Machine learning-based stiffness optimization of digital composite metamaterials with desired positive or negative Poisson's ratio

Mechanical metamaterials such as auxetic materials have attracted great interest due to their unusual properties that are dictated by their architectures.However,these architected materials usually have low stiffness because of the bending or rotation deformation mechanisms in the microstructures.In this work,a convolutional neural network(CNN)based self-learning multi-objective optimization is performed to design digital composite materials.The CNN models have undergone rigorous training using randomly generated two-phase digital composite materials,along with their corresponding Poisson's ratios and stiffness values.Then the CNN models are used for designing composite material structures with the minimum Poisson's ratio at a given volume fraction constraint.Furthermore,we have designed composite materials with optimized stiffness while exhibiting a desired Poisson's ratio(negative,zero,or positive).The optimized designs have been successfully and efficiently obtained,and their validity has been confirmed through finite element analysis results.This self-learning multi-objective optimization model offers a promising approach for achieving comprehensive multi-objective optimization.

Diagnostic value of FIB-4, aspartate aminotransferaseto-platelet ratio index and liver stiffness measurement in hepatitis B virus-infected patients with persistently normal alanine aminotransferase

AIM To assess the diagnostic value of FIB-4, aspartate aminotransferase-to-platelet ratio index(APRI), and liver stiffness measurement(LSM) in patients with hepatitis B virus infection who have persistently normal alanine transaminase(PNALT).METHODS We enrolled 245 patients with chronic hepatitis B: 95 in PNALT group, 86 in intermittently elevated alanine transaminase(PIALT1) group [alanine transaminase(ALT) within 1-2 × upper limit of normal value(ULN)], and 64 in PIALT2 group(ALT > 2 × ULN). All the patients received a percutaneous liver biopsy guided by ultrasonography. LSM, biochemical tests, and complete blood cell counts were performed.RESULTS The pathological examination revealed moderate inflammatory necrosis ratios of 16.81%(16/95), 32.56%(28/86), and 45.31%(28/64), and moderate liverfibrosis of 24.2%(23/95), 33.72%(29/86), and 43.75%(28/64) in the PNALT, PIALT1, and PIALT2 groups, respectively. The degrees of inflammation and liver fibrosis were significantly higher in the PIALT groups than in the PNALT group(P < 0.05). No significant difference was found in the areas under the curve(AUCs) between APRI and FIB-4 in the PNALT group; however, significant differences were found between APRI and LSM, and between FIB-4 and LSM in the PNALT group(P < 0.05 for both). In the PIALT1 and PIALT2 groups, no significant difference(P > 0.05) was found in AUCs for all comparisons(P > 0.05 for all). In the overall patients, a significant difference in the AUCs was found only between LSM and APRI(P < 0.05).CONCLUSION APRI and FIB-4 are not the ideal noninvasive hepatic fibrosis markers for PNALT patients. LSM is superior to APRI and FIB-4 in PNALT patients because of the influence of liver inflammation and necrosis.

Debris flow impact on flexible barrier: effects of debrisbarrier stiffness and flow aspect ratio

Conventionally,flexible barriers are rated based on their ability to resist a free-falling boulder with a particular input energy.However,there is still no well-accepted approach for evaluating performance of flexible barrier under debris flow impact.In this study,a large-nonlinear finite-element model was used to back-analyze centrifuge tests to discern the effects of impact material type,barrier stiffness,and flow aspect ratio(flow height to flow length)on the reaction force between the impacting medium and flexible barrier.Results show that,in contrast to flexible barriers for resisting rockfall,the normal impact force induced by the highly frictional and viscous debris is insensitive to barrier stiffness.This is because the elongated distributions of kinetic energy are mainly dissipated by the internal and boundary shearing,and only a small portion is forwarded to the barrier.Furthermore,a new stiffness number is proposed to characterize the equivalent stiffness between a debris flow or a boulder,and a flexible barrier.Under the circumstance of an extremely elongated debris flow event,i.e.,low aspect ratio,the load on a barrier is dominated by the static component and thus not sensitive to the barrier stiffness.

Stiffness Distribution and Aeroelastic Performance Optimization of High-Aspect-Ratio Wings

The relationship between stiffness distribution and aeroelastic performance for a beam-frame model and a3-D model is investigated based on aeroelastic optimization of global stiffness design for high-aspect-ratio wings.The sensitivity information of wing spanwise stiffness distribution with respect to the twist angle at wing tip,the vertical displacement at wing tip,and the flutter speed are obtained using a sensitivity method for both models.Then the relationship between stiffness distribution and aeroelastic performance is summarized to guide the design procedure.By using the genetic/sensitivity-based hybrid algorithm,an optimal solution satisfying the strength,aeroelastic and manufacturing constraints is obtained.It is found that the summarized guidance is well consistent with the optimal solution,thus providing a valuable design advice with efficiency.The study also shows that the aeroelastic-optimization-based global stiffness design procedure can obtain the optimal solution under multiple constraints with high efficiency and precision,thereby having a strong application value in engineering.

Measurement of Vibrational Stiffness and Air Damping of Damselfly Wings

A simple cantilever beam vibration test method made of biomorph and insect wing, were used to measure the vibrational stiffness and the air damping of insect wings. Vibration tests were performed in vacuum pressures to atmosphere and the wing stiffness and air damping factor were measured. The test method was found to be a viable method for measuring wing stiffness, natural frequencies and mode shapes. The vibrational deformation of the insect wings was found to be combination of bending and torsion because of unsymmetrical geometry of wing. The measured stiffness (K) of damselfly wings varied from 0.18 to 0.31 N/m and the air damping ratio ranged from 0.72 to 0.79. The undamped natural frequency (fn) at 13 kPa varied from 249 to 299 Hz and at atmosphere it varied from 168 to 198 Hz.

Variable stiffness control algorithm for high-rising buildings of closely spaced frequencies based on wavelet transforms

To establish the algorithm of SAT-TMD system with the wavelet transform(WT),the modal mass participation ratio is proposed to distinguish if the high-rising structure has the characteristic of closely distributed frequencies.A time varying analytical model of high-rising structure such as TV-tower with the SAT-TMD is developed.The proposed new idea is to use WT to identify the dominant frequency of structural response in a segment time,and track its variation as a function of time to retune the SAT-TMD.The effectiveness of SAT-TMD is investigated and it is more robust to change in building stiffness and damping than that of the TMD with a fixed frequency corresponding to a fixed mode frequency of the building.It is proved that SAT-TMD is particularly effective in reducing the response even when the building stiffness is changed by ±15%;whereas the TMD loses its effectiveness under such building stiffness variations.

一种小动静比橡胶隔振器仿真与试验

通过隔振器内部结构设计,在保证承载能力的同时降低橡胶隔振器的动态刚度。开展橡胶隔振器的静态变形、在给定静变形下的动态仿真分析,表明该隔振器在静态力-位移曲线上具有负斜率特性,对应隔振器内部结构的失稳变形;进行隔振器的静压试验、动态试验和隔振系统中的振动传递试验,结果表明,静态力-位移曲线试验结果呈现负斜率特性,在静态曲线上不同工作点处的固有频率呈现负斜率处最小的特性。

基于管柱动力学的特深井通井组合刚度评价方法与应用

我国特深井钻井正处于快速发展阶段,受井眼尺寸大和深度深的影响,钻井过程中极易出现微井斜、扩径缩径和井壁台阶等,导致套管下入困难,因此通井钻具组合评价尤为重要。目前,现场使用的评价方法常忽略了管柱下入过程中与井壁的碰撞和变形,导致通井效果时好时坏。为了辅助现场施工,文章基于全井管柱动力学模型和HHT-α数值计算方法提出了一种能够模拟管柱下入的动态过程的通井钻具组合的评价方法,该方法考虑了实测井眼轨迹、井眼扩大率和下入阻力等要素。通过某特深井三开下套管前的钻具组合进行实例计算分析,表明井眼扩大率和套管串下入阻力对通井能力和套管下入影响较大,当井眼扩大率较小且套管串下入阻力较大时,需要使用刚度较大的多扶正器通井钻具组合。本方法可以实现实测井况条件下比较精准的刚度对比模拟,也成功指导了案例井?486 mm套管的安全下入。

考虑永久位移的锚索框架减震锚头弹簧组件合理刚度研究

强震作用下预应力框架锚索可能出现内锚段松脱、锚索拉断等震害,在锚头处设置弹簧是一种新型抗震措施,而弹簧刚度的合理选取对改善锚索受力至关重要。建立在锚头处设置弹簧预应力锚索框架的加固基岩-覆盖层边坡三维数值模型,研究边坡在不同峰值加速度、不同持时地震波作用下响应规律,调整锚索-弹簧串联体系等效刚度大小,分析坡体永久位移和锚索轴力减载比随弹簧刚度的非线性变化特征;以控制边坡位移及锚索减载效果为目标,提出弹簧组件的合理刚度确定方法。研究表明:随弹簧刚度降低,缓冲减震作用逐渐显著;坡顶水平加速度受刚度变化影响较小,但当弹簧刚度低于临界值后边坡位移及弹簧变形量急剧增加;以边坡永久位移实际调查经验限值为首要控制条件,结合位移、弹簧峰值行程随刚度变化拟合“直-曲分界点”曲线,以共同确定弹簧刚度下限;同理,依据减载比拟合曲线轴力削减明显区段得出刚度上限,以保证一定工程经济性。针对算例模型取永久位移10 cm、拟合曲线曲率k小于0.002 k max作为直曲分界判断依据,得0.4 g~0.6g强震下弹簧刚度区间为(2.5,3.8)kN/mm,研究方法可为边坡预应力锚固工程抗震设计提供参考。

减振制品用天然橡胶在存储过程中结构与性能的演变

采用长期储存和加速老化相结合的方法对减振制品用天然橡胶(NR)在存储过程中结构与性能的演变进行研究。结果表明,随着老化温度的升高和老化时间的延长,NR硫化胶的硬度、100%定伸应力、300%定伸应力、储能模量和损耗模量均逐渐增大且总体变化趋势与交联密度的变化趋势相近,拉伸强度和扯断伸长率的变化率均呈现整体下降趋势,而损耗因子则迅速降低并趋于稳定,动静刚度比呈现先降低后缓慢上升的趋势;热空气老化对NR硫化胶减振性能的影响较小。

斜齿轮时变啮合刚度改进算法及影响因素分析

在势能法基础上,基于切片积分原理,考虑齿根过渡曲线方程,提出一种改进的斜齿时变啮合刚度计算方法。该方法考虑了真实齿根过渡曲线参数方程,修正了渐开线齿廓的积分区间,与有限元方法的对比结果验证了算法的有效性,减小了时变啮合刚度的计算误差。在此方法基础上,分析了齿宽、螺旋角、齿数和模数对时变啮合刚度的影响。结果表明,时变啮合刚度均值受齿宽影响较大,近似成线性关系;受螺旋角、齿数影响较小;螺旋角增大,均值以较小幅度波动性变化;中心距一定时,齿数增大,时变啮合刚度缓慢增大;齿轮参数改变会影响重合度;轴向重合度为整数时,时变啮合刚度波动值较小。

炭黑N330用量对天然橡胶阻尼性能的影响

研究炭黑N330用量对天然橡胶(NR)阻尼性能的影响。结果表明:随着炭黑用量的增大,NR混炼胶的FL,Fmax和Fmax-FL增大,t90缩短;NR硫化胶的邵尔A型硬度、100%定伸应力、300%定伸应力、拉伸强度和撕裂强度增大,拉断伸长率和回弹值减小;NR硫化胶低温下的损耗因子(tanδ)峰值减小,高温下的tanδ呈增大趋势。随着温度的升高,NR硫化胶的tanδ减小,且炭黑N330用量大的NR硫化胶的tanδ减小速率大于炭黑N330用量小的NR硫化胶,表明高温下炭黑N330用量大的NR硫化胶的阻尼性能对温度变化更敏感。随着炭黑N330用量的增大,NR硫化胶的静刚度和动/静刚度比都增大,表明炭黑N330用量增大对NR硫化胶的阻尼性能有提升作用。

基于调研数据的部分斜拉桥设计关键参数研究

针对部分斜拉桥设计关键参数缺少足够规范指导的问题,通过对国内外的部分斜拉桥进行设计主要参数调研统计,并与现有相关规范、科研成果进行对比研究,讨论了部分斜拉桥各型结构体系适用性、拉索布置形式,总结给出拉索倾角和间距、边主跨比、塔跨比、索梁刚度比和无索区长度等重要设计参数取值范围。结果表明,拉索倾角一般为11°~35°,主梁上拉索间距一般采用3.5 m~8 m,塔上的拉索间距一般采用0.5 m~1.2 m;3跨布置时,边主跨比一般集中在0.41~0.70,塔跨比宜采用1/3~1/13,索梁刚度比宜小于30;主跨跨中无索区长度与跨径之比宜取0.06~0.21。

高速铁路无砟轨道斜拉桥刚度理论分析与设计参数研究

为研究高速铁路无砟轨道斜拉桥的刚度特征以及合理结构设计参数,采用理论公式推导、数值分析等手段,得到结构设计参数刚度影响解析公式及其影响规律,用于指导桥梁设计。首先引入索梁活载刚度比的概念,通过建立斜拉桥主梁多点支承连续梁的三弯矩方程得到主梁刚度,通过考虑主塔变形和拉索伸长量得到斜拉索等效刚度,进而给出斜拉桥整体刚度、索梁活载刚度比和主梁跨中挠度的理论公式,并基于理论公式得出主梁惯性矩、斜拉索倾角、塔高以及拉索面积对斜拉桥整体刚度的影响。以广湛铁路东平水道特大桥主桥(67.5+60+60+350+60+60+67.5)m钢箱混合组合梁无砟轨道斜拉桥为例,通过对比跨中活载位移理论解与数值解验证本文理论公式的准确性,进而采用理论公式分析结构设计参数对桥梁刚度的影响。研究结果表明:随拉索倾角增大或者塔高增加,主梁的跨中位移先减小后增大,索梁活载刚度比先增大再减小,拉索倾角为29.2°~73.9°时主梁跨中位移达到最小值36.4 cm,此时索梁活载刚度比为15.4。随着主梁的梁高增加,主梁跨中位移减小,索梁活载刚度比减小,但是减小幅度较为缓慢。拉索面积增大,主梁跨中活载位移减小,索梁活载刚度比增加。提出了桥梁合理梁高、塔高和斜拉索规格及拉索面积。桥梁结构静动力检算、桥上轨道静态高低不平顺验算以及无砟轨道受力与层间压缩量等检算结果均表明,该桥具有良好的安全性和行车平稳性。

大跨度铁路连续梁拱组合桥梁结构拱梁合理刚度比研究

大跨度连续梁拱组合桥梁具有良好的受力性能与经济效应,其中,拱梁刚度比是该类桥型的1个重要设计参数,而以往对拱梁刚度比的研究不够深入和全面。为了解拱梁刚度比对大跨度铁路连续梁拱组合桥梁的影响,以正在建设的福州乌龙江大桥为例,研究拱梁刚度比对桥梁的结构受力特性、梁拱荷载分担和工程量的影响。结果表明:拱梁刚度比对拱肋轴力及主梁、拱脚和拱顶处弯矩影响显著。当刚度比<1:73或>1:44时弯矩最大增长接近50%,拱梁刚度比的增加,更有利于发挥拱肋的性能;但在拱梁刚度比≥1:44时,拱肋刚度增加对整体受力性能提升有限,而当拱梁刚度比≤1:73时,主梁承受活载增长过快,增加了结构发生局部破坏或整体倒塌的风险。综合考虑,此类桥型拱梁刚度比合理取值范围应在1:44~1:73。工程中为了获得更好的经济效应,可适当减小主梁刚度或拱肋刚度。

地震作用下钢筋混凝土圆形空心墩有效刚度研究

基于52个圆形空心墩拟静力试验数据及等效屈服位移三分量模型,分析了既有有效刚度公式对圆形空心墩的适用性;讨论了圆形空心墩有效刚度的主要影响因素,以轴压比、纵筋率、剪跨比及空心率为控制因素,标定了圆形空心墩有效刚度计算公式;并通过实尺寸及缩尺寸桥墩滞回曲线的模拟验证了建议公式的有效性。研究表明:绝大多数有效刚度既有公式在平均意义上严重高估了圆形空心墩的有效刚度,且变异系数较大;圆形空心墩有效刚度随轴压比、纵筋率和剪跨比的增大而增大,随空心率的增大而减小,剪跨比与空心率的耦合效应可通过等效剪跨比考虑,且有效刚度随等效剪跨比的增大而增大;建议公式给出的有效刚度计算值与试验值之比的均值及变异系数分别为1.04和0.21,与既有有效刚度公式相比,均值更合理而且离散性更小,提高了圆形空心墩有效刚度的预测精度;建议公式计算的有效刚度能较准确地模拟实尺寸及缩尺寸圆形空心墩拟静力滞回曲线,及相应的首次屈服刚度、加卸载刚度。

轮胎碎片对黏土剪切特性影响研究

【目的】为解决黏土工程性质差和废旧轮胎利用率低的问题,【方法】将轮胎碎片加入黏土中,开展轮胎碎片-黏土界面直剪试验,探究了轮胎碎片含量对黏土剪切性能的影响。随后进行了一系列循环剪切试验,分析不同轮胎碎片含量、竖向应力和剪切幅值对轮胎碎片混合土剪切界面剪切刚度和阻尼比的影响。【结果】结果表明,轮胎碎片的掺入可以增大黏土抗剪强度、内摩擦角、阻尼比和剪切刚度,但会降低黏聚力。其中,5%轮胎碎片含量混合土具有最优的抗剪强度和较好的剪切刚度与阻尼比。竖向应力和剪切幅值对混合土剪切界面剪切刚度和阻尼比影响较大,且随着竖向应力和剪切幅值的增大,混合土剪切界面剪切刚度增大,阻尼比下降。【结论】结果证明,轮胎碎片的掺入可以使黏土呈现一定的隔震减震性能,但何种轮胎碎片掺量能使黏土具有相对较优的抗剪强度和减震性能,以及轮胎碎片的加筋作用机理还需进一步研究。

某388m超高层建筑的侧移刚度与整体稳定性分析

通过某带伸臂桁架和环带桁架加强层的边筒-周边框架体系超高层结构工程设计实例,讨论了高层建筑结构侧移刚度和整体稳定性分析方法。指出在计算结构侧移刚度时应考虑楼板面内有限刚度、地下室结构构件变形、混凝土抗侧向力结构构件开裂刚度折减及二阶效应,从而得到更准确的结构侧移变形和最低阶整体屈曲荷载因子。提出了利用弯矩-曲率分析方法,并根据构件实际受力状态,迭代计算混凝土抗侧向力结构构件的开裂刚度折减系数。比较了中国、欧盟及美国规范关于整体稳定性分析的计算方法,提出在进行整体稳定性分析时,应考虑各种不利因素,从而准确评估二阶效应。讨论了高层建筑在正常使用极限状态下层间位移角限值的取值,并介绍了美国规范对层间位移角的计算方法和限值的建议。详细论述了当前中国规程中关于侧移刚度分析的不足并提出建议,以期为相关类似工程设计提供参考。

某部分框支剪力墙结构等效刚度比控制分析

转换层下部结构刚度对部分框支剪力墙结构的竖向刚度规则性和地震剪力传递突变程度有较大影响,等效刚度比是衡量转换层下部结构相对刚度的重要设计指标。本文对8度区某部分框支剪力墙结构等效刚度比的控制进行分析讨论。首先,通过转换层下部结构高度线性插值对等效刚度比进行修正,其次,通过调整转换层层高和转换层下部结构剪力墙厚度两种方式,分析不同等效刚度比对结构整体指标、构件受力和结构整体抗震性能的影响。结果表明,等效刚度比越大,转换层与其上一层的层间位移角比和有害层间位移角比越小,等效刚度比小于1.0时,转换层与其上一层的有害位移角比突变增大;设置转换层对转换层以上3层的地震剪力分配产生了较大影响;框支框架的相对刚度比越大,水平力传递突变程度越小;随着等效刚度比增加,结构主要屈服部位由结构底部转移至转换层以上部位。

隔震支座布置对地铁上盖多塔楼隔震结构的抗震性能影响分析

隔震支座的布置方式影响整隔震层的刚度,从而影响隔震效果。基于铅芯橡胶支座刚度占比和偏心率,确定了隔震支座的布置方式,研究了不同隔震支座布置方式对地铁上盖多塔楼隔震结构地震响应的影响。研究了隔震结构的减震系数、楼层加速度、层间位移角、楼层位移和楼层剪力的变化规律。研究结果表明,铅芯橡胶支座刚度占比在一定范围内变化,对于隔震层及盖上结构,减震系数增大,楼层加速度、楼层位移、层间位移角和楼层剪力减小;对于盖下结构,减震系数和楼层剪力增大,楼层加速度和层间位移角减小,楼层位移几乎不变;偏心率在一定范围内变化,对于隔震层及盖上结构,减震系数和楼层加速度增大,楼层位移、层间位移角和楼层剪力减小;对于盖下结构,减震系数增大,层间位移角和楼层剪力减小;楼层加速度和楼层位移几乎没有变化。