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通讯作者:

张小龙,E-mail:xiaolongzhang@stdu.edu.cn

中图分类号:O342

文献标识码:A

文章编号:1672-6553-2023-21(7)-028-010

DOI:10.6052/1672-6553-2023-086

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目录contents

    摘要

    负泊松比超材料结构作为一种新型智能材料与结构,精确计算超材料结构在大应变下的非线性力学性能对其在工程中的潜在应用具有重要意义.本文在弧形内凹负泊松比结构中加入直杆,设计了一类直杆增强型直曲耦合内凹超材料结构;利用能量法推导出了曲边内凹蜂窝结构的横/纵向等效泊松比与等效弹性模量的解析表达式,讨论结构各参数对结构等效泊松比与等效弹性模量的影响.考虑几何非线性大变形,建立了曲边内凹负泊松比结构的有限元模型,并与线性模拟结果对比,验证了解析表达式的正确性.结果表明,等效泊松比与等效弹性模量均随变形增大而变化,且变形越大差异越明显,大变形下须考虑几何非线性;利用谐响应分析计算结构的加速度级和加速度振级落差,凸显所设计超材料结构的减振性能;分析结构整体减振性能,发现其随层数增加逐渐增大;不随频率变化,在低频范围内对激励产生的响应能够起到抑制作用.因此,合理的设计超材料微结构对结构的低频振动具有很好的抑制作用,对负泊松比超材料减振结构设计具有一定的参考意义.

    Abstract

    As a new type of smart materials and structures, the accurate prediction of the nonlinear mechanical properties of metamaterial structures with negative Poisson's ratio under large strains is of great significance for their potential applications in engineering. In this paper, a class of metamaterial structures enhanced by straight struts are designed by adding straight struts to ratio arc-shaped structures with negative Poisson's ratio. The analytical expressions of the transverse/longitudinal equivalent Poisson's ratio and equivalent elastic modulus of the concave honeycomb structure are derived by energy method. The effects of structural parameters on the equivalent Poisson's ratio and equivalent elastic modulus of the structure are discussed. Considering the geometric nonlinearity under large deformation, a finite element model of re-entrant arc-shape structure with negative Poisson's ratio is established. The correctness of the analytical expression is verified by comparing with the linear simulation results. The results show that the equivalent Poisson's ratio and equivalent elastic modulus both varies with the increase of deformation. The differences increase with the increased deformation. Therefore, geometric nonlinearity analysis must be considered in the case of large deformation. The vibration reduction performance of the designed metamaterial structure is calculated by using the harmonic response analysis. The analysis shows that the overall damping performance of the structure increases gradually with the increase of the number of layers. The overall vibration reduction performance of the structure does not change with the frequency, and the response to excitation can play a very good inhibitory effect in the low frequency range. Therefore, the rational design of metamaterial microstructure has a good effect on the low frequency vibration suppression of structures, and has a certain reference significance for the design of vibration reduction metamaterial with negative Poisson's ratio.

  • 引言

  • 力学超材料是一类人工设计的具有反常力学性能的新结构/材料,通过微观结构设计可获得优异的力学性能[1].具有拉胀效应的负泊松比超材料由于具有优异的可设计性、能量吸收性能、抗断裂、抗冲击等性能,在车辆工程、防护设备、航空航海等工程领域展现出广泛的应用前景[23].

  • 典型的负泊松比超材料,包括星型结构、箭头形、内凹多边形、弧形内凹等[4-6]获得了大量研究.基于以上基本基元结构范式的组合和创新设计,多种复合型负泊松比超材料构型不断的被提出[7-9].学者对其平面弹性、静力学特性和动态力学性能开展了充分研究.如Evans等[10]科学家建立了一种理论模型用于推导负泊松比结构的弹性变形下的泊松比、剪切模量与拉伸模量的经典理论.复合型负泊松比超材料展现出比单胞元结构更优的力学性能.目前多数结构的弹性行结构分析主要聚焦于线弹性小变形阶段[3].然而,随着负泊松比结构的深入研究,仅对结构进行线弹性小变形分析已不能满足实际要求[11].如李扬等[12]基于结构非线性特性的影响、轻量化及驱动载荷范围,对柔性鱼骨结构的中心板厚度的尺寸设计提出更为合理的设计方法.基于几何非线性结构大变形理论模型比线性理论方法精度更高的反映结构的变形情况[1314].运用几何非线性,国内外学者研究了典型负泊松比结构中蜂窝结构的等效弹性模量、弹塑性变形和结构参数化设计[15].Gao等[16]建立用于预测双V结构的非线性弹性模量与泊松比.因此,将非线性变形理论引入组合超材料结构中,将会进一步提高超材料结构的变形预测精度.

  • 负泊松比超材料结构由于变形精确可控和可设计性强,在吸声、隔振和减振等领域逐渐受到关注.学者对内六角蜂窝型负泊松比超材料隔的隔振性能、夹芯板的振动特性和减振性能等研究,获得了优异的隔振减振超材料蜂窝结构[17-19].如将复合材料基元结构构建为蜂窝超材料结构,有望提高此类结构的减振性能.

  • 本文将直杆引入弧形内凹结构获得一类直杆增强型的复合内凹弧形超材料结构,研究其在非线性分析中的力学性能.利用能量法推导结构的线弹性等效弹性模量和泊松比的解析表达式,并与有限元模拟结果进行比较.讨论非线性等效弹性模量和等效泊松比与结构几何参数之间的关系.初步探究以直杆增强内凹结构胞元构建的蜂窝超材料结构的减振性能.

  • 1 结构设计

  • 图1 竖直向直杆增强内凹结构单胞元

  • Fig.1 Represented unit-cell of the vertical straight enhanced re-entrant-arc structure

  • 图1是新设计胞元模型,由竖直直杆和内凹弧形耦合而成,尺寸如图.胞元在水平与竖直方向周期性排列组合可获得相应的蜂窝结构.结构参数尺寸如下:上下水平板跨度为2lY轴方向壁板长度为h,两侧圆弧部分的半径为r,对应圆心角为2θ,圆弧中心连接杆长度l,胞元Z方向的长度为t.

  • 2 结构等效泊松比和弹性模量

  • 2.1 Y轴方向受载

  • 胞元结构受到任意方向的荷载都会产生变形.当结构所受荷载较小时,在结构的屈服极限内产生弹性变形,结构的泊松比与弹性模量等力学性能由胞元的几何参数确定.采用能量法理论对直杆增强内凹结构胞元的力学性能进行分析.结构进行简化和受力分析如图2.胞元模型受到压力内部产生压缩、抗剪和抗弯的变形,与弯曲变形相比,杆件的轴力和剪力变形量很小,因此,仅考虑结构的弯曲变形.边界条件简化为施加在结构对称两端的荷载MN处,如图2(a)所示.根据结构对称性,模型可简化为1/4结构进行受力分析,如图2(b)所示,竖直杆一端为固定端,圆弧另一端为自由端C.受力简化为C点处的载荷F0和未知弯矩M0的作用.根据C处变形协调条件有

  • δ11X1+Δ1F=0
    (1)
  • 式中,Δ1F为施加垂直载荷时C处产生的转角[图2(c)];δ11C上单位弯矩引起C处的横截面的转角[图2(d)].

  • 当仅在C处施加垂直载荷F0时,AB杆和BC杆的力矩分布为My)和M-y;当在C端仅受单元弯矩时,AB杆和BC杆的力矩分布为Mφ)和M-φ,则有δ11Δ1F.

  • δ11=rr+h M-(y)2EIdy+0θ M-(φ)2EIrdφ=h+rθEI
    (2)
  • Δ1F=ΔBC+ΔAB=rr+h M (y) M- (y) EIdy+0θ M (φ) M- (φ) EIrdφ=Fr2EI (θ-sinθ) +FrhEI

  • I=dt312
    (3)
  • 式中的E表示材料的弹性模量,I表示截面对中性轴线的惯性矩.

  • 将公式(2)和(3)代入公式(1),可得自由端C处的未知弯矩M0表达如下

  • X1=M0=-Δ1Fδ11=-Fr(rθ-rsinθ+h)(r+h)
    (4)
  • 当自由端C处竖直方向作用的载荷时,用单位荷载法,可得1/4胞元结构沿Y轴方向上的位移形变Δyy

  • 图2 Y轴方向受载时的受力分析

  • Fig.2 Force analysis loading along r Y-axis

  • Δyy=rr+h M(y)M(y)¯EIdy+0θ M(φ)M(φ)¯EIrdφ=rr+h -Fr-X1(-r)EIdy+0θ -Fr(1-cosφ)-X1[-r(1-cosφ)]EIrdφ=Fr2(4h+3r-4rcosθ+rcos2θ)-4M0r(h+rcosθ-r)EI
    (5)
  • 垂直载荷载荷F0和弯矩M0同时作用于自由端时,ABBC杆的弯矩分布为My)和Mφ);当只有水平单元力在自由端起作用时[如图2(e)所示],在杆件ABBC处的弯矩分别为Mφ)和M-φ;此时,当垂直荷载F0作用于结构自由端时,利用单位力法可得X轴方向上的位移变形Δyx

  • Δyx=rr+h M(y)M-(y)EIdy+0θ M(φ)M-(φ)EIrdφ=rr+h -Fr-X1(-y)EIdy+0θ -Fr(1-cosφ)-X1[-rsinφ]EIrdφ=Fr4rh+2h2+3r2-4r2cosθ+r2cos2θ-2M02rh+h2-2r2cosθ+2r24EI
    (6)
  • 2.2 X轴方向受载

  • 结构左右连接杆顶点处受到X方向的荷载2F0作用时如图3,取1/4胞元结构,自由端C处受水平方向载荷F0和未知弯矩M0.由自由端的变形协调条件求自由端的未知弯矩大小与方向,具体表达式如下:

  • Δ1F=ΔAB+ΔBC=rr+h M(y)M-(y)EIdy+0θ M(φ)M-(φ)EIrdφ=rr+h FyEIdy+0θ Fr2sinφEIdφ=F2rh+h2+Fr2(1-cosθ)EI
    (7)
  • δ11=rr+h M(y)2¯EIdy+0θ M(φ)2¯EIrdφ=h+rθEI
    (8)
  • 由自由端C处截面的转角变形协调条件可得未知弯矩的表达式如下:

  • X1=M0=-Δ1Fδ11=-F2rh+h2+Fr2(1-cosθ)h+rθ
    (9)
  • 利用单位力法,求得1/4胞元结构分别在水平单位荷载与竖直单位荷载作用下,在X轴和Y轴方向的位移

  • Δxy=rr+h M(y)M-(y)EIdy+0θ M(φ)M-(φ)EIrdφ=rr+h Fy+M0rEIdy+0θ Frsinφ+M0r2(1-cosφ)EIdφ=Fr4rh+2h2+3r2-4r2cosθ+r2cos2θ+M0r(h+rθ-rsinθ)4EI
    (10)
  • Δxx=rr+h M(y)M-(y)EIdy+0θ M(φ)M-(φ)EIrdφ=rr+h Fy+M0yEIdy+0θ Frsinφ+M0rsinφEIrdφ=2F3r2h+3rh2+h3+2r3θ-r3sin2θ+3M02rh+h2-2r2cosθ+2r26EI
    (11)
  • 图3 整体胞元结构X轴方向受载示意图

  • Fig.3 Schematic diagram of integral cell structure loading along X axis

  • 2.3 等效泊松比与等效弹性模量

  • 根据Y轴方向受荷载时1/4胞元结构的X轴、Y轴方向的位移变形公式式(5)、式(6),可得胞元的等效弹性模量E-y与等效泊松比vy

  • E-y=σyεyy=F0(r+h)Δyy(2l-r)t
    (12)
  • νy=-εyxεyy=-Δyx(r+h)Δyy(2l-r)
    (13)
  • 式中εyyεyx为单元结构沿Y轴加载时Y方向、X方向的应变;σy代表Y方向施加载荷时单位结构的应力.

  • X轴向载荷作用下结构单元的等效弹性模量和等效泊松比,由式(10)与式(11)可得胞元的等效弹性模量E-y与等效泊松比vx

  • E-x=σxεxx=F0(2l-r)Δxxt(r+h)
    (14)
  • νx=-εxyεxx=-Δxy(2l-r)Δxx(r+h)
    (15)
  • 从以上结果分析可知,在小形变范围内,组合结构的等效弹性模量和等效泊松比受结构参数和材料参数的影响.

  • 3 结果分析与讨论

  • 3.1 解析解与有限元模拟结果对比

  • 根据公式(13)和(15),带入胞元参数得到结构的等效泊松比和弹性模量的解析解.同时利用ANSYS软件对结构的平面弹性性能进行数值模拟,提取结构的变形参数,可得到直杆增强内凹复合结构的等效泊松比和等效弹性模量的模拟值.采用不锈钢为基材参数:弹性模量E=206GPa,泊松比v=0.33;弧角θ=90°,厚度t=0.8mm,圆弧半径r=4mm,竖直直杆h=2mm,上下水平直杆2l=12mm,水平连接杆长度l=6mm.

  • 模拟结构平面变形,对模型约束Z方向的变形;当模拟结构X向受载时,对结构水平直杆两端施加水平对称荷载,同时底部完全固定,顶部在X方向上约束变形,Y方向上自由活动;当结构受Y方向荷载时,在顶部和底部中心竖直施加对称荷载,对底部完全约束,顶部约束X方向变形,对水平两直杆约束Y方向变形.因此得到在XY方向的等效泊松比和弹性模量分别随弧形半径r、圆弧弧角θ、竖直直杆h变化的图,如图4~图6所示.

  • 由图4可以看出,随弧角的增大,模型在Y方向受载时等效泊松比增大,在X向受载时等效泊松比vx逐渐减小且减小趋势较缓,可以看出随着弧角的增加,等效泊松比在沿水平连接杆方向是逐渐减小的,沿垂直于胞元连接杆方向是逐渐增加的.

  • 图4 结构等效泊松比随弧角θ的变化

  • Fig.4 Variations of the structural equivalent Poisson's ratio with arc angle θ

  • 图5 结构等效泊松比随半径r的变化

  • Fig.5 Variations of the structural equivalent Poisson's ratio with radius r

  • 由图5可以看出,解析解与理论计算值变化趋势相同,随半径的增加,模型X向受载时等效泊松比vx逐渐减小,Y向受载时等效泊松比vy逐渐增大.

  • 改变杆长h的尺寸模型变化趋势由图6可以看出,随着杆长的增加,模型等效泊松比均在沿杆长方向即Y轴方向逐渐减小,而垂直杆长方向逐渐增加.

  • 图6 结构等效泊松比随杆长h的变化

  • Fig.6 Variations of the structural equivalent Poisson's ratio with the length of the linking strut h

  • 图7 结构等效弹性模量随弧角θ的变化

  • Fig.7 Variations of the equivalent elastic modulus of the structure with arc angle θ

  • 图8 结构等效弹性模量随半径r的变化

  • Fig.8 Variations of the equivalent elastic modulus of the structure with radius r

  • 如图7和图8所示,在改变结构胞元的弧角或半径时,解析解与理论值变化趋势相同,随参数的增大,无论是X向受载还是Y向受载,模型等效弹性模量变化趋势均为逐渐减小.

  • 3.2 非线力学性能分析

  • 精确预测组合结构的非线性行为可更准确地描述和控制结构变形.基于大挠度梁建模的几何非线性问题已经得到了深入研究[2021]. 但是解析模型通常需要借助椭圆积分等近似解析法来求解[22].最近,Wang等[15]基于大挠度梁的增宽双V结构的非线性理论模型解与非线性有限元模型和实验结果非常吻合. Sinwoo Jeong和Hong Hee Yoo[23]也报道了用于优化设计的非线性梁理论模型可以被相应的有限元模型准确地验证.为了有效地研究所设计的超材料组合结构的非线性力学性能,考虑其几何非线性特性,在采用Beam188单元的模型基础上建立了数值有限元模型. 该单元适用于高细比和低细比的柱,以及线性和大应变非线性的应用.边界条件与相应的线性模型相同.

  • 3.2.1 非线性等效泊松比

  • 竖直向直杆增强内凹结构的等效泊松比对比结果如图9所示.由图可知,考虑结构非线性时,随位移的增大,结构的等效泊松比数值逐渐减小,当沿X方向受载位移等于8mm时,线性等效泊松比为-0.65,非线性等效泊松比为-0.74.当受载逐渐增加时,线性位移和非线性位移的差别逐渐增大,并随着Y方向的固定位移逐渐增大,结构的非线性等效泊松比随之增大,X方向变化趋势与之相反.上述结果表明,组合超材料结构承受大变形时,必须考虑结构的几何非线性效应,才能更精确的描述结构的变形行为.

  • 图9 竖直向直杆增强内凹结构等效泊松比线性与非线性对比(a)X向受载和Y向受载(b)

  • Fig.9 The comparison of linear and nonlinear equivalent negative Poisson's ratio of the vertical straight strut structure (a) loading along X direction (b) loading along Y direction

  • 3.2.2 非线性等效弹性模量

  • 为研究大变形情况下,结合非线性效应对结构的等效弹性模量的影响,在0~10mm内对结构加载.图10为模型考虑几何非线性情况下等效弹性模量随位移增大的变化图.如图10(a)所示,组合内凹结构在X方向受载时,结构等效弹性模量随X方向加载位移的增大呈先增大后减小趋势;在Y方向受载时,结构等效弹性模量随加载位移的增大而逐渐减小.当加载位移为6mm时,线性等效弹性模量是0.175GPa,非线性弹性模量是0.191GPa,表明此类超材料结构需进一步考虑结构的几何非线性特性,才能精确预测结构的平面力学非线性性能.

  • 图10 竖直向直杆增强内凹结构等效弹性模量线性与非线性对比(a)X向受载和(b)Y向受载

  • Fig.10 The comparison of linear and nonlinear equivalent elastic modulus of the vertical straight strut structure (a) loading along X direction (b) loading along Y direction

  • 4 减振性能研究

  • 为进一步研究所设计结构的潜在应用,将所设计的胞元结构设计为蜂窝层状阵列结构,初步探究阵列结构的减振特性.谐响应分析是计算结构在一种或多种频率下的响应,并得到频率响应曲线.谐响应分析主要有三种方法:模态叠加法、完全法和缩减法.基于完全法,利用ANSYS对模型进行谐响应分析,获得结构的频率响应曲线,分析结构的位移与频率曲线,提取响应数据,获得加速度级与振级落差值.

  • 4.1 有限元模型

  • 对胞元结构周期性排列,分别设计出4×4、6×6、8×8、10×10四种阵列蜂窝,结构有限元模型如图11所示分别对结构上端面中心施加简谐激励,动荷载均取1000N;激振频率取1~500Hz,步长为5Hz;;阻尼比取3%.在模型对称轴中心位置按不同层级分别选取一定数量的连接点作为评价点,评价点分布截图如图11(a)~(d)所示.

  • 图11 评价点分布示意图(a)4×4阵列,(b)6×6阵列,(c)8×8阵列和(d)10×10阵列

  • Fig.11 Evaluation point distribution (a) 4×4 array (b) 6×6 array (c) 8×8 array (d) 10×10 array

  • 4.2 减振性能分析

  • 选取加速度级与振级落差作为主要的评价指标:

  • Lai(dB)=20lgaia0
    (16)
  • Lp=Lai-Laj=20lgaiaj
    (17)
  • 其中加速度参考值a0=10-6m/s2aiaj分别为评价点ij处的加速度值.结构的减振效果可以通过加速度振级落差表征,加速度振级落差越大,减振效果越好.

  • 图12 6×6阵列评价点加速度级与频率图

  • Fig.12 6×6 array displacement and frequency plots of evaluation points

  • 在有限元软件中对模型进行谐响应分析,取模型各评价点求其位移随频率变化曲线函数,对时间求二次导得其加速度级,得到得评价点处加速度值,计算得加速度级与频率曲线.图12是6×6阵列评价点加速度级随着频率变化图,取其峰值点计算得到该评价点加速度级.在1~500 Hz 计算频段内,越靠近底端的位置加速度级越小,表明底端结构在简谐激励下变形小,振动能量被负泊松比结构吸收,动能转化为应变能,加速度衰减显著,减振效果显著.

  • 提取各评价点的加速度级,根据公式(16)和(17)计算各个方案的相邻两层胞元和最外部两层胞元间的振级落差,振级落差结果如图13(a)~(d)所示.

  • 图13 加速度振级落差曲线(a)4×4 阵列(b)6×6 阵列(c)8×8 阵列和(d)10×10 阵列

  • Fig.13 Acceleration vibration level difference curve (a) 4×4 array (b) 6×6 array (c) 8×8 array and (d) 10×10 array

  • 对比分析图13,模型的振级落差在1~500Hz内低频率处都有一段较为平缓的减振效果稳定的频率范围,并且四个方案所表现出的振级落差随层级变化趋势基本相似,除上层与底层胞元间的振级落差外,其他相邻层级间的振级落差均随层级增大而增大,表明随层级的增加,减振效果逐渐提升.表明随层级的增加,减振效果逐渐增加,结构整体吸能和减振效果良好.尤其是4×4和6×6结构,几乎所有频段内,相邻两层之间的振级落差均大于零.

  • 四个方案内除外部两层间的振级落差外,其余相邻层级间的振级落差大部分在-15~15 dB范围内,层数增加而振级落差均集中在此范围内,说明随层数的增加,模型的减振性能依旧稳定.所有结构最低点均具有很好的减振效果.进一步分析表明,随层级的增加,振级落差在低频处的变化越来越平稳,且峰值逐步后移,可以看出减振频段区间逐渐增加.

  • 最低评价点的加速度级是整体模型减振效果的重要体现,减振结构设计的最终目的是尽可能减小最低评价点的加速度级,四种方案1~500Hz频率范围内的加速度级结果如图14所示.对比不同方案间的数据结果可见,随着激励频率的增大,能量密度增大,加速度级变大;由于结构的负泊松比特性,结构呈现反常变形规律,使得结构具有更优的能量吸收特性.胞元层数增加时,最低评价点的加速度级第一阶响应峰值依次减小,低频范围内具有稳定减振能力的频段区间逐渐变宽.

  • 图14 加速度级曲线

  • Fig.14 Acceleration level curve

  • 5 结论

  • (1)利用能量法推导出结构等效泊松比和等效弹性模量解析解,并用数值模拟验证了理论推导结果的正确性.研究参数对结构等效泊松比与等效弹性模量的影响.当直杆杆长变化时,结构等效泊松比在沿杆长方向随参数增加而减小,垂直杆长方向逐渐增加;等效弹性模量变化趋势与等效泊松比相反.

  • (2)考虑结构的几何非线性时,结构发生大变形时其等效泊松比与等效弹性模量预测准确性大幅提高.在直杆增强方向结构的等效弹性模量随变形增加而单调减小,而垂直于直杆增强方向结构等效弹性模量随变形增加而增加到一定值后逐渐减小.因此,可以看出此类超材料结构需进一步考虑结构的几何非线性特性,才能精确预测结构的平面力学非线性性能.

  • (3)通过对四种方案进行谐响应分析可以看出,随层级的增加,减振效果逐渐增加,负泊松比结构有助于振动能量吸收.且所设计的超材料蜂窝结构在1~500Hz低频范围内减振区间逐渐增加,设计的超材料结构具有较好的减振特性,尤其适用于低频减振.

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