THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

466 16. Ultrasonicsthe mechanical resistance is given byR m = σ Au= ρcAin terms of the stress, radiating surface velocity u, material density ρ, and soundpropagation speed c. For the electrical Q factor, electrical capacitance C e betweentransducer electrodes must be taken into consideration. At resonant frequenciesthe only effective mechanical impedance is R m . ThusQ e∼ = Ce ω r R = C e ω rR mα 2 T= π 2 /2k 2 eQ mMagnetostrictive TransducersMagnetostriction occurs in ferromagnetic materials and certain nonmetals thatare termed ferrites. When a magnetic field is applied, a bar of ferromagnetic orferrimagnetic material undergoes a change in length. Conversely, a mechanicalstress applied to the bar will cause a change in intensity of magnetization. Theformer effect was discovered by Joule in 1847 and the converse effect by Villari in1868.Magnetostriction occurs prominently in materials such as iron, nickel, andcobalt. Whether there occurs an increase or decrease in length fully depends on thenature of the material as well as on the strength of the applied magnetic field. Thechange in length does not depend on the direction of the magnetic field. The magnitudeof the strain varying as a function of the applied magnetic field is shownin Figure 16.5 for four different materials, viz. cast cobalt, permendur, nickel,and iron. Figure 16.6 shows strain varying as a function of magnetic polarization.The magnetostrictive effect generally decreases with a rise in temperature anddisappears altogether at the Curie temperature.Figure 16.5. Mechanical strain as a function of magnetic field.