Penetration and cratering experiments of graphite by 0.5-mm diameter steel spheres at various impact velocities

Cratering experiments have been conducted with 0.5-mm diameter AISI 52100 steel spherical projectiles and 30-mm diameter, 15-mm long graphite targets. The latter were made of a commercial grade of polycrystalline and porous graphite named EDM3 whose behavior is known as macroscopically isotropic. A two-stage light-gas gun launched the steel projectiles at velocities between 1.1 and 4.5 km s 1. In most cases, post-mortem tomographies revealed that the projectile was trapped, fragmented or not, inside the target. It showed that the apparent crater size and depth increase with the impact velocity. This is also the case of the crater volume which appears to follow a power law significantly different from those constructed in previous works for similar impact conditions and materials. Meanwhile, the projectile depth of penetration starts to decrease at velocities beyond 2.2 km s 1. This is firstly because of its plastic deformation and then, beyond 3.2 km s 1, because of its fragmentation. In addition to these three regimes of penetration behavior already described by a few authors, we suggest a fourth regime in which the projectile melting plays a significant role at velocities above 4.1 km s 1. A discussion of these four regimes is provided and indicates that each phenomenon may account for the local evolution of the depth of penetration

Dynamic cratering of graphite : experimental results and simulations

The cratering process in brittle materials under hypervelocity impact (HVI) is of major relevance for debris shielding in spacecraft or high-power laser applications. Amongst other materials, carbon is of particular interest since it is widely used as elementary component in composite materials. In this paper we study a porous polycrystalline graphite under HVI and laser impact, both leading to strong debris ejection and cratering. First, we report new experimental data for normal impacts at 4100 and 4200 m s-1 of a 500-μm-diameter steel sphere on a thick sample of graphite. In a second step, dynamic loadings have been performed with a high-power nanosecond laser facility. High-resolution X-ray tomographies and observations with a scanning electron microscope have been performed in order to visualize the crater shape and the subsurface cracks. These two post-mortem diagnostics also provide evidence that, in the case of HVI tests, the fragmented steel sphere was buried into the graphite target below the crater surface. The current study aims to propose an interpretation of the results, including projectile trapping. In spite of their efficiency to capture overall trends in crater size and shape, semi-empirical scaling laws do not usually predict these phenomena. Hence, to offer better insight into the processes leading to this observation, the need for a computational damage model is argued. After discussing energy partitioning in order to identify the dominant physical mechanisms occurring in our experiments, we propose a simple damage model for porous and brittle materials. Compaction and fracture phenomena are included in the model. A failure criterion relying on Weibull theory is used to relate material tensile strength to deformation rate and damage. These constitutive relations have been implemented in an Eulerian hydrocode in order to compute numerical simulations and confront them with experiments. In this paper, we propose a simple fitting procedure of the unknown Weibull parameters based on HVI results. Good agreement is found with experimental observations of crater shapes and dimensions, as well as debris velocity. The projectile inclusion below the crater is also reproduced by the model and a mechanism is proposed for the trapping process. At least two sets of Weibull parameters can be used to match the results. Finally, we show that laser experiment simulations may discriminate in favor of one set of parameters

A User Friendly Phase Detection Methodology for HPC Systems' Analysis

International audienceA wide array of today's high performance computing (HPC) applications exhibits recurring behaviours or execution phases throughout their run-time. Accurate detection of program phases allows reconfiguring the system for a better power/performance trade off; and can reduce the simulation time of programs by identifying regions of code whose performance is critical to the entire program. Program phases are also reflected in different behaviours the system goes through or system phases, which can be used as an alternative means of program phase detection for users lacking expertise. In this paper, we present an execution vector based (EV-based) phase detection, which is an on-line methodology for detecting phases in the behaviour of a HPC system and determining execution points that correspond to these phases. We also present a methodology for defining a small set of EVs representative of the system's behaviour over a fixed period of time and show that EV-based phase detection identifies recurring phases. Our methodology is illustrated with benchmarks and a real life application

Inhibition of NF-κB-mediated signaling by the cyclin-dependent kinase inhibitor CR8 overcomes pro-survival stimuli to induce apoptosis in chronic lymphocytic leukemia cells

Purpose: Chronic lymphocytic leukemia (CLL) is currently incurable with standard chemotherapeutic agents, highlighting the need for novel therapies. Overcoming proliferative and cytoprotective signals generated within the microenvironment of lymphoid organs is essential for limiting CLL progression and ultimately developing a cure. Experimental Design: We assessed the potency of cyclin-dependent kinase (CDK) inhibitor CR8, a roscovitine analog, to induce apoptosis in primary CLL from distinct prognostic subsets using flow cytometry–based assays. CLL cells were cultured in in vitro prosurvival and proproliferative conditions to mimic microenvironmental signals in the lymphoid organs, to elucidate the mechanism of action of CR8 in quiescent and proliferating CLL cells using flow cytometry, Western blotting, and quantitative real-time PCR. Results: CR8 was 100-fold more potent at inducing apoptosis in primary CLL cells than roscovitine, both in isolated culture and stromal-coculture conditions. Importantly, CR8 induced apoptosis in CD40-ligated CLL cells and preferentially targeted actively proliferating cells within these cultures. CR8 treatment induced downregulation of the antiapoptotic proteins Mcl-1 and XIAP, through inhibition of RNA polymerase II, and inhibition of NF-κB signaling at the transcriptional level and through inhibition of the inhibitor of IκB kinase (IKK) complex, resulting in stabilization of IκBα expression. Conclusions: CR8 is a potent CDK inhibitor that subverts pivotal prosurvival and proproliferative signals present in the tumor microenvironment of CLL patient lymphoid organs. Our data support the clinical development of selective CDK inhibitors as novel therapies for CLL

Strings in Gravimagnetic Fields

We provide a complete solution of closed strings propagating in Nappi-Witten space. Based on the analysis of geodesics we construct the coherent wavefunctions which approximate as closely as possible the classical trajectories. We then present a new free field realization of the current algebra using the gamma, beta ghost system. Finally we construct the quantum vertex operators, for the tachyon, by representing the wavefunctions in terms of the free fields. This allows us to compute the three- and four-point amplitudes, and propose the general result for N-point tachyon scattering amplitude.Comment: final version, 29 pages + 4 app

Is there still a need for prophylactic intra-abdominal drainage in elective major gastro-intestinal surgery?

SummaryProphylactic drainage of the abdominal cavity after gastro-intestinal surgery is widely used. The rationale is that intra-abdominal drainage enhances early detection of complications (gastro-intestinal leakage, hemorrhage, bile leak), prevents collection of fluid or pus, reduces morbidity and mortality, and decreases the duration of hospital stay. However, dogmatic attitudes favoring systematic drain placement should be questioned. The aim of this review was to evaluate the evidence supporting systematic use of prophylactic abdominal drainage following gastrectomy, pancreatectomy, liver resection, and rectal resection. Based on this review of the literature: (i) there was no evidence in favor of intra-peritoneal drainage following total or sub-total gastrectomy with respect to morbidity-mortality, nor was it helpful in the diagnosis or management of leakage, however the level of evidence is low, (ii) following pancreatic resection, data are conflicting but, overall, suggest that the absence of drainage is prejudicial, and support the notion that short-term drainage is better than long-term drainage, (iii) after liver resection without hepatico-intestinal anastomosis, high level evidence supports that there is no need for abdominal drainage, and (iv) following rectal resection, data are insufficient to establish recommendations. However, results from the French multicenter randomized controlled trial GRECCAR5 (NCT01269567) should provide new evidence this coming year. Accumulating data support that systematic drainage of the abdominal cavity in digestive surgery is a non-beneficial and obsolete practice, except following pancreatectomy where the consensus appears to indicate the usefulness of short-term drainage. While the level of evidence is high for liver resections, new randomized controlled trials are awaited regarding gastric, pancreatic and rectal surgery

On Four-Point Functions of Half-BPS Operators in General Dimensions

We study four-point correlation functions of half-BPS operators of arbitrary weight for all dimensions d=3,4,5,6 where superconformal theories exist. Using harmonic superspace techniques, we derive the superconformal Ward identities for these correlators and present them in a universal form. We then solve these identities, employing Jack polynomial expansions. We show that the general solution is parameterized by a set of arbitrary two-variable functions, with the exception of the case d=4, where in addition functions of a single variable appear. We also discuss the operator product expansion using recent results on conformal partial wave amplitudes in arbitrary dimension.Comment: The discussion of the case d=6 expanded; references added/correcte

Valve-sparing aortic root replacement using the reimplantation (David) technique:a systematic review and meta-analysis on survival and clinical outcome

BACKGROUND: Current guidelines recommend valve-sparing aortic root replacement (VSRR) procedures over valve replacement for the treatment of root aneurysm. The reimplantation technique seems to be the most widely used valve-sparing technique, with excellent outcomes in mostly single-center studies. The aim of this systematic review and meta-analysis is to present a comprehensive overview of clinical outcomes after VSRR with the reimplantation technique, and potential differences for bicuspid aortic valve (BAV) phenotype. METHODS: We conducted a systematic literature search of papers reporting outcomes after VSRR that were published since 2010. Studies solely reporting on acute aortic syndromes or congenital patients were excluded. Baseline characteristics were summarized using sample size weighting. Late outcomes were pooled using inverse variance weighting. Pooled Kaplan-Meier (KM) curves for time-to-event outcomes were generated. Further, a microsimulation model was developed to estimate life expectancy and risks of valve-related morbidity after surgery. RESULTS: Forty-four studies, with 7,878 patients, matched the inclusion criteria and were included for analysis. Mean age at operation was 50 years and almost 80% of patients were male. Pooled early mortality was 1.6% and the most common perioperative complication was chest re-exploration for bleeding (5.4%). Mean follow-up was 4.8±2.8 years. Linearized occurrence rates for aortic valve (AV) related complications such as endocarditis and stroke were below 0.3% patient-year. Overall survival was 99% and 89% at 1- and 10-year respectively. Freedom from reoperation was 99% and 91% after 1 and 10 years, respectively, with no difference between tricuspid and BAVs. CONCLUSIONS: This systematic review and meta-analysis shows excellent short- and long-term results of valve-sparing root replacement with the reimplantation technique in terms of survival, freedom from reoperation, and valve related complications with no difference between tricuspid and BAVs