|NDT.net - August 2002, Vol. 7 No.08|
3nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Nov 14-16, 2001, Seville Spain
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.
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.
|Table 1: Block number and chemical composition in terms of key elements in weight percent (wt%).|
|Block Nr.||Sample Nr.||Block Nr.||Sample Nr.|
|Table 2: Specimen number related to condition and originating block.|
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 (ֶ2 - ֶ1), 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).|
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 = xmax – xmin; 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.
|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
|Table 4: Percentage variation of standard deviation to mean value on RMS Barkhausen values.|
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.
|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 (TT3.1J), 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.|
|Table 6: Variation of RMS Barkhausen signal, transition temperature, between the fresh and irradiated samples of the different model alloy blocks.|
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.
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