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.

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。

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。

防屈曲支撑框架抗震性能影响因素的模拟分析

本文旨在研究防屈曲支撑(BRB)平面布置方式和抗侧刚度比两种综合因素对防屈曲支撑钢筋混凝土框架(BRBF)的抗震性能影响,以及框架中BRB的合理布置方案。通过采用OpenSees软件建立了4、8和12层的BRBF空间模型,并进行双向地震作用下的结构弹塑性时程分析,对比分析3种BRB布置方式与5种抗侧刚度比的组合工况下结构抗震性能,基于分析结果综合判定BRB最优布置方案。结果表明:在外框架、内框架和全框架的3种支撑布置方式中地震损伤程度的控制效果分别可达30%、40%和60%,且效果随抗侧刚度比的增加而递增;BRB的设置显著增加了底柱的最大轴力,其中全框架布置方式在3种支撑方式中表现出最优的降低结构竖向反应的效果。BRB最优布置方案组合为内框架布置与刚度比1.0和全框架布置与刚度比2.5。研究成果可为每层采用相同BRB布置方式的规则钢筋混凝土框架结构的加固设计提供参考。

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。

串列联动双圆柱流致摆振试验研究

提出了串列联动双圆柱流致摆振理念,构建了摆振运动方程,分析了受力与运动规律,建立了摆振试验装置,并基于室内试验探究了串列联动双圆柱流致摆振特性,研究旨在为流致振动发电设备设计提供参考。研究表明:串列联动双圆柱摆振响应表现为涡激振动规律;间距比对摆振振幅和锁定区间影响较大,当间距比为4时振幅最小且响应中出现“再增长区”,当间距比为6时振幅最大,当间距比为8时锁定区间最大;系统刚度对摆振响应趋势影响不大,但不同刚度下摆振达到的最大振幅差异明显,该试验最大弧长比A^(*)为0.98,出现在1 400 N/m刚度工况。

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。

大重合度直齿轮动载特性分析

针对大重合度直齿轮啮合齿数与普通直齿轮间的差异性变化,以一对大重合度直齿轮和普通直齿轮为研究对象,通过有限元法求解齿轮时变啮合刚度,建立大重合度直齿轮啮合耦合型动力学模型,利用Newmark-β时域积分求解齿轮系统动态响应,对其进行时频域分析并与普通直齿轮进行对比,获得了大重合度直齿轮在不同转速、误差影响下的动载系数变化规律。结果表明,大重合度直齿轮啮合刚度与普通齿轮有较大差异,其动态响应结果波动幅值较普通直齿轮有明显降低,提升了系统稳定性;大重合度直齿轮动载系数与普通直齿轮相比有明显下降,并且随着转速的升高和误差的增大出现不同程度的增加,其中在共振转速区域范围内增幅明显。

变位设计对斜齿轮系统动态特性影响研究

为了研究不同变位设计下斜齿轮的动态特性,以标准斜齿轮和变位斜齿轮为对比研究对象,基于拉格朗日动力学建立了斜齿轮传动系统动力学模型;采用解析法求解变位斜齿轮时变啮合刚度,分析了不同变位系数对啮合刚度的影响;将不同变位设计下的斜齿轮时变啮合刚度与动力学模型结合,分析了不同变位设计对斜齿轮系统动态特性的影响。结果表明,斜齿轮正传动修正能够减小时变啮合刚度,从而减小啮合力;负传动修正能够增大时变啮合刚度,从而增大啮合力。研究可为后续斜齿轮传动的设计和优化提供一定的参考。

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。

双向循环荷载作用下胶-砂粒径比对橡胶砂剪切特性影响

橡胶砂作为一种新型廉价的材料,常被用作路基、挡墙等的填料,然而鲜有研究涉及其在双向循环荷载作用下的动力学特性。采用大型动态直剪仪对不同胶-砂粒径比的橡胶砂进行了一系列双向循环荷载作用下的直剪试验,研究了粒径比、法向循环荷载幅值、水平剪切频率对橡胶砂剪切特性的影响。研究结果表明:法向循环荷载作用下,橡胶砂的循环峰值剪切应力随粒径比的增大而增大,最终剪缩量随着粒径比的增大而减小;在剪切应力上,大粒径比的橡胶砂对法向循环荷载幅值的变化更加敏感,且随着法向循环荷载幅值的增大,大粒径比的橡胶砂剪胀更明显;在不同的水平剪切频率下,试样的剪切应力随剪切时间呈不同形式的周期性变化,最大循环剪切应力随水平剪切频率的增大而减小;橡胶砂的剪切刚度和阻尼比均与粒径比呈正相关,即大粒径比的橡胶砂更能提高路基在循环荷载作用下的稳定性。

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。

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。

混凝土结构设计中常见的稳定问题

混凝土结构的稳定问题受重视程度不及钢结构,本文对比中国、欧洲、美国和新西兰的混凝土设计规范中关于结构整体稳定、构件稳定的条文,并结合示例和公式推导,给出相关设计建议。对于整体稳定,刚重比数值受侧向合力位置、建筑重力大小及位置的影响,不应简单地以刚重比的数值来评判不同建筑的整体稳定性。对于构件稳定,分析了几个常见而易被忽视的问题。穿层柱及支撑连桥的框架柱宜考虑二阶效应;对于大跨度梁和长悬臂梁可借鉴外国规范,控制梁的高宽比和侧向无支撑长度;通过数值屈曲分析来反算柱子计算长度时,应正确施加荷载;国内对墙肢的稳定限制较严,对比墙肢的稳定等效荷载与轴压比限值荷载,给出了楼梯间外墙和L形剪力墙的截面由稳定控制的临界高度。