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1.
以蒙皮材料Uretek3216L为对象开展了膜材的双轴循环拉伸力学性能试验研究.首先,通过一系列双轴循环试验获得了膜材的应力应变数据,并对其非线性、各向异性等性质进行了探讨分析.然后,运用应变残差最小二乘法,计算出不同应力比组合的弹性常数.最后,分析了应力比的组合方法及正交互补性质对膜材弹性常数的影响规律.结果表明,经纬向纱线卷曲形态的不均衡性是膜材正交异性特征的主要原因.膜材的弹性常数受应力比组合方法及正交互补性质的影响显著,且弹性模量随名义应力比的改变而有规律地变化.在结构设计中,需要根据具体受力状态及应力比范围来确定膜材的弹性常数,以提高设计分析的精确性;且考虑到弹性常数的多变性,采用2组界限弹性常数比仅采用1组更可取. 相似文献
2.
3.
Rate-dependent constitutive model of poly（ethylene terephthalate） for dynamic analysis 总被引：1，自引：0，他引：1
Uniaxial tensile testing at strain rates ranging from 10-3 to 10-1 s-1 was carried out to study the rate-dependent me-chanical behavior for poly（ethylene terephthalate） （PET） used in the packaging industry. The experimental results show that a rate-dependent plastic behavior exists for PET material. The value of the yield strength was found to increase with the increasing strain rate. A new constitutive model based on the improved Cowper-Symonds rate-dependent constitutive model is proposed to describe the mechanical behavior of PET material in the strain rate ranging from 10-3 to 10-1 s-1, providing more accurate material data for the subsequent simulation analysis of drop test and dynamic buckling. The predictions obtained using the proposed model are compared with experimental results of the improved Cowper-Symonds model. The simulating results of the proposed model agree well with the experimental data. For a low strain rate, the predictions of this model are more precise than those obtained using the improved Cowper-Symonds model. This confirms that the new constitutive model is suitable for describing the me-chanical behavior of PET material at a low strain rate and modeling impact problem. 相似文献
4.
Median noise barriers, like parallel noise barriers, can be employed to reduce the impact of traffic on roadside communities via the direct propagation path. The performance of different shapes of median barriers was compared using reactive and passive surfaces and a 2D boundary element method (BEM). In the case of reactive surfaces, quadratic residue diffusers (QRDs) and primitive root diffusers (PRDs) were used on the top and stem surfaces of median barriers. To introduce passive barriers, two different absorbent materials including fibrous material and a grass surface with flow resistivity of 20 000 and 2500 kg/(s·m2), respectively, were similarly applied. The effect of thin absorptive barriers was similar at lower frequencies and better at mid and high frequencies to that of their equivalent rigid barriers. More improvement was achieved by covering the top surface of thick barriers with grass rather than with fibrous material. The performance of QRD and PRD barriers where the diffuser was located on the top surface was more frequency dependent than that of barriers coated with fibrous material. A comparison of the average A-weighted insertion loss in the thick barriers showed that the greatest improvement (2.59 dB (A)) was achieved using a barrier of 30-cm thickness covered with grass. 相似文献
5.
The failure mode and adhesion of thermal barrier coating (TBC) 8YSZ (ZrO2+8% (w/w) Y2O3) deposited on NiCoCrAlTaY bond coat by atmospheric plasma spraying were investigated. A grooved modified three-point bending specimen that can generate a single interface crack to facilitate the control of crack growth was adopted for testing, which was conducted at the ambient temperature of 100 °C. The morphology and composition of fractured surfaces were examined by means of a scanning electron microscopy (SEM) and an energy disperse spectroscopy (EDS). Images and spectrum show that cracks are initiated and propagated exclusively within YSZ layer adjacent to top/bond coat interface. The load-displacement curves obtained exhibit similar shapes that indicate two distinct stages in crack initiation and stable crack growth. Finite element analyses were performed to extract the adhesion strength of the TBCs. The delamination toughness of the plasma-sprayed 8YSZ coatings at 100 °C, in terms of critical strain energy release rate Gc, can be reliably obtained from an analytical solution. 相似文献
6.
The definition of J-integral of interfacial crack was introduced. The three-point bending tests were carried out to obtain the critical loading
values when the interfacial crack initiation occurred between coatings and substrates. The finite element analysis (FEA) was
adopted to analyze the stress distribution in the specimens and compute the J-integral of the interfacial crack between LX88A coating and Chinese Q345 steel. The results showed that the average value
of critical J-integral is 0.70 N/m, which can be taken as the fracture parameter to evaluate the interfacial fracture behavior for the
three-point bending specimens of LX88A coating/Q345 steel system.
Supported by Tianjin Natural Science Foundation (No.08JCYBJC09100), New Teacher Research Fund for the Doctoral Program of
Higher Education of China (No.20070056096) and New Century Outstanding Talented Person Plan of China.
XU Lianyong, born in 1975, male, Dr. 相似文献
7.
Diffusion tensor imaging of the brain in patients with Alzheimer＇s disease and cerebrovascular lesions 总被引：1，自引：0，他引：1
Background： Recent autopsy study showed a high incidence ofcerebrovascular lesions in Alzheimer＇s disease （AD）. To assess the impact of cerebrovascular pathology in AD, we used diffusion tensor imaging （DTI） to study AD patients with and without cerebrovascular lesions. Materials and Methods： Conventional and DTI scans were obtained from 10 patients with probable AD, 10 AD/V patients （probable AD with cerebrovascular lesions） and ten normal controls. Mean diffusivity （D） and fractional anisotropy （FA） values of some structures involved with AD pathology were measured. Results： D value was higher in AD patients than in controls in hippocampus and the cingulate gyrus. In AD/V patients, increased D value was found in the same structures and also in the thalamus and basal ganglia compared to controls. There was a significant difference of D value between AD and AD/V patients. FA value reduced in the white matter of left inferior temporal gyrus and in the bilateral middle cingulate gyrus in patients with AD/V compared with controls. The MMSE （mini-mental state examination） score significantly correlated with FA value in the right hippocampus （r=0.639, P〈0.019）, in the right anterior cingulate gyrus （r=0.587, P〈0.035） and in left parahippocampal gyrus （r=0.559, P〈0.047）. Conclusion： Cerebrovascular pathology had stronger impact on the D value than the AD pathology alone did. Elevated D value in thalamic and basal ganglia may contribute to cognitive decline in AD/V patients. Reduced FA values in AD/V patients may indicate that cerebrovascular pathology induced more severe white matter damage than the AD pathology alone did. 相似文献
8.
INTRODUCTION Failure of reinforced and pre-stressed concrete structures is initiated in many instances by cracking of plain concrete. The post-cracking softening behavior of concrete subjected to tension has been realized for many years (Li et al., 1993). However, the resistance of cracked concrete is generally ignored in conven-tional design practice. Until recently it has become increasingly evident that it is necessary to account for the post-cracking resistance of concrete when per-f… 相似文献
9.
An experimental and model-based study of the effect of rich air/fuel ratios （AFRs） and temperature on the NOx slip of a lean NOx trap （LNT） was conducted in a lean-burn gasoline engine with an LNT after-treatment system. The emissions of the engine test bench and the inlet temperature of the LNT were used as the major inlet boundary conditions of the LNT. The engine periodically operated between a constant lean AFR of 23 with alterable rich AFRs of 10, 11, 12, 13, and 14. A decrease in the rich AFR of the engine strengthened the desorption atmosphere in the LNT, an effect closely related to the number of reductants, and further heightened the NOx desorption of the LNT, but with a penalty in fuel consumption. To eliminate that penalty, the inlet boundary conditions of the LNT were varied by adjusting the inlet temperature within a range between 200 ℃ and 400℃. An increase in inlet temperature heightened the NOx desorption of the LNT, and a NOx breakthrough occurred after the inlet tem- perature exceeded 390 ℃. To control NOx breakthrough, the inlet temperature can be adjusted to offset the strong desorption atmosphere in the LNT commonly created by a rich AFR. 相似文献
10.
Silicon carbide (SiC) composites were prepared by hot-press sintering from α-SiC starting powders with BaAl2Si2O8 (BAS). The effects of additives on densification, microstructure, flexural strength, and fracture behavior of the liquid phase sintered (LPS) SiC composites were investigated. The results show that the served BAS effectively promotes the densification of SiC composites. The flexural strength and fracture toughness of the SiC composites can reach a maximum value of 454 MPa and 5.1 MPa·m1/2, respectively, for 40% (w/w) BAS/SiC composites. SiC grain pullout, crack deflection, and crack bridging were main toughening mechanisms for the sintered composites. 相似文献
11.
Cun-jian MIAO Jin-yang ZHENG Xiao-zhe GAO Ze HUANG A-bin GUO Du-yi YE Li MA 《浙江大学学报(A卷英文版)》2013,14(1):31-37
Cold-stretched pressure vessels from austenitic stainless steels(ASS) are widely used for storage and transportation of liquefied gases,and have such advantages as thin wall and light weight.Fatigue is an important concern in these pressure vessels,which are subjected to alternative loads.Even though several codes and standards have guidelines on these pressure vessels,there are no relevant design methods on fatigue failure.To understand the fatigue properties of ASS 1.4301(equivalents include UNS S30400 and AISI 304) in solution-annealed(SA) and cold-stretched conditions(9% strain level) and the response of fatigue properties to cold stretching(CS),low-cycle fatigue(LCF) tests were performed at room temperature,with total strain amplitudes ranging from -.4% to -.8%.Martensite transformations were measured during the tests.Comparisons on cyclic stress response,cyclic stress-strain behavior,and fatigue life were carried out between SA and CS materials.Results show that CS reduces the initial hardening stage,but prolongs the softening period in the cyclic stress response.Martensite transformation helps form a stable regime and subsequent secondary hardening.The stresses of monotonic and cyclic stress-strain curves are improved by CS,which leads to a lower plastic strain and a much higher elastic strain.The fatigue resistance of the CS material is better than that of the SA material,which is approximately 1?03 to 2?04 cycles.The S-N curve of the ASME standard for ASS is compared with the fatigue data and is justified to be suitable for the fatigue design of cold-stretched pressure vessels.However,considering the CS material has a better fatigue resistance,the S-N curve will be more conservative.The present study would be helpful in making full use of the advantages of CS to develop a new S-N curve for fatigue design of cold-stretched pressure vessels. 相似文献
12.
Sheng-hong ZHU ;Li-jia ZHOU ;Hong JIANG ;Rong-jun CHEN ;Chuan LIN ;Shi FENG ;Juan JIN ;Jiang-hua CHEN ;Jian-yong WU 《Journal of Zhejiang University. Science. B》2014,15(8):735-742
研究目的:在小鼠模型中利用吲哚美辛阻断COX-1/2通路,探讨非甾体类抗炎药对肾缺血再灌注损伤的保护作用。创新要点:非甾体类抗炎药被认为具有肾毒性,本研究首次在小鼠模型中探讨非甾体类抗炎药对肾缺血再灌注损伤的保护作用。研究方法:小鼠左侧肾蒂夹闭后,通过腹腔注射不同剂量的吲哚美辛,在肾缺血再灌注损伤24小时后,获取血液和肾脏标本。利用酶联免疫(ELISA)试剂盒测定血清肌酐和细胞因子浓度来评估肾功能,肾组织样本进行苏木精-伊红染色和免疫组化分析。重要结论:腹腔注射吲哚美辛5 mg/kg组的小鼠血清肌酐值与对照组相比显著降低,肾小管损伤也显著减轻(见图1和2);腹腔注射5和7 mg/kg吲哚美辛组的小鼠血清肾肿瘤坏死因子-α和白介素-6的浓度显著降低(见图3a和3b);腹腔注射3和5 mg/kg吲哚美辛组的小鼠血清血栓素B2与6-酮前列腺素F1α的比值明显降低(见图3c);腹腔注射5 mg/kg吲哚美辛组小鼠肾组织COX-1和COX-2染色较弱(见图4)。因此,吲哚美辛对小鼠肾缺血再灌注损伤的作用与其剂量相关,在某个特定的剂量范围内具有肾保护作用。吲哚美辛对小鼠肾缺血再灌注损伤的保护作用与阻断COX-1/2有关。 相似文献
13.
Kelly R. Fisher Kathy Hirsh‐Pasek Nora Newcombe Roberta M. Golinkoff 《Child development》2013,84(6):1872-1878
Shape knowledge, a key aspect of school readiness, is part of early mathematical learning. Variations in how children are exposed to shapes may affect the pace of their learning and the nature of their shape knowledge. Building on evidence suggesting that child‐centered, playful learning programs facilitate learning more than other methods, 4‐ to 5‐year‐old children (N = 70) were taught the properties of four geometric shapes using guided play, free play, or didactic instruction. Results revealed that children taught shapes in the guided play condition showed improved shape knowledge compared to the other groups, an effect that was still evident after 1 week. Findings suggest that scaffolding techniques that heighten engagement, direct exploration, and facilitate “sense‐making,” such as guided play, undergird shape learning. 相似文献
14.
Staggered pattern perforations are introduced to isolated isothermal plates, vertical parallel isothermal plates, and vertical
rectangular isothermal fins under natural convection conditions. The performance of perforations was evaluated theoretically
based on existing correlations by considering effects of ratios of open area, inclined angles, and other geometric parameters.
It was found that staggered pattern perforations can increase the total heat transfer rate for isolated isothermal plates
and vertical parallel plates, with low ratios of plate height to wall-to-wall spacing (H/s), by a factor of 1.07 to 1.21, while only by a factor of 1.03 to 1.07 for vertical rectangular isothermal fins, and the magnitude
of enhancement is proportional to the ratio of open area. However, staggered pattern perforations are detrimental to heat
transfer enhancement of vertical parallel plates with large H/s ratios. 相似文献
15.
RH和RH-KTB精炼过程中钢液流动和混合特性的研究 总被引：7，自引：0，他引：7
The flow and mixing characteristics of molten steel during the vacuum circulation refining,including RH(Ruhrstahl-Heraeus) and RH-KTB(Ruhrastahl-Heraeus-Kawasaki top blowing) processes,were investigated on a 1/5 linear scale water model of a 90t multifunction RH degasser,The circulation rate was directly and more accurately determined,using a new method by which the more reliable results can be obtained.The fluid flow pattern and flow field in the ladle were demonstrated,observed and analyzed.The mixing time of liquid in the ladle was measured using electrical conductivity method.The residence time distribution in the RH model was obtained by tracer response technique.The influence of the main technological and geometric factors,including the gas top blowing*(KTB) operation,was examined.The results indicated that the circulation rate of molten steel in the RH degasser can be fairly precisely calculated by the formula:Q1p=0.0333Qg^0.26Du^0.69Dd^0.80(t/min),where Qg-the lifting gas flow rate(NL/min);Du and Dd-the inner diameters of the up and down-snorkels(cm),respectively.The maximum value of circulation rate of molten steel in the case of the 30cm diameters either of the up-and down-snorkels for the RH degasser(the “Saturated” rate) is approximately 31 t/min.the corresponding gas flow rate is 900NL/min.Blowing gas into the vacuum chamber through the top lance like KTB operation does not markedly influence the circulatory flow and mixing charateristics of the RH process under the conditions of the present work.There exist a major loop and a large number of small vortices and eddies in the ladle during the RH refining process.A liquidliquid two-phase flow is formed between the descnding stream from the down-snorkel and the liquid around the stream.All of these flow situation and pattern will strongly influence and determine the mixing and mass transfer in the ladle during the refining.The correlation between the mixing time and the stirring energy density is τmε^0.50 for the RH degasser.The mixing time rapidly shortens with an increase in the lifting gas flowrate.At a same gas flow rate,the mixing times with the up-and down-snorkel diameters either of 6 and 7cm are essentially same.The 30cm diameters either of the up-and down-snorkels for the RH degasser would be reasonalble.The concentration-time curve showed that three circulation cycles are at least needed for complete mixing of the liquid steel in the RH degasser. 相似文献
16.
This paper discusses stress intensity factor (SIF) calculations for surface cracks in round bars subjected to combined torsion
and bending loadings. Different crack aspect ratios, a/b, ranging from 0.0 to 1.2 and relative crack depths, a/D, ranging from 0.1 to 0.6 were considered. Since the loading was non-symmetrical for torsion loadings, a whole finite element
model was constructed. Then, the individual and combined bending and torsion loadings were remotely applied to the model.
The equivalent SIF method, F*EQ, was then used explicitly to combine the individual SIFs from the bending and torsion loadings. A comparison was then carried
out with the combined SIF, F*FE, obtained using the finite element analysis (FEA) under similar loadings. It was found that the equivalent SIF method successfully
predicted the combined SIF for Mode I. However, discrepancies between the results determined from the different approaches
occurred when F
III was involved. It was also noted that the predicted F*FE using FEA was higher than the F*EQ predicted through the equivalent SIF method due to the difference in crack face interactions. 相似文献
17.
1 Introduction We consider the finite digraphs that do not havemultiple arcs or loops , but pairs of opposite arcs areallowed.LetD=(V,A) be a digraph , whereVisthe set of vertices andAis the set of arcs .If there isan arc (x,y) fromxtoy,thenthe vertexxis … 相似文献
18.
以福寿螺为研究对象,测定其在Zn^2＋浓度分别为0、1、2、5、10mg／L时,于不同暴露时间（12、24、48、96h）内,肝脏超氧化物歧化酶（SOD）、过氧化氢酶（CAT）和丙二醛（MDA）活性的变化．结果表明：低浓度（1、2mg／L）的Zn^2＋处理组在24h内对福寿螺肝脏SOD活性起到诱导作用;随着时间的延长活性逐渐减弱,在96h时下降至最低值,极显著低于对照组水平;而中、高浓度（5、10mg／L）处理组酶活性均低于对照组．在各浓度组的胁迫下,CAT活性在12h时升高到最大值,之后随着时间的延长均表现为下降趋势．MDA含量在整个试验期间均处于诱导状态,并且与重金属浓度成正比,在10mg／L96h时达到最大值．Zn^2＋会对福寿螺产生明显的氧化胁迫,SOD、CAT和MDA指标能有效评价Zn^2＋对福寿螺的氧化损伤． 相似文献
19.
《学校用计算机》2013,30(1-2):85-104
Summary PCLogo and Geometer's Sketchpad are powerful tools that may be used by mathematics teachers who want to integrate technology with geometry instruction in the elementary classroom. The purpose of this study was to examine the usefulness of PCLogo and Geometer's Sketchpad to stimulate thinking about geometric concepts in elementary age children. We used a collective case study design that included four cases, two girls (ages 8 and 10) and two boys (ages 10 and 11). All participants were trained to use PCLogo and Geometer's Sketchpad to construct geometric shapes and measure the attributes of the shapes. After the training, participants used these technologies as tools to stimulate thinking about geometric concepts. As a result of our observations of children's performances as they reasoned about geometric concepts, we developed a learning model for teaching children about geometry. 相似文献
20.
The flat dilatometer test (DMT) has the potential to be a useful tool in the evaluation of liquefaction potential of soils.
In practice, it is necessary to carefully examine existing DMT-based methods for evaluating liquefaction potential. We conducted
the DMT and cone penetration test (CPT) in high liquefaction potential areas to examine the existing DMT-based methods for
liquefaction potential evaluation. Specifically, the DMT and CPT were conducted side-by-side at each of six in-situ sites,
and thus it is feasible to utilize those test results to validate the existing DMT-based methods. The DMT parameter, horizontal
stress index (K
D), is used as an indicator for estimating liquefaction resistance of soils in terms of cyclic resistance ratio (CRR). The
analysis results revealed that the existing K
D-based liquefaction evaluation methods would overestimate the CRR of soils, which leads to overestimation of the factor of
safety against liquefaction. Also, the estimations of DMT-K
D values by using the CPT-q
c as well as the correlation between DMT-K
D and CPT-q
c proposed by the previous studies would be significantly smaller than field measurements. The results reflected that further
validation of the existing DMT-based methods for liquefaction evaluation is desirable. 相似文献