impacts of metals. To assess the significance of these variations in field sediments, sediments spiked with cadmium, copper, lead, nickel, and zinc were deployed in Narragansett Bay for four months and recolonization by benthic organisms investigated. In surface sediments, concentrations of AVS decreased with time whenever AVS exceeded SEM but remained unchanged when AVS was less than SEM; in subsurface sediments, concentrations increased slightly. Concentrations of total SEM decreased with time only in surface sediments in which SEM exceeded AVS. Metals were not detected in interstitial waters of sediments where AVS exceeded SEM but were significant when SEM exceeded AVS and followed the order of solubilities of their sulfides. Concentrations in interstitial waters decreased with time, but exceeded U.S. Environmental Protection Agency chronic water quality criteria for 56 to 119 d. After 119 d, faunal assemblages in all treatments were similar to controls. Lack of biological response was related to vertical distributions of AVS and SEM. Biological exposure took place in near-surface sediments, where AVS exceeded SEM in even the highest treatments. Therefore, concentrations of metals in interstitial waters were low and consequently biological impacts were undetectable.