Magnetostriction

The magnetostriction , also called piezomagnetism , indicates the property which have the ferromagnetic materials to become deformed under the effect of a Magnetic field.

The phenomenon was discovered for the first time by James Prescott Joule in 1847, which showed its existence by measuring the lengthening of a sample of Fer subjected to a magnetic field.

When the material is magnetized until saturation, the maximum deformation produced by magnetostriction is generally about the Micromètre per centimetre of material. Thus, iron presents a weak deformation relative of to the more 0,2  μm/cm whereas alloys of iron and rare earths like the Terbium or the Dysprosium can reach relative deformations of more than 15  μm/cm under a magnetic field of: 1 M A/m.

The ferromagnetic materials exhibent also a magnetostrictive effect reverses , called magnetomechanical Effet, which is characterized by the modification of the magnetic Susceptibilité in the presence of forced mechanical in material.

Applications

According to the type of effect considered, direct or opposite magnetostrictive effect, magnetostriction can be exploited to build actuators or electromechanical pick-ups converting electrical energy into mechanical energy and reciprocally.

The principal uses of magnetostriction concern:

  • the emission of low frequency sounds in mileu underwater (Sonar)
  • the generation of ultrasounds (medical applications or industrial)
  • the realization of linear motors (used for example on Machine tool)
  • the measurement of force or couple
  • the active control of the noise and the vibrations, by simultaneously using the opposite effect for measurement of the vibrations, and the direct effect to carry out the correct action

A magnetostrictive Transducteur is typically made up of a Solénoïde crossed by a running of excitation being used to generate a variable magnetic field, inside of which is placed a bar out of ferromagnetic material, the whole being locked up in a cylinder ensuring the closing of the magnetic circuit. In order to ensure of bidirectional displacements, a static magnetizing of material is obtained either by means of a permanent Aimant, or while making circulate in the solenoid a D.C. current which comes to be superimposed on the variable operate current. In addition, a mechanical prestressing is generally applied to the ferromagnetic bar by means of a Boulon, this in order to make work the transducer in compression, and thus always to guarantee the transmission of the mechanical efforts to the interface of the transducer and load applied.

The ferromagnetic materials employed can be iron, the Nickel, or of the alloys Aluminum - iron (Alfenol) or nickel Cobalt, although the material most usually used either the Terfenol-D, because of its magnetostrictive good performances.

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