Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 51675111, 51775123)
Received Date:03 September 2019
Accepted Date:21 September 2019
Available Online:27 November 2019
Published Online:05 December 2019
Abstract:One of the challenges relating to acoustic metamaterials is to achieve a tunable performance without modifying the structure. In this paper, we propose two types of acoustic metamaterials with a magnetorheological elastomer (MRE), and their tunable band gap structures and the transmission spectra are investigated by the finite element method (FEM). The MRE acts as a cladding layer, and its shear modulus can be changed by an externally applied magnetic field. The cell resonance frequency of acoustic metamaterial is changed. The band gap structures and the transmission spectra of the two kinds of acoustic metamaterials are calculated under various magnetic fields, and it is found that the frequency and width of band gap, the maximum attenuation frequency and transmission loss of transmission spectrum increase with externally applied magnetic field intensity increasing. Meanwhile, two types of the mass-spring models are used to estimate the band gap frequencies of the two kinds of acoustic metamaterials. The FEM results are in good agreement with the estimation results. In addition, the effects of material parameters of core and shell and filling rate on the band gap and transmission spectrum are also studied. The effects of core material parameters on the band gap and transmission spectrum of single-layer acoustic metamaterial are analyzed. It is found that the core mass has an effect on the band gap frequency and width, and the elastic parameter of the core affects the transmission loss of the transmission spectrum. The influences of core and shell material parameter on the band gap and transmission spectrum of double-layer acoustic metamaterial is analyzed by the control variable method. The results show that the core and shell mass affect the band gap frequency, width and pass-band width, and the elastic parameter of the core and the shell affect the transmission loss of the transmission spectrum. As the filling rate increases, the band gap frequency and width of the single- and double-layer MRE acoustic metamaterial increase, the maximum attenuation frequency of the transmission spectrum does not change, and the transmission loss increases. These results will greatly contribute to the application of acoustic metamaterials to controlling the active noise and vibration. Keywords:acoustic metamaterials/ magnetorheological elastomer cladding layers/ bandgap and transmission spectra/ magnetic field intensity
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2.1.声学超材料结构
两种声学超材料的元胞结构如图1所示, 第一种声学超材料元胞结构为单层MRE包裹纯铁圆柱体(芯体)组成局域共振单元结构, 如图1(a)所示, 将其植入基体材料, 按周期结构排列可构成单包覆层MRE声学超材料; 第二种声学超材料元胞结构为内层MRE (内包覆层)包裹纯铁圆柱体 (芯体), 在内层MRE外包裹一层纯铁圆柱壳体, 在圆柱壳体外再包裹一层MRE (外包覆层), 构成双包覆层MRE局域共振单元结构, 如图1(b)所示, 同样将其植入基体材料按周期结构排列, 构成双包覆层MRE声学超材料. 两种声学超材料的基体材料均为环氧树脂, 且局域共振单元均按照正方晶格结构排列. 第一不可约布里渊区如图1(c)所示. 构成声学超材料的具体材料参数如表1所列. 图 1 MRE包覆层声学超材料元胞结构和不可约布里渊区 (a) 单包覆层MRE声学超材料; (b) 双包覆层MRE声学超材料; (c) 正方晶格第一不可约布里渊区 Figure1. Schematic of the MRE acoustic metamaterial cells and the irreducible first Brillouin zone: (a) Single-layer MRE acoustic metamaterial; (b) double-layer MRE acoustic metamaterial; (c) the irreducible first Brillouin zone of square lattices.
材料
密度$\rho /{\rm{kg}} \cdot {{\rm{m}}^{ - 3}}$
拉梅常数
λ/GPa
μ/GPa
纯铁
7900
115.38
76.92
环氧树脂
1180
4.52
1.59
MRE
3009
6.26×10-3
0.4×10-3
表1构成MRE声学超材料的材料参数 Table1.Material properties of the MRE acoustic metamaterials.
单包覆层MRE声学超材料, 其元胞结构如图1(a)所示, 当外部磁场作用时, MRE包覆层的剪切模量可随磁场变化, 因此其带隙结构可以通过改变磁场强度进行调节. 图2为MRE包覆层在没有磁场(H = 0 kOe), 以及磁场强度为H = 6 kOe作用下的带隙结构, 以说明磁场对带隙结构的影响. 图 2 单包覆层MRE声学超材料带隙结构 (a) 磁场强度H = 0; (b) 磁场强度H = 6 kOe Figure2. The band gap structures of single-layer MRE acoustic metamaterial: (a) Magnetic field intensity of H = 0; (b) magnetic field intensity of H = 6 kOe.
对于单包覆层MRE声学超材料我们关注其第一阶带隙, 因为第一阶带隙频率较低, 且带隙较宽. 如图2(a)所示, 当没有外部磁场作用时, 带隙频率为488—1114 Hz; 当磁场强度为H = 6 kOe时, 带隙结构如图2(b)所示, 带隙频率为607—1368 Hz. 由以上计算可知, 有磁场作用于包覆层MRE时, 声学超材料带隙的下边界与上边界频率都随之升高. 在图2中还可以看到在高频处还有一窄带隙, 但由于其频率高, 且带隙窄, 很难将其应用于实际工程领域. 由图2还可以发现, 声学超材料的带隙为多条平直线, 具有典型的局域共振特性. 为了进一步理解声学超材料的带隙机理, 下面讨论在图2(a)中带隙边界处标出的A, B两点的位移向量场. 图3为A, B两点的位移向量场, 由图3(a)中可以看到, 在带隙下边界的A点处, 当弹性波在声学超材料中传播时为芯体在振动, 而基体保持静止; 由图3(b)中可以看到, 在带隙上边界处B点, 为基体在振动, 芯体只有轻微振动, 且芯体与基体运动方向相反, 此时, MRE包覆层可以视为弹簧, 而芯体与基体可视为集中质量, 芯体与基体以相对振动的方式发生共振. 图3中的箭头表示基体和芯体的相对运动方向. 图 3 单包覆层MRE声学超材料带隙边界位移场 (a) 带隙下边界; (b) 带隙上边界 Figure3. Displacement field of the band gap boundaries of the single-layer MRE acoustic metamaterial: (a) The lower boundary of the band gap; (b) the upper boundary of the band gap.
根据位移向量场, 单包覆层MRE声学超材料可简化为图4所示的质量-弹簧模型[49]来描述其带隙下边界和上边界的振动模式. 其中${m_1}$为元胞的芯体质量, ${m_2}$为基体质量, 弹簧k为MRE包覆层的等效刚度, 其具体定义见文献[49]. 在带隙下边界处, ${m_1}$在弹簧k的作用下, 发生共振(对应图3(a)). 在带隙上边界处, m1和m2在弹簧k的作用下, 以相对振动的方式发生共振(对应图3(b)). 图 4 单包覆层MRE声学超材料的质量-弹簧模型 (a) 带隙下边界; (b) 带隙上边界 Figure4. The mass-spring model of single-layer MRE acoustic metamaterial: (a) Lower boundary of band gap; (b) upper boundary of band gap.
$f_\alpha ^0$为磁场强度$H = {\rm{0\;kOe}}$时的带隙边界. 由以上仿真结果可知, 单包覆层MRE声学超材料可以通过改变磁场强度来改变其带隙结构. 现在分析磁场强度连续变化时声学超材料的带隙变化情况. 磁场强度由0—10 kOe连续变化时, 单包覆层MRE声学超材料的带隙变化如图5(a)所示, 其带隙频率从488—754 Hz变化到1114—1723 Hz. 由此可知, 随着磁场强度增强, 带隙下边界与上边界频率都呈上升趋势, 但其上边界频率变化较大, 宽度增加, 这是因为MRE的剪切模量随着磁场强度增强而增加. 图5(b)为随着磁场强度增强, 由(8)式计算的${{{f_\alpha }}/{f_\alpha ^0}}$随磁场强度变化曲线与有限元法计算的${{{f_\alpha }}/{f_\alpha ^0}}$随磁场强度变化曲线的对比图, 从图中可以看出, 有限元计算结果与质量-弹簧模型方法具有很好的一致性. 图 5 单包覆层MRE声学超材料带隙随磁场强度变化及有限元法与质量-弹簧模型对比 (a) 磁场强度H = 0—10 kOe; (b) 有限元法与质量弹簧-模型对比 Figure5. Dependence of the band gap boundaries on the applied magnetic field of single-layer MRE acoustic metamaterial and a comparison of the FEM and the mass-spring model for (a) magnetic field intensity of H = 0–10 kOe, (b) comparison of the FEM and mass-spring model.
图 6 单包覆层MRE声学超材料带隙随材料和结构参数变化 (a) 随芯体质量增加变化; (b) 随填充率增加变化 Figure6. The band gap of single-layer MRE acoustic metamaterial changes with (a) the core mass and (b) the filling rate.
双包覆层MRE声学超材料的元胞结构如图1(b)所示, 图7为双包覆层MRE声学超材料在没有磁场($H = {\rm{0\;kOe}}$), 以及磁场强度为$H = {\rm{6\;kOe}}$作用下的带隙结构, 以说明磁场对双包覆层MRE声学超材料带隙结构的影响. 图 7 双包覆层MRE声学超材料带隙结构 (a) H = 0; (b) H = 6 kOe Figure7. The band gap structures of the double-layer MRE acoustic metamaterial: (a) H = 0; (b) H = 6 kOe.
与单包覆层MRE声学超材料相比, 双包覆层MRE声学超材料有两阶宽频带隙, 在单包覆层MRE声学超材料中存在的高频窄带隙消失. 如图7(a)所示, 当没有磁场(H = 0 kOe)作用时, 双包覆层MRE声学超材料的第一阶带隙频率为374—771 Hz, 第二阶带隙频率为1365—1670 Hz. 当磁场强度为H = 6 kOe时, 带隙结构如图7(b)所示, 此时第一阶带隙频率为465—959 Hz, 第二阶带隙频率为1698—2077 Hz, 与没有磁场作用时相比, 第一阶与第二阶带隙频率升高, 宽度增加. 下面进一步考察第一阶与第二阶带隙边界处的位移向量场, 图7(a)中标识的A, B, C, D, E点的位移向量场如图8所示. 图8(a)为第一阶带隙下边界A点的位移向量场, 在第一阶带隙下边界处为芯体在振动, 而基体保持静止; 第一阶带隙上边界B点处, 位移向量场如图8(b)所示, 芯体与基体的运动方向相反, 其与单包覆层MRE声学超材料的带隙上边界处位移向量场相似; 第二阶带隙下边界C点的位移向量场如图8(c)所示, 最大位移发生在包覆层位置, 壳体与芯体的运动方向相反; 第二阶带隙上边界D点的位移向量场如图8(d)所示, 为壳体与基体的运动方向相反, 可以看出壳体、芯体和基体相当于集中质量, 而包覆层相当于弹簧. 图7中E点的位移向量场如图8(e)所示, 从图中可以看到, 基体与芯体都不运动, 仅内、外包覆层扭转运动, 由于没有水平和竖直方向运动, 因此不能产生带隙, 所以高频窄带隙消失. 图 8 双包覆层MRE声学超材料带隙边界处位移向量场 (a) 第一阶带隙下边界; (b) 第一阶带隙上边界; (c) 第二阶带隙下边界; (d) 第二阶带隙上边界; (e) 图7(a)中E点 Figure8. Displacement field of the band gap boundaries of the double-layer MRE acoustic metamaterial: (a) The lower boundary and (b) the upper boundary of the first band gaps; (c) the lower boundary and (d) the upper boundary of the second band gap; (e) point E in Fig.7 (a).
由以上分析可知, 声学超材料的带隙数量随着包覆层数量的增加而增多, 这是因为存在更多的局域共振模式. 带隙数量增多可以使声学超材料更好地应用于振动与噪声的控制. 由双包覆层MRE声学超材料的位移向量场, 可将双包覆层MRE声学超材料简化为如图9所示的质量-弹簧模型[50], 可以用质量-弹簧模型进一步估算其带隙频率. 其中${m_1}$为元胞的芯体质量, ${m_2}$为壳体质量, ${m_3}$为基体质量, 弹簧${k_1}$和${k_2}$分别为MRE内、外包覆层的等效刚度, 其具体定义见文献[50]. 在双包覆层MRE声学超材料的两阶带隙下边界处, ${m_1}$和${m_2}$在弹簧${k_1}$和${k_2}$的作用下, 发生共振. 在两阶带隙上边界处, ${m_1}$, ${m_2}$和${m_3}$在弹簧${k_1}$和${k_2}$的作用下, 以相对振动的方式发生共振. 图 9 双包覆层MRE声学超材料质量-弹簧模型 (a) 第一阶与第二阶带隙下边界; (b) 第一阶与第二阶带隙上边界 Figure9. The mass-spring model of the double-layer MRE acoustic metamaterial: (a) Lower boundary of the first and second band gap; (b) upper boundary of the first and second band gap.
当磁场强度由0—10 kOe连续变化时, 双包覆层MRE声学超材料的带隙变化如图10(a)所示, 第一阶带隙频率从374—771 Hz变化到575—1187 Hz; 第二阶带隙频率从1365—1670 Hz变化到2099—2567 Hz. 由此可知, 随着磁场强度增强两阶带隙下边界与上边界频率都呈上升趋势, 带隙宽度增加, 这是因为内、外包覆层MRE的剪切模量随着磁场强度增强而增加. 图 10 双包覆层MRE声学超材料带隙随磁场强度变化及有限元法与质量-弹簧模型对比 (a) 磁场强度H = 0—10 kOe; (b) 有限元法与质量-弹簧模型对比 Figure10. Dependence of the band gap boundaries on the applied magnetic field of double-layer MRE acoustic metamaterial and a comparison of the FEM and mass-spring model for (a) magnetic field intensity of H = 0–10 kOe, (b) comparison of the FEM and mass-spring model.