Generalized asymptotic expansions for the wavenumbers in infinite flexible in vacuo orthotropic cylindrical shells

Analytical expressions are found for the wavenumbers and resonance frequencies in flexible, orthotropic shells using the asymptotic methods. These expressions are valid for arbitrary circumferential orders n. The Donnell–Mushtari shell theory is used to model the dynamics of the cylindrical shell. Initially, an in vacuo   cylindrical isotropic shell is considered and expressions for all the wavenumbers (bending, near-field bending, longitudinal and torsional) are found. Subsequently, defining a suitable orthotropy parameter ϵϵ, the problem of wave propagation in an orthotropic shell is posed as a perturbation on the corresponding problem for an isotropic shell. Asymptotic expressions for the wavenumbers in the in vacuo   orthotropic shell are then obtained by treating ϵϵ as an expansion parameter. In both cases (isotropy and orthotropy), a frequency-scaling parameter (ηη) and Poisson's ratio (νν) are used to find elegant expansions in the different frequency regimes. The asymptotic expansions are compared with numerical solutions in each of the cases and the match is found to be good. The main contribution of this work lies in the extension of the existing literature by developing closed-form expressions for wavenumbers with arbitrary circumferential orders n in the case of both, isotropic and orthotropic shells. Finally, we present natural frequency expressions in finite shells (isotropic and orthotropic) for the axisymmetric mode and compare them with numerical and ANSYS results. Here also, the comparison is found to be good.

► Using Donnell–Mushtari shell theory orthotropy is posed as a perturbation to isotropy.
► Asymptotic expansions are found for wavenumbers in an infinite orthotropic shell.
► Resonance frequencies for orthotropic shells are calculated.
► The calculated resonances are compared with literature and Ansys.