implementation of small punch testing and automated ball

IMPLEMENTATION OF SMALL PUNCH TESTING AND AUTOMATED BALL INDENTATION IN THE PROCESS OF IRRADIATED NPP MATERIALS

DEGRADATION EVALUATION

Radim Kopriva UJV Rez, a. s.

Integrity and Technical Engineering Division Hlavni 130, Rez, 25068 Husinec, Czech Republic

Ivana Eliasova UJV Rez, a. s.

Integrity and Technical Engineering Division Hlavni 130, Rez, 25068 Husinec, Czech Republic

Milos Kytka

UJV Rez, a. s. Integrity and Technical Engineering Division

Hlavni 130, Rez, 25068 Husinec, Czech Republic

ABSTRACT

In the terms of nuclear power plant operational life management, current trend of components lifetime extension requires precise and credible information of structural material degradation. Present-day standard conventional methods of mechanical testing are usually based on the use of large specimens and higher consumption of testing material, whose availability and volume is often limited.

For determination of material properties, sampling of the necessary volume of material is in most cases connected with affecting the integrity or even destruction of the assessed component. Moreover, several components are not usually covered surveillance programs, e.g. reactor pressure vessel internals.

Innovative testing methods of Small Punch Testing (SPT) and Automated Ball Indentation Test (ABIT) are based on the determination of material properties from miniaturized testing specimens and their semi-destructive approach is very promising for the possibility of present data base of irradiated materials testing results enlargement and enable the option of component in-situ testing (ABI testing).

Paper describes the process of implementation of Small Punch Testing and Automated Ball Indentation Testing techniques in the evaluation of mechanical properties of irradiated NPP structural materials degradation. Presentation depicts all necessary steps carried out at the Hot Cell facility of UJV Rez, a. s. (Integrity and Technical Engineering Division) for the employment of these innovative testing techniques to the portfolio of accredited mechanical tests performed within the frame of surveillance programs of WWER type reactors -

e.g. preliminary design, modification of the testing equipment, fractography analyses etc.

Detailed comparison between the results of standard conventional testing techniques and SPT and ABIT for both initial unirradiated and irradiated state for the material A533B (JRQ) is included for determination of suitability of used methods for testing of materials within the surveillance programs of WWER type reactors.

INTRODUCTION

From 1970, ÚJV Řež, a. s., Division of Integrity and Technical Engineering, operates the fleet of 51 hot cells situated in the three floors of the Radiochemistry building. In the first period of operation until 1978, considerable attention was paid to verifying operational abilities of fuel elements. After the year 1980, most of activities are focused on the determination of degradation of NPP irradiated structural materials, especially from WWER components. At present, main purpose of the facility is to be support of Czech (Dukovany NPP, Temelin NPP), Slovak and several Ukrainian NPPs within the frame of their RPV surveillance programs. Besides the portfolio of standard accredited mechanical tests (impact testing, fracture toughness, etc.) is laboratory also focused on the development of innovative testing methods and their employment to the process of determination of irradiated material operational degradation – within Czech grant projects, EC framework programs and IAEA CRPs.

Paper presents results of the project “Development of innovative semi-destructive method of high active material evaluation for nuclear reactor components lifetime assessment”,

Proceedings of the 2016 24th International Conference on Nuclear Engineering ICONE24

June 26-30, 2016, Charlotte, North Carolina

ICONE24-60620

1 Copyright © 2016 by ASME

that was focused on the preparation of certified procedure for quantification of mechanical properties of structural materials of WWER type reactor components using the miniaturized specimens. Project was realized from 2013 to 2015 in cooperation with the Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department of Material Science.

NOMENCLATURE Rp0,2 – yield strength [MPa] Rm – ultimate tensile strength [MPa] Fe – force on the yield strength determined by small

punch tests [kN]. h0 – initial thickness of small punch specimen [mm]

MATERIALS Project was focused on confirming the suitability of the

ABIT for the determination of mechanical properties of structural materials of WWER type reactors components. For the purposes of the project testing matrix was prepared with important materials of WWER-440 type reactors – base material 15Ch2MFA, weld material 10ChMFT, reactor pressure vessel cladding 08Ch19N10G2B and IAEA correlation monitor A533B (JRQ) [1]. Paper depicts correlations of conventional test methods with results of innovative testing methods obtained for the material A533B (JRQ), which is used as a reference material in surveillance programs of the Czech nuclear power plants.

TESTING AND EVALUATION

Results of static tensile tests One of the first steps within the testing was to prepare data

base of standard static tensile tests results for subsequent preparation of correlations with perspective methods of semi-destructive testing, SPT and ABIT.

For the realization of tests, tensile testing specimens were prepared with 4 mm diameter and 20 mm gauge length in the transverse orientation in original block of material 5JRQ52. Tests were carried out at room temperature and +265 °C in accordance with the ISO 6892 [2] using video extensometer for precise measurements of elongation. Results are summarized in the Table 1, Annex A.

Results of Small Punch Tests Small punch test method is based on the testing of thin

clamped specimens using semi spherical punch with the measurement of the specimen deflection. Scheme of the test is shown on the Fig. 1. Obtained results can be correlated with standard tensile test results.

Small punch tests were carried out on the electro mechanical testing machine INSTRON in accordance with CWA15627 document [3] on the round specimens with the diameter of 8 mm

and thickness of 0.5 mm (prepared with electric discharge machining and polishing).

Evaluated yield strength data were from SPT obtained by empirical correlations between SP and standardized test results on the UJV Rez large database of base materials. It was observed that the nature of the load – deflection curve (Fig. 2) varies significantly with the specimen thickness h0. The effect of test specimen thickness can be eliminated by correlating yield stress with parameter Fe/h0

2. Results of tests are summarized in Table 2, Annex A.

Results of ABIT Method is based on the principle of strain controlled multiple

indentations at a single penetration location on a polished surface by a small spherical indenter (various diameters from 0.508 to 2.5 mm).

Fig. 1 – Configuration of the testing apparatus (1 -specimen, 2 - punch, 3 - receiving die, 4 – clamping die)

Fig. 2 – Recorded load – specimen deflection curve of a small punch test sample

Fe


Fig. 3 – ABI indentation profile during force application and after force removal (complete unloading)

Instrumented test method is fully automatic and does not require the diameter of the indentation to be measured after testing. Scheme of the test is shown on the Fig. 3., example of test results on Fig. 4.

For the purposes of the project, new electro mechanical testing machine INSTRON 5967 (modified to 10 kN load in 30 kN reinforced frame) was bought and installed into semi-hot cell.

Fig. 4 – Example of loading diagram of ABI test

Experimental data were evaluated in several steps. For the yield strength assessment, for each loading cycle was determined force and the depth of indent ht during load application (Fig. 3). From the depth of indent parameter dt is calculated (idealized diameter of contact area) and is defined by the formula (1):

2 · · (1)

All data points of the dependence are fitted by a curve defined by the Mayer’s law (2) - parameter A with the Mayer’s index m is determined by the linear regression:

· (2)

Parameter A is used for the calculation of yield strength approximation (3), where is the parameter for evaluated material:

, · (3)

Final value of the yield strength is calculated from the formula (4), where parameters a and b are determined by the calibration for the evaluated type of material.

, · , (4)

For the ultimate strength evaluation, for each load cycle is determined a pair of values εp and σt where εp is logarithmic plastic deformation and σt is proportional to the actual stress in one-dimensional homogeneous deformation. If the resulting stress-strain curve is expressed by the exponential equation (5) it is possible to use obtained strain hardening coefficient K and strain hardening exponent n to estimate the ultimate strength approximation (6).

· (5)

· (6)

Calculation of the ultimate tensile strength is carried out in an analogous manner as for the yield strength (7), where parameters c and d are determined by the calibration for the evaluated type of material.

· (7)

Evaluated results of ABI testing for both unirradiated and irradiated specimens were correlated with the results from standard tensile testing. Summarized results for the various types of indenter diameters are in the Table 3, Annex A.

CONCLUSIONS Based on obtained results on the IAEA reference material

A533B (JRQ), correlation between standard tensile test and ABI tests was prepared as well as correlation with Small Punch test results (Fig. 4, Fig. 5)

Both semi-destructive testing methods showed to be very promising for assessment of structural irradiated NPP materials degradation. Evaluated results of ABIT and SPT show very good correlation with results from standard tensile testing.


At present, for the employment of the method into standard portfolio of laboratory test techniques is necessary to enlarge the experimental data volume for different types of used indenters, materials and to perform testing in temperature wider range from -190 °C to +350 °C.

Fig. 4 – Correlation with standard tensile test results (unirradiated specimens)

Fig. 5 – Correlation with standard tensile test results (unirradiated specimens)

ACKNOWLEDGEMENT This paper includes results created within the project

TA03011266 “Development of innovative semi-destructive method of high active material evaluation for nuclear reactor components lifetime assessment”.

REFERENCES

[1] IAEA TECDOC 1230 «Reference Manual on the IAEA JRQ Correlation Monitor Steel for Irradiation Damage Studies», IAEA, 2001

[2] ISO 6892 - Metallic materials - Tensile testing

[3] CWA15627 - Small Punch Test Method for Metallic Materials, 2007

[4] Haggag F.M., Wang J.A., Sokolov M.A., Murty K.L.: Use of portable/in situ stress-strain microprobe system to measure stress-strain behaviour and damage in metallic materials and structures, in Non-traditional methods of sensing stress, strain and damage in materials and structures, ASTM STP 1318, Lucas and Stubbs eds. ASTM, Philadelphia, 1997


ANNEX A

EXPERIMENTAL DATA

Tab. 1 – Tensile test results for the correlation with SPT and ABIT

Tab. 2 – Results of Small Punch Testing (SPT)

Tab. 3 – Results of Automated Ball Indentation Testing

Number of specimens

Test Temperature [°C]

Neutron fluence (E > 0.5 MeV)

[1018 cm-2]

Average Rp0,2 [MPa]

Average Rm [MPa]

Data used for the correlation with

3 24 unirradiated 476.9 615.4 SPT

3 265 unirradiated 428.6 580.7 SPT

3 24 unirradiated 481.7 424.0 ABIT

3 265 unirradiated 424.0 581.1 ABIT

3 24 12.5 546.7 694.7 SPT

3 265 12.5 498.4 660.0 SPT

3 24 130.0 649.1 765.0 ABIT

3 265 130.0 584.9 708.2 ABIT

Number of specimens

Test Temperature [°C]


[1018 cm-2]

Average Fe/(h0)

2 [MPa]

Average Rp0,2

[MPa]

Average Fm/(um .ho)

[MPa]

Average Rm

[MPa] 15 24 unirradiated 852.8 478.9 2399.3 619.2

15 265 unirradiated 785.8 433.8 2194.9 566.7

3 24 12.5 1363.7 589.8 2632.3 728.8

3 265 12.5 990.8 506.6 2411.3 666.1

Test Temperature

[°C]

Indenter Diameter

[mm]


[1018 cm-2]

Average Rp0,2

[MPa]

Average Rm

[MPa]

24

2.500 unirradiated 489.0 609.0




265




24 2.500 130.0 571.0 738.0

0.762 130.0 597.0 735.0

265 2.500 130.0 513.0 630.0

0.762 130.0 507.0 611.0


implementation of small punch testing and automated ball

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