Central Research Institute of Electric Power Industry

Utilization of Miniature C(T) specimens for the fracture toughness evaluation of RPV steels by the Master Curve Method

Materials Science Research Laboratory

EPRI International Light Water Reactor Materials Reliability Conference and Exhibition 2016

August 2016, Hyatt Regency McCormick Place, Chicago, IL, USA

Masato Yamamoto, Naoki Miura and Naoki Soneda

1

Background

If we can use Mini-C(T) specimens, which can be machined from broken Charpy specimens, additional fracture toughness data can be obtained from existing surveillance material.

The Master Curve (MC) method can compensate the specimen size effect.

It was confirmed that T0 values obtained using Mini-C(T) specimens are consistent to those obtained by larger C(T) specimens up to 4T-C(T) specimens in three different heats. (ASME J. of PVT 134-021402)

2August, 2016

Overall goal: Establish and standardize a test procedure to obtain T0 values

using Mini-C(T) specimens machined from broken Charpy halves.

The Master Curve method

August, 2016 3

Fracture toughness (KJc) distribution can be described with specific Weibull distribution.

Unique parameter To for indexing KJc distribution and its temperature trend.

Specimen size dependency of fracture toughness can be compensated.

T - T1 0 0

(oC )

-150 -100 -50 0 50

KIc -

1T

(M

Pa

·m1/2)

0

50

100

150

200

250

300

350

T - T1 0 0

(oF )

-300 -200 -100 0 100

KIc -

1T

(k

si·

in1/2)

0

50

100

150

200

250

300

350

1%

3%

5%

50%

95%97%

99%

KIc (m ed )

1T= 27 .3+ 63 .6exp [0 .01056 (T -T 100)], ks iｷin .

1 /2

H S S T -01 suba rc w e ld , N =8 , T100

= -104 .8oF

A 533B c l. 1 suba rc w e ld , N =8 , T100

= -58oF

A 533B c l. 1 w e ld , N =10 , T100

= -151 .6oF

A 533B w e ld -H A Z , N =6 , T100

= -131 .8oF

A 508 c l.2 , N =12 , T100

= -119 .2oF

A 508 c l.2 , N =9 , T100

= -69 .8oF

A 508 c l.2 , N =10 , T100

= -5 .8oF

A 533B c l.1 , N =13 , T100

= -74 .2oF

H S S T -01 , N =17 , T100

= -2 .2oF

H S S T -02 , N =69 , T100

= -18 .4oF

H S S T -03 , N =9 , T100

=28 .4oF

KJc(med ) = 30 + 70exp 0.019 T -To( )éë ùû, MPa

T-T0, degF

T-T0, degC

KJc

(1Teq),

Pa√m

KJc

(1Teq),

ksi√

in

Mini-C(T) specimenDimension

4mm thickness C(T) specimen, whose principal dimensions (B, W, a0, L, 2H) are the same as those standardized in ASTM E1921.

Specimen size (4 x 10 x 9.6 mm)Small enough to be machined maximum 4 specimens from a broken Charpy half.

2.2±

0.0

42

.2±

0.0

4

( )

3

a0 (4±0.4)

W (8±0.04)

L (10±0.08)

D (f2±0.04)

2H

(9.6±

0.0

8)

B (4±0.08)

0.8 at side surface

N

4August, 2016

Development and standardization of Mini-C(T) testand evaluation technique

Basic applicability of Mini-C(T) technique

Round robin testing Standardization of testing and evaluation standard

Implementation of Mini-C(T) technique in surveillance program

Un-irradiated RPV base metal

PVT2012, vol. 134, 021402 (2012)

ASTM STP1576, STP157620140020 (2015)

Revision ofASTM E1921 (2016) and JEAC4216(2015)

(2016~?)

Un-irrad. weld and HAZ

PVP2015-45545PVP2016-63762(2015, 2016)

Small RR among HZDR, SCK/CEN, EPRI, CRIEPI

Irrad. base metal

JAEA project (2015-)CRIEPI project(2015~2018)

Independent projects on various irradiated specimens?

Irrad. weld and HAZ

JAEA project?

Completed, In progress, Planning

Steps

5August, 2016

Objective

Understand “the size effect” of Mini-C(T)for the practical application

How many Mini-C(T) specimens will be needed to evaluate the reference temperature, T0?

What test temperature will be the most appropriate selection?

August, 2016 6

ProcedureCarry out the Monte-Carlo simulation with assuming the Weibull fracture toughness distribution of the Master Curve method.

1. Generate fracture toughness (KJc) data set sampled from the Master Curve Weibull distribution.

2. Estimate To and check its accuracy to the true value.

3. Repeat above two steps up to 1000 times to obtain the statistics of To values.

August, 2016 7

Sampling of fracture toughness data The cumulative probability of KJc in the Master Curve

Pf=1-exp{-[(KJc-20)/(K0-20)]4}

KJc : Individual fracture toughness

K0 : Parameter related to To

Two materials (plate and weld) are simulated

To: -115 degC (SQV2A base metal, unirradiated)

To: -77 degC (SQV2A weld metal, unirradiated)

August, 2016

8Yamamoto et al, ASME PVP2015-45545, (2015) Random number source: Mersenne

twister

Specimen size effect on KJc(limit)

August, 2016 9

Lower KJc(limit) for smaller specimens.

KJc data exceeding KJc(limit) are “Invalid” and to be censored.

Increasing number of invalid KJc will lead to

Larger fraction of invalid T0 .

Less accurate estimation of T0 .

In order to minimize invalid KJc, possible test temperature window has to be narrower in Mini-C(T) than the larger ones.

MC – KJc(limit) relationship before and after irradiation

August, 2016 10

E= 210-0.063(T-20), σy (unirrad) = 84 +271 exp {110 / (T +273.15)}Miura, N. and Soneda, N., ASME, Journal of Pressure Vessel Technology,Vol. 134, 021402, (2010)

ΔTo =0.70 ΔσySokolov, M.A., and Nanstad, R. K., ASTM STP 1325,(1999)

Analytical conditions

Clarify the effect of KJc(limit) for 4mm-T specimen in To evaluation.

Number of specimens: N=8, 9, 10, 11, 12, 13, 14, 16, 20 and 50

To true value: -115 (base metal), -77 (weld metal) degC

Shift of To due to irradiation : ΔTo = 0, 100 degC

To determination procedure : ASTM E1921-15

Specimen : Mini-C(T) (10x9.6x4 mm C(T) specimen)

Number of trials : 1000

August, 2016 11

Distribution of estimated To

August, 2016 12

80% of To

within 5 ˚C70% of To

within 10 ˚C

60% of To

within 5 ˚CVery few valid To

Estimated To is 10˚C

higher than true value

Selection of T is very important.

T-To=-30˚C

T-To= 0˚C

Fraction of valid T0 to 1000 trials, base metal (To (unirrad) =-115 ˚C)

August, 2016 13

Invalid

due to low

num

ber

of

valid

KJc

T-T

0<

-50

T = To- 30 ˚C will be a good choice of test temperature.

Irradiation gives no remarkable influence on fraction of valid To .

Fra

ction

ofva

lidT

oto

10

00

tria

ls

Fra

ction

ofva

lidT

oto

10

00

tria

ls

T-To , ˚C T-To , ˚C

Fraction of valid T0 to 1000 trials, weld metal (To (unirrad) =-77 ˚C)

August, 2016 14

Minimum 8 to 10 KJc data are required to obtain valid To for the given To range.

Fra

ction

ofva

lidT

oto

10

00

tria

ls

Fra

ction

ofva

lidT

oto

10

00

tria

ls

T-To , ˚C T-To , ˚C

Average of To, standard deviation of To, (base metal)

August, 2016 15

Higher test temperature results in higher To due to increase of invalid KJc data.

Standard deviation is about 5 ˚C and insensitive to test temperature.

Average of To, standard deviation of To, (weld metal)

August, 2016 16

Summary of Monte-Carlo analyses Selection of test temperature is important.

T = To - 30 ˚C can be recommended. Higher temperature may results in higher possibility of invalid To, or higher To than

the true value.

Lower temperature may slightly increase the chance of invalid To due to test temperature range requirement.

N = 8 to 10 KJc data in proper test temperature are adequate to valid To

determination for the material in the initial To range of -115 to -77 degC.

To will have ±5 degC error as the standard deviation with limited number of specimens in single data set (N=8 to 16).

August, 2016 17

Possible procedure for Mini-C(T) MC evaluation

August, 2016 18

Procedure

1. Use 2 or 3 specimens to estimate the right test temperature (T=T0 - 30 degC).

2. Continue 8 tests at around T=T0 - 30 degC. (8 to 11 in total of 1. and 2.)

3. If invalid KJc was obtained, continue tests until the number of valid KJc data isbecoming 8 or upper (This may require a few more specimens).

In total, 10 to 12 specimens will be required as the minimum set of specimens for estimating one To value.

Some more specimens may required due to;

Non-straight pre-crack front.

Test failure : bend or brake of loading pins, detachment of clip gauge, stick of pin and specimen due to ice buidup, ductile crack growth.

3 broken Charpy halves