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1.
This paper presents a new analytical criterion for brittle failure of rocks and heavily over-consolidated soils. Griffith’s model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith’s criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as half of the critical distance. This fracture criterion is known as the point method, and is part of the theory of critical distances, which is utilised in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, σ 0, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, σ c and σ t. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils (σ c/σ t = 3–50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low-confining stresses. 相似文献
2.
In coastal populations of invertebrates and fishes, the distribution of discrete subpopulations is influenced by adult and larval dispersal, as well as by the effects of habitat heterogeneity on site fidelity or connectivity. Here, we examine evidence for spatial structure of sea perch, Helicolenus percoides, populations among four fjords in the Fiordland region of southwestern New Zealand. We examine patterns in adult morphology, length-at-age, δ13C and δ15N of muscle tissue, and trace elemental composition of whole otoliths as proxies for population isolation among the four inner fjord regions. A multivariate analysis of morphometrics reveals significant differences among populations from each of the four sites, suggesting existence of four distinct subpopulations. These patterns are consistent with observed differences in δ13C and δ15N, and length-at-age estimates among the four subpopulations. Differences in whole otolith concentrations of Sr, Ba, Mg and Li, and high classification scores based on the whole otolith elemental fingerprint are also consistent with significant subdivision among areas. Patterns across all four markers are consistent with discrete subpopulation structure of adult sea perch among the four study sites. These data indicate that the newly implemented network of marine protected areas in Fiordland is likely to contain discrete populations of sea perch. 相似文献
3.
The Thukela Bank, KwaZulu-Natal, supports a diverse ecosystem and South Africa’s only prawn fishery. Oceanographic studies suggest riverine input is not important for the biology of this system, whereas biological studies suggest the contrary, with prawn catches increasing with increased fluvial run-off. The aim of this study was to determine (i) the importance of riverine and marine organic matter for the Thukela Bank food web; and (ii) whether there are seasonal changes in the Thukela River stable isotope values, and, if so, whether these are reflected in the isotope values of demersal organisms. Estuarine organic matter, sediments and demersal organisms were collected from several sites across the bank in the wet and dry seasons of 2008, 2009 and 2010. Marine particulate organic matter was also collected in 2010 and analysed for δ13C and δ15N, as well as C/N ratios. There were strong seasonal changes in isotopic values of organic matter and fauna, especially faunal δ13C. There was an apparent time-lag in organisms assimilating riverine organic matter isotopic values, with the isotopic signature of demersal organisms reflecting that of riverine organic matter from the previous season, which is likely the result of tissue turnover time. In 2010, Thukela Bank sediment organic matter was of riverine origin and this maintained the demersal food web. We conclude that Thukela River organic matter is an important input to the food web of the Thukela Bank, indicating that any future damming of the catchment area could have serious consequences for this ecosystem. 相似文献
4.
This article deals with the morpho- and lithodynamic role of ice in shaping the Arctic coasts. Taking the Pechora Sea as an example, the direct and indirect impact of ice on the sea floor and shores is described. Active gouging by ice floes, ice-rafting of sedimentary material, formation of strudel scour depressions and frozen rocks on shallows are all direct impacts. The indirect impact of ice that arrests the coastal zone for most of the year is a considerable reduction of the overall energy of waves and currents. Based on the performed analysis, we plot a schematic zonation of the Pechora Sea according to the probable impact of ice on the shores and seafloor. 相似文献
5.
The diet of hoki was determined from examination of stomach contents of 1992 fish of 26–112 cm total length (TL) sampled at depths of 209–904m on Chatham Rise, New Zealand, from summer research trawl surveys and seasonal commercial fishing trawls, during 2004–2008. Prey was predominantly euphausiids, mesopelagic fishes and natant decapods. Multivariate analyses using distance-based linear models, non-parametric multi-dimensional scaling and similarity percentages indicated that the best predictors of diet variability were the position of the fish in relation to the subtropical front (STF), fish size and longitude. Pasiphaeids were more important to the north of the STF, and sternoptychid fishes and euphausiids more important in the STF convergence area. Euphausiids and sternoptychid fishes were important for smaller hoki (26–55 cm TL), myctophid fishes and natant decapods for larger hoki, and macrourids for the largest hoki (>84 cm TL). The longitudinal effect was characterised by pasiphaeids, euphausiids and sternoptychids to the west, and myctophids in the centre of Chatham Rise. Feeding activity was analysed using generalised additive models, and was found to vary with time of day, sample source (research or commercial), longitude, hoki size and depth. The variability in diet suggested hoki forage opportunistically within their preferred habitat and biological limits. 相似文献
6.
Cianelli D Manfra L Zambianchi E Maggi C Cappiello A Famiglini G Mannozzi M Cicero AM 《Marine environmental research》2008,65(4):325-337
Produced formation waters (PFWs), a by-product of both oil and gas extraction, are separated from hydrocarbons onboard oil platforms and then discharged into the sea through submarine outfalls. The dispersion of PFWs into the environment may have a potential impact on marine ecosystems. We reproduce the initial PFW-seawater mixing process by means of the UM3 model applied to offshore natural gas platforms currently active in the Northern Adriatic Sea (Mediterranean Sea). Chemical analyses lead to the identification of a chemical tracer (diethylene glycol) which enables us to follow the fate of PFWs into receiving waters. The numerical simulations are realized in different seasonal conditions using both measured oceanographic data and tracer concentrations. The numerical results show the spatial and temporal plume development in different stratification and ambient current conditions. The analytical approach measures concentrations of the diethylene glycol at a maximum sampling distance of 25 m. The results show a good agreement between field observations and model predictions in the near-field area. The integration of numerical results with chemical analyses also provides new insight to plan and optimize PFW monitoring and discharge. 相似文献
7.
The rough pen shell Pinna rudis Linnaeus, 1758 (family Pinnidae), a mollusc with an Atlantic–Mediterranean distribution, is able to live in coarse sandy substrates. Considering its shell structure and ecological characteristics, P. rudis can enhance biodiversity by providing a substrate for settlement on its shell. For this reason, we compared the diversity of benthic taxa around P. rudis shells with the species diversity on P. rudis shells, at Matiota Beach, São Vicente Island, Cabo Verde. We sampled an area of 900 m2 and recorded data in situ to estimate the population size of P. rudis and the epibiotic and benthic community diversity. The average density of P. rudis estimated in the sampled area was 6.6 ind. 100 m–2 and the highest density was found at between 2 and 3 m depth, mostly in biogenic and sandy substrates. The epibiotic species diversity on P. rudis shells was significantly higher than the species diversity in the microhabitat around the shells. The P. rudis shell seems to play an important role in increasing the biodiversity of the ecosystem, with some species found only as epibionts on P. rudis. 相似文献
8.
Marco Barbera Fabio Gastaldello Luisa Sciortino Simonpietro Agnello Gianpiero Buscarino Alfonso Collura Nicola La Palombara Ugo Lo Cicero Nicola Sartore Andrea Tiengo Salvatore Varisco Anna Maria Venezia 《Experimental Astronomy》2016,42(2):179-197
After more than 15 years of operation of the EPIC camera on board the XMM-Newton X-ray observatory, we have reviewed the status of its Thin and Medium filters. We have selected a set of Thin and Medium back-up filters among those still available in the EPIC consortium and have started a program to investigate their status by different laboratory measurements including: UV/VIS transmission, Raman scattering, X-Ray Photoelectron Spectroscopy, and Atomic Force Microscopy. Furthermore, we have investigated the status of the EPIC flight filters by performing an analysis of the optical loading in the PN offset maps to gauge variations in the optical and UV transmission. We both investigated repeated observations of single optically bright targets and performed a statistical analysis of the extent of loading versus visual magnitude at different epochs. We report the results of the measurements conducted up to now. Most notably, we find no evidence for change in the UV/VIS transmission of the back-up filters in ground tests spanning a 2 year period and we find no evidence for change in the optical transmission of the thin filter of the EPIC-pn camera from 2002 to 2012. We point out some lessons learned for the development and calibration programs of filters for X-ray detectors in future Astronomy missions. 相似文献
9.
Reproduction by sexual or asexual viviparity is a common phenomenon in some anemone species. In this short communication, the origin of the brooded young of Actinia equina and A. schmidti from the Portuguese shore was investigated. DNA was extracted from 56 brooding adult Actinia sp. and the nuclear gene that codes for the 28S ribosomal subunit was sequenced. Species identity was then assessed using GenBank. In total, 50 individuals were A. schmidti, five were A. equina and one had a hybrid origin. Three adult anemones (the hybrid, one A. equina and one A. schmidti) possessed two different 28S sequences and so their offspring was selected for further analysis using the same molecular procedure. Each brooded polyp was found to possess the exact same sequence as its parent, strongly suggesting the asexual origin of broods in A. equina and A. schmidti. 相似文献
10.