Search

Ascidians are rapidly emerging biofouling organisms that generally occur on the natural and artificial substratum of the marine environment. However, very few studies have been reported on their species diversity and distribution patterns. This present investigation focuses on the seasonal impact on ascidians diversity. The distributions of ascidians were recorded by the collection of samples in six sampling stations from Andaman and Nicobar Islands. A total of 43 species of ascidians (32 colonial, 11 solitary) were recorded and most of the species are under the Didemnidae family. Various diversity indexes were utilized to examine the diversity, richness and evenness of species and to look at the assorted variety among tests and their likeness regarding species composition. The greatest (3.227) individuals were observed during summer season in Pigeon Island and the lowest (2.063) number of species were recorded during the monsoon season in Haddowharf harbour. The Margalef richness (d) fluctuated between in the range of 2.033 and 5.53 during the monsoon in Haddowharf harbour and Carbyns Cove beach in summer seasons. The Pielou's evenness record (J′) changed from 0.7409 to 0.9713. Diversity indexes were recorded as followed the order of 0.9642, 0.9552, 0.9482 and 0.9292 at Carbyns Cove shoreline, Burmanella, Pigeon Island and Campbell Bay individually during summer season. Present findings in the cluster examination uncovered that the species at various stations have framed four gatherings. Most elevated species rates were spread in summer. Field observation shows that species distribution is able to tolerate temperature ranges of 26.9–30.3°C and salinity of 32.1–34.64 ppt. Occasional examples of space occupation by colonial ascidians corresponded with seasonal changes in seawater temperature and salinity. [ABSTRACT FROM AUTHOR]

Copyright of Brazilian Journal of Biology is the property of Instituto Internacional de Ecologia and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract Acacia mangium (Willd., 1806) (Fabales: Fabaceae) is a fast growing, rustic, pioneer species, with potential to fix nitrogen, and for programs to recover degraded areas. The objective was to evaluate the distribution and the functional diversity of interactions and the K-dominance of arthropod groups on A. mangium saplings. The number of individuals of eleven species of phytophagous insects, three bee species, and fourteen natural enemy species were highest on the adaxial leaf surface of this plant. Abundance, diversity and species richness of phytophagous insects and natural enemies, and abundance and species richness of pollinators were highest on the adaxial A. mangium leaf surface. The distribution of five species of sap-sucking hemipterans and six of protocooperating ants (Hymenoptera), with positive interaction between these groups, and three bee species (Hymenoptera) were aggregated on leaves of A. mangium saplings. Aethalion reticulatum (L.) (Hemiptera: Aethalionidae) and Bemisia sp. (Hemiptera: Aleyrodidae); Brachymyrmex sp. and Camponotus sp. (Hymenoptera: Formicidae); and Trigona spinipes Fabricius (Hymenoptera: Apidae) were the most dominant phytophagous insects, natural enemies, and pollinators, respectively, on A. mangium leaves. Knowledge of preferred leaf surfaces could help integrated pest management programs.

Abundance–Biomass Comparison (ABC) models, first described for marine benthic macrofauna, have been applied successfully to other marine and terrestrial/freshwater fauna but never to ecotonal communities. In particular, to our knowledge, ABC models have not been applied to hyporheic communities. This study represents the first application of ABC models to hyporheic assemblages. We aimed at testing the effectiveness of ABC models in describing the perturbation of hyporheic communities subjected to an existing/known disturbance. To this end, we applied the models to the hyporheic community of an Apennine creek, where the hyporheic waters of the upstream stretch were uncontaminated, whereas those of the downstream stretch were contaminated by ammonium. We also tested separated models for the summer and winter periods to account for potential variability due to season. ABC models provided a satisfactory description of the hyporheic community changes due to ammonium by showing the abundance dominance curve overlying that of the biomass in the downstream stretch contrarily to what was observed in the upstream stretch. However, ABC models did not highlight any significant seasonal effects. Our results showed that ABC models have the potential to be used as assessment tools for ecological quality of hyporheic zones in temperate regions.

In the past two decades, crack-tip mechanics has been studied increasingly using full-field techniques. Within these techniques, Digital Image Correlation (DIC) has been most widely used due to its many advantages, to extract important crack-tip information, including Stress Intensity Factor (SIF), crack opening displacement, J-integral, T-stress, closure level, plastic zone size, etc. However, little information is given in the literature about the experimental setup that provides best estimations for the different parameters. The current work aims at understanding how the experimental conditions used in DIC influence the crack-tip information extracted experimentally. The influence of parameters such as magnification factor, the position of the images with respect the crack-tip and size of the subset used in the correlation is studied. The influence is studied in terms of SIF by using Williams’ model. In this regard, cyclic loading on a fatigue crack in a compact tension (CT) specimen, made of aluminium 2024-T351 alloy, has been applied and the surface deformation around the crack-tip has been examined. The comparison between nominal and experimental values of K I showed that the effect of subset size on the measured K I is negligible compared to the effect of the field of view and the position of the area of interest. [ABSTRACT FROM AUTHOR]

Abstract: Fracture of solid materials is a complex multiscale process that essentially occurs due to macroscopic propagation of cracks formed by hierarchical evolution of atomic scale breaking of interatomic bonds. Because it depends only on the initial crack size and the applied stress, the critical stress intensity factor (Kc ) is commonly used as the single parameter to predict whether a mechanically stressed brittle material will fracture by crack propagation or not. In this study, we have shown that when a crack is highly constrained in a nanoscale brittle solid, Kc becomes gage-length dependent, hence, it may not be used as the single parameter for fracture prediction. Classical molecular dynamics simulations have been used to conduct fracture simulations of center-cracked brittle single crystal sodium chloride nanosolids. Infinitely rigid end tabs were modeled to generate boundary constraints on the specimen. The degree of constraints was varied by varying the gage length between the tabs. Our study reveals that for a given crack length, the fracture strain increases as the gage length decreases. Such gage-length dependency of failure strains suggests that the critical stress intensity factor, a measure of material fracture toughness, can be affected by boundary constraints. We have also measured the K-dominance zone size for different degree of constraints and found the K-dominance zone decreases as gage length decreases. It is thus inferred that Kc alone cannot fully capture the fracture force. [Copyright &y& Elsevier]

K-dominance of static crack tip in functionally gradient materials (FGMs) with a crack oriented along the direction of the elastic gradient is studied through coherent gradient sensing (CGS), digital speckle correlation method (DSCM) and finite element method (FEM). In the direction of crack propagation, the shear modulus has a linear variation with constant mass density and Poisson's ratio. First, the CGS and DSCM governing equations related to the measurements and the elastic solutions at mode I crack in FGMs are obtained in terms of the stress intensity factor, material constants and graded index. Secondly, two kinds of FGMs specimens and one homogenous specimen are prepared to observe the influences of the property variation on the K-dominance. Then, CGS and DSCM experiments using three-point-bending of FGMs and homogenous beams are performed. Thirdly, based on the results of the experiments, the stress intensity factors of three kinds of specimens are calculated by CGS and DSCM. Meanwhile, the stress intensity factors are obtained by FEM. Finally, comparing the results from CGS, DSCM and FEM, the K-dominance of mode-I static crack tip in FGMs is discussed in detail. [ABSTRACT FROM AUTHOR]

Abstract: This work describes the effect of material gradation (parallel to the crack plane) on stress intensity factors and K-dominance, i.e. the dominance of the singular region, of fracture specimens; SE(T), SE(B) and C(T). The extent of K-dominance is investigated by comparing the actual stress field with the Williams’ asymptotic stress field. Linear-elastic finite element analyses are performed using graded elements which incorporate graded material properties at the element level. Material gradation and crack geometry are systematically varied to perform the parametric study. Results reveal that the effect of material gradation on K I is most pronounced when a short crack is located on the stiffer side of the fracture specimen. For a given specimen and crack geometry, the extent of K-dominance yields a curve with a peak point at a certain material gradation. Results of the present study provide valuable insight into the K-dominance of FGMs. [Copyright &y& Elsevier]