stress gradient
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Precise prediction of mechanical behavior of thin films at the nanoscale requires techniques that consider size effects and fabrication-related issues. Here, we propose a test methodology to estimate the Young’s modulus of nanometer-thick films using micromachined bilayer cantilevers. The bilayer cantilevers which comprise a well-known reference layer and a tested film deflect due to the relief of the residual stresses generated during the fabrication process. The mechanical relationship between the measured residual stresses and the corresponding deflections was used to characterize the tested film. Residual stresses and deflections were related using analytical and finite element models that consider intrinsic stress gradients and the use of adherence layers. The proposed methodology was applied to low pressure chemical vapor deposited silicon nitride tested films with thicknesses ranging from 46 nm to 288 nm. The estimated Young’s modulus values varying between 213.9 GPa and 288.3 GPa were consistent with nanoindentation and alternative residual stress-driven techniques. In addition, the dependence of the results on the thickness and the intrinsic stress gradient of the materials was confirmed. The proposed methodology is simple and can be used to characterize diverse materials deposited under different fabrication conditions.

Heat treatment is an important stage in the technology of rolling rolls, due to the fact that the thermal stresses that occur during rapid or uneven heating summing up with the rather high residual stresses after casting, create a risk of cracking. Goal. The aim is improving the quality of rolling rolls by varying the modes of heat treatment. Therefore, it is important to assess the level of thermal stress. Method. Evaluation of thermal stresses arising in heating and cooling in the heat treatment process that summing up to the relatively high residual stresses after casting, creates the risk of fractures. The profile of the distribution of chromium in the cross section of the working layer at each time under the action of the stress gradient that occurs during heat treatment of the roll is obtained by calculation. Results To ensure a minimum temperature difference between the surface and the core, it is necessary to reduce the heating and cooling rate, as well as increase the duration of exposure at a given temperature. Reducing the cooling rate from 17 to 3.7°C/h decreases the temperature difference at the surface and in the center of the roll and the intensity of thermal stresses from 29 to 7 MPa. It is established that the rate of heating and cooling should not exceed 10– 15° C/h, and exposure to annealing should be at least 5–7 hours. Scientific novelty. The modes of heat treatment of rolling rolls with a high-chromium cast iron working layer are designed by estimating the level of thermal stresses. The profile of distribution of chromium after various modes of heat treatment is calculated analytically and its mode at which the most uniform distribution of chromium on section of a working layer remains is offered. Practical significance. The developed technique allows to calculate analytically the profile of distribution of chromium after various modes of heat treatment and to choose such a mode at which the most uniform distribution of chromium remains on the section of a working layer.

Rotating parts of aeroengines need to have a high speed margin according to the civil aviation airworthiness regulations. Previous studies on burst speed are based on mechanical properties of standard specimens. In this paper, a new method for predicting burst speed by means of a tensile test of a simulative specimen is proposed, and the predicted results are compared with the traditional method. The results show that the stress gradient of the designed simulative specimen and the assessment location of vortex reducer are in good agreement, which indicates that they have similar stress characteristics. The burst speed predicted by the new method is greater than the traditional method. Both prediction methods can provide a reference for such a structure in the design stage. In addition, the overspeed test of a vortex reducer is carried out, and the results verify that it still has sufficient strength reserves at 120% relative speed.

Abstract It remains unclear how competitive exclusion is avoided between two ecologically, economically, and culturally important codominant grass species in the tallgrass prairie of the Great Plains, Andropogon gerardii and Sorghastrum nutans. These functionally similar C4 grasses appear to coexist despite considerable niche overlap, and asymmetric competition and drought tolerance in favor of A. gerardii. According to the stress gradient hypothesis, it may be that the sum of interactions between these species, which is typically negative (competitive) due to similar resource requirements, shifts to positive (facilitative) as abiotic stress increases. For instance, if the canopy cover of the stronger competitor reduces losses of subcanopy humidity or shallow soil moisture, recruitment of S. nutans tillers may be extended further into the drought event than would occur in the absence of A. gerardii. As later months of the growing season are drier on average where these species are codominant, such a mechanism may enable S. nutans to recover from early season asymmetric competition and stabilize their codominance. We tested this hypothesis in a greenhouse experiment in which we manipulated community composition and water availability in the latter half of the growing season. We found no evidence that a shift from a negative to positive interaction occurs, with each species performing similarly in mixed communities and monocultures. The similarities of the two species in their functional traits and responses to water limitations may limit such a shift in interaction net effects and suggests that other mechanisms are determining coexistence of these co-occuring C4 grasses.

Abstract This paper presents the simulation and theoretically calculation results of a shunt switch with Electro-mechanical modelling and stress gradient characteristics. The analysis is done with three membrane structures such as plane beam, incorporated with and without perforations, and non-uniform meander type beam, these are simulated in the COMSOL Multi-physics tool. The various Modal analyses are carried out for different values of residual stress gradients such as different structures, materials, and beam thickness. These analyzes are described by the fact that higher stress gradient values ​​are undesirable for switching. By analysing all the results we have observed that the stress analysis for a shows that non-uniform meandered switch experiences maximum stress of 35.6 MPa, and center deflection of 0.06 MPa/μm, the deformations of the beam which is the least among the considered switches.

<p>Current research has emphasized the need to identify and quantify the effects of positive and negative interactions (both direct and indirect) between species, taking into account the influence of abiotic conditions and spatial scales. In this research it is particularly challenging to adequately assess and predict the impact of introduced species on native communities. This study examined interactions of introduced and native species on coastal sand dunes in New Zealand. Substantial areas of this habitat have become dominated by the highly competitive exotic sand-binder marram grass (Ammophila arenaria) and other exotic weeds, replacing native species, including the native eco-engineering spinifex (Spinifex sericeus). First, I examined direct interactions (competition and facilitation) between marram and spinifex along an abiotic stress gradient where experimental plots were subject to different restoration management techniques. In my large-scale dune experiment I planted 2475 spinifex seedlings in three different treatments (bare sand, live marram, dead marram) including an unplanted control, and monitored the plots for one year along exposed marram grass-dominated dune fields near Whanganui, West Coast, North Island/New Zealand. The stress gradient hypothesis predicts facilitation will be greatest where stress is most severe. I hypothesized facilitation of spinifex plantings and other self-colonizing plants in dead sprayed marram compared to live marram and a change of interaction between spinifex and marram grass along an abiotic stress gradient. Spinifex survival was not significantly different across treatments, but the interaction between treatment and location from the sea as well as pre-existing marram cover were significant predictors of plant growth in a linear mixed effect model. Exotic weeds such as Senecio elegans, Conyza canadensis, dandelions and legumes were facilitated by sprayed marram grass relative to abundances in live marram grass, while grasses other than marram grass and spinifex performed best in bare sand, in the absence of competitors. For S. elegans, abundances were higher closer to the sea. Spinifex growth was greatest in plots closest to the sea. Comparing spinifex growth in live marram grass and dead marram showed similar patterns at the fore, mid and back dune, but sprayed maram showed better facilitation of spinifex. For restoration plantings it is only recommended to plant into dead marram if the site is not weed prone as weeds were facilitated in the same way as spinifex and in some instances appeared to hinder spinifex growth. Crucial for a successful conversion from marram to spinifex is that the initial marram cover is not too high prior to spraying- lower densities allow for better spinifex growth. Second, I examined indirect competition with a survey of natural populations and a common garden experiment at a shingle beach. I was interested in determining the influence of plant density at different spatial scales as well as plant morphology on insect abundances. I studied indirect interactions by choosing native and introduced Senecio spp. as host plants for insect colonization and incorporated fine scale plant density (50cm radius circle, area = 0.8 m²) and coarse scale plant density (6m quadrat annulus, area = 32 m²) as predictors for insect colonization in addition to plant morphology. I surveyed Senecio spp. over a period of three growth seasons and conducted an experiment where I manipulated densities of pairs of species of either introduced Senecio elegans, S. skirrhodon and native S. lautus. My survey of natural populations and the manipulative field experiment show a negative impact of high conspecific and heterospecific plant density on the colonization of the seed head predator fly Sphenella fascigera. Along with plant density, plant size was a significant predictor of insect abundances. Higher densities of introduced S. elegans indirectly facilitated S. lautus at fine spatial scales by reducing the incidence of S. fascigera. This supports the resource dilution hypothesis which predicts higher insect herbivore numbers on isolated resource patches. Thus, my results provide empirical evidence for apparent facilitation of a native plant by an introduced plant via a shared herbivore. For future assessments of the impacts of invasive species it will be important to consider the net-outcome of direct and indirect competitive and facilitative interactions. In particular, for restoration purposes in stressful environments removal of invasive species may have to occur in a carefully controlled manner taking into account the abiotic conditions and spatial scales at which interactions occur.</p>