Shaoyun Wang
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Mechanics · 2024-10-16

Rayleigh-Ritz method

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Bold letters means matrix or vectors: A\boldsymbol A

Bold letters with subscripts means the ii th row and jj th column block matrix in matrix A\boldsymbol A: Aij\boldsymbol A_{ij}

The kk th row and ll th column element in ii th row and jj th column block matrix Aij\boldsymbol A_{ij} in matrix A\boldsymbol A: (Aij)kl(A_{ij})_{kl}

0.1 Eq(14)

U(ξ,η,ζ)=∑i=1I1∑j=1J1∑k=1K1Bijk1Pi(ξ)Pj(η)Pk(ζ),V(ξ,η,ζ)=∑i=1I2∑j=1J2∑k=1K3Bijk2Pi(ξ)Pj(η)Pk(ζ),W(ξ,η,ζ)=∑i=1I3∑j=1J3∑k=1K3Bijk3Pi(ξ)Pj(η)Pk(ζ).\begin{array}{c} U(\xi, \eta, \zeta)=\sum_{i=1}^{I_1} \sum_{j=1}^{J_1} \sum_{k=1}^{K_1} B^1_{i j k} P_{i}(\xi) P_{j}(\eta) P_{k}(\zeta), \\ V(\xi, \eta, \zeta)=\sum_{i=1}^{I_2} \sum_{j=1}^{J_2} \sum_{k=1}^{K_3} B^2_{i j k} P_{i}(\xi) P_{j}(\eta) P_{k}(\zeta), \\ W(\xi, \eta, \zeta)=\sum_{i=1}^{I_3} \sum_{j=1}^{J_3} \sum_{k=1}^{K_3} B^3_{i j k} P_{i}(\xi) P_{j}(\eta) P_{k}(\zeta). \end{array}

We use the following relation (i,j,k)=(1,1,1),(1,1,2),…,(1,1,K),(1,2,1),…,(1,2,K),…,(1,J,K),…,(I,J,K)→p=1,2,3…,IJK(i,j,k)=(1,1,1),(1,1,2),\ldots, (1,1,K), (1,2,1), \ldots, (1,2,K), \ldots, (1,J,K), \ldots, (I,J,K) \rightarrow p = 1,2,3 \ldots, IJK to transform the counting indices (i,j,k)(i,j,k) to a new index pp. Then, we collect the coefficients Bijkm,m=1,2,3B^m_{i j k}, m=1,2,3 into new vectors Am\boldsymbol A_m where the ppth element (Am)p=Bijkm(\boldsymbol A_m)_p=B^m_{ijk}. And we collect all the coefficients into a big vector which is A=(A1,A2,A3)\boldsymbol A = (\boldsymbol A_1,\boldsymbol A_2,\boldsymbol A_3).Therefore, we have

U(ξ,η,ζ)=∑i=1I1∑j=1J1∑k=1K1(A1)pPi(ξ)Pj(η)Pk(ζ),V(ξ,η,ζ)=∑i=1I2∑j=1J2∑k=1K3(A2)pPi(ξ)Pj(η)Pk(ζ),W(ξ,η,ζ)=∑i=1I3∑j=1J3∑k=1K3(A3)pPi(ξ)Pj(η)Pk(ζ).\begin{array}{c} U(\xi, \eta, \zeta)=\sum_{i=1}^{I_1} \sum_{j=1}^{J_1} \sum_{k=1}^{K_1} (A_1)_p P_{i}(\xi) P_{j}(\eta) P_{k}(\zeta), \\ V(\xi, \eta, \zeta)=\sum_{i=1}^{I_2} \sum_{j=1}^{J_2} \sum_{k=1}^{K_3} (A_2)_p P_{i}(\xi) P_{j}(\eta) P_{k}(\zeta),\\ W(\xi, \eta, \zeta)=\sum_{i=1}^{I_3} \sum_{j=1}^{J_3} \sum_{k=1}^{K_3} (A_3)_p P_{i}(\xi) P_{j}(\eta) P_{k}(\zeta). \end{array}

##Eq 16

Π=Vmax⁡−Tmax⁡→L=Vmax⁡−Tmax⁡\Pi=V_{\max }-T_{\max } \rightarrow L=V_{\max }-T_{\max }

0.2 Eq 17

∂L∂Bijk1=0,∂L∂Bijk2=0,∂L∂Bijk3=0.\frac{\partial L}{\partial B^1_{i j k}}=0, \frac{\partial L}{\partial B^2_{i j k}}=0, \frac{\partial L}{\partial B^3_{i j k}}=0 .

By using the index transform, we have

∂L∂(A1)p=0,∂L∂(A2)p=0,∂L∂(A3)p=0.\frac{\partial L}{\partial (A_1)_p}=0, \frac{\partial L}{\partial (A_2)_p}=0, \frac{\partial L}{\partial (A_3)_p}=0 .

0.3 Eq21

[Π11Π12Π13Π21Π22Π23Π31Π32Π33][A1A2A3]=[000].\left[\begin{array}{ll} \boldsymbol \Pi_{11} & \boldsymbol \Pi_{12} & \boldsymbol \Pi_{13} \\ \boldsymbol \Pi_{21} & \boldsymbol \Pi_{22} & \boldsymbol \Pi_{23}\\ \boldsymbol \Pi_{31} & \boldsymbol \Pi_{32} & \boldsymbol \Pi_{33} \end{array}\right] \left[\begin{array}{l} {\boldsymbol A_1} \\ {\boldsymbol A_2} \\ {\boldsymbol A_3} \end{array}\right] =\left[\begin{array}{c} {\boldsymbol 0} \\ {\boldsymbol 0} \\ {\boldsymbol 0} \end{array}\right] .

where

(Πij)pq=∂2L∂(Ai)p∂(Aj)q(\Pi_{ij})_{pq} = \frac{\partial^2 L}{\partial (A_i)_p \partial (A_j)_q}

In fact,

Π=K−ω2M\boldsymbol \Pi= \boldsymbol K -\omega^2 \boldsymbol M

and

(Kij)pq=∂2Vmax∂(Ai)p∂(Aj)q(K_{ij})_{pq} = \frac{\partial^2 V_{\rm max}}{\partial (A_i)_p \partial (A_j)_q} ω2(Mij)pq=∂2Tmax∂(Ai)p∂(Aj)q\omega^2(M_{ij})_{pq} = \frac{\partial^2 T_{\rm max}}{\partial (A_i)_p \partial (A_j)_q} Π11=∂L∂(A1)p=Ec2λ(1+v)∫−11∫−11∫−11{v1−2v(εˉξξ+εˉηη+εˉζζ)λPl(ξ)∂Pm(η)∂(η)Pn(ζ)+λεˉηηPl(ξ)∂Pm(η)∂(η)Pn(ζ)+12[εˉξη∂Pl(ξ)∂(ξ)Pm(η)Pn(ζ)+λγεˉηζPl(ξ)Pm(η)∂Pn(ζ)∂(ζ)]}dξdηdζ+Eˉc2λ(1+vˉ)∫−11∫−11∫11+2cˉc{vˉ1−2vˉ(εˉξξ+εˉηη+εˉζζ)⋅λPl(ξ)∂Pm(η)∂(η)Pn(ζ)+λεˉηηPl(ξ)∂Pm(η)∂(η)Pn(ζ)+12[εˉξη∂Pl(ξ)∂(ξ)Pm(η)Pn(ζ)+λγεˉηζPl(ξ)Pm(η)∂Pn(ζ)∂(ζ)]}dξdηdζ−{ρ8abcω2∫−11∫−11∫−11V[Pl(ξ)Pm(η)Pn(ζ)]dξdηdζ+ρˉ8abcω2∫−11∫−11∫11+2cˉcV[Pl(ξ)Pm(η)Pn(ζ)]dξdηdζ}\begin{aligned} \Pi_{11}=\frac{\partial L}{\partial (A_1)_p}=& \frac{E c}{2 \lambda(1+v)} \int_{-1}^{1} \int_{-1}^{1} \int_{-1}^{1}\left\{\frac { v } { 1 - 2 v } \left(\bar{\varepsilon}_{\xi \xi}+\bar{\varepsilon}_{\eta \eta}\right.\right.\\ &\left.+\bar{\varepsilon}_{\zeta \zeta}\right) \lambda P_{l}(\xi) \frac{\partial P_{m}(\eta)}{\partial(\eta)} P_{n}(\zeta)+\lambda \bar{\varepsilon}_{\eta \eta} P_{l}(\xi) \frac{\partial P_{m}(\eta)}{\partial(\eta)} P_{n}(\zeta) \\ &+\frac{1}{2}\left[\bar{\varepsilon}_{\xi \eta} \frac{\partial P_{l}(\xi)}{\partial(\xi)} P_{m}(\eta) P_{n}(\zeta)\right.\\ &\left.\left.+\frac{\lambda}{\gamma} \bar{\varepsilon}_{\eta \zeta} P_{l}(\xi) P_{m}(\eta) \frac{\partial P_{n}(\zeta)}{\partial(\zeta)}\right]\right\} d \xi d \eta d \zeta \\ &+\frac{\bar{E} c}{2 \lambda(1+\bar{v})} \int_{-1}^{1} \int_{-1}^{1} \int_{1}^{1+\frac{2 \bar{c}}{c}}\left\{\frac{\bar{v}}{1-2 \bar{v}}\left(\bar{\varepsilon}_{\xi \xi}+\bar{\varepsilon}_{\eta \eta}+\bar{\varepsilon}_{\zeta \zeta}\right)\right.\\ & \cdot \lambda P_{l}(\xi) \frac{\partial P_{m}(\eta)}{\partial(\eta)} P_{n}(\zeta)+\lambda \bar{\varepsilon}_{\eta \eta} P_{l}(\xi) \frac{\partial P_{m}(\eta)}{\partial(\eta)} P_{n}(\zeta) \\ &+\frac{1}{2}\left[\bar{\varepsilon}_{\xi \eta} \frac{\partial P_{l}(\xi)}{\partial(\xi)} P_{m}(\eta) P_{n}(\zeta)\right.\\ &\left.\left.+\frac{\lambda}{\gamma} \bar{\varepsilon}_{\eta \zeta} P_{l}(\xi) P_{m}(\eta) \frac{\partial P_{n}(\zeta)}{\partial(\zeta)}\right]\right\} d \xi d \eta d \zeta \\ &-\left\{\frac{\rho}{8} a b c \omega^{2} \int_{-1}^{1} \int_{-1}^{1} \int_{-1}^{1} V\left[P_{l}(\xi) P_{m}(\eta) P_{n}(\zeta)\right] d \xi d \eta d \zeta\right.\\ &\left.+\frac{\bar{\rho}}{8} a b c \omega^{2} \int_{-1}^{1} \int_{-1}^{1} \int_{1}^{1+\frac{2 \bar{c}}{c}} V\left[P_{l}(\xi) P_{m}(\eta) P_{n}(\zeta)\right] d \xi d \eta d \zeta\right\} \end{aligned}