3nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Nov 14-16, 2001, Seville Spain

Introduction

The Model Alloy project, carried out by the JRC (Joint Research Centre) within the frame of the European Network AMES (Ageing Material Evaluation System), has the objective to study the influence of elements, i.e. Phosphorus, Cooper and Nickel on the mechanical properties of steels by different content alloys. As these model alloys are a representative of a large spectrum of ferritic steels with parametric variation of elements known to play a significant role in material characteristic and degradation, they provide an opportunity also to study the elements influence on the steels when exposed to degradation by irradiation.

The knowledge of mechanical property changes due to irradiation is included in in-service inspections, nowadays part of the lifetime management studies of nuclear power plants. The mechanical properties changes are followed by destructive and non-destructive tests. There is a continuous highlight on developing cost effective and reliable non-destructive testing to structural integrity assurance of nuclear components.

This paper presents the results of micromagnetic measurements, Barkhausen signal measurements, in the model alloys sub-size Charpy-V specimens at fresh and irradiated conditions. The Barkhausen signal parameter is the root-mean-square value. Statistical analysis is performed on the Barkhausen results with influence of different settings.

The samples were irradiated in the LYRA rig at the High Flux Reactor in the Petten (Netherlands) site of the Joint Research Centre.

Irradiation effects on the mechanical properties were studied through transition curve.

The relationships of Barkhausen signal to chemical composition, mechanical and physical properties have been carried out on the results.

TESTED MATERIAL

The samples were 3×4×27 mm, sub-size Charpy-V specimens. The irradiated samples received a dose of ~10×10^-3 dpa at 270°C.

The classification of the specimen of the model alloys was in accordance to element compositions of the originating block producing numbered specimens at fresh or irradiated condition.

The table 1 presents respectively the block number and chemical composition in term of the main parameter elements, i.e. Nickel, Copper and Phosphorus, and table 2 shows sample number related to condition and blocks.

Block No	Cu wt%	Ni wt%	P wt%	Block No	Cu wt%	Ni Wt%	P wt%
175	0.110	1.14	0.010	444	0.110	1.2	0.001
177	0.390	1.2	0.002	445	0.110	1.22	0.002
178	0.400	1.2	0.009	633	0.005	0.005	0.002
179	0.006	1.98	0.001	635	0.005	0.007	0.029
180	0.110	1.97	0.001	636	0.100	0.009	0.004
181	0.110	1.98	0.006	637	0.100	0.006	0.012
183	0.400	1.98	0.002	638	0.100	0.007	0.035
184	0.410	1.99	0.008	639	0.400	0.004	0.002
439	0.110	0.20	0.039	641	0.990	0.003	0.002
441	0.400	0.71	0.001	642	0.39	0.005	0.031
443	0.006	1.21	0.001	643	0.98	0.004	0.011
Table 1: Block number and chemical composition in terms of key elements in weight percent (wt%).

Block Nr.	Sample Nr.		Block Nr.	Sample Nr.
	Fresh	Irradiated		Fresh	Irradiated
175	690	669	444	614	609
177	732	729	445	644	639
178	764	757	633	28	24
179	794	*	635	88	*
180	838	819	636	105	101
181	854	849	637	150	127
183	914	909	638	180	159
184	960	936	639	*	199
439	*	461	641	*	253
441	540	519	642	299	279
443	584	579	643	330	301
Table 2: Specimen number related to condition and originating block.

* Sample not available

Test Procedure

The main device is a typical yoke-coil equipment used to generate and captured the so-called Barkhausen jumps, bursts or signals. The device is provided by AEKI (Atomic Energy Institute, Hungary) through a researching cooperation with JRC.

The probe consists of a wounded coil inducing magnetic flux in iron electromagnet yoke with a secondary coil placed among its pole legs, see figure 1. The probe is an open electromagnetic circuit closed by specimen or sampled material, in our case sub-size Charpy-V specimen.

The sub-size Charpy-V specimen is positioned in the center of the coil with the specimen touching the yoke poles, closing the circuit. The specimen notch points upwards.

The magnetic flux going through the yoke is called the excitation or applied magnetic field. This magnetic applied field is adjusted by applied excitation, current, on the wounded coil. The adjustment is done by a relative value (applied excitation relative value) proportional to current intensity.

A coil is used to capture the Barkhausen signals, induced voltage, generated by the AC-applied field produced by AC sinusoidal current in the yoke wounded coil.

The electromagnet yoke with 17 mm distance between magnetic poles, centers a pick up (or sensing coil type) probe with dimensions of 10 mm external diameter and 5 mm internal diameter sensitive coil.

As it can be seen from the probe and sample dimensions the magnetizing poles and the pick up coil are bigger than the sample. Therefore, applied magnetic flux leakage occurs in the probe arrangement being captured by the sensing coil providing background noise to Barkhausen measurements. Therefore the Barkhausen RMS values between fresh and irradiated can be used as a relative and not absolute value. However, the device can control the magnetic flux within the specimen by fluxing detection keeping it constant to the selected relative value and so this feature was used. It measures the applied flux given by primary coil (the yoke wounded) and the flux entering on the yoke and calculated the difference between coil voltage 2 and 1 (Æ₂ - Æ₁), representative of difference between produced magnetic flux and flux passing through specimen which controls the magnetic flux variation due to flux leakage and specimen properties.

The used applied excitation frequency is 10 Hz and the filter to capture the Barkhausen signal is a 500 kHz high-pass band.

Fig 1: Schematic arrangement of the probe (side view).

Experimental Determination of Measurement Error and Deviations by Statistical Analysis

Device reliability, measurement repeatability and the effect of background noises are important factors to take into account in any non-destructive testing measurements. One of the tools is the statistical analysis of the results at different settings once factor levels can be changed or choose to ones that minimize deviations and errors. The statistical analysis was performed in the Barkhausen measurements and it is presented in this section.

Repeated measurements at same specimen by same device settings were done as well on different excitation levels and mode to provide statistical calculation to repeatability and influence of the generating excitation.

Deviation and error of the measuring system were investigated by measurements at different relative applied excitation values in different specimens and conditions.

The spread (variability) of the data values were measured by the range value:

R = x_max – x_min; the largest minus the smallest data value,and by standard deviation (SD).

The standard deviation is calculated as follows:




Var is the population variance given by:




n is number of data points, and is the mean.

The standard error of the mean (SEM) is calculated as follows:




The statistical analysis on RMS Barkhausen signal, possible measure of the dispersion, or scatter, of the measurements taken at different specimens and applied excitation levels are given on the table 3. The applied excitation levels are given by setting relative applied excitation (Rel. Exc.) value.

Specimen	Rel. Exc.	RMS Bark Mean V	SD V	SEM V	xmin V	xmax V	R V	n	State
732	600	0.959	0.109	0.016	0.70	1.17	0.46	45	Fresh
794	400	0.422	0.021	0.003	0.38	0.45	0.06	45	Fresh
88	200	0.407	0.015	0.001	0.37	0.45	0.07	298	Fresh
960	300	0.384	0.011	0.002	0.36	0.40	0.04	26	Fresh
732	300	0.386	0.013	0.002	0.37	0.41	0.04	52	Fresh
159	300	0.492	0.013	0.003	0.465	0.517	0.05	17	Irradiated
199	300	0.379	0.012	0.002	0.357	0.4	0.04	29	Irradiated
253	300	0.358	0.008	0.002	0.345	0.377	0.03	30	Irradiated
279	300	0.347	0.008	0.002	0.333	0.366	0.03	26	Irradiated
936	300	0.356	0.014	0.003	0.332	0.378	0.05	20	Irradiated
Table 3: Statistical calculation on RMS Barkhausen signal to different specimens and applied excitation.

R range value, xmax largest value, xmin smallest value, SD standard deviation, Var population variance, SEM standard error of the mean value and n number of measurements

In order to easier comparison, percentage variation of measurements was calculated on Barkhausen RMS mean values and besides results.

The standard deviation related to the mean value of the RMS Barkhausen signal, in percentage variation, was calculated and they are given in the table 4. The relationship being:

SD variation (in %) = SD * 100 / Mean value

Specimen	Rel. Exc.	SD variation (%)
732	600	11.4
794	400	5
960	300	2.4
732	300	3.4
253	300	2.2
159	300	2.6
199	300	3.2
279	300	2.3
936	300	3.9
88	200	3.7
Table 4: Percentage variation of standard deviation to mean value on RMS Barkhausen values.

Results

By statistical analysis and amplifier limitation the 301 Exc. Rel., relative applied excitation, value is chosen to evaluate the RMS Barkhausen signal of the model alloys samples in fresh and irradiated conditions. The measurements are presented in table 5.

Block No.	Specimen	Barkhausen RMS, Volt	Rel. Exc.
175	690 F	0.413	301
175	669 I	0.334	301
177	732 F	0.374	301
177	729 I	0.352	301
178	764 F	0.376	301
178	757 I	0.316	301
180	838 F	0.409	301
180	819 I	0.309	301
181	854 F	0.478	301
181	849 I	0.318	301
183	914 F	0.386	301
183	909 I	0.322	301
184	960 F	0.351	301
184	936 I	0.296	301
441	540 F	0.34	301
441	519 I	0.302	301
443	584 F	0.355	301
443	579 I	0.316	301
444	614 F	0.354	301
444	609 I	0.287	301
445	644 F	0.358	301
445	639 I	0.329	301
633	28 F	0.43	301
633	24 I	0.25	301
636	105 F	0.37	301
636	101 I	0.292	301
637	150 F	0.381	301
637	127 I	0.329	301
638	180 F	0.353	301
638	159 I	0.342	301
642	299 F	0.31	301
642	279 I	0.287	301
643	330 F	0.482	301
643	301 I	0.298	301
Table 5: RMS Barkhausen signal results for fresh and irradiated specimens for the different blocks (F = fresh, I = Irradiated)

The overview of the results can be better visualized in figure 2 where RMS Barkhausen signal measured in irradiated condition are visibly lower than the fresh ones for all specimens of the different blocks.

Fig 2: The RMS Barkhausen signal measured in fresh and irradiated specimens at 301 Rel. Exc. value.

The mechanical results are given as the Charpy transition temperature at 3.1 J (TT_3.1J), the measurements and variation between irradiated and fresh to the diverse blocks are given on table 6; see Figure 3.

Fig 3: Transition temperature as function of Cu, Ni and P.

Block No.	RMS Barkhausen Variation (%)	Shift DTT3.1J (°C)
175	19	121
177	5.9	172
178	16	184
180	24	130
181	33	182
183	17	174
184	16	222
441	11	171
443	11	123
444	19	128
445	8.1	115
633	42	111
636	21	31
637	14	29
638	3.1	103
642	7.4	143
643	38	191
Table 6: Variation of RMS Barkhausen signal, transition temperature, between the fresh and irradiated samples of the different model alloy blocks.

Conclusions

The present paper shows the results obtained by the Model Alloy project, carried out by the JRC (Joint Research Centre) within the frame of the European Network AMES (Ageing Material Evaluation System).

The objective of the project is to study the influence of elements, i.e. Phosphorus, Cooper and Nickel on irradiation embrittlement.

The samples were irradiated at 270 °C in the LYRA rig at the High Flux Reactor in the Petten (Netherlands) site of the Joint Research Centre.

The mechanical properties changes are followed by destructive and non-destructive tests and this paper in particular presents the results of micromagnetic measurements, Barkhausen signal measurements, in the model alloys sub-size Charpy-V specimens at fresh and irradiated conditions.

The Barkhausen signal measurements, RMS values, for Mini-Charpy specimens are found to change with irradiation on the model alloys for the used setting: low applied excitation amplitude of 10 Hz sinusoidal wave.

Decrease of the RMS Barkhausen noise values from 3 to 42 % are observed for the different irradiated model alloys; investigation of the influence of Cu, P and Ni are being carried out and preliminary results show that greater sensitivity of Barkhausen signal to irradiation is found on alloys with higher Nickel or Copper content. This fact can be related to the Nickel and Copper effects on the lattice structure, strain hardening.

The significant scattering of the Barkhausen measurements can be attribute to the fact that the probe was originally built up to Charpy and not Mini-Charpy specimens so there was a considerable flux leakage affecting the pick up signal. A pick up coil system as shown here is not recommendable to Mini-Charpy specimens and it would be necessary to use a different type, e.g. encircling coil or lateral grip poles.

Further works will be done through developing new Barkhausen probe to this specific use, analyzing the signal captured by digital oscilloscope, more detailed studies on the excitation and frequency effects as well attempts to measure the applied magnetic field on the specimens (Hall probe and different coil arrangements on yoke) and others magnetic properties.

References

1. "The European Network AMES", Sevini F, Debarberis L and Davies M, 19-23 June 2000, 6th International Conference on Materials Issues in Design, Manufacturing and Operation of NPP Equipment, St. Petersburg, Russia
2. "MODEL ALLOYS PROJECT Material Characterisation at the Ames Laboratory", Acosta B., Debarberis L., Beers M., McGirl C. and Kryukov. A:, IAM, JRC Petten, July 2000.
3. "IRRADIATION REPORT LYRA-03 Experiment"’ Acosta B, Beers M, Debarberis L and Sordon G, CEC-JRC-Institute for Advanced Materials, Petten, 1999.
4. Experimental Studies of Copper, Phosphorus and Nickel effect on RPV Model Alloys at Two Different Fluences" Debarberis L, Törrönen K, Sevini F, Acosta B, Kryukov A, Nikolaev Y and Valo M, VTT, 19-23 June 2000, 6^th International Conference on Materials Issues in Design, Manufacturing and Operation of NPP Equipment, St. Petersburg, Russia
5. "The effect of Nickel, Phosphorus and Copper in Irradiation Embrittlement of RPV Steel Model Alloys", Debarberis L, NATO International Workshop on RP Vessel Embrittlement, September 2000, Varna, Bulgaria

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