Statics, vibration, and buckling of sandwich plates with metamaterial cores characterized by negative thermal expansion and negative Poisson's ratio

This paper proposes a three-dimensional(3D)Maltese cross metamaterial with negative Poisson’s ratio(NPR)and negative thermal expansion(NTE)adopted as the core layers in sandwich plates,and aims to explore the relations between the mechanical responses of sandwich composites and the NPR or NTE of the metamaterial.First,the NPR and NTE of the metamaterial are derived analytically based on energy conservation.The effective elastic modulus and mass density of the 3D metamaterial are obtained and validated by the finite element method(FEM).Subsequently,the general governing equation of the 3D sandwich plate under thermal environments is established based on Hamilton’s principle with the consideration of the von Kármán nonlinearity.The differential quadrature(DQ)FEM(DQFEM)is utilized to obtain the numerical solutions.It is shown that NPR and NTE can enhance the global stiffness of sandwich structures.The geometric parameters of the Maltese cross metamaterial significantly affect the responses of the thermal stress,natural frequency,and critical buckling load.

A Novel Bi-Level VSC-DC Transmission Expansion PlanningMethod of VSC-DC for Power System Flexibility and Stability Enhancement

Investigating flexibility and stability boosting transmission expansion planning(TEP)methods can increase the renewable energy(RE)consumption of the power systems.In this study,we propose a bi-level TEP method for voltage-source-converter-based direct current(VSC-DC),focusing on flexibility and stability enhancement.First,we established the TEP framework of VSC-DC,by introducing the evaluation indices to quantify the power system flexibility and stability.Subsequently,we propose a bi-level VSC-DC TEP model:the upper-level model acquires the optimal VSC-DC planning scheme by using the improved moth flame optimization(IMFO)algorithm,and the lower-level model evaluates the flexibility through time-series production simulation.Finally,we applied the proposedVSC-DC TEPmethod to the modified IEEE-24 and IEEE-39 test systems,and obtained the optimalVSCDC planning schemes.The results verified that the proposed method can achieve excellent RE curtailment with high flexibility and stability.Furthermore,the well-designed IMFO algorithm outperformed the traditional particle swarm optimization(PSO)and moth flame optimization(MFO)algorithms,confirming the effectiveness of the proposed approach.

Warm hydroforming of magnesium alloy tube with large expansion ratio

Process of warm tube hydroforming was experimentally investigated for forming an AZ31B magnesium alloy tubular part with a large expansion ratio. Effects of temperature on the mechanical properties and formability were studied by uniaxial tensile test and hydraulic bulge test. Total elongation increases with temperature up to 250℃, but uniform elongation and maximum expansion ratio get the highest value at 175℃. Different axial feeding amounts were applied in experiments to determine the reasonable loading path. A preform with useful wrinkles was then realized and the tubular part with an expansion ratio of 50% was formed. Finally, mechanical condition to produce useful wrinkles is deduced and the result illustrates that useful wrinkles are easier to be obtained for tube with higher strain hardening coefficient value and tubular part with smaller expansion ratio.

Investigation on Noise Control Technology of Ultra-High Expansion Ratio Turbine

Aerodynamic noise control is one of the key technologies in the development of stealth technology of torpedo. The noise producing mechanism in an ultra-high expansion ratio turbine with partial admission were discussed. Wide wakes originating from non-outlet region between elliptical nozzles,circumferential non-uniformity owing to partial admission design and complex shock system formed in narrow space between nozzles and rotors are three main sources of aerodynamic noise in this turbine. Thus,two kinds of aerodynamic noise control methods were put forward,namely adding axial gap between the nozzle and rotor,nozzle-down,bilateral repair and trailing edge blowing. Steady annulus simulation results showed that the aerodynamic noise was significantly reduced by more than 10 dB after optimization,which had little impact on the aerodynamic performance of the turbine.

Experimental and Numerical Research on Deformation Behavior of Thin-Walled and Large Expansion Ratio Guide Vane Liner in Hydroforming Process

Some tube hydroforming process tests and further research work were conducted to manufacture hollow guide vane liners( made of super alloy GH3030).The relative thickness( t0/ OD) of the tubular blank is approximately 0. 01,and the maximum expansion ratio( Dmax/ OD) of the needed part is more than 40%,and the length to diameter ratio of the expansion regionis more than 3. 0. It is very hard to manufacture this kind of ultra-thin-wall,curved axis and large expansion ratio tubular part without fracture and wrinkles. The success of the process is highly dependent on useful wrinkles with appropriate internal pressure and axial feeding. A simplified finite element model and a theoretical model are used for detecting the deformation behavior and forming laws. Further study results demonstrate that the useful wrinkles do not appear at the same time and middle-wrinkles need bigger axial force than tube-end-wrinkles and feeding-wrinkles. The wrinkles can transfer bigger axial force after its wave peak has come into contact with the die inner surface. The thickness thinning rate of the element at the peak is bigger than that at the trough. With the increase of the axial and hoop stress ratio,the critical buckling stress also increases. Microstructure examination results show that the grain size in the maximum thinning zone has been stretched and refined after the large deformation and annealing treatment.The process is feasible and the finished part is qualified.

Development and evaluation of a hot-rolled 780 MPa steel sheet with an ultra-high expansion ratio

This paper explores the development of a 780 MPa hot-rolled high-strength steel with an ultra-high hole expansion ratio(HER) by using a nanoprecipitation-controlled technology.Systematic analysis and evaluation of an industrially produced steel sheet have been performed to investigate the microstructure, nanoprecipitates, tensile properties, HER,bendability, and forming limit diagram.The newly developed 780 MPa hot-rolled high-strength steel sheet is composed of a fully ferritic microstructure of approximately 5 μm with precipitates of approximately 4-5 nm in ferrite grain interiors.The yield strength and tensile strength can reach above 700 and 780 MPa, respectively.Moreover, the fractured elongation is higher than 19% in the transversal direction, and the average HER exceeds 70%.Furthermore, the newly developed 780 MPa high-strength steel has good bendability reaching R/t=0.2 at 90°.Compared with the conventional 780 MPa high-strength steel, the newly developed 780 MPa high-strength steel exhibits superior forming ability, which is suitable for the production of complex components.High-cycle fatigue indicates that the fatigue limit of the newly developed high-strength steel is 430 MPa under a stress ratio of r=-1,indicating good fatigue properties.The excellent combined mechanical properties of the newly developed 780 MPa high-strength steel are attributed to the grain-refined ferritic microstructure with nanoprecipitates in ferrite grain interiors.

An experimental study of salt expansion in sodium saline soils under transient conditions

Salt expansion in sulfate saline soils that are widely distributed in northwestern China causes serious infrastructural damages under low-temperature conditions. However, the mechanism of salt expansion under low temperatures is not clear. In this study, we conducted a series of cooling experiments combined with salt crystallization to study this mechanism, and employed an ionic model to calculate the supersaturation ratio of the solution. During the experiments, the strength and the process of salt expansion were examined under different cooling rates and various crystal morphologies. The relationship between temperature and supersaturation ratio under transient conditions was also considered. Results indicate that the initial supersaturation ratio of a sodium sulfate solution is closely related to environmental conditions, and that this ratio decreases with slowing the cooling rates and stabilizing the crystal forms. Higher initial supersaturation ratios lead to an increased non-steady-state zone, resulting in less salt expansion. On the other hand, chloride ion content has a distinct influence on the crystallization supersaturation ratio of the sodium sulfate solution, and higher chloride ion content can inhibit salt expansion in sodium saline soils. These findings help explain salt expansion mechanisms in complex conditions such as seasonally frozen soils, and thus help search for improved methods of preventing salt expansion in sulfate saline soils.

Experimental study on expansion and cracking properties of static cracking agents in different assembly states

Rolled static cracking agent(RSCA)can solve the intractable problem of traditional bulk static cracking agent(BSCA)in engineering applications.This paper innovatively studies the rational water-cement ratio of BSCA and the immersion soaking time of RSCA under the condition of controlling temperature.Through the expansion and cracking performance experiments,the development characteristics of expansion pressure,the cracking effect of the single-hole specimen and the performance of hole spraying prevention under the action of BSCA and RSCA were compared and analyzed.The results show that:(1)The volume growth rate of static cracking agent decreases with the increase of water-cement ratio,and the fluidity increases with the increase of water-cement ratio.The rational water-cement ratio for BSCA application is 0.3,and the rational immersion time of RSCA is 2-2.5 min;(2)Under the bore diameters of 30,35,40 and 45 mm,the expansion pressure of BSCA with a water-cement ratio of 0.3 is 38.2,52.3,61.5 and 68 MPa,and the expansion pressure of RSCA immersed in water for 2.5 min is 43.5,58.8,69.5 and 75.1 MPa,respectively.Among them,the development speed of expansion pressure of BSCA is higher than that of RSCA,and the arrival time of the peak expansion pressure of RSCA is 1.7 times that of BSCA;(3)The crack initiation speed of single-hole specimen under the action of RSCA is 10.3%lower than that under the action of BSCA,but the cracking speed of the former is 72.6%higher than that of the latter;(4)The hole spraying occurs in BSCA under the bore diameter of 50,55 and 60 mm,while the hole spraying occurs in RSCA under the bore diameter of 60 mm.In terms of bore diameter,the hole spraying prevention of the RSCA is better than that of BSCA.The research results enrich the static blasting technology and provide data support and theoretical reference for field application.

Factors influencing accuracy of free swelling ratio of expansive soil

Many geotechnical structures,such as the subgrade of high-speed railway,are extremely sensitive to micro deformations.As one of the most commonly used indexes in China to evaluate the potential swelling level of expansive soils,the effectiveness and accuracy of free swelling ratio should be highly required.However,due to the deficiency of geotechnical test regulations for the free swelling ratio test,non-negligible variation difference is often observed among the test results of the same type of soil samples.Thus,a series of laboratory tests are conducted to figure out the influences of soil particle size,initial soil temperature,and wet-dry circulation on the free swelling ratio of expansive soils.The results show that the initial soil temperature exerts an obvious influence on free swelling ratio and a relative weak influence on soil mass of expansive soil with the micro soil particle size(d<0.075 mm),and the correlation becomes unclear when soil particle size is within the range of 0.075 mm≤d<0.500 mm.A larger particle size of expansive soils induces a larger free swelling ratio and soil mass in the soil measuring cup regardless of initial soil temperature.However,the enlarging amplitude decreases as the particle size of expansive soils increases.There is a progressive enlargement of free swelling ratio at the first two wet-dry cycles and then it attenuates gradually when the number of wetdry cycles further increases.

Experimental research on the mixed sand ratio and initial dry density of weathered sand improved expansive soil free load swelling rate

In this paper, through the indoor free load swelling rate test, expansive soil in a section of a first- class highway reconstruction project in Yichang City was studied. It emphatically analyzed the interrelations among free load swelling rate, non-load time, the proportion of mixed sand and initial dry density. Experimen- tal studies have shown that： Free load swelling deformation is mainly divided into three stages of rapid expan- sion, slow expansion and final stability; when the initial dry density is constant, free load swelling rate of the weathered sand modified soil will reduce rapidly before they slow down with the increase of sand proportion, and weathered sand modified soil free load swelling rate is not sensitive to the large amount of sand mixed; in the same mixed sand ratio, weathered sand modified soil free load swelling rate increases rapidly with the in- crease of initial dry density, there is a good linear correlation between them. To take appropriate control of the initial dry density during the expansive soil subgrade construction helps to reduce its swelling deformation and ensures the stability of the embankment.

Experimental Study on the Expansion Behavior of Full Aggregate Steel Slag Concrete

To improve the utilization rate of steel slag,this paper investigates the expansion behavior of Full Aggregate Steel Slag Concrete(FASSC).Orthogonal experiment was performed to analyze the effects of sand ratio,water-cement ratio,steel slag sand content,replacement particle size of steel slag sand,coarse steel slag content,and replacement particle size of coarse steel slag on the expansion rate of FASSC.Test results demonstrated that the expansion rate of FASSC decreased with a gradual increase in the replacement particle size of steel slag sand or the coarse steel slag content.With a gradual increase in the water-cement ratio or the steel slag sand content,the expansion rate of FASSC first increased and then decreased.As the sand ratio increased,the expansion rate of FASSC first decreased and then increased.Among these,the steel slag sand content and the replacement particle size of steel slag sand significantly affected the expansion rate of FASSC.The mix proportions of SSC for self-stressing CFST and shrinkage compensating SSC were suggested according to the experimental research.Additionally,a formula for approximately predicting the expansion rate of FASSC was proposed based on the single factor weighted fitting.

Unsaturated expansive soil fissure characteristics combined with engineering behaviors

The relationship among the surface fissure ratio, moisture content, seepage coefficient and deformation modulus of field unsaturated expansive soil in Nanning, Guangxi Province, China, was obtained by a direct or indirect method. Digital images of expansive soil of the surface fissure with different moisture contents were analyzed with the binarization statistic method. In addition, the fissure fractal dimension was computed with a self-compiled program. Combined with in situ seepage and loading plate tests, the relationship among the surface fissure ratio, moisture content, seepage coefficient and deformation modulus was initially established. The surface fissure ratio and moisture content show a linear relation, ＂y=-0.019 1x＋1.028 5＂ for rufous expansive soil and ＂y=-0.07 1x＋2.610 5＂ for grey expansive soil. Soil initial seepage coefficient and surface fissure ratio show a power function relation, ＂y=1× 10^-9exp（15.472x）＂ for rufous expansive soil and ＂y=5× 10^-7exp（4.209 6x）＂ for grey expansive soil. Grey expansive soil deformation modulus and surface fissure ratio show a power fimction relation of ＂y=3.935 7exp（0.993 6x）＂. Based on the binarization and fractal dimension methods, the results show that the surface fissure statistics can depict the fissure distribution in the view of two dimensions. And the evolvement behaviors of permeability and the deformation modulus can indirectly describe the developing state of the fissure. The analysis reflects that the engineering behaviors of unsaturated expansive soil are objectively influenced by fissure.

The Hydration of Blended Cement at Low W/B Ratio

The hydration process,hydration product and hydration heat of blended cement paste mixed with mineral admixture and expansive agent at low W/B ratio are studied by XRD,thermo analysis,and calorimetry instrument,and they were compared with those of pure cement paste.The results show that pure cement and blended cement at low W/B ratio have the same types of hydration products,but their respective amounts of hydration products of various blended cements at same ages and the variation law of the amount of same hydration products with ages are different;The joint effect of tumefaction of gel-ettringite due to water absorption and the expansive pressure on the pore and rift caused by the crystalloid ettringite is the impetus of the volume expansion of cement paste,and the former effect is much greater than the latter one.

单组分湿固化型聚氨酯混合料的体积膨胀特性

为研究单组分湿固化型聚氨酯混合料的体积膨胀特性,提出了基于游标卡尺的体积膨胀特性表征方法,分析了体积膨胀后的宏微观形貌,研究了不同因素作用下体积膨胀的变化规律,以及体积膨胀对空隙率的影响,并提出了抑制体积膨胀的方法.结果表明:聚氨酯混合料会因空隙率较小,致使固化反应产生的二氧化碳气体难以完全逸出,在混合料内部会产生数量较多的宏微观气孔结构,1个马歇尔试样可产生高达1.5 mm的膨胀变形;提高压实温度、采用二次压实或添加氢氧化钙,均会显著降低混合料的体积膨胀,且对路用性能也有一定提升;增大胶石比,混合料的体积膨胀单调递增,空隙率则先减小后增大.为控制体积膨胀,建议压实温度为45~65℃,且胶石比不宜过大,添加氢氧化钙时,其质量分数不宜超过1%.

典型控流调压阀后突扩体的压力特性

为了研究突扩体对控流调压阀门的增压减振作用,以典型控流调压阀门(活塞式调流阀和固定式锥形阀)为研究对象,采用物模试验和三维数值模拟相结合的研究方法,重点对2类阀门在不同突扩体型、阀门开度及空化强度下的阀后流态及管壁压力特性进行了观测分析,探究突扩比与阀后稳定压力的定量关系,试验阀门通径为150 mm,阀门直径为150~370 mm,突扩体长度为1000 mm.结果表明,突扩体能提升阀后管壁时均压力、减弱压力脉动的基本原理是水垫层效应.综合物模与数模试验结果,拟合得到了突扩比与阀后管壁稳定时均压力的关系式.突扩体可以提升阀门的工作背压,提升程度随突扩比的增大而渐缓.当突扩比为4.00时,阀后管壁稳定时均压力的增幅已非常平缓,趋于稳定.不同阀型试验结果差异小,说明突扩体的增压减振作用具有普适性.研究结果可供输水调压工程阀门段管道系统设计参考借鉴.

扩底桩竖向承载特性及群桩效应研究

为探明扩底群桩竖向承载特性,采用扩底桩筏结构,开展单桩(扩径比分别为1.5、2.0、2.5、3.0)和桩筏(扩径比为2)基础静载模型试验,分析其荷载传递规律,并建立桩间距为3.75、4.00、4.50、5.00倍桩直径时扩底桩筏基础有限元模型,研究桩间距对群桩效应影响。结果表明:扩底单桩和扩底桩筏结构荷载沉降曲线均为缓变型;扩底桩极限承载力随扩径比增大而逐渐增大,扩径比2.5~3.0时极限承载力增幅变缓,建议扩底桩扩径比取2.5~3.0;由于桩土共同沉降,桩间土压缩,桩土作用更充分,扩底桩筏基础中心桩侧摩阻力荷载分担比比扩底单桩增大了17.71%;群桩效应系数随桩间距增大而逐渐增大,桩间距为4.5~5.0倍桩直径时群桩效应系数增幅较小,群桩效应较弱,建议扩底桩筏基础桩间距取值不小于4.5倍桩直径。

不同截面类型玻纤增强PA6的性能

对比研究了扁平玻纤及圆形玻纤增强尼龙6 (PA6)复合材料的力学性能、流动性能、收缩翘曲情况、摩擦性能及线膨胀系数(CLTE)。结果表明,随着玻纤含量增加,扁平玻纤及圆形玻纤增强PA6复合材料力学性能均增加,但熔体流动速率(MFR)降低;相同玻纤含量下,与圆形玻纤相比,扁平玻纤增强PA6复合材料的拉伸强度、弯曲强度及弯曲弹性模量较低,简支梁缺口冲击强度及MFR较高。在玻纤含量30%的条件下,不同扁平玻纤及圆形玻纤增强PA6复合材料性能存在差异,通过收缩率测试分析,扁平玻纤复合材料比圆形玻纤具有更低的收缩率及横纵收缩比,扁平比越高,横纵收缩比越低,翘曲度越小;通过MFR及注塑螺旋线测试,发现扁平玻纤复合材料具有更好流动性、更高流长比,扁平比越高优势越明显,有利于成型加工;通过滑动摩擦测试结果看出,扁平玻纤复合材料具有更低摩擦系数,随着扁平比增大,摩擦系数越低;通过热机械分析检测,扁平玻纤复合材料在垂直流动方向CLTE低于圆形玻纤,且流动方向和垂直流动方向CLTE差异也较小。扁平玻纤增强材料能改善传统圆形玻纤增强体系的一些不足,优化产品性能。

基于NOMA-OFDM的雷达通信一体化波形设计

针对基于正交频分复用的雷达通信一体化波形设计中,通信信息的随机性会影响雷达性能以及过高峰均包络比(PAPR)可能导致信号在射频端出现严重失真的问题,文中提出了利用非正交多址将雷达信号与通信信号在功率域中分开,以降低通信信息随机性对雷达性能的影响,并利用主动星座图扩展(ACE)对叠加信号进行PAPR抑制的波形设计方法。首先,分析了传统功率分配原则导致叠加星座出现混叠的原因,并给出在不同调制阶数的最佳功率分配;其次,从理论上分析了ACE技术对叠加信号进行PAPR抑制的可行性;最后,仿真结果表明,在不同调制方式和不同信噪比下,当功率分配最佳时能同时达成雷达信号高检测率和通信符号最低误码率。ACE技术的使用同时保证了优良的雷达性能和通信端的低误码率,对叠加信号的PAPR有较好的抑制效果。

CFB飞灰压浆料的制备及高强微膨胀机理研究

利用循环流化床燃煤固硫灰(CFB飞灰)的活性与膨胀性制备压浆料,研究了CFB飞灰掺量和水胶比对压浆料工作性能、强度和膨胀率的影响;CFB飞灰与矿粉以不同比例复配,对比研究了CFB飞灰压浆料的高强微膨胀特性,并采用XRD与SEM分析验证。结果表明,随CFB飞灰替代水泥掺量的增加,压浆料的需水性增强,抗压强度降低,20%(质量分数)CFB飞灰的试样28 d抗压强度与基样接近,可达85.8 MPa,而抗折强度先升高后降低,膨胀率一直升高。随用水量的增加,压浆料的膨胀率降低但均为正值。随CFB飞灰取代矿粉率的提高,压浆料的工作性能降低,28 d抗压强度先升高后降低,其中20%(质量分数)CFB飞灰、10%(质量分数)矿粉复合的试样28 d抗压强度最高,为99.8 MPa,膨胀率一直升高。通过CFB飞灰与矿粉复合配制了各项性能均满足铁标要求的压浆料。CFB飞灰不仅自身水化产生膨胀,还能激活矿粉活性,产生复合增强作用。

基于响应面法的膨胀性充填体强度演化规律及配比优化

针对某铁矿采场充填接顶率低和接顶效果差的问题,通过添加复合膨胀剂可实现采场主动接顶.采用响应面法,借助Design-Expert软件中的Box-Behnken design(BBD)方法,开展了膨胀性充填体强度配比试验,研究了料浆质量分数、胶固粉质量分数和复合膨胀剂质量分数对膨胀性充填体抗压强度的影响规律.结果表明,单一因素及各因素间交互作用对膨胀性充填体抗压强度均有显著影响,其中胶固粉质量分数影响最大,复合膨胀剂质量分数次之,料浆质量分数最小;料浆质量分数与胶固粉质量分数的交互作用对充填体的早、中期抗压强度影响最为显著,胶固粉质量分数与复合膨胀剂质量分数的交互作用对充填体的后期抗压强度影响最为显著.通过目标规划法确定了膨胀性充填体最优配比:料浆质量分数为69%,胶固粉质量分数为10%,复合膨胀剂质量分数为3×10^(-4).利用X射线衍射(XRD)和扫描电镜(SEM)分析发现,复合膨胀剂可使充填体的内部结构疏松而引起膨胀,提高充填接顶率.