Thermal residual stresses and stress distributions under tensile and compressive loadings of short fiber reinforced metal matrix composites

The thermal residual stresses and the stress distributions of short fiber reinforced metal matrix composite under tensile and compressive loadings were studied using large strain axisymmetric elasto plastic finite element method. It is demonstrated that the thermal residual stresses can result in asymmetrical stress distributions and matrix plasticity. The thermal residual stresses decrease the stress transfer in tension and enhance the stress transfer in compression. The fiber volume fraction has more important effects on the thermal residual stresses and the stress distributions under tensile and compressive loadings than the fiber aspect ratio and the fiber end distance. [

On temporal and spatial distribution of seismic apparent stresses in Yunnan area

The method of radiation energy （ER） of the earthquake wave measured by the peak velocity （r·v） of the ground motion is applied to a detailed study on the temporal and spatial distribution of the seismic apparent stresses （σa） for the moderate and small earthquakes and two aftershock sequences in Yunnan area. The results show that there exists an obvious non-homogeneity for the seismic apparent stresses in the spatial distribution. The concentrated regions of the high apparent stresses are related to the active places of the moderate and small earthquakes. Before the Dayao M=6.2 earthquake, there was a period in which the apparent stresses were high and the value was 5 times of the average value, 0.25 MPa. The relatively high values of apparent stresses distribute around the epicentral area of the major shock and nearby. It indicates that the variation characteristics of the apparent stresses can be taken as a new kind of criterion for the earthquake-risk forecast. Usually the ratio of the apparent stresses of the aftershock sequence σaA to the ones σaM of main shock is less than 1.0.

Distribution of Stresses in Particulate Half Space under Surface Pressure

Reducing the linear system of two first order equilibrium equations involving normal stress σ(ρ,θ) and shearing stress v(ρ,θ), by elimination, to two decoupled second order equations in σ and v, we find that, for pressure only case, v(ρ,θ) vanishes in the half space. Consequently, the second order equation in σ can be simplified. In the language of linear system analysis, the medium(system) function, characterizing the mechanical behavior of a particulate medium in pressure only case, is obtained from the simplified second order equation ( 2 ρ+ 2 θ)σ(ρ,θ)=0 and can be inverted to give impulse reponse explicitly. Thus, response σ α(ρ,θ) may be computed directly from input, i.e., the surface pressure φ α(ρ) , by integration. Some explicit formulas for transmission problems, including response to input of strip linearly increasing pressure, are given in the paper.

Effect of Localized Scorch on the Transport and Distribution of Exogenous Jasmonic Acid in Vicia faba

Exogenous jasmonic acid (JA) has been showed to be able to induce stomatal closure in Vicia faba L. in previous investigations. The transport and distribution of 3H-JA affected by localized scorch on V. faba seedling were studied with radioisotope technique. The results showed that 3H-JA could be transported up or down at the rate of 4-5 cm·min -1 following feeding into root or shoot tip. The transport of 3H-JA in shoot reached a relative stable rate at 30 min after being fed through root. Wounding by scorch in the youngest leaf caused an increase in the transport of 3H-JA from root to shoot and enhanced the distribution of 3H-JA in the wounded leaf. However, distribution of 3H-JA in unwounded leaves increased after 5 h being fed through the youngest leaf. It was noticed that wounding improved accumulation of 3H-JA in abaxial epidermis. Consistent results were obtained: wounding prevented transport of 3H-JA out from the youngest leaf to root; These observations suggest that JA plays an important role as a defense signal and might be involved in the regulation of the stomatal movement in response to wounding stress.

Analytical Research of Temperature Distribution and Thermal Stresses within Circular Micro-hotplates

Micro-hotplate （MHP） technology is one key part in the manufacturing of gas sensors. The pursuit of analytical solutions for the temperature distribution and also thermal stresses within the MHP is of intrinsic scientific interest. In this study, analytical solutions for the temperature field, and both radial and tangential stresses and van Mises stress for circular MHP were obtained. Two geometries were considered： one had a circular heater at the center and the other had a circular heater at the center and an annular heater within the membrane part. Internal heat generation was incorporated in the energy equation for the MHP and different values of convection heat transfer coefficient were used at the upper and lower surfaces of the MHP. It has been shown that the MHP with two heaters can provide more uniform temperature field compared with the MHP with one heater. The main objective of this work is to provide an exact analytical solution for thermal stresses within the circular micro-hcater with a simple geometry as a benchmark, from mathematical point of view, against which the accuracy of new numerical schemes can be checked. To make sure that the analytical procedure is correct, the analytical results are checked against numerical solutions derived from finite element simulation. Since the analytical models for the temperature field and especially for the thermal stresses of MHP ace seldom investigated in the literature, the obtained results are believed to facilitate the design and performance evaluation of MHPs as well.

An Analytical Study on Distribution of Statically Determinate Stresses in Particulate Half Space

General solution of stresses solved from the two dimensiona l system of equilibrium equations in Cartesian coordinates is characterized by the presence of two families of characteristic lines along which initial stresses and discontinuities in them are transmitted intact far down to infinity.This is against our intuition and not verifiable by experimental findings. For the fundamental case of infinite uniform pressure on the upper surface,a comparison between solutions from equilibrium equations in Cartesian coordinates and from those in polar coordinates is carried out in details.The semi infinite characteristic lines in the former are bent up to exponential spirals with both ends on the upper surface in the latter.Thus,the transmission pattern from solution in polar coordinates comes closer to actual situation.However,in polar reference frame,the solution for distribution of stresses in particulate half space under surface strip pressure or so can then only be obtained from boundary value problem of second order partial differential equation.

Finite element analysis of stress distribution and burst failure of SiC_f/Ti-6Al-4V composite ring

A three-dimensional cyclic symmetry finite element model of titanium-matrix composites（TMCs） ring was developed to investigate the stress distribution and burst failure. The effects of fiber volume fractions, reinforced areas, thermal residual stresses and two different temperatures on stress distribution were studied. The burst speed was obtained through analyzing the hoop tensile stresses under a series of rotating speeds. The results indicate that at the two different temperatures, the influences of fiber volume fractions and reinforced areas on stress level and distribution are different. Some proposals are provided for the structure design of the TMCs ring. With regard to thermal residual stresses, a larger reinforced area is an advisable choice for design of the ring at higher temperature.

Effect of dynamic loading orientation on fracture properties of surrounding rocks in twin tunnels

For expedited transportation,vehicular tunnels are often designed as two adjacent tunnels,which frequently experience dynamic stress waves from various orientations during blasting excavation.To analyze the impact of dynamic loading orientation on the stability of the twin-tunnel,a split Hopkinson pressure bar(SHPB)apparatus was used to conduct a dynamic test on the twin-tunnel specimens.The two tunnels were rotated around the specimen’s center to consider the effect of dynamic loading orientation.LS-DYNA software was used for numerical simulation to reveal the failure properties and stress wave propagation law of the twin-tunnel specimens.The findings indicate that,for a twin-tunnel exposed to a dynamic load from different orientations,the crack initiation position appears most often at the tunnel corner,tunnel spandrel,and tunnel floor.As the impact direction is created by a certain angle(30°,45°,60°,120°,135°,and 150°),the fractures are produced in the middle of the line between the left tunnel corner and the right tunnel spandrel.As the impact loading angle(a)is 90°,the tunnel sustains minimal damage,and only tensile fractures form in the surrounding rocks.The orientation of the impact load could change the stress distribution in the twin-tunnel,and major fractures are more likely to form in areas where the tensile stress is concentrated.

In-situ thermal Raman mapping and stress analysis of CNT/CF/epoxy interfaces

A study of the interfacial behavior and internal thermal stress distribution in fiber-reinforced composites is essential to assess their performance and reliability.CNT/carbon fiber(CF)hybrid fibers were constructed using electrophoretic deposition.The interfacial properties of CF/epoxy and CNT/CF/epoxy composites were statistically investigated and compared using in-situ thermal Raman mapping by dispersing CNTs as a Raman sensing medium(CNT_(R))in a resin.The associated local thermal stress changes can be simulated by capturing the G'band position distribution of CNT_(R) in the epoxy at different temperatures.It was found that the G'band shifted to lower positions with increasing temperature,reaching a maximum difference of 2.43 cm^(−1) at 100℃.The interfacial bonding between CNT/CF and the matrix and the stress distribution and changes during heat treatment(20-100℃)were investig-ated in detail.This work is important for studying thermal stress in fiber-reinforced composites by in-situ thermal Raman mapping technology.

Boulder-induced form roughness and skin shear stresses in a gravel-bed stream

Boulder spacing in mountain rivers and near-wake flow zones within the boulder array is very useful for fish habitat and growth of aquatic organisms.The present study aims to investigate how the boulder array and spacing influence the near-bed flow structures in a gravel-bed stream.Boulders are staggered over a gravel-bed stream with three different inter-boulder spacing namely(a)large(b)medium and(c)small spacing.An acoustic Doppler velocimeter was used for flow measurements in a rectangular channel and the results were compared with those acquired from numerical simulation.The time-averaged velocity profiles at the near-wake flow zones of boulders experience maximum flow retardation which is an outcome of the boulder-induced form roughness.The ratio of velocity differences associated to form and skin roughness and its positive magnitude reveals the dominance of form roughness closest to the boulders.Form roughness computed is 1.75 to 2 times higher than the skin roughness at the near-wake flow region.In particular,the collective immobile boulders placed at different inter-boulder spacings developed high and low bed shear stresses closest to the boulders.The low bed shear stresses characterised by a secondary peak developed at the trough location of the boulders is attributed to the skin shear stress.Further,the spatial averaging of time-averaged flow quantities gives additional impetus to present an improved illustration of fluid shear stresses.The formation of form-induced shear stress is estimated to be 17%to 23%of doubleaveraged Reynolds shear stress and partially compensates for the damping of time-averaged Reynolds shear stress in the interfacial sub-layer.The quadrant analysis of spatial velocity fluctuations depicts that the form-induced shear stresses are dominant in the interfacial sub-layer and have no significance above the gravel-bed surface.

Carbon allocation in Picea jezoensis:Adaptation strategies of a non-treeline species at its upper elevation limit

Understanding the physiological adaptations of non-treeline trees to environmental stress is important to understand future shifts in species composition and distribution of current treeline ecotone.The aim of the present study was to elucidate the mechanisms of the formation of the upper elevation limit of non-treeline tree species,Picea jezoensis,and the carbon allocation strategies of the species on Changbai Mountain.We employed the^(13)C in situ pulse labeling technique to trace the distribution of photosynthetically assimilated carbon in Picea jezoensis at different elevational positions(tree species at its upper elevation limit(TSAUE,1,700 m a.s.l.)under treeline ecotone;tree species at a lower elevation position(TSALE,1,400 m a.s.l.).We analyzed^(13)C and the non-structural carbohydrate(NSC)concentrations in various tissues following labeling.Our findings revealed a significant shift in carbon allocation in TSAUE compared to TSALE.There was a pronounced increase inδ^(13)C allocation to belowground components(roots,soil,soil respiration)in TSAUE compared to TSALE.Furthermore,the C flow rate within the plant-soil-atmosphere system was faster,and the C residence time in the plant was shorter in TSAUE.The trends indicate enhanced C sink activity in belowground tissues in TSAUE,with newly assimilated C being preferentially directed there,suggesting a more conservative C allocation strategy by P.jezoensis at higher elevations under harsher environments.Such a strategy,prioritizing C storage in roots,likely aids in withstanding winter cold stress at the expense of aboveground growth during the growing season,leading to reduced growth of TSAUE compared to TSALE.The results of the present study shed light on the adaptive mechanisms governing the upper elevation limits of non-treeline trees,and enhances our understanding of how non-treeline species might respond to ongoing climate change.

Geological risk assessment of traffic engineering construction among 7.0-8.5 magnitude earthquake areas:Practice from the Sichuan-Tibet transport corridor in the eastern Tibetan Plateau

At least 13 active fault zones have developed in the Ya'an-Linzhi section of the Sichuan-Tibet transport corridor,and there have been undergone 17 MS≥7.0 earthquakes,the largest earthquake is 1950 Chayu MS 8.5 earthquake,which has very strong seismic activity.Therefore,carrying out engineering construction in the Sichuan-Tibet transport corridor is a huge challenge for geological technological personnel.To determining the spatial geometric distribution,activity of active faults and geological safety risk in the Sichuan-Tibet transport corridor.Based on remote sensing images,ground surveys,and chronological tests,as well as the deep geophysical and current GPS data,we investigated the geometry,segmentation,and paleoearthquake history of five major active fault zones in the Ya'an-Linzhi section of the Sichuan-Tibet transport corridor,namely the Xianshuihe,Litang,Batang,Jiali-Chayu and Lulang-Yigong.The five major fault zones are all Holocene active faults,which contain strike-slip components as well as thrust or normal fault components,and contain multiple branch faults.The Selaha-Kangding segment of the Xianshuihe fault zone,the Maoyaba and Litang segment of the Litang fault zone,the middle segment(Yigong-Tongmai-Bomi)of Jiali-Chayu fault zone and Lulang-Yigong fault zone have the risk of experiencing strong earthquakes in the future,with a high possibility of the occurrence of MS≥7.0 earthquakes.The Jinsha River and the Palong-Zangbu River,which is a high-risk area for geological hazard chain risk in the Ya'an-Linzhi section of the Sichuan-Tibet transport corridor.Construction and safe operation Ya'an-Linzhi section of the Sichuan-Tibet transport corridor,need strengthen analysis the current crustal deformation,stress distribution and fault activity patterns,clarify active faults relationship with large earthquakes,and determine the potential maximum magnitude,epicenters,and risk range.This study provides basic data for understanding the activity,seismicity,and tectonic deformation patterns of the regional faults in the Sichuan-Tibet transport corridor.

Nonlinear Analysis of Axial-load and Stress Distribution for Threaded Connection

Analytical method for the distributions of axial-load and stress is based on elastic assumption, but the threaded connections are often in plastic deformation stage in practice. Meanwhile the strain in the threaded connection is difficult to measure. So it is necessary to study the reliable numerical method. At present neither the convergence analysis of the computational results nor the elastic-plastic analysis in the loading-unloading process are studied. In this paper, von Mises plasticity and kinematic hardening model is used to describe the material response. A new convergence criterion for nonlinear finite element analysis of the loading-unloading process is proposed. An axisymmetric finite element model according to the proposed convergence criterion is developed and used to analyze the distributions of axial-load and stress. It can be conclude that the stress distribution analysis is more dependent on the mesh density than the axial-load distribution analysis. The stress distribution result indicates that with increasing of applied load, the engaged threads close to the nut-bearing surface become plastic firstly. The axial-load distribution result reveals that the load percentage carried by single thread depends on the position of thread and load intensity. When the load is relatively small, the applied load is mainly carried by the engaged threads near the nut-bearing surface, when the load is larger, the differences of percentages for all threads become small. The proposed convergence analyzing procedure is applicable for other nonlinear analyses. The obtained distributions of axial-load and stress can be a reference of engineering application.

Effects of neutral salt and alkali on ion distributions in the roots,shoots,and leaves of two alfalfa cultivars with differing degrees of salt tolerance

The effects of neutral salt and alkali on the ion distribution were investigated in two alfalfa （Medicago sativa L.） cultivars, including Zhongmu 1, a high salt-tolerant cultivar, and Algonquin, a low salt-tolerant cultivar. The alkali stress expressed more serious growth inhibition than the neutral salt stress at the same Na＋ concentration. Compared with Algonquin, Zhongmu 1 did not exhibit a higher alkali tolerance under the Na2CO3-NaHCO3 treatment with the low Na＋ concentration （50 mmol L-l）. The alkali increased the accumulation of Na＋, Ca2＋, and Mg2＋ in the root and changed the Ca2＋ and Mg2＋ balance in the entire alfalfa plant. The salt and alkali stresses decreased the K＋ and Fe3＋ contents of the roots and leaves, the root Mn2＋ content, and the shoot Zn2＋ content, but they increased the Fe3＋ accumulation of the shoots, the shoot and leaf Cu2＋ contents, and the leaf Zn2＋ content in both alfalfa cultivars. Based on the results obtained under the conditions of this experiment, we found that the salt and alkali stresses reduced the plant growth in both alfalfa cultivars, while the alkali caused a stronger stress than the neutral salt in alfalfa. Thus, we conclude that under hydroponic conditions, the deleterious effects of the alkali on plants are due to the distribution change of some trophic ion balance in the roots, shoots, and leaves of the plants by causing of Na＋, CO3^2-, and/or HCO3- stresses.

Stress distribution and surrounding rock control of mining near to the overlying coal pillar in the working face

The occurrence of overlying coal pillar(OCP)exerts a strong effect on the stress and strain distribution of the surrounding rock in the stope.In this paper,the stress distribution characteristics are analyzed via the numerical calculation with the account of OCP presence or absence.In addition,this study revealed the joint effect of side pressure relief area of the goaf and stress concentration in OCP on the final stress distribution.Furthermore,the rules of abutment stress distribution affected by three influencing factors,namely horizontal-vertical distances between OCP and working face and buried depth of OCP,are analyzed.The functional model linking the peak stress of surrounding rock with the above influencing factors is developed.The field application of the above results proved that the rib spalling and deformation of a 2.95 m-high and 5.66 m-wide roadway could be efficiently controlled by rationally adjusting working states of the support,and adopting the hydraulic prop coordinated with the p type metal beam and anchor cable to strengthen the surrounding rock of working face and roadway,respectively.The proposed measures are considered appropriate to satisfy the safe operation requirements.

A Modification to Vertical Distribution of Tidal Flow Reynolds Stress in Shallow Sea

Tidal flow is a periodic movement of unsteady and non-uniform, which has acceleration and deceleration process obviously, especially in coastal shallow waters. Many researches show that vertical distribution of tidal flow Reynolds stress deviated from linear distribution. The parabolic distribution of the tidal flow Reynolds stress was proposed by Song et al. （2009）. Although the model fills better with field observations and indoor experimental data, it has the lower truncated series expansion of tidal flow Reynolds stress, and the description of the distribution is not very comprehensive By introducing the motion equation of tidal flow and improving the parabolic distribution established by Song et al. （2009）, the cubic distribution of the tidal flow Reynolds stress is proposed. The cubic distribution is verified well by field data （Bowden and Fairbairn, 1952; Bowden et al., 1959; Rippeth et al., 2002） and experimental data （Anwar and Atkins, 1980）, is consistent with the numerical model results of Kuo et al. （1996）, and is compared with the parabolic distribution of the tidal flow Reynolds stress. It is shown that this cubic distribution is not only better than the parabolic distribution, but also can better reflect the basic features of Reynolds stress deviating from linear distribution downward with the tidal flow acceleration and upward with the tidal flow deceleration, for the foundation of further study on the velocity profile of tidal flow.

Study of Stress and Strain Distributions of First Pass Conventional Spinning Under Different Roller-trace

Based on simplified axisymmetrical forming model, a elasto-plastic FEM simulation system of multi-pass conventional spinning is developed. Taking the typical draw-spinning as the study object, and establishing reasonable mechanics model, research on the first pass of spinning process is carried out with FEM system developed. The distributions of the stress and strain are obtained by three types of roller-trace curves: straight line, involute curves and quadratic curves. The results are as follows: (1) The values of equivalent stress and strain are the lowest under involute curve compared to other two curves, and they change relatively small and decrease with the increase of radius. The values of equivalent stress and strain is the highest under quadratic curves, and increase with the increase of radius. (2) The value of radial stress is smallest under involute curve, and is the largest under straight line. Value of radial stress is often used as the criterion of cracking limit, so its distribution laws can provide references for studying the condition of cracking in multi-pass conventional spinning under different roller-trace. (3) Tangential stress is compressive stress. Absolute value of tangential stress is the smallest under involute curve, and values of tangential stress are close between other two curves. The distribution laws of tangential stress can serve as a significant guide to research the critical condition of wrinkling in multi-pass conventional spinning under different roller-trace. (4) The reduction of thickness is the smallest under involute curve. The distribution of the thickness strain is very unequal under quadratic curves. The results obtained can provide references for selecting reasonable roller-trace in multi-pass conventional spinning.

Stress distribution rule of roadway affected by overhead mining in gently inclined coal seams group

In light of the severe deformation and destruction of the district raise tunnel in the mining area at the northern part of the Lubanshan colliery, by the theoretic analysis and numerical simulation, both the mining stress distribution in seams group and the deformation and destruction mechanism of floor district raise were investigated. The results show that, at the maximum vertical distance of 40 m, the abutment stress has an influence on the recovery of 2# and 3# coal seam and 8# coal seam at distance of 30 m. As a result, the recovery of 8# is rather than those of 2# or 3# coal seam, which contributes to the deformation and destruction of the district raise surrounding rock. The major factors affecting the abutment stress include the mining depth, mining height, residual gob space, adjacent working faces and short spacing coal seam recovery.

Statistical analysis of distribution patterns of coal seams in fold zones in Northwest China

The mechanisms for rock bursts occurrences in fold zones are complex, and the redistribution of in-situ stresses is closely related to the complexity of the structures. Analysis of the geomorphology of fold structures and changes of coal thickness can help identify zones prone to rock bursts to improve safety and productivity in coal mines. This study investigated the distribution characteristics of fold structures in coal seams in fold zones in four mines in northwest China. Geometrical characteristics of fold structures in coal seams and changes of coal thickness were analysed, based on comprehensive evaluation indexes,such as the length–width ratio of folds, interlimb angle, ratio P1 of projected width of fold limbs to that of the hinge zone, curvature ratio P2, the maximum curvature and amplitude. The statistical analysis of the four coal mines shows that the length–width ratio of folds changed from 0.78 to 2.03 and the maximum curvature of cross sections of folds was less than 0.04. The curvature ratio of cross section of a fold in the structure was no more than 1.4 and the interlimb angles of cross sections of 89% of folds were larger than 150°. Gentle fold structures were dominant and the specific geological morphologies were domes or basins. The isopleth of coal thickness above the coal mines showed a fluctuation trend similar to the contour line of the floor of coal seams. The coal thickness in an anticline area was smaller than that in the neighboring syncline area. Therefore, the overall variation of coal thickness in the mining areas was likely to have a relation with the direction of the regional principal stress.

Analytical model and application of stress distribution on mining coal floor

Given the analysis of underground pressure, a stress calculation model of coal floor stress has been established based on a theory of elasticity. The model presents the law of stress distribution on the relatively fixed position of the mining coal floor： the extent of stress variation in a fixed floor position decreases gradually along with depth, the decreasing rate of the vertical stress is clearly larger than that of the horizontal stress at a specific depth. The direction of the maximum principal stress changes gradually from a vertical direction to a horizontal direction with the advance of the working face. The deformation and permeability of the rock mass of the coal floor are obtained by contrasting the difference of the principal stress established from theoretical calculations with curves of stress-strain and permeability-strain from tests, which is an important mechanical basis for preventing water inrush from confined aauifers.