Tham khảo tài liệu 'nuclear power control, reliability and human factors part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Non-Destructive Testing for Ageing Management of Nuclear Power Components 319 Ma Fig. 10. Incremental permeability profile curves documenting the influence of mechanical stress steel quality X20Cr13 The incremental permeability profile-curve pA Ht as a function of a controlled applied magnetic field Ht is a well defined property of the material and independent of the magnetic prehistory as long as HMax HC and AH HC. The frequency fA of the incremental field AH is a parameter for selecting the depth of the analyzed near surface zone fA should be chosen such that fA 100 X f where f is the frequency of the applied field Ht controlling the hysteresis. pA Ht is measured as eddy current impedance parallel to the hysteresis reversals. The hysteresis is modulated by the alternating field AH excited by the eddy current coil. The spatial resolution is the same as that for eddy current coils. Figure 9 shows the hysteresis with the inner loops performed by the above mentioned modulation. By definition pA Ht is proportional to the inclination of each individual inner loop touching the hysteresis for magnetic field values Ht. In Figure 10 pA Ht profile-curves are presented indicating the characteristic measuring parameters as function of mechanical stresses. The dynamic or incremental magnetostriction profile-curve EÀ Ht is the intensity of ultrasound which is excited and received by an EMAT Electro-Magnetic-AcousticTransducer Salzburger 2009 for instance by measuring a back-wall echo caused by magnetostrictive excitation as a function of the applied field Ht controlling the hysteresis. The incremental alternating field AH in this case is excited by the EMA - transmitter using a pulsed current. The magnetostriction is modulated. Figure 11 upper part The spatial resolution -depending on the transmitter design - is of the order of 5 mm. In order to achieve such a spatial resolution an EMA - receiver was designed to transform the ultrasonic signal into an electrical signal only .
