Magneto-mechanical actuation of bonded ferromagnetic fibre arrays

Strongly bonded assemblies of metallic fibres constitute an interesting class of highly porous, permeable materials. A high degree of control can be exercised over their properties, by tailoring the fibre architecture so as to achieve specified void contents, fibre connectivity and fibre orientation distributions. There is also scope for introducing controlled heterogeneity and gradient structures. It is possible, by using relatively strong fibres, and generating appropriate fibre–fibre joint geometries, to produce material with relatively high tensile strength and toughness, facilitating usage in various load-bearing applications. Furthermore, if ferromagnetic fibres are employed, then the material can be actuated by the imposition of a magnetic field, with the fibres becoming magnetised along their length and tending to align parallel with the applied field. The resultant deformation of the fibre array generates a shape change, which can be predicted for a given fibre orientation distribution and fibre segment aspect ratio (inter-joint distance over fibre diameter). Moreover, a non-magnetic (matrix) material located in the inter-fibre space will be mechanically strained by these fibre deflections. This was proposed in a previous publication as a possible mechanism for bone growth stimulation by magnetic field application, for example by making the surface layers of a prosthetic implant from a ferromagnetic fibre array, into which bone cell growth would occur. In the present paper, analyses are presented for prediction both of conventional elastic constants exhibited by bonded fibre arrays and of novel magneto-mechanical elastic constants. These analyses also allow identification of conditions for the onset of inelastic behaviour. Comparisons are made with experimental data, relating to nominally isotropic fibre arrays, with and without the presence of relatively compliant matrices. It is confirmed that a simple modelling approach can give fairly reliable indications of how the material will behave, under both mechanical and magnetic loading.