Search

The focus of this study, using results obtained with hollow specimens of the alloy Al 2024 T3 (AlCuMg2 T3) with a one‐sided bore, was to determine if, under multiaxial loading, the fatigue strength in a notch with a uniaxial stress state can simply be assessed without the application of any multiaxial strength hypothesis. In the fatigue critical location, the bore edge, only a superimposition of normal stress components, resulting from axial loading or/and torsion, occurs. In the case of out‐of‐phase loading, the resulting local uniaxial stress is even lower than the stress under in‐phase loading and leads to an increase of fatigue life to crack initiation under the non‐proportional loading, which is not observed in the case of multiaxial stressing/straining of the material itself. As the local stress state in the crack initiation site is uniaxial, the application of a multiaxial strength hypothesis, for the failure criterion of crack initiation (l=0.2 mm), is not necessary. Furthermore, the benefit of non‐homogenous stress distributions, which allow locally higher stresses than the levels given by the Woehler‐line for unnotched specimens, was underlined by considering highly‐stressed, material‐volume‐related support factors. [ABSTRACT FROM AUTHOR]

The influence of various combinations of residual stress, composition and grain interaction gradients in polycrystalline materials with cubic symmetry on energy‐dispersive X‐ray stress analysis is theoretically investigated. For the evaluation of the simulated sin2ψ distributions, two different strategies are compared with regard to their suitability for separating the individual gradients. It is shown that the separation of depth gradients of the strain‐free lattice parameter a0(z) from residual stress gradients σ(z) is only possible if the data analysis is carried out in section planes parallel to the surface. The impact of a surface layer z* that is characterized by a direction‐dependent grain interaction model in contrast to the volume of the material is quantified by comparing a ferritic and an austenitic steel, which feature different elastic anisotropy. It is shown to be of minor influence on the resulting residual stress depth profiles if the data evaluation is restricted to reflections hkl with orientation factors Γhkl close to the model‐independent orientation Γ*. Finally, a method is proposed that allows the thickness of the anisotropic surface layer z* to be estimated on the basis of an optimization procedure. [ABSTRACT FROM AUTHOR]

The way strong environmental gradients shape multispecific assemblages has allowed us to examine a suite of ecological and evolutionary hypotheses about structure, regulation and community responses to fluctuating environments. But whether the highly diverse co-occurring microorganisms are shaped in similar ways as macroscopic organisms across the same gradients has yet to be addressed in most ecosystems. Here, we characterize intertidal biofilm bacteria communities, comparing zonation at both the "species" and community levels, as well as network attributes, with co-occurring macroalgae and invertebrates in the same rocky shore system. The results revealed that the desiccation gradient has a more significant impact on smaller communities, while both desiccation and submersion gradients (surge) affect the larger, macroscopic communities. At the community level, we also confirmed the existence of distinct communities within each intertidal zone for microorganisms, similar to what has been previously described for macroorganisms. But our results indicated that dominant microbial organisms along the same environmental gradient exhibited less differentiation across tidal levels than their macroscopic counterparts. However, despite the substantial differences in richness, size and attributes of co-occurrence networks, both macro- and micro-communities respond to stress gradients, leading to the formation of similar zonation patterns in the intertidal rocky shore. [ABSTRACT FROM AUTHOR]

This review paper covers a topic of significant importance in micro- and nano-systems development and manufacturing, specifically the residual stresses in deposited thin-film material layers and methods to control or mitigate their impact on device behavior. A residual stress is defined as the presence of a state of stress in a thin-film material layer without any externally applied forces wherein the residual stress can be compressive or tensile. While many material properties of deposited thin-film layers are dependent on the specific processing conditions, the residual stress often exhibits the most variability. It is not uncommon for residual stresses in deposited thin-film layers to vary over extremely large ranges of values (100% percent or more) and even exhibit changes in the sign of the stress state. Residual stresses in deposited layers are known to be highly dependent on a number of factors including: processing conditions used during the deposition; type of material system (thin-films and substrate materials); and other processing steps performed after the thin-film layer has been deposited, particularly those involving exposure to elevated temperatures. The origins of residual stress can involve a number of complex and interrelated factors. As a consequence, there is still no generally applicable theory to predict residual stresses in thin-films. Hence, device designers usually do not have sufficient information about the residual stresses values when they perform the device design. Obviously, this is a far less than ideal situation. The impact of this is micro- and nano-systems device development takes longer, is considerably more expensive, and presents higher risk levels. The outline of this paper is as follows: a discussion of the origins of residual stresses in deposited thin-film layers is given, followed by an example demonstrating the impact on device behavior. This is followed by a review of thin-film deposition methods outlining the process parameters known to affect the resultant residual stress in the deposited layers. Then, a review of the reported methods used to measure residual stresses in thin-films are described. A review of some of the literature to illustrate the level of variations in residual stresses depending on processing conditions is then provided. Methods which can be used to control the stresses and mitigate the impact of residual stresses in micro- and nano-systems device design and fabrication are then covered, followed by some recent development of interest. [ABSTRACT FROM AUTHOR]

In this study, the failure mechanisms of Cu-Cu joints under thermal cycling were investigated. Two structures of dielectrics (PBO/underfill/PBO and SiO2) were employed to seal the joints. Stress gradients induced in the joints with the different dielectrics were simulated using a finite element method (FEM) and correlated with experimental observations. We found that interfacial voids were forced to move in the direction from high stress regions to low stress ones. The locations of migrated voids varied with the dielectric structures. Under thermal cycling, such voids were likely to move forward to the regions with a small stress change. They relocated and merged with their neighboring voids to lower the interfacial energy. [ABSTRACT FROM AUTHOR]

The objective of this experiment is to investigate the behaviour of lightweight aggregate concrete (LWAC) under compression and with stress gradients. Experimental program contained three sets of LWAC which were used for production of 21 prisms. Lightweight aggregate argillite slate, called Stalite, from North Carolina had been used. The sets differed in using dry (0.10% moisture content) or saturated (7.9% moisture content) aggregate. The third set included a small amount of polyvinyl alcohol fibres (PVA). The geometry of the prisms were 100 × 140 × 480 mm (width × length × height). Prismatic samples were loaded centrically and eccentrically in compression.

This review paper covers a topic of significant importance in micro- and nano-systems development and manufacturing, specifically the residual stresses in deposited thin-film material layers and methods to control or mitigate their impact on device behavior. A residual stress is defined as the presence of a state of stress in a thin-film material layer without any externally applied forces wherein the residual stress can be compressive or tensile. While many material properties of deposited thin-film layers are dependent on the specific processing conditions, the residual stress often exhibits the most variability. It is not uncommon for residual stresses in deposited thin-film layers to vary over extremely large ranges of values (100% percent or more) and even exhibit changes in the sign of the stress state. Residual stresses in deposited layers are known to be highly dependent on a number of factors including: processing conditions used during the deposition; type of material system (thin-films and substrate materials); and other processing steps performed after the thin-film layer has been deposited, particularly those involving exposure to elevated temperatures. The origins of residual stress can involve a number of complex and interrelated factors. As a consequence, there is still no generally applicable theory to predict residual stresses in thin-films. Hence, device designers usually do not have sufficient information about the residual stresses values when they perform the device design. Obviously, this is a far less than ideal situation. The impact of this is micro- and nano-systems device development takes longer, is considerably more expensive, and presents higher risk levels. The outline of this paper is as follows: a discussion of the origins of residual stresses in deposited thin-film layers is given, followed by an example demonstrating the impact on device behavior. This is followed by a review of thin-film deposition methods outlining the process parameters known to affect the resultant residual stress in the deposited layers. Then, a review of the reported methods used to measure residual stresses in thin-films are described. A review of some of the literature to illustrate the level of variations in residual stresses depending on processing conditions is then provided. Methods which can be used to control the stresses and mitigate the impact of residual stresses in micro- and nano-systems device design and fabrication are then covered, followed by some recent development of interest.

The size effect on the fatigue performance of 18CrNiMo7‐6 alloy steel is investigated. From rotating bending fatigue tests at room temperature, the relationship between the stress amplitude and the number of cycles of specimens with different diameters is obtained. The surface topography, residual stress, and fracture surface of the specimen are measured. The results show that the fatigue strength increases with increasing diameter of the specimen. The fracture of the small specimens tends to be brittle fracture, whereas the fracture of the large specimens is inclined to be ductile fracture. The effects of the surface condition, relative stress gradient (RSG), residual stress, and fatigue crack propagation life on the fatigue performance of the alloy are discussed. The reasons for the size effect are interpreted in terms of the change in the RSG caused by residual stress, the trend of the RSG significantly changes with changing specimen size under different stress levels. The crack propagation life of the large specimens is longer than those of the small specimens. [ABSTRACT FROM AUTHOR]

Stress gradients influence many ecosystem processes and properties, including ecosystem recovery from and resistance to disturbance. While recent analytical approaches have advanced multivariate metrics of ecosystem resilience that allow quantification of conceptual resilience models and identification of thresholds of state change, these approaches are not often translated to landscape scales.Using natural and restored salt marshes in Louisiana, USA, we quantified plant community recovery and resistance metrics along flooding stress gradients. n‐dimensional hypervolumes of plant community biomass and structure were simulated using field data collected from disturbance‐recovery experiments. The relationships between multivariate resilience metrics and flooding stress gradients were then mapped at community‐ and landscape‐relevant scales by scaling with airborne‐derived data across the region.Greater pre‐disturbance abiotic stress decreased live below‐ground, but not above‐ground, biomass, and ultimately led to lower post‐disturbance total recovery, recovery rates and resistance of plant communities. Vegetated plots flooded >52% of the time transitioned to an alternative, unvegetated state after disturbance. Mapping revealed differences in spatial patterns of resilience—highlighting low, interior marsh edges as especially vulnerable to the combination of chronic flooding stress and acute disturbance. At the landscape scale, approximately half of the area (48%) is vulnerable to state change after pulse disturbances.Synthesis. Ultimately, we quantify the ball‐and‐cup conceptual model for a salt marsh ecosystem and its alternative state, mudflat. We find that increasing abiotic stress due to climate change diminishes ecosystem resilience, but the interaction with common episodic disturbances is necessary to reveal transitions to alternative states and quantify state change thresholds. Quantifying and mapping resilience and where alternative states may exist in this fashion improves ecologists' ability to investigate the mechanisms of stress gradient control on emergent ecosystem properties while providing spatially explicit resources for managing ecosystems according to their projected resilience. [ABSTRACT FROM AUTHOR]

A novel statistical approach to model size effects in very high cycle fatigue (VHCF) is proposed in the paper. The statistical distributions of the VHCF life and of the size of the defect leading to VHCF failure (critical defect) are identified in the paper through the weakest‐link principle. The statistical distributions are able to account for the stress gradients that are present in a loaded material volume. An efficient procedure for estimating the parameters involved in the statistical distributions is also shown. The proposed statistical models are finally validated through experimental data taken from the literature. The experimental validation proves the fitting capability of the proposed models that outperform the traditional models based on the 90% risk‐volume. [ABSTRACT FROM AUTHOR]

This paper concerns the algorithm of transition piezoelectric elements for adaptive analysis of electro-mechanical systems. In addition, effectivity of the proposed elements in such an analysis is presented. The elements under consideration are assigned for joining basic elements which correspond to the mechanical models of either the first or higher order, while the electric model is of arbitrary order. In this work, three variants of the transition models are applied. The first one assures continuity of displacements between the basic models and continuity of electric potential between these models, as well. The second transition piezoelectric model guarantees additional continuity of the stress field between the basic models. The third transition model additionally enables continuous change of the strain state between the basic models. Based on the mentioned models, three types of the corresponding transition finite elements are introduced. The applied finite element approximations are h p q / h p -adaptive ones, which allows element-wise changes of the element size parameter h, and the element longitudinal and transverse orders of approximation, respectively, p and q, depending on the error level. Numerical effectiveness of the models and their approximations is investigated in the contexts of: ability to remove high stress gradients between the basic and transition models, and convergence of the numerical solutions for the model problems of piezoelectrics with and without the proposed transition elements. [ABSTRACT FROM AUTHOR]

X‐Ray diffraction microstrain characterization is a technique which enables the quantification of threading dislocations by measuring the radial microstrain field surrounding these defects. This work demonstrates how to accurately perform these measurements on a broad range of GaN samples. In particular, for GaN grown on Si substrates with large stress gradient through the layer, the measurements need to be performed on the (20–25) reflection to avoid being affected by a macrostrain influence. A comparison of this characterization with cathodoluminescence and X‐ray diffraction lattice misorientations measurements shows differences in the measurement depth, so the results are affected by the various evolutions of dislocations densities through the layers of the different samples. Independently of the measurement depth, the analysis also highlights a dependence of the X‐ray measurements results on the type of sample, in particular the starting substrate. Microstrain measurements appear therefore as being well suited for the comparison of threading dislocations densities between similar GaN layers. It can be used to replace lattice misorientations measurements, or be associated with them to obtain an edge versus screw dislocations ratio. [ABSTRACT FROM AUTHOR]

Questions: Recent studies have suggested that response patterns of species and phylogenetic diversity may differ. Here, we address the following questions: What are the most important drivers and is there a difference in the responses to environmental drivers between species and phylogenetic diversity? If so, which ecological mechanisms determine these patterns and will different habitat types host plants of different evolutionary lineages? Location: Czech Republic. Methods: We used a unique data set of 419 permanent plots in thermophilous temperate deciduous forests. Vegetation of the herbaceous layer was sampled along a wide range of environmental gradients. Soil characteristics were measured in the laboratory from field‐collected samples. Topographic variables were derived from a digital elevation model. Results: Communities of shaded habitats on moist fertile soils were characterized by phylogenetically older lineages (pteridophytes and ancient lineages of lilioids and dicots) resulting in a higher phylogenetic diversity. On the other hand, dry oligotrophic habitats were distinguished by short‐lived light‐demanding species (some asterids) and showed higher species, but relatively lower phylogenetic diversity. Besides simple effects of soil, light availability and topographic properties, interactions between the factors played an important role. Conclusions: Our results indicate that both types of biodiversity were mainly driven by environmental stress created by the interplay among factors. Patterns of phylogenetic diversity suggest that historical factors, i.e., the shifting of species' habitats at the evolutionary scale, could also provide plausible explanations. [ABSTRACT FROM AUTHOR]

It is increasingly acknowledged that stressors can resonate across the boundaries between ecosystems. Salt marshes, vast areas shaped by ocean-shore interactions, constitute prime examples of ecosystems where multiple stress factors arising from one ecosystem act on the local community of another ecosystem. Although it is generally recognized that zonation of plant communities on salt marshes is strongly affected by marine stress factors associated with frequent flooding (salinity, anoxia), it is largely unknown what the isolated and interacting effect are of these different stressors. This calls for experiments to disentangle the relative effects of these single and interacting multiple stressors. In this study, we determined the single and interacting effects of two main abiotic stress factors on salt marshes: salinity and anoxia (as a result of flooding) and one biotic stress factor: soil compaction (as a result of livestock grazing) on the growth of the twelve dominant salt marsh plant species, using a full-factorial experiment. To link the experimental work to distributions of natural plant communities along a natural stress gradient, we related our experimental results to observed plant species distributions on a salt marsh that is exposed to all these three stressors. Salinity strongly affected ten species with two high-marsh species not surviving the highest salinity levels whereas anoxia only consistently affected growth of four species. Interestingly, we observed no synergistic effect of anoxia and salinity in salt marsh plants. Moreover, we observed a trade-off between the amount of aerenchyma and mechanical strength, indicating that species vary in their resistance to soil compaction. Overall, our results suggest that salinity is a major determinant of plant species composition on the salt marsh, followed by anoxia. The importance of soil compaction depends on salt marsh elevation: on the low marsh, increased oxygen supply by aerenchyma seems to outweigh resistance against mechanical stress whereas on the anaerobe low marsh, the reverse applies. Using the experimental data to predict cover of plant species in the field, our results suggest that the combination of plant responses to the various stressors may be a powerful predictor for explaining the plant composition on the salt marsh.

Environmental gradients play a key role in shaping diversity in tropical forests. However, we have a little understanding of how evolutionary diversity is affected by gradients and the role of niche persistence in flooded forests in dry biomes. Here, we assessed the evolutionary diversity across a flooding gradient in the Caatinga Domain of South America. We established 120 plots across four tributaries of the São Francisco River, eastern Brazil, consisting of 72 plots in flooded, 24 in occasionally flooded, and 24 in unflooded forests. We computed richness, phylogenetic diversity (PD), mean nearest taxon distance (MNTD), and mean pairwise phylogenetic distance (MPD) and their standardized effect sizes (ses.PD, ses.MNTD, and ses.MPD). We found low richness, low PD, and high MNTD values in flooded forests relative to unflooded and occasionally flooded forests. MPD did not differ across the flooding gradient. The standardized effect size metrics were higher in flooded forests. Despite the unflooded and occasionally flooded forests being rich in terms of species and correlated phylogenetic structure, flooded forests showed more lineage diversity than expected by chance. We assessed whether this pattern could be driven by resprouting ability testing its phylogenetic signal. Resprouting is randomly distributed across phylogeny, but plant communities are likely assembled from random draws of the resprouters' lineage pool. Quantifying evolutionary diversity across flooding gradients in dry environments brought new insights to how the same environmental filters may lead to disparate patterns of evolutionary diversity and the role of response traits in allowing certain clades to persist in flooded habitats. [ABSTRACT FROM AUTHOR]

• FEM is developed for prismatic tapered web plates with simply supported edges. • ANN models are developed for predicting the elastic critical buckling coefficients. • The effects of tapering ratios, plate aspect ratios, and stress gradients are investigated. • A user-friendly Graphic User Interface (GUI) program is designed. Tapered steel plate girders are commonly used in large span industrial structures and composite bridges. The tapered thin steel web plates under stress gradients in such structures may be susceptible to elastic local buckling. An efficient and robust Artificial Neural Network (ANN) model is developed and presented in this paper for predicting the elastic critical buckling coefficients of prismatic tapered steel webs under stress gradients. More than 4000 samples are used in extensive parametric analyses of tapered web plates with simply supported edges. The ANN model recognizes important features associated with tapered plates, including the tapering ratio of the plate, aspect ratio, and stress gradient. A dataset containing 4665 records is developed for the training, validation, and testing of the ANN model developed. Two main ANN models are developed. The first model consists of a single hidden layer with neurons varying from 5 to 11. The second model consists of two hidden layers, with 8 neurons in the first layer and 6 neurons in the second layer. The accuracy of these models is thoroughly evaluated. It is found that the developed ANN model with two-hidden layers has superior performance than other models. The developed ANN can be employed to accurately predict the elastic critical buckling coefficients of prismatic tapered steel webs under stress gradients. [ABSTRACT FROM AUTHOR]

ABSTRACT Seagrass meadows are vital ecosystems in coastal zones worldwide, but are also under global threat. One of the major hurdles restricting the success of seagrass conservation and restoration is our limited understanding of ecological feedback mechanisms. In these ecosystems, multiple, self-reinforcing feedbacks can undermine conservation efforts by masking environmental impacts until the decline is precipitous, or alternatively they can inhibit seagrass recovery in spite of restoration efforts. However, no clear framework yet exists for identifying or dealing with feedbacks to improve the management of seagrass ecosystems. Here we review the causes and consequences of multiple feedbacks between seagrass and biotic and/or abiotic processes. We demonstrate how feedbacks have the potential to impose or reinforce regimes of either seagrass dominance or unvegetated substrate, and how the strength and importance of these feedbacks vary across environmental gradients. Although a myriad of feedbacks have now been identified, the co-occurrence and likely interaction among feedbacks has largely been overlooked to date due to difficulties in analysis and detection. Here we take a fundamental step forward by modelling the interactions among two distinct above- and belowground feedbacks to demonstrate that interacting feedbacks are likely to be important for ecosystem resilience. On this basis, we propose a five-step adaptive management plan to address feedback dynamics for effective conservation and restoration strategies. The management plan provides guidance to aid in the identification and prioritisation of likely feedbacks in different seagrass ecosystems. [ABSTRACT FROM AUTHOR]

Aim We aimed to assess the contribution of marginal habitats to the tree species richness of the Mata Atlântica (Atlantic Forest) biodiversity hotspot. In addition, we aimed to determine which environmental factors drive the occurrence and distribution of these marginal habitats. Location The whole extension of the South American Atlantic Forest Domain plus forest intrusions into the neighbouring Cerrado and Pampa Domains, which comprises rain forests ('core' habitat) and five marginal habitats, namely high elevation forests, rock outcrop dwarf-forests, riverine forests, semideciduous forests and restinga (coastal white-sand woodlands). Methods We compiled a dataset containing 366,875 occurrence records of 4,431 tree species from 1,753 site-checklists, which were a priori classified into 10 main vegetation types. We then performed ordination analyses of the species-by-site matrix to assess the floristic consistency of this classification. In order to assess the relative contribution of environmental predictors to the community turnover, we produced models using 26 climate and substrate-related variables as environmental predictors. Results Ordination diagrams supported the floristic segregation of vegetation types, with those considered as marginal habitats placed at the extremes of ordination axes. These marginal habitats are associated with the harshest extremes of five limiting factors: temperature seasonality (high elevation and subtropical riverine forests), flammability (rock outcrop dwarf-forests), high salinity ( restinga), water deficit severity (semideciduous forests) and waterlogged soils (tropical riverine forests). Importantly, 45% of all species endemic to the Atlantic Domain only occur in marginal habitats. Main conclusions Our results showed the key role of the poorly protected marginal habitats in contributing to the high species richness of the Atlantic Domain. Various types of environmental harshness operate as environmental filters determining the distribution of the Atlantic Domain habitats. Our findings also stressed the importance of fire, a previously neglected environmental factor. [ABSTRACT FROM AUTHOR]

Different X-ray measurement and data evaluation concepts are presented, which allow for residual stress analysis in thin films with pronounced gradients in chemical composition. These gradients lead to a variation in the strain-free lattice parameter a0 with respect to the film thickness and superimpose the lattice strain induced by the film's inherent stresses. Non-consideration of a0( z) gradients is shown to lead to considerable errors (`ghost stresses') in the residual stress depth profiles. With the simulated example of a TiC xN1− x film with a pronounced carbon gradient, the first part of this series introduces four approaches, which permit the separation of residual stress and composition depth distributions at different levels of approximation. They are based on lattice spacing depth profile measurements performed in either the sin2ψ mode or the scattering vector mode, or in combinations of these two scanning modes. Depending on the approach used for separating the residual stress and composition gradients, angle- or energy-dispersive diffraction has to be applied, employing monochromatic X-ray sources available in the laboratory or either white high-energy synchrotron radiation or the Bremsstrahlung of conventional X-ray tubes, respectively. The methods introduced here assume a biaxial residual stress state within the film. For a triaxial residual stress state with σ33 ≠ 0, a separation of stress and composition gradients is not straightforward, because an a0( z) gradient cannot be distinguished from the hydrostatic part of the stress tensor. [ABSTRACT FROM AUTHOR]

The different approaches for separating residual stress and composition gradients introduced in the first part of this series [] are demonstrated with the experimental example of a graded Ti(C,N) coating layer deposited by a modified high-temperature chemical vapour deposition process on a cemented carbide substrate. The coating layer features a depth gradient in the lattice parameter in the strain-free direction of the biaxial stress state, ψ*, hkl, and tensile residual stresses σ|| which are nearly uniform over the coating thickness but drop down significantly towards the free surface. On the assumption that the out-of-plane stress component σ33 can be neglected, the gradient is related to the variation in the chemical composition with depth. Therefore, the example considered here corresponds to case ( d) of possible combinations of residual stress and composition gradients discussed in the first part of this series. The comparison of the results achieved by means of the different methods reveals the importance of choosing appropriate experimental conditions that fit best to the sample to be investigated. For the case of thin-film analysis, it is shown that the X-ray information depth is the crucial parameter which should match the film thickness. [ABSTRACT FROM AUTHOR]

In alpine river networks, water source (i.e. snow, ice or groundwater) plays a major role in influencing flow regimes, benthic habitat and macro-invertebrate community structure. Across these systems, a natural stress gradient can be conceptualized, from rivers fed exclusively by meltwater (harsh habitat) to those with no melt input (relatively benign, groundwater fed). However, despite the current context of rapid glacier retreat, our understanding of linkages between meltwater contributions, physico-chemical habitat and biodiversity remains limited. To address this research gap, habitat characteristics and macro-invertebrate community structure were studied at 26 sites (five river basins) in the French Pyrénées across a meltwater gradient from 0% to 99%. A combination of generalized regression models and multivariate analyses showed that the stress gradient was associated with the following: (i) linear responses of key physico-chemical habitat variables, in particular bed stability, (ii) unimodal responses at the community level (e.g. richness and total density peaks at 40-60% meltwater contribution), and (iii) both unimodal and monotonic responses at the level of individual taxa. Sites characterized by high contributions of meltwater, although species poor, were important for beta diversity because of their specialist endemic fauna. Our findings suggest that continued glacier and snowpack retreat due to expected future climate change are likely to lead to more homogeneous alpine river habitats (i.e. reduced meltwater-groundwater stress gradient breadth). As a result, increased alpha diversity is expected as previously harsh habitats become more favourable; however, an associated decrease in beta diversity is likely as glacial stream specialists become replaced by generalists. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The whole powder pattern decomposition procedure is an established tool for the analysis of diffraction patterns with overlapping reflections. Its adapted application to data obtained in asymmetric diffraction geometry was shown to yield depth-resolved results of biaxial residual stress states, e.g. of thin film materials. Yet, with increasing coating thickness and in presence of pronounced stress gradients, this approach overestimates the magnitudes of the residual stresses present. The current work aims at overcoming this by including the variations in X-ray penetration depth encountered during the experiment into the applied refinement strategy. A proof of concept is obtained by measuring stress states in molybdenum thin films as a model material using the proposed approach in comparison to classical X-ray residual stress evaluation techniques. [ABSTRACT FROM AUTHOR]

This paper investigates notch mechanics under multiaxial fatigue loading conditions with respect to notch stress–strain estimation rules and stress gradient effects. A pseudo stress-based plasticity modeling technique, incorporating a structural yield surface concept, was applied to predict local stress distributions for a 2024-T3 aluminum alloy notched tubular specimen, as well as for two different AISI 1141 steel alloy notched shaft specimen geometries. Stress–strain predictions were generated for several nominal loading conditions and compared to nonlinear FEA solutions. Most predictions were found to be within ±15% error for loading levels relevant to typical fatigue loading applications. To evaluate stress gradient models, fatigue life predictions were performed for each specimen geometry using Neuber’s rule with fatigue notch factor, as well as several different interpretations of Theory of Critical Distances (TCD) approaches. The effect of critical distance value on life predictions was also studied. The Fatemi–Socie critical plane damage parameter was used to calculate fatigue lives. While the TCD approaches were found to provide improved multiaxial fatigue data correlation when compared to fatigue notch factor, negligible differences between the different TCD approaches were observed. Life prediction trends were found to be consistent regardless of notch geometry and material, thus suggesting some generality of the findings. [ABSTRACT FROM AUTHOR]

Multireflection grazing-incidence X-ray diffraction (MGIXD) was used to determine the stress- and strain-free lattice parameter in the surface layer of mechanically treated (polished and ground) tungsten and austenitic steel. It was shown that reliable diffraction stress analysis is possible only when an appropriate grain interaction model is applied to an anisotropic sample. Therefore, verification of the X-ray stress factors (XSFs) was accomplished by measuring relative lattice strains during an in situ tensile test. The results obtained using the MGIXD and standard methods (χ and ω geometries) show that the Reuss and free-surface grain interaction models agree with the experimental data. Moreover, a new interpretation of the MGIXD results was proposed and applied for the first time to measure the probability of stacking faults as a function of penetration depth for a polished and ground austenitic sample. The XSF models verified in the tensile test were used in the analysis of residual stress components. [ABSTRACT FROM AUTHOR]

Highlights: [•] There is large scatter in the measured fatigue failure location. [•] A gradient adjusted stress describes the fatigue failure location best. [•] The fatigue lives are generally longer for the larger notch. [•] Competing notch tests are not captured well by any of the four models. [Copyright &y& Elsevier]

Past studies have shown that the strength of top-down herbivore control on plant physiological performance, abundance and distribution patterns can shift with abiotic stress, but it is still unclear whether herbivores generally exert stronger effects on plants in stressful or in nonstressful environments., One hypothesis suggests that herbivores' effects on plant biomass and fitness should be strongest in stressful areas, because stressed plants are less able to compensate for herbivore damage. Alternatively, herbivores may reduce plant biomass and fitness more substantially in nonstressful areas, either because plant growth rates in the absence of herbivory are higher and/or because herbivores are more abundant and diverse in nonstressful areas., We test these predictions of where herbivores should exert stronger effects by measuring individual performance, population size structure and densities of a common subshrub, Hibiscus meyeri, in a large-scale herbivore exclosure experiment arrayed across an aridity gradient in East Africa., We find support for both predictions, with herbivores exerting stronger effects on individual-level performance in arid (stressful) areas, but exerting stronger effects on population size structure and abundance in mesic (nonstressful) areas. We suggest that this discrepancy arises from higher potential growth rates in mesic areas, where alleviation of herbivory leads to substantially more growth and thus large changes in population size structure. Differences in herbivore abundance do not appear to contribute to our results., Synthesis. Our work suggests that understanding the multiple facets of plant response to herbivores (e.g. both individual performance and abundance) may be necessary to predict how plant species' abundance and distribution patterns will shift in response to changing climate and herbivore numbers. [ABSTRACT FROM AUTHOR]

Abstract: Indicator species are species that readily reflect some measure of habitat characteristics and have become an increasingly appealing tool in environmental monitoring. Traditionally, habitat conditions were derived based on the presence of specific indicator species, but the absence of indicator species may be just as informative. To evaluate the importance of presence vs. absence of indicator species for characterizing habitats across environmental gradients, we evaluated the interactions of zooplankton and acid-stress in 244 boreal lakes. We adopted the statistical methods proposed by Dufrene and Legendre (1997) to identify presence and absence indicator species to characterize high, intermediate and low acid-stress lake categories. Presence indicator species (identified by the statistical analysis) for highly stressed lakes were not entirely appropriate because further evaluation identified them as ubiquitous generalists. In contrast, absence indicators for highly stressed lakes were more appropriate as these habitat specialists were specifically absent from this category of lakes. On the other hand, presence indicators for the low acid-stress category were largely habitat specialists and therefore appropriate indicators. However there were no presence or absence indicators for lakes at intermediate acid-stress level. Thus the combined use of both presence and absence indicators is recommended to characterize habitats across a stress gradient. To evaluate if the successful application of this combined approach is dependent on a stress gradient, we applied the same analyses to a sub-set of uninfluenced (non-sensitive) lakes representing three different environmental conditions. This approach identified statistically significant presence and absence indicator species for all three different reference conditions. Yet, the absence-based approach was not essential under unstressed conditions, as presence indicator species were habitat specialists for all reference categories. Finally, this study also emphasizes the importance of meaningful ecological traits of species in order to ensure the appropriateness of statistically selected indicator species. [Copyright &y& Elsevier]

Background and Aims Populations of dioecious flowering plants commonly exhibit heterogeneity in sex ratios and deviations from the equilibrium expectation of equal numbers of females and males. Yet the role of ecological and demographic factors in contributing towards biased sex ratios is currently not well understood. Methods Species-level studies from the literature were analysed to investigate ecological correlates of among-population sex-ratio variation and metapopulation models and empirical data were used to explore the influence of demography and non-equilibrium conditions on flowering sex ratios. Key Results The survey revealed significant among-population heterogeneity in sex ratios and this was related to the degree of sampling effort. For some species, sex-ratio bias was associated with the proportion of non-reproductive individuals, with greater male bias in populations with a lower proportion of individuals that were flowering. Male-biased ratios were also found at higher altitudes and latitudes, and in more xeric sites. Simulations and empirical data indicated that clonal species exhibited greater heterogeneity in sex ratios than non-clonal species as a result of their slower approach to equilibrium. The simulations also indicated the importance of interactions between reproductive mode and founder effects, with greater departures from equilibrium in clonal populations with fewer founding individuals. Conclusions The results indicate that sex-based differences in costs of reproduction and non-equilibrium conditions can each play important roles in affecting flowering sex ratios in populations of dioecious plants. [ABSTRACT FROM PUBLISHER]

Abstract: The optical method of Digital Gradient Sensing (DGS) is extended to the study of fracture mechanics and impact mechanics problems. DGS is based on the elasto-optic effect exhibited by transparent materials subjected to non-uniform state of stress causing the angular deflection of light rays propagating through the material. Under plane stress conditions, the deflections of light rays can be related to two orthogonal in-plane stress gradients. In this paper, the principle of the method is presented first, followed by crack and punch experiments on PMMA plates for quantifying stress gradients near stress risers. The crack-tip stress intensity factors under both quasi-static and dynamic loading conditions are extracted from DGS measurements and are in good agreement with analytical and finite element results. The problem of a square-punch impacting the edge of a PMMA sheet is also studied using the new method by exploiting punch-tip–crack-tip analogy. The dynamic punch-tip stress intensity factors are extracted from the optical measurements and are again in good agreement with the ones from a complementary finite element analysis of the problem. [Copyright &y& Elsevier]

mechanical analysis of the conductive keratoplasty on hyperopic eyes has been carried out, and the attention has been focused on incorporating the actual viscoelastic properties of the human corneal tissue and on the stress gradients induced by the intervention. By avoiding unnecessary complications which may obscure the essential behaviour of the model, the results are in very good agreement with the clinical and experimental findings and suggest that the major role in the commonly observed decrease of the initial degree of the refractive correction might be played by the stress gradients at the intervention spots, which are likely to influence the wound-healing. The study aims to contribute some firm mechanical roots to the predictability of the outcome of an increasingly popular technique that, notwithstanding several advantages with respect to ablative interventions, at present cannot be considered completely satisfactory. [ABSTRACT FROM AUTHOR]

Unpalatable plant species often act as biotic refuges by protecting neighboring plants from herbivores. This positive interaction can increase functional diversity in grazed ecosystems by protecting species sensitive to grazing. While many studies investigate pair-wise interactions between benefactors and beneficiaries, few show that these interactions result in community composition effects. We studied the effect of an unpalatable plant, Juncus effusus, on wetland plant communities in grazed and ungrazed plots. We tested the following predictions: (1) Juncus would increase plant functional diversity in grazed wetlands; (2) Juncus would have significant effects on community composition; and (3) the effects of Juncus on other species would change across the grazing gradient. We found that Juncus preserved functional diversity in grazed wetland communities by protecting species that decrease with grazing pressure. In multivariate analyses, grazing was the strongest driver of species composition but we found significant effects of Juncus on both vegetation change and species composition in grazed plots. Interactions with Juncus were facilitative in grazed plots and competitive in ungrazed plots, but did not vary along the grazing gradient (0.15-1.7 cows/ha). These results indicate that Juncus has significant community composition effects in grazed wetland plant communities. Understanding the effects of plant interactions such as these at the community level is essential to applying plant interactions to restoration or management. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a series of depositions of titanium nitride (TiN) films on M2 and D2 steel substrates were conducted in a Triode Magnetron Sputtering chamber. The temperature; gas flow and pressure were kept constant during each run. The substrate bias was either decreased or increased in a sequence of steps. Residual stress measurements were later conducted through the grazing X-ray diffraction method. Different incident angles were used in order to change the penetration depth and to obtain values of residual stress at different film depths. A model described by Dolle was adapted as an attempt to calculate the values of residual stress at each incident angle as a function of the value from each individual layer. Stress results indicated that the decrease in bias voltage during the deposition has produced compressive residual stress gradients through the film thickness. On the other hand, much less pronounced gradients were found in one of the films deposited with increasing bias voltage. [Copyright &y& Elsevier]

Secondary succession is impacted by both biotic and abiotic forces, but their relative importance varies due to environmental drivers. Across estuarine salinity gradients, physical stress increases with salinity, and biotic stresses are greater at lower salinities. In southern New England tidal marshes spanning a landscape-scale salinity gradient, we experimentally examined the effects of physical stress and consumer pressure by mammalian herbivores on secondary succession in artificially created bare patches. Recovery was slower in marshes exposed to full-strength seawater, where physical stress is high. Compared to full-strength salt marshes, recovery in low-salinity marshes was much faster and was influenced by small-mammal consumers. At lower salinities, small mammals selectively ate and prevented the establishment of several native and two invasive, nuisance species (Typha angustifolia and Phragmites australis) but were unable to control the expansion of established P. australis stands. By controlling the establishment of competitively dominant species and the trajectory of secondary succession in low-salinity marshes, small mammals may play a cryptic keystone role in estuarine plant communities and are a critical, overlooked consideration in the conservation and management of estuarine marshes. [ABSTRACT FROM AUTHOR]

Abstract: Dune systems are characterized by strong gradients of physical stress, with blowing sand and salt spray decreasing with distance from the ocean, and soil nutrients increasing. In this study we ask how soil microbial community composition and biomass change along transects away from the ocean, and whether these changes are regulated by abiotic stress or by resource availability. We collected bulk soil from under three plant species representative of the dune front, back, and flat: Ammophila breviligulata, Rosa rugosa, and Myrica pensylvanica. The biomass and composition of microbial communities were examined using phospholipid fatty acid (PLFA) analysis under patches of dominant vegetation, and in paired bare plots. We found that microbial biomass was strongly correlated with soil C, and thus the presence of vegetation. Community composition, on the other hand, varied with abiotic stresses, especially soil salinity. These variables in turn depended on distance from the shore, and were ameliorated in some cases by vegetation. These findings demonstrate that biomass and community composition are influenced by different environmental variables. Thus, relationships between biomass and composition are unlikely to be readily predicted on the basis of a single resource. [Copyright &y& Elsevier]

Abstract: X-ray residual stress analysis on multilayered coating systems is a quite difficult and demanding procedure. To obtain information on both, the individual sublayers the coating consists of and the interfacial substrate region, it is necessary to apply different methods which are complementary with respect to the accessible information depth. Based on the concept of an ‘equivalent thickness’ for describing angle-dispersive diffraction in multilayer structures, a method is proposed that allows for the evaluation of steep intra — as well as interlayer stress gradients within the upper sublayers of multilayer coating systems. Furthermore, energy-dispersive diffraction is shown suitable to detect the residual stress distribution in the near interface substrate zone beneath the coatings. The applicability of the approaches introduced here is demonstrated by the example of cemented carbide WC/Co cutting tools being coated by chemical vapor deposition with sequences of Al2O3/TiCN sublayers. [Copyright &y& Elsevier]

There is currently no consensus on how physical and biological factors affect competitive intensity. Tests of whether competitive intensity varies along axes of environmental change have commonly been conducted in systems with a single strong environmental gradient, such as productivity, a soil resource, or an environmental stress. Frequently, these same axes are associated with changes in species composition, yet few studies have asked whether shifts in the identity of competitors affect competitive intensity. We ask whether resources (nutrients, water), stressors (heavy metals, Ca:Mg ratio), productivity (aboveground biomass), or species identity (an ordination axis of plant community composition) were the best predictors of the intensity of competition in a heterogeneous grassland landscape that included multiple independent environmental gradients. The reproductive fitness of six annual plant species was measured in the presence and absence of competitors and used to calculate relative interaction intensity (RII). We found that RII was best predicted by community composition. Nutrient availability was also important, and a post hoc test showed that competitive intensity was best explained by the combined effects of community composition and nutrient availability. We argue that community composition may be the most effective metric for predicting competitive intensity in many ecosystems because it includes both the competitive effects of the local community and information about covarying environmental characteristics. [ABSTRACT FROM AUTHOR]

This paper reports the first direct experimental measurement of electromigration-induced stress in aluminum interconnects, using a series of microrotating stress sensors. The build-up of stress gradients in interconnect metallization, which is concomitant with backstress, has been previously investigated theoretically, but experimental verification using optical or X-ray techniques has proven more difficult. These initial results show a compressive-stress gradient along the line of stress sensors, consistent with that predicted by conventional mass-transport theory. Additionally, these measurements are in qualitative agreement with the mathematical model proposed by Korhonen et al. (1993), with a reduced-stress and increased-diffusion coefficient. These differences are discussed, and the causes postulated. The limited resolution of previous techniques restricts their ability to obtain a detailed characterization, whereas, in principle, this new technique can be scaled to the end of the International Technology Roadmap for Semiconductors. These preliminary findings suggest that the presented technique will provide a valuable tool for the investigation of back-end-of-line interconnect stress in the future. [ABSTRACT FROM PUBLISHER]

Abstract: Strips of carbon 2D braided textile composites (2DTBC) were used as test specimens to investigate the failure mechanisms associated with combined bending/compression displacement control loading. Experiments were conducted by eccentrically loading the strips to determine the types of failure mechanisms. The test data were used to postulate a strength criterion for failure under combined bending/compression loading. A linear relationship between the compressive strains and the bending strains at failure was determined and this observation was captured by a hybrid finite element simulation method that combines micromechanical and macromechanical modeling. [Copyright &y& Elsevier]

Ecosystem engineers are organisms whose presence or activity alters their physical surroundings or changes the flow of resources, thereby creating or modifying habitats. Because ecosystem engineers affect communities through environmentally mediated interactions, their impact and importance are likely to shift across environmental stress gradients. We hypothesize that in extreme physical environments, ecosystem engineers that ameliorate physical stress are essential for ecosystem function, whereas in physically benign environments where competitor and consumer pressure is typically high, engineers support ecosystem processes by providing competitor- or predator-free space. Important ecosystem engineers alleviate limiting abiotic and biotic stresses, expanding distributional limits for numerous species, and often form the foundation for community development. Because managing important engineers can protect numerous associated species and functions, we advocate using these organisms as conservation targets, harnessing the benefits of ecosystem engineers in various environments. Developing a predictive understanding of engineering across environmental gradients is important for furthering our conceptual understanding of ecosystem structure and function, and could aid in directing limited management resources to critical ecosystem engineers. [ABSTRACT FROM AUTHOR]

Over 50 years of whisker research (cadmium, zinc, and tin) has not resulted in consensus about whisker formation fundamentals for metal films. New analytical tools have recently provided new insights into microstructural changes that occur during whisker formation. Integration of these newer observations with historical data led the authors to propose an Integrated Theory of Whisker Formation. The Integrated Theory incorporates physical attributes such as microstructure and internal stress states. Particular emphasis is placed on recrystallization, grain boundary diffusion, film-substrate interdiffusion (i.e., the Kirkendall effect), and stress gradients. The Integrated Theory does not require dislocation mechanisms for material transport to the whisker location. Material is transported to a whisker grain by the surrounding grain boundary network under the driving impetus of positive stress gradients. Transported atoms then move from the grain boundary network into the whisker grain. This movement into the whisker grain pushes the free surface of the whisker grain upward and, thereby, grows the whisker structure. [ABSTRACT FROM PUBLISHER]

A conspicuous long-term consequence of hypertension is a thickening of the arterial wall, which many suggest returns the circumferential wall stress toward its normal value. This thickening results from an increase in smooth muscle and extracellular matrix, with the associated growth and remodeling processes depending on a host of regulatory signals that likely include the altered mechanical environment. Although the precise mechanotransduction pathways remain unknown, we propose that vasoconstriction may be an early response of the arterial wall to a step-change in pressure. In particular, computations suggest that such a response can decrease the magnitude and transmural gradients of the pressure-induced wall stresses and return the mean wall shear stress toward its homeostatic value. Such an initial ‘compensatory vasoconstriction’ could also help set into motion subsequent growth and remodeling responses due to growth regulatory characteristics of the vasoactive molecules (e.g., nitric oxide, endothelin-1, angiotensin-II). Although the consequences of growth and remodeling have been the focus of prior biomechanical and histological studies, early responses dictate subsequent developments and therefore deserve increased attention in vascular biomechanics and mechanobiology. [Copyright &y& Elsevier]

The Weibull parameter m of the strength distribution of ceramics under high stress gradients differs from that for moderate stress gradients. This is shown for contact loading. Bars were loaded by oppositely concentrated forcers via rollers. For most investigated materials, measured contact strengths showed strongly reduced Weibull exponents compared with those from 4-point bending tests. This was the reason for a study, in which the effective volumes and surfaces for the two tests were compared and the influence of the strong stress gradients was considered. Under the assumption of the Weibull theory being valid, the effective surfaces and volumes were computed for the normal stress and the energy release rate criteria. In the second part, it will be shown that the strongly non-homogeneous stresses lead to a reduced Weibull exponent. [Copyright &y& Elsevier]

The effects of stress gradients in beryllium surface layers on traditional X-ray stress measurements are investigated by relationship analysis of d vs. (sin ψ)2 plots with stress gradient in the surface layers of beryllium. The results show that over the range of (sin ψ)2≤0.5, there are significant effects of stress gradient on the measurement results. The stress measurement error resulting from the stress gradient is decreased using a vanadium target and high ψ range. [Copyright &y& Elsevier]

Positive interactions often play an important role in structuring plant communities and increasing biological diversity. Using three scales of resolution, we examine the importance of a long-lived desert tree, ironwood (Olneya tesota), in structuring plant communities and promoting biological diversity in the Sonoran Desert. We examined the positive effects of Olneya canopies of different sizes on plant communities in mesic and xeric habitats throughout the central Gulf Coast subregion of Sonora, Mexico. In xeric sites, Olneya canopies had strong positive effects on plant richness and abundance, and small positive effects on the size of plants, underscoring the role of facilitation in extreme environments. In mesic sites, Olneya canopies had very little effect on perennials and a negative effect on ephemeral richness, suggesting predominantly competitive effects in this less stressful environment. Overall, Olneya canopies increased biological diversity where abiotic stress was high, but did not increase diversity in more mesic areas. Thus Olneya canopies caused consistent shifts in plant-community structure among xeric and mesic sites, but not when these landscapes were combined. Benefactor size also mediated positive interactions, with larger Olneya canopies supporting larger perennials in both xeric and mesic sites. Thus stress gradients and benefactor size both influenced the balance of facilitative and competitive effects under nurse-plant canopies, and the spatial scale at which facilitative effects shape community structure. [ABSTRACT FROM AUTHOR]