Effects of crack tip geometry on dislocation emission and cleavage: A possible path to enhanced ductility

We present a systematic study of the effect of crack blunting on subsequent crack propagation and dislocation emission. We show that the stress intensity factor required to propagate the crack is increased as the crack is blunted by up to thirteen atomic layers, but only by a relatively modest amount for a crack with a sharp 60$^\circ$ corner. The effect of the blunting is far less than would be expected from a smoothly blunted crack; the sharp corners preserve the stress concentration, reducing the effect of the blunting. However, for some material parameters blunting changes the preferred deformation mode from brittle cleavage to dislocation emission. In such materials, the absorption of preexisting dislocations by the crack tip can cause the crack tip to be locally arrested, causing a significant increase in the microscopic toughness of the crack tip. Continuum plasticity models have shown that even a moderate increase in the microscopic toughness can lead to an increase in the macroscopic fracture toughness of the material by several orders of magnitude. We thus propose an atomic-scale mechanism at the crack tip, that ultimately may lead to a high fracture toughness in some materials where a sharp crack would seem to be able to propagate in a brittle manner. Results for blunt cracks loaded in mode II are also presented.Comment: 12 pages, REVTeX using epsfig.sty. 13 PostScript figures. Final version to appear in Phys. Rev. B. Main changes: Discussion slightly shortened, one figure remove

EMISSION, ABSORPTION AND QUANTUM YIELDS IN THE C2N2A1Σ−←1Σ+C_{2}N_{2} A^{1}\Sigma^{-} \leftarrow {^{1}}\Sigma^{+} TRANSITION

Author Institution: Department of Chemistry, Howard University WashingtonWe have measured the absorption, emission end photodissociation spectra of cyanogen in the 220 and 210 nm regions near the $4_{O}{^{1}}$ and $1_{O}{^{1}}4_{O}{^{1}}$ bands of the $A^{1}\Sigma^{-} \leftarrow x^{1}\Sigma^{+}$ system. The spectra were excited with a frequency doubled Nd-YAG pumped dye laser. The doubled light was Reman shifted in high pressures of hydrogen gas. Emission spectra were measured by an unfiltered VUV photomultiplier. The emission spectra did not show all lines that appeared in the absorption spectra and almost all of the intensity was in the $1_{O}{^{n}}4_{O}{^{1}}$ progression. The radiative lifetime of the emission was 660 ns. We used laser induced fluorescence to measure the relative yield of CN radicals as a function of photolysis wavelength. Above the dissociation threshold the yield generally followed the absorption spectrum. A few nascent quantum state distributions of CN were measured. Energy in the CN fragments appeared to be statistically distributed. These results can be understood if some of the excited $C_{2}N_{2}$ is crossing over into a non-dissociating triplet manifold. We are attempting to establish quantum yields for dissociation and emission. If possible fluorescence spectra will also be measured. Dispersed fluorescence spectra will identify the emitting state

THE FLUORESCENCE SPECTRA OF C2N2C_{2}N_{2}

Author Institution: Department of Chemistry, Howard University; Department of Chemistry, Ballistic Research Laboratory; Department of Chemistry, Howard UniversityDispersed fluorescence spectra have been measured following excitation of all of the non-dissociative levels of the $A^{1}\Sigma^{-}_u$ levels of ethanedinitrile $C_{2}N_{2}$. The spectra are characterized by long progressions in the $v_{1}$ and $v_{4}$ modes. The longest wavelength fluorescence terminates on ground state vibrational levels more than $14,000 cm^{-1}$ above the origin. In addition the radiative lifetimes have an inverse dependence on the amount of excitation in the $v_{4}$ mode, which may indicate a crossing to high lying levels of the $x^{1}\Sigma^{+}_g$ electronic state

SULF1/SULF2 splice variants differentially regulate pancreatic tumour growth progression.

This study highlights the highly dynamic nature of SULF1/SULF2 splice variants in different human pancreatic cancers that regulate the activities of multiple cell signalling pathways in development and disease. Most pancreatic tumours expressed variable levels of both SULF1 and SULF2 variants including some expression during inflammation and pancreatitis. Many ductal and centro-acinar cell-derived pancreatic tumours are known to evolve into lethal pancreatic ductal adenocarcinomas but the present study also detected different stages of such tumour progression in the same tissue biopsies of not only acinar cell origin but also islet cell-derived cancers. The examination of caerulein-induced pancreatic injury and tumorigenesis in a Kras-driven mouse model confirmed the activation and gradual increase of SULF1/SULF2 variants during pancreatitis and tumorigenesis but with reduced levels in Stat3 conditional knockout mice with reduced inflammation. The significance of differential spatial and temporal patterns of specific SULF1/SULF2 splice variant expression during cancer growth became further apparent from their differential stimulatory or inhibitory effects on growth factor activities, tumour growth and angiogenesis not only during in vitro but also in vivo growth thus providing possible novel therapeutic targets

Analysis of whole-genome microarray replicates using mixed models

Motivation: Microarray experiments are inherently noisy. Replication is the key to estimating realistic fold-changes despite such noise. In the analysis of the various sources of noise the dependency structure of the replication needs to be taken into account

Closing Barts: community and resistance in contemporary UK hospital policy

Debates concerning the nature and extent of hospital provision in London, England are longstanding. Reviews in the 1990s have focused on a perceived over-provision and recommended rationalisation. This paper explores the representations of place which emerged in the discourses surrounding the possible closure of St Bartholomew's Hospital (Barts), London. Through a discourse analysis of official and unofficial reports, Parliamentary debates, press releases, campaign material and coverage in the London Evening Standard and other newspapers, we assess resistance to closure and the construction of communities dedicated to the retention of Barts. Four different representations of Bart's are identified: as community resource, as a site of expertise, as a heritage symbol and as a site pertinent to the identities of Londoners. The effectiveness of these different strategies is considered and their positioning and use within the 'Campaign for Barts' is evaluated. We conclude that, notwithstanding the potential to present the (possibly temporary) retention of Barts as a recognition of its status as a locus of particular medical expertise, the potency of this health care facility as a symbol both of London and of medical tradition was the crucial factor in its reprieve