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Harmonic frequency analysis of acoustic Barkhausen noise on neutron irradiated materialCheul-Muu Sim, Seung-Sik Park, Young-Yeal Song, Duck-Gun Park, Kee-Ok Chang
Korea Atomic Energy Research Institute
P.O BOX 105, Yusong, Daejon, 305-600, Korea
In relation to a non-destructive evaluation of irradiation damages of micro-structure interstitial, void and dislocation, the changes in the hysteresis loop and Barkhausen noise amplitude and the harmonics frequency due to a neutron irradiation were measured and evaluated. The Mn-Mo-Ni low alloy steel of RPV was irradiated to a neutron fluence of 2.3´1019 n/cm2 (E³1 MeV) at 288 °C. The saturation magnetization of neutron irradiated metal did not change. The neutron irradiation caused the coercivity to increase, whereas susceptibility to decrease. The amplitude of Barkhausen noise parameters associated with the domain wall motion were decreased by a neutron irradiation. The spectrum of Barkhausen noise is analyzed with an applied frequency of 4 Hz and 8 Hz, sampling time of 50 msec and 20msec. The harmonic frequency shows 4 Hz, 8 Hz, 12 Hz and 16 Hz reflected from an unirradiated specimen. On the contrary, the harmonic frequency disappeared on the irradiated specimen.
Monitoring the irradiation induced degradation of light water RPV(reactor pressure vessel) and research reactor HANARO is of primary concern to operating nuclear reactors in relation to the safety and the prediction of a life time. The degradation known as embrittlement has long been evaluated by using approved models and guidelines together with mechanical tests, such as Charpy impact tests and tensile tests through reactor surveillance programs[1,2] . Results from the models and surveillance program, however, do not always provide enough accurate information regarding the exact materials conditions about reactors. Further problem regarding the current surveillance program can be realized in limited number for the basis of life extension. Recently, several attempts have been made to utilize the measurements of magnetic parameter as a nondestructive testing method for examining the material state of reactor under operation [3,4,5,6,7] .
These investigations are based on the idea that microstructure changes due to irradiation inherently affect the magnetic domain wall movement and consequently affect the nature and magnitude of magnetic parameters . Until now, most of these examinations have been performed to explore interrelationship among the changes in the material and mechanical parameters such as grain size, microstructure, hardness, tensile properties and fracture toughness, etc. and magnetic parameters like Barkhausen noise and hysteresis loop due to irradiation.. In the present work, the changes and magnetic parameters related to Barkhausen noise amplitude, Barkhausen noise harmonics frequency and hysteresis loop parameters due to neutron irradiation were measured and analyzed to investigate the interrelationship between these parameters on RPV steel. Results showed a possibility that a magnetic measurement may be used for the nondestructive evaluation of material degradation.
Materials used in the present study were obtained from a surveillance test of operating reactor. The total accumulated fluence was 2.3´1019 n/cm2(E³1 MeV). The saturation magnetization, coercivity and susceptibility were measured by vibrating sample magnetometer. To minimize demagnetizing field effect, specimens were made in 3mm diameter disk. The block diagram for the Barkhausen noise parameters is shown in figure 1. An U-shaped ferrite magnet core was used to magnetize the specimen. The electromagnet was placed along the length of the specimen. A magnetic field was applied to the specimen by supplying a sinusoidal current of 4 Hz, 8 Hz to the electromagnet. An encircling sensing coil wound around the specimen was used to measure the magnetic induction in the specimen. The Barkhausen noise signal detected by the pick-up coil was amplified. The data were processed by a computer via a storage digital oscilloscope.
|Fig 1: Block diagram of Barkhausen noise measurement equipment|
3.1 Effect of neutron irradiation in magnetic
Figure 2 shows the comparison of the hysteresis loop for both an unirradiated base metal and an irradiated base metal. After neutron irradiation, specimen's hysteresis loop showed a clockwise rotation relative to that of unirradiated specimen. This means a reduction of relative susceptibility. The susceptibility of base metal decreases 50%. It is believed that the decrease of susceptibility comes from the interference of domain walls with the defects, such as strain, inclusions, lattice imperfection, or small amount of impurities. The saturation magnetization is known to be insensitive to structure in the sense that it does not depend on the details of fine structure. Therefore, saturation magnetization didn't show any change due to neutron irradiation and showed constant saturation magnetization of about 208 emu/g. The coercivity of base metal showed an increase from 8.8 Oe to 11.9 Oe. The coercivity can be explained by using the equation of Gyorgy et al . According to the equation, the coercivity can be expressed as :
where S is function of the size and density of precipitates, A is the exchange constant, K is the anisotropy constant, Ms is the saturation magnetization and t is the thickness. When A, K, Ms and t are known to be variables in the equation above, coercivity is able to be determined from the size and density of precipitates. Therefore, the formation of defect and the second phase due to a neutron irradiation bring in an increase of the coercivity. Same results were revealed in the work of Narayan that the size and shape of the second phase grain affect magnetic field and, in particular, coercivity in high strength and low alloy iron.
Fig 2: Comparison of hysteresis loops for base and weld metal with does of 2.3x1019n/cm2|
A Barkhausen noise waveform is in comparison between the irradiation and un-irradiation for base metal with a magnetic field, 15, 30, 45, 60, and 90 Oe, respectively. The Barkhausen noise signals reflect the statistical nature of both the number of event and individual pulse height via the degree of overlapping in the detector coil, in which the individual pulse is signified by the irreversible wall motion. Generally, it is known as Barkhausen noise amplitude decreases when the motion of domain wall is impeded by a retarding force. Therefore, the decrease of Barkhausen noise amplitude in neutron irradiated specimen is attributed to the hindrance of the domain wall induced by defect clusters. Figure 3 show a comparison of the Barkhausen noise waveform both the pre-irradiation and post -irradiation for base metal with a magnetic field, 4Hz.
|Fig 3: Magnetic Barkhausen waveforms of base metals at a magnetizing field of amplitude of 45 Oe|
3.2 Spectrum analysis of Barkhausen noise
The spectrum analysis is a useful method for a sort of random noise embedded with a deterministic signal to extract a certain characteristics of the specimen. In order to observe the harmonics frequency of the applied AC current, the spectrum analysis of the BN signal of specimen is performed. This method is able to supply the characteristic of the harmonics frequency as well as the fundamental frequency. The spectrum of input signals is represented as the equation of (2).
where f( n ) is the signal of time domain. f is represented as the AC signal.
where w = 2p f. In the experiment, frequency 4Hz, 8Hz and magnetic 45 Oe is adopted to meet the impedance-match. In case of 3 order model of harmonics, the output is depicted as below(4)
Where k0 - k3 is the harmonic parameter, A1 - A3 is an amplitude. The output spectrum involves the DC components, fundamental frequency and harmonics frequency. If w1, w2the input signals exist, the equation is expressed as the below.
where n and m results in n + m £ 3.
The spectrum of Barkhausen noise is analyzed with an applied frequency 4 Hz, 8 Hz of sampling time 50 msec, 20 msec. The spectrum of an applied magnetic field 4 Hz is plotted with the leakage phenomena. The leakage phenomena is resulting from the finite computation different from the equation 3-4 . In case of 4 Hz, the harmonic frequency shows at 4 Hz, 8 Hz and 12 Hz dominantly reflected from a pre-irradiation specimen in figure 4. In case of 8 Hz, the harmonic frequencies are shown at 8 Hz, 16 Hz dominantly echoed from the a pre-irradiation specimen. The harmonic frequency, however, disappeared on the irradiation specimen possibly due to the hindrance of the domain wall induced by the defect clusters in figures 5,[8,9]. The analysis of the harmonic frequency is a useful tool for the monitoring degradation induced by the irradiation.
|Fig 4: The harmonics frequency of an applied current 4 Hz on the unirradiated base metal|
|Fig 5: The non-harmonics frequency of an applied current 4 Hz on the irrdiated base metal|
Irradiation effects of neutron irradiated Mn-Mo-Ni low alloy steel were measured and compared using magnetic measurements. The saturation magnetization of neutron irradiated samples did not change. The neutron irradiation caused the coercivity to increase, whereas susceptibility to decrease. Barkhausen noise parameters associated with the domain wall motion were decreased by neutron irradiation. Barkhausen noise energy and susceptibility have increased while coercivity have decreased. The harmonic frequency show at 4 Hz, 8 Hz and 12 Hz reflected from an unirradiation specimen. The harmonic frequencies, however, disappeared on an irradiation specimen due to the hindrance of the domain wall induced by the defect clusters. The harmonic frequency can be a useful tool for the monitoring degradation induced by the irradiation. The results suggest that the magnetic measurements may be used as a promising nondestructive evaluation method in monitoring degradation of the reactor material such as a SA-508 of PWR-RPV steel and a Zr4 of HANARO-reactor due to a neutron irradiation.
This work was supported by the neutron beam utilization research as a part of a long term nuclear project of MOST
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