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Hybrid-Trefftz displacement elements for three-dimensional elastodynamics

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Abstract

In this paper the formulation of hybrid-Trefftz displacement finite elements for transient problems in three-dimensional elastic media is derived. The mathematical model is derived from the classical theory of elasticity. The governing domain and boundary equations are discretized in time using a wavelet basis expansion to yield a series of spectral problems which only depend on space. Displacements are the main approximation in the domain of the element (displacement model) and tractions are approximated on the essential boundaries (hybrid formulation). The displacement trial functions are constrained to satisfy exactly the domain equations (Trefftz constraint), and consist of one family of compression waves and two families of shear waves. The problem is reduced to solving an algebraic system whose unknowns are the weights of the displacement bases. Strains and stresses are derived from the displacement approximations through the compatibility and elasticity equations. The formulation is implemented as a new module in FreeHyTE, an open-source and user-friendly Trefftz platform. Numerical tests have been carried out with the new 3D FreeHyTE module implemented with the functions derived in this paper and the results are satisfactory.

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Acknowledgements

This work was funded by Fundação para a Ciência e a Tecnologia (MCTES) through national funds (PIDDAC) under the R &D Unit “Civil Engineering Research and Innovation for Sustainability (CERIS)”, reference UIDB/04625/2020 and through research project CEN-DynaGEO, reference PTDC/EAM-GTC/29923/2017.

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Appendices

Appendix A: Spherical coordinates system

The spherical coordinates system used in this work is shown in Fig. 19. The three coordinates are: the radial distance r (where \(0\leqslant r<\infty \)), the polar angle \(\theta \) (where \(0\leqslant \theta \leqslant \pi \)) and the azimut angle \(\varphi \) (where \(0\leqslant \varphi < 2 \pi \)).

Fig. 19
figure 19

Spherical coordinates system

The gradient, divergent and curl operators are

$$\begin{aligned} \varvec{\nabla }= & {} \begin{pmatrix} \frac{\partial }{\partial r} \\ \frac{1}{r} \frac{\partial }{\partial \theta } \\ \frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } \end{pmatrix} \end{aligned}$$
(78)
$$\begin{aligned} \varvec{\nabla }^*= & {} \begin{pmatrix} \frac{1}{r^2} \frac{\partial }{\partial r} r^2&\frac{1}{r \sin \theta } \frac{\partial }{\partial \theta } \sin \theta&\frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } \end{pmatrix} \end{aligned}$$
(79)
$$\begin{aligned} \varvec{{\tilde{\nabla }}}= & {} \begin{pmatrix} 0 &{} -\frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } &{} \frac{1}{r \sin \theta } \frac{\partial }{\partial \theta } \sin \theta \\ \frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } &{} 0 &{} -\frac{1}{r} \frac{\partial }{\partial r} r \\ -\frac{1}{r} \frac{\partial }{\partial \theta } &{} \frac{1}{r} \frac{\partial }{\partial r} r &{} 0 \end{pmatrix} \end{aligned}$$
(80)

and the scalar and vector Laplacian operators are

$$\begin{aligned} \nabla ^2= & {} \frac{1}{r^2} \frac{\partial }{\partial r} \left( r^2 \frac{\partial }{\partial r}\right) + \frac{1}{r^2 \sin \theta } \frac{\partial }{\partial \theta } \left( \sin \theta \frac{\partial }{\partial \theta }\right) \nonumber \\&+ \frac{1}{r^2 \sin ^2 \theta } \frac{\partial ^2}{\partial \varphi ^2} \end{aligned}$$
(81)
$$\begin{aligned} \varvec{\nabla }^2= & {} \begin{pmatrix} \nabla ^2 - \frac{2}{r^2} &{} - \frac{2}{r^2 \sin \theta } \frac{\partial }{\partial \theta } \sin \theta &{} -\frac{2}{r^2 \sin \theta } \frac{\partial }{\partial \varphi } \\ \frac{2}{r^2} \frac{\partial }{\partial \theta } &{} \nabla ^2 - \frac{1}{r^2 \sin ^2 \theta } &{} -\frac{2 \cos \theta }{r^2 \sin ^2 \theta } \frac{\partial }{\partial \varphi } \\ \frac{2}{r^2 \sin \theta } \frac{\partial }{\partial \varphi } &{} \frac{2 \cos \theta }{r^2 \sin ^2 \theta } \frac{\partial }{\partial \varphi } &{} \nabla ^2 - \frac{1}{r^2 \sin ^2 \theta } \end{pmatrix} \end{aligned}$$
(82)

Operators \(\varvec{{\mathcal {D}}}\) and \(\varvec{{\mathcal {D}}}^*\) are

$$\begin{aligned} \varvec{{\mathcal {D}}}= & {} \begin{pmatrix} \frac{2}{r} + \frac{\partial }{\partial r} &{} -\frac{1}{r} &{} -\frac{1}{r} &{} \frac{\cot \theta }{r} + \frac{1}{r} \frac{\partial }{\partial \theta } &{} \frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } &{} 0 \\ 0 &{} \frac{\cot \theta }{r} + \frac{1}{r}\frac{\partial }{\partial \theta } &{} -\frac{\cot \theta }{r} &{} \frac{3}{r} + \frac{\partial }{\partial r} &{} 0 &{} \frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } \\ 0 &{} 0 &{} \frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } &{} 0 &{} \frac{3}{r} + \frac{\partial }{\partial r} &{} \frac{2 \cot \theta }{r} + \frac{1}{r} \frac{\partial }{\partial \theta } \end{pmatrix} \nonumber \\ \end{aligned}$$
(83)
$$\begin{aligned} \varvec{{\mathcal {D}}}^*= & {} \begin{pmatrix} \frac{\partial }{\partial r} &{} 0 &{} 0 \\ \frac{1}{r} &{} \frac{1}{r} \frac{\partial }{\partial \theta } &{} 0 \\ \frac{1}{r} &{} \frac{\cot \theta }{r} &{} \frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } \\ \frac{1}{r} \frac{\partial }{\partial \theta } &{} -\frac{1}{r} + \frac{\partial }{\partial r} &{} 0 \\ \frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } &{} 0 &{} -\frac{1}{r} + \frac{\partial }{\partial r} \\ 0 &{} \frac{1}{r \sin \theta } \frac{\partial }{\partial \varphi } &{} -\frac{\cot \theta }{r}+\frac{1}{r} \frac{\partial }{\partial \theta } \end{pmatrix} \end{aligned}$$
(84)

Appendix B: Domain equations in spherical coordinates

In the spherical coordinates system defined in Appendix A, the spectral equilibrium Eq. (15) yields,

$$\begin{aligned}&\frac{\partial \sigma _{rr}}{\partial r} + \frac{1}{r} \frac{\partial \sigma _{r \theta }}{\partial \theta } + \frac{1}{r \sin \theta } \frac{\partial \sigma _{r \varphi }}{\partial \varphi }\nonumber \\&\quad + \frac{1}{r} \left( 2 \sigma _{rr} - \sigma _{\theta \theta } - \sigma _{\varphi \varphi } + \cot \theta \sigma _{r \theta } \right) + \omega ^2 \rho u_r = 0 \end{aligned}$$
(85)
$$\begin{aligned}&\frac{\partial \sigma _{r \theta }}{\partial r} + \frac{1}{r} \frac{\partial \sigma _{\theta \theta }}{\partial \theta } + \frac{1}{r \sin \theta } \frac{\partial \sigma _{\theta \varphi }}{\partial \varphi } \nonumber \\&\quad + \frac{1}{r} \left[ \left( \sigma _{\theta \theta } - \sigma _{\varphi \varphi } \right) \cot \theta + 3 \sigma _{r \theta } \right] + \omega ^2 \rho u_{\theta } = 0 \end{aligned}$$
(86)
$$\begin{aligned}&\frac{\partial \sigma _{r \varphi }}{\partial r} + \frac{1}{r} \frac{\partial \sigma _{\theta \varphi }}{\partial \theta } + \frac{1}{r \sin \theta } \frac{\partial \sigma _{\varphi \varphi }}{\partial \varphi }\nonumber \\&\quad + \frac{1}{r} \left( 2 \sigma _{\theta \varphi } \cot \theta + 3 \sigma _{r \varphi } \right) + \omega ^2 \rho u_{\varphi } = 0 \end{aligned}$$
(87)

Moreover, the spectral compatibility Eq. (16) yields,

$$\begin{aligned} \epsilon _{rr}= & {} \frac{\partial u_r}{\partial r} \end{aligned}$$
(88)
$$\begin{aligned} \epsilon _{\theta \theta }= & {} \frac{1}{r} \left( \frac{\partial u_{\theta }}{\partial \theta } + u_r \right) \end{aligned}$$
(89)
$$\begin{aligned} \epsilon _{\varphi \varphi }= & {} \frac{1}{r \sin \theta } \left( \frac{\partial u_{\varphi }}{\partial \varphi } + u_r \sin \theta + u_{\theta } \cos \theta \right) \end{aligned}$$
(90)
$$\begin{aligned} \epsilon _{r \theta }= & {} \frac{1}{2} \left( \frac{1}{r} \frac{\partial u_r}{\partial \theta } + \frac{\partial u_{\theta }}{\partial r} - \frac{u_{\theta }}{r} \right) \end{aligned}$$
(91)
$$\begin{aligned} \epsilon _{r \varphi }= & {} \frac{1}{2} \left( \frac{1}{r \sin \theta } \frac{\partial u_r}{\partial \varphi } + \frac{\partial u_{\varphi }}{\partial r} - \frac{u_{\varphi }}{r} \right) \end{aligned}$$
(92)
$$\begin{aligned} \epsilon _{\theta \varphi }= & {} \frac{1}{2 r} \left( \frac{1}{\sin \theta } \frac{\partial u_{\theta }}{\partial \varphi } + \frac{\partial u_{\varphi }}{\partial \theta } - u_{\varphi } \cot \theta \right) \end{aligned}$$
(93)

Finally, the spectral elasticity Eq. (17) yields,

$$\begin{aligned} \sigma _{rr}= & {} \left( \lambda + 2 \mu + \alpha ^2 M \right) \epsilon _{rr}\nonumber \\&+ \left( \lambda + \alpha ^2 M \right) \left( \epsilon _{\theta \theta } + \epsilon _{\varphi \varphi } \right) \end{aligned}$$
(94)
$$\begin{aligned} \sigma _{\theta \theta }= & {} \left( \lambda + 2 \mu + \alpha ^2 M \right) \epsilon _{\theta \theta }\nonumber \\&+ \left( \lambda + \alpha ^2 M \right) \left( \epsilon _{rr} + \epsilon _{\varphi \varphi } \right) \end{aligned}$$
(95)
$$\begin{aligned} \sigma _{\varphi \varphi }= & {} \left( \lambda + 2 \mu + \alpha ^2 M \right) \epsilon _{\varphi \varphi } \nonumber \\&+ \left( \lambda + \alpha ^2 M \right) \left( \epsilon _{rr} + \epsilon _{\theta \theta } \right) \end{aligned}$$
(96)
$$\begin{aligned} \sigma _{r \theta }= & {} 2 \mu \epsilon _{r \theta } \end{aligned}$$
(97)
$$\begin{aligned} \sigma _{r \varphi }= & {} 2 \mu \epsilon _{r \varphi } \end{aligned}$$
(98)
$$\begin{aligned} \sigma _{\theta \varphi }= & {} 2 \mu \epsilon _{\theta \varphi } \end{aligned}$$
(99)

Appendix C: P-waves functions

Substituting the compression waves potential (37) into Eq. (30), the three components of the P-wave displacements in spherical coordinates are

$$\begin{aligned} u_{r_{p}}= & {} \frac{Y_n^m \left( \theta , \varphi \right) }{r} \left[ n j_n \left( \beta _{p} r \right) - \beta _{p} r j_{n+1} \left( \beta _{p} r \right) \right] \end{aligned}$$
(100)
$$\begin{aligned} u_{\theta _{p}}= & {} \frac{- j_n \left( \beta _{p} r \right) }{r \sin \theta }\nonumber \\&\left[ \left( n+1 \right) \cos \theta \ Y_n^m \left( \theta , \varphi \right) - \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \right] \end{aligned}$$
(101)
$$\begin{aligned} u_{\varphi _{p}}= & {} \frac{ {\hat{\imath }} m}{r \sin \theta } j_n \left( \beta _{p} r \right) Y_n^m \left( \theta , \varphi \right) \end{aligned}$$
(102)

where

$$\begin{aligned} \Lambda = \sqrt{ \frac{ \left( 2n+1 \right) \left( n+m+1 \right) \left( n-m+1 \right) }{2n+3}} \end{aligned}$$
(103)

The stresses of the P-wave are obtained substituting Eqs. (100)–(102) into Eqs. (88)–(99)

$$\begin{aligned} \sigma _{rr_p}= & {} \frac{Y_n^m \left( \theta , \varphi \right) }{\left( 1-\nu \right) r^2} \Big [\left( 2\mu n \left( n-1 \right) \left( 1-\nu \right) \right. \nonumber \\&\left. - \beta _p^2 r^2 \left[ \lambda + 2\mu -2\nu \left( \lambda +\mu \right) \right] \right) j_n \left( \beta _p r \right) \nonumber \\&+ 4\mu \left( 1-\nu \right) \beta _p r j_{n+1} \left( \beta _p r \right) \Big ] \end{aligned}$$
(104)
$$\begin{aligned} \sigma _{\theta \theta _p}= & {} \frac{1}{\left( 1-\nu \right) r^2 \sin ^2 \theta } \Bigg (\Bigg [\Big (2\mu \left( 1-\nu \right) \left( m^2+n+1 \right) \nonumber \\&+ \left[ \lambda \left( 2\nu -1 \right) \beta _p^2 r^2 \right. \nonumber \\&\left. - 2\mu \left( 1-\nu \right) \left( n^2+n+1 \right) \right] \sin ^2 \theta \Big ) j_n \left( \beta _p r \right) \nonumber \\&- 2\mu \left( 1-\nu \right) \beta _p r \sin ^2\theta j_{n+1} \left( \beta _p r \right) \Bigg ] Y_n^m \left( \theta , \varphi \right) \nonumber \\&-2\mu \left( 1-\nu \right) \cos \theta \Lambda j_n \left( \beta _p r \right) Y_{n+1}^m \left( \theta , \varphi \right) \Bigg ) \end{aligned}$$
(105)
$$\begin{aligned} \sigma _{\varphi \varphi _p}= & {} \frac{1}{2 \left( 1-\nu \right) r^2 \sin ^2 \theta } \Bigg [\Bigg (\Big [ \lambda \left( 2\nu -1 \right) \beta _p^2 r^2\nonumber \\&- 2\mu \left( 1-\nu \right) \left( 2 m^2+1 \right) \nonumber \\&+ \left[ \lambda \left( 1-2\nu \right) \beta _p^2 r^2 \right. \nonumber \\&\left. -2\mu \left( 1-\nu \right) \left( 2n+1 \right) \right] \cos \left( 2\theta \right) \Big ] j_n \left( \beta _p r \right) \nonumber \\&- 4 \mu \left( 1-\nu \right) \beta _p r \sin ^2 \theta j_{n+1} \left( \beta _p r \right) \Bigg ) Y_n^m \left( \theta , \varphi \right) \nonumber \\&+ 4\mu \left( 1-\nu \right) \cos \theta \Lambda j_n \left( \beta _p r \right) Y_{n+1}^m \left( \theta , \varphi \right) \Bigg ] \end{aligned}$$
(106)
$$\begin{aligned} \sigma _{r \theta _{p}}= & {} \frac{2\mu }{r^2 \sin \theta } \left[ \left( 1-n \right) j_n \left( \beta _{p} r \right) + \beta _{p} r j_{n+1} \left( \beta _{p} r \right) \right] \nonumber \\&\left[ \left( n+1 \right) \cos \theta Y_n^m \left( \theta , \varphi \right) - \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \right] \end{aligned}$$
(107)
$$\begin{aligned} \sigma _{r \varphi _{p}}= & {} \frac{2 {\hat{\imath }} m \mu }{r^2 \sin \theta } Y_n^m \left( \theta , \varphi \right) \nonumber \\&\left[ \left( n-1 \right) j_n \left( \beta _{p} r \right) - \beta _{p} r j_{n+1} \left( \beta _{p} r \right) \right] \end{aligned}$$
(108)
$$\begin{aligned} \sigma _{\theta \varphi _{p}}= & {} \frac{-2 {\hat{\imath }} m \mu }{r^2 \sin ^2 \theta } j_n \left( \beta _{p} r \right) \nonumber \\&\left[ \left( n+2 \right) \cos \theta Y_n^m \left( \theta , \varphi \right) - \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \right] \end{aligned}$$
(109)

Appendix D: S-waves functions

The components of the S1 and S2-waves displacements are obtained substituting the potentials (42)–(47) into Eq. (31)

$$\begin{aligned} u_{r_{s_1}}= & {} \frac{{\hat{\imath }} n \left( n+1 \right) }{2mr \sin ^2 \theta } \left[ \cos \left( 2 \theta \right) -1 \right] j_n \left( \beta _s r \right) Y_n^m \left( \theta , \varphi \right) \end{aligned}$$
(110)
$$\begin{aligned} u_{\theta _{s_1}}= & {} \frac{{\hat{\imath }}}{mr \sin \theta } \left[ \left( n+1 \right) j_n \left( \beta _s r \right) - \beta _s r j_{n+1} \left( \beta _s r \right) \right] \nonumber \\&\left[ \left( n+1 \right) \cos \theta Y_n^m \left( \theta , \varphi \right) - \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \right] \end{aligned}$$
(111)
$$\begin{aligned} u_{\varphi _{s_1}}= & {} \frac{Y_n^m \left( \theta , \varphi \right) }{r \sin \theta } \left[ \left( n+1 \right) j_n \left( \beta _s r \right) - \beta _s r j_{n+1} \left( \beta _s r \right) \right] \end{aligned}$$
(112)
$$\begin{aligned} u_{r_{s_2}}= & {} \frac{{\hat{\imath }}}{2m \beta _s r^2 \sin ^2 \theta } \Bigg (\Big [\left( 2n^2-n-1 \right) \left( \cos \left( 2 \theta \right) -1 \right) j_n \left( \beta _s r \right) \nonumber \\&+ 2 \left( n-1 \right) \beta _s r \sin ^2 \theta j_{n+1} \left( \beta _{s_{s}} r \right) \Big ] \cos \theta Y_n^m \left( \theta , \varphi \right) \nonumber \\&+ 2 \sin ^2 \theta \left[ \left( n-1 \right) j_n \left( \beta _s r \right) \right. \nonumber \\&\left. - \beta _s r j_{n+1} \left( \beta _s r \right) \right] \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \Bigg ) \end{aligned}$$
(113)
$$\begin{aligned} u_{\theta _{s_2}}= & {} \frac{{\hat{\imath }}}{2m \beta _s r^2 \sin \theta } \Bigg [\Big ([2m^2+n+\beta _s^2 r^2 \nonumber \\&+ \left( 2n^2+n- \beta _s^2 r^2 \right) \cos \left( 2\theta \right) ] j_n \left( \beta _s r \right) \nonumber \\&- \beta _s r \left( n+2+n \cos \left( 2\theta \right) \right) j_{n+1} \left( \beta _s r \right) \Big ) Y_n^m \left( \theta , \varphi \right) \nonumber \\&- 2 \cos \theta \left( n j_n \left( \beta _s r \right) \right. \nonumber \\&\left. - \beta _s r j_{n+1} \left( \beta _s r \right) \right) \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \Bigg ] \end{aligned}$$
(114)
$$\begin{aligned} u_{\varphi _{s_2}}= & {} \frac{1}{\beta _sr^2 \sin \theta } \Bigg (\Big [\left( 2n+1 \right) j_n \left( \beta _s r \right) \nonumber \\&- \beta _s r j_{n+1} \left( \beta _s r \right) \Big ] \cos \theta Y_n^m \left( \theta , \varphi \right) \nonumber \\&- \Lambda j_n \left( \beta _s r \right) Y_{n+1}^m \left( \theta , \varphi \right) \Bigg ) \end{aligned}$$
(115)

The stresses of the S1 and S2-waves are calculated substituting Eqs. (110)–(115) into Eqs. (88)–(99)

$$\begin{aligned} \sigma _{rr_{s_1}}= & {} \frac{2\mu {\hat{\imath }} n \left( n+1 \right) }{mr^2} Y_n^m \left( \theta , \varphi \right) \left[ \left( 1-n \right) j_n \left( \beta _s r \right) \right. \nonumber \\&\left. + \beta _s r j_{n+1} \left( \beta _s r \right) \right] \end{aligned}$$
(116)
$$\begin{aligned} \sigma _{\theta \theta _{s_1}}= & {} \frac{\mu {\hat{\imath }}}{mr^2 \sin ^2 \theta } \Bigg [\Big (\left( n+1 \right) [n^2-2m^2-n\nonumber \\&-1- \left( n^2+n+1 \right) \cos \left( 2\theta \right) ] j_n \left( \beta _s r \right) \nonumber \\&+ \beta _s r \left[ 2m^2-n^2+1\right. \nonumber \\&\left. + \left( n+1 \right) ^2 \cos \left( 2\theta \right) \right] j_{n+1} \left( \beta _s r \right) \Big ) Y_n^m \left( \theta , \varphi \right) \nonumber \\&+ 2 \cos \theta \left[ \left( n+1 \right) j_n \left( \beta _s r \right) \right. \nonumber \\&\left. - \beta _s r j_{n+1} \left( \beta _s r \right) \right] \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \Bigg ] \end{aligned}$$
(117)
$$\begin{aligned} \sigma _{\varphi \varphi _{s_1}}= & {} \frac{\mu {\hat{\imath }}}{mr^2 \sin ^2 \theta } \Bigg [\Big (\left( n+1 \right) \left[ 2m^2+1\right. \nonumber \\&\left. + \left( 2n+1 \right) \cos \left( 2 \theta \right) \right] j_n \left( \beta _s r \right) \nonumber \\&- \beta _s r \left[ 2m^2+n+1\right. \nonumber \\&\left. + \left( n+1 \right) \cos \left( 2\theta \right) \right] j_{n+1} \left( \beta _s r \right) \Big ) Y_n^m \left( \theta , \varphi \right) \nonumber \\&- 2 \cos \theta \left[ \left( n+1 \right) j_n \left( \beta _s r \right) \right. \nonumber \\&\left. - \beta _s r j_{n+1} \left( \beta _s r \right) \right] \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \Bigg ] \end{aligned}$$
(118)
$$\begin{aligned} \sigma _{r \theta _{s_1}}= & {} \frac{\mu {\hat{\imath }}}{mr^2 \sin \theta } \left[ \left( 2n^2-2-\beta _s^2 r^2 \right) j_n \left( \beta _s \right) \right. \nonumber \\&\left. + 2 \beta _s r j_{n+1} \left( \beta _s r \right) \right] \nonumber \\&\left[ \left( n+1 \right) \cos \theta Y_n^m \left( \theta , \varphi \right) - \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \right] \end{aligned}$$
(119)
$$\begin{aligned} \sigma _{r \varphi _{s_1}}= & {} \frac{\mu }{r^2 \sin \theta } Y_n^m \left( \theta , \varphi \right) \left[ \left( 2n^2-2-\beta _s^2 r^2 \right) j_n \left( \beta _s r \right) \right. \nonumber \\&\left. + 2 \beta _s r j_{n+1} \left( \beta _s r \right) \right] \end{aligned}$$
(120)
$$\begin{aligned} \sigma _{\theta \varphi _{s_1}}= & {} \frac{-2\mu }{r^2 \sin ^2 \theta } \left[ \left( n+1 \right) j_n \left( \beta _s r \right) - \beta _s r j_{n+1} \left( \beta _s r \right) \right] \nonumber \\&\left[ \left( n+2 \right) \cos \theta Y_n^m \left( \theta , \varphi \right) - \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \right] \end{aligned}$$
(121)
$$\begin{aligned} \sigma _{rr_{s_2}}= & {} \frac{2\mu {\hat{\imath }}}{m\beta _s r^3} \Big [ \left( \left[ \beta _s^2 r^2 + n \left( 3-2n \right) +2 \right] j_n \left( \beta _s r \right) \right. \nonumber \\&\left. + \beta _s r \left( n-2 \right) j_{n+1} \left( \beta _s r \right) \right) \left( n-1 \right) \cos \theta Y_n^m \left( \theta ,\varphi \right) \nonumber \\&+ \left( \left[ n \left( n-3 \right) +2-\beta _s^2 r^2 \right] j_n \left( \beta _s r \right) \right. \nonumber \\&\left. + 4 \beta _s r j_{n+1} \left( \beta _s r \right) \right) \Lambda Y_{n+1}^m \left( \theta ,\varphi \right) \Big ] \end{aligned}$$
(122)
$$\begin{aligned} \sigma _{\theta \theta _{s_2}}= & {} \frac{\mu {\hat{\imath }}}{m \beta _s r^3 \sin ^2{\theta }} \Bigg (\Big [\Big ( 1-4m^2\left( n+1 \right) \nonumber \\&-n \left[ n \left( 3-2n \right) +1+\beta ^2 r^2 \right] \nonumber \\&- \left( 1+n \left[ 1+n\left( 2n-1 \right) \right. \right. \nonumber \\&\left. \left. -\beta ^2 r^2 \right] \right) \cos \left( 2\theta \right) ) j_n \left( \beta _s r \right) \nonumber \\&+ \beta _s r \left( 2m^2+1-n \left( n-3 \right) \right. \nonumber \\&\left. + \left[ 1+n \left( n-1 \right) \right] \cos \left( 2\theta \right) \right) j_{n+1} \left( \beta _s r \right) \Big ] \cos \theta Y_n^m \left( \theta ,\varphi \right) \nonumber \\&+ \Big (\left[ 2m^2-1+n\left( 3-n \right) + \beta ^2 r^2 \right. \nonumber \\&\left. + \left( 1-n+n^2-\beta ^2 r^2 \right) \cos \left( 2\theta \right) \right] j_n \left( \beta _s r \right) \nonumber \\&+ 2 \beta _s r \left( \cos \left( 2\theta \right) -2 \right) j_{n+1} \left( \beta _s r \right) \Big ) \Lambda Y_{n+1}^m \left( \theta ,\varphi \right) \Bigg ) \end{aligned}$$
(123)
$$\begin{aligned} \sigma _{\varphi \varphi _{s_2}}= & {} \frac{\mu {\hat{\imath }}}{m\beta _s r^3 \sin ^2{\theta }} \Bigg (\Big [\Big (2 \left[ 2m^2\left( n+1 \right) - n^2+n \right] \nonumber \\&+ 1 + \beta _s^2 r^2 + \left( 4n^2-1-\beta ^2 r^2 \right) \cos \left( 2\theta \right) \Big ) j_n \left( \beta _s r \right) \nonumber \\&+ \beta r \left[ \left( 1-2n \right) \cos \left( 2\theta \right) \right. \nonumber \\&\left. -2m^2 -3 \right] j_{n+1} \left( \beta _s r \right) \Big ] \cos \theta Y_n^m \left( \theta ,\varphi \right) \nonumber \\&+ \left( \left[ \left( 1-2n \right) \cos \left( 2 \theta \right) -2m^2-1 \right] j_n \left( \beta _s r \right) \right. \nonumber \\&\left. + 2 \beta _s r \cos \left( 2 \theta \right) j_{n+1} \left( \beta r \right) \right) \Lambda Y_{n+1}^m \left( \theta ,\varphi \right) \Bigg ) \end{aligned}$$
(124)
$$\begin{aligned} \sigma _{r \theta _{s_2}}= & {} \frac{\mu {\hat{\imath }}}{2m\beta _s r^3 \sin \theta } \Bigg (\Big [\Big [2 \left( n-2 \right) \left( 2m^2+n \right) - 3 \beta _s^2 r^2 \nonumber \\&+ \left[ 2n^2 \left( 2n-3 \right) +\beta ^2 r^2 \right. \nonumber \\&\left. -2n \left( \beta _s^2 r^2 +2 \right) \right] \cos \left( 2 \theta \right) \Big ] j_n \left( \beta _s r \right) \nonumber \\&+ \beta _s r (2n \left( n+4 \right) +4 \left( 3-m^2 \right) -\beta ^2 r^2 \nonumber \\&+ \left[ 2n \left( 2-n \right) + \beta ^2 r^2 \right] \cos \left( 2 \theta \right) ) j_{n+1} \left( \beta _s r \right) \Big ] Y_n^m \left( \theta , \varphi \right) \nonumber \\&+ 2 \cos \theta \left( \left[ 2n \left( 2-n \right) + \beta _s^2 r^2 \right] j_n \left( \beta _s r \right) \right. \nonumber \\&\left. -6 \beta _s r j_{n+1} \left( \beta _s r \right) \right) \Lambda Y_{n+1}^m \left( \theta ,\varphi \right) \Bigg ) \end{aligned}$$
(125)
$$\begin{aligned} \sigma _{r \varphi _{s_2}}= & {} \frac{\mu }{\beta _s r^3 \sin \theta } \Bigg [\Big (\left( 2 \left( 2n^2-3n-2 \right) - \beta ^2 r^2 \right] j_n \left( \beta _s r \right) \nonumber \\&+ 2 \left( 2-n \right) \beta _s r j_{n+1} \left( \beta _s r \right) \Big ) \cos \theta Y_n^m \left( \theta ,\varphi \right) \nonumber \\&+ 2 \left[ \left( 2-n \right) j_n \left( \beta _s r \right) \right. \nonumber \\&\left. + \beta _s r j_{n+1} \left( \beta _s r \right) \right] \Lambda Y_{n+1}^m \left( \theta ,\varphi \right) \Bigg ] \end{aligned}$$
(126)
$$\begin{aligned} \sigma _{\theta \varphi _{s_2}}= & {} \frac{\mu }{2\beta _sr^3\sin ^2 \theta } \Bigg (\Big [\Big (\left[ \beta _s^2 r^2 - 2 \left( 2n^2+3n+1 \right) \right] \cos \left( 2\theta \right) \nonumber \\&- \left[ 2 \left( 2m^2+3n+1 \right) + \beta _s^2r^2 \right] \Big ) j_n \left( \beta _s r \right) \nonumber \\&+ 2 \beta _s r \left[ n+3+ \left( n+1 \right) \cos \left( 2\theta \right) \right] j_{n+1} \left( \beta _s r \right) \Big ] Y_n^m \left( \theta ,\varphi \right) \nonumber \\&+ 4 \cos \theta \left[ \left( n+1 \right) j_n \left( \beta _s r \right) \right. \nonumber \\&\left. - \beta _s r j_{n+1} \left( \beta _s r \right) \right] \Lambda Y_{n+1}^m \left( \theta , \varphi \right) \Bigg ) \end{aligned}$$
(127)

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Climent, N., Moldovan, I.D. & Bendea, E.D. Hybrid-Trefftz displacement elements for three-dimensional elastodynamics. Comput Mech 70, 1083–1105 (2022). https://doi.org/10.1007/s00466-022-02224-4

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