TAILIEUCHUNG - Nondestructive Evaluation and Quality Control (1998) Part 4

Tham khảo tài liệu 'nondestructive evaluation and quality control (1998) part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 10 Hysteresis loops of the material under two magnetomotive forces HB and HRF Fig. 11 RF permeability as a function of the alternating magnetomotive force HB at a fixed value of the RF magnetomotive force HRF. The numbers on the curves correspond to numbers on the curve of HB in Fig. 10. The variation in pRF as a function of HB is the basis for the magabsorption phenomenon. Because the RF permeability RF changes as HB is varied sinusoidally the energy absorbed by the material from the RF coil also varies as a function of HB. These variations in the absorption of RF energy by the material can also cause changes in the impedance of the RF coil. For example if a cylindrical sample is placed in the coil as shown in Fig. 12 the change in coil impedance AZ is related to the energy absorbed according to Eq 3. Fig. 12 Basic magabsorption circuit composed of an inductance L with the sample core in a biasing field HB and tuned to resonance with capacitance C. The circuit is fed from an RF voltage e E cos . t The variations in coil impedance from magabsorption involve a resistive change AR and a change in inductance AL. Both of these changes can be related to the RF permeability pRF. Because the RF hysteresis loop has a shape other than a straight line that is an ellipse if HRF is HRF exp i ữt the RF induction is BRF exp i ữt 0 . Therefore the RF permeability is a complex variable

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