Spatial deformation of a ferromagnetic elastic rod

TL;DR

This paper analyzes the complex deformation behaviors of ferromagnetic elastic rods under magnetic fields and mechanical loads, revealing bifurcations, buckling modes, and unique magnetoelastic effects through Hamiltonian phase space analysis.

Contribution

It introduces a Hamiltonian framework for three-dimensional ferromagnetic rod deformation, identifying bifurcations and localized buckling modes influenced by magnetoelastic coupling.

Findings

Purely elastic and hard ferromagnetic rods undergo supercritical Hamiltonian Hopf bifurcation.

Soft ferromagnetic rods exhibit bifurcation only within a specific magnetoelastic parameter range.

Localized buckling modes show non-collinear extended segments due to magnetoelastic effects.

Abstract

Ferromagnetic elastic slender structures offer the potential for large actuation displacements under modest external magnetic fields, due to the magneto-mechanical coupling. This paper investigates the phase portraits of the Hamiltonian governing the three-dimensional deformation of inextensible ferromagnetic elastic rods subjected to combined terminal tension and twisting moment in the presence of a longitudinal magnetic field. The total energy functional is formulated by combining the Kirchhoff elastic strain energy with micromagnetic energy contributions appropriate to soft and hard ferromagnetic materials: magnetostatic (demagnetization) energy for the former, and exchange and Zeeman energies for the latter. Exploiting the circular cross-sectional symmetry and the integrable structure of the governing equations, conserved Casimir invariants are identified and the Hamiltonian is reduced to a single-degree-of-freedom system in the Euler polar angle. Analysis of the resulting phase portraits reveals that purely elastic and hard ferromagnetic rods undergo a supercritical Hamiltonian Hopf pitchfork bifurcation, whereas soft ferromagnetic rods exhibit this bifurcation only within a restricted range of the magnetoelastic parameter, $0<\tilde{K}_{dM}<1/8$. Both helical and localized post-buckling configurations are analyzed, and the corresponding load-deformation relationships are systematically characterized across a range of loading scenarios. Localized buckling modes, corresponding to homoclinic orbits in the Hamiltonian phase space, are constructed numerically. In contrast to the purely elastic case, the localized configurations of soft ferromagnetic rods exhibit non-collinear extended straight segments, a geometrically distinctive feature arising directly from the magnetoelastic coupling.