Influence of Hydrides upon the Fatigue Initiation Behavior ......Suggests hydrides interact with plasticity in some way Possible reasons for reduction: Hydride may enable earlier initiation

© 2019 Rolls-RoyceOFFICIAL

Influence of Hydrides upon the Fatigue Initiation Behavior of Irradiated Zircaloy-2

Dr Pete Honniball1, Lucile Cogez2, Chas Gee1

May 2019

19th International Symposium on Zirconium in the Nuclear Industry

1. Rolls-Royce, PO Box 2000, Derby, DE21 7XX

2. Canadian Nuclear Laboratories, Chalk River, ON K0J 1J0, Canada

© 2019 Rolls-RoyceOFFICIAL2

Background – Zirconium Fatigue

Cyclic / dynamic loading of fuel rods

• Power / thermal variations through life

• Transport of spent fuel

Consequently, fatigue assessment of Zrcomponents are required

(even if real loads/strains are small)



IrradiationHardening

Previous testing has considered …

Conflicting results on importance of irradiation upon

fatigue



Hydrogen


Hydrogen effects often tested using unirradiatedmaterial

Literature to-date suggests limited influence of hydrogen upon fatigue curves



Hydride Alignment


Unclear effect in unirradiated material

Limited study of worst case hydride alignment in irradiated material



IrradiationHardening Hydrogen

Hydride Alignment


Individually there isn’t a clear effect of each of these factors

This study aims to understand how the combination might influence fatigue behaviour



IrradiationHardening Hydrogen

Hydride Alignment

Irradiation hardening may affect:

• Degree of hydride alignment

and

• Hydride fracture behaviourIndividually there isn’t a clear effect of each of these factors

This study aims to understand how the combination might influence fatigue behaviour


Testing Programme

Rolls-Royce experimental programme undertaken by Canadian Nuclear Laboratories at Chalk River, Ontario

Notched fatigue tests to determine:

Is fatigue initiation behaviour influenced by preferentially orientated hydrides in irradiated material

Irradiated weld material (Zircaloy-2)• Unhydrided behaviour (and sensitivity to T and frequency)

• Influence of hydrides (250 ppm hydrogen)

• Varying degrees of pre-alignment at notch root


Experimental Work - Material

TProof Strength

(MPa)UTS

(MPa)

25°C 780 801

260°C 564 569

Microstructure and texture typical of welded Zr

Region of Interest:Electron Beam weld

15-20 ppm initial hydrogen

Neutron fluence3-5 x1025 nm-2

4.3-7.2dpa

Strain (%)


Experimental Work - Specimens

Side view

Top View

Region of Interest:Electron Beam weld

22 mm

Two notch geometries studied

U Notch V Notch

250 ppm 55 ppm


Testing Approach

HH

Cold

Thermal gradientDrive hydrogen from surface to

be notched

NotchInto hydride free region

HH

1 2 3

Hydrides formed during hydriding process are potentially highly susceptible to failure (overload fracture - Shek et al., Cui et al.)

We felt such failures would be an artefact of hydriding process

To avoid this the following approach was taken

Thermo-Mechanical Cycling

Hot

U notch specimens

only

4

Fatigue Cycling


Thermal Gradient Treatment Developed

Hot

Cold

Thermal gradientDrive hydrogen from surface to

be notched

HH

1

For the 250 ppm U notch testing

Hydrogen driven away from region due to be notched

Constant thermal gradient applied ~340°C for ~150 hours

Example H distribution (after shorter treatment)

U notch specimens

only


Notching

Notches broached into material

Pickling used to remove broaching damage to ensure underlying material is being tested

U Notches pickled to remove surface

damage

NotchInto hydride free region

2


Thermo-Mechanical Pre-conditioning

0

100

200

300

400

500

600

700

800

900

1000

0

50

100

150

200

250

300

350

0 50 100 150

Pe

ak N

otc

h R

oo

t El

asti

c St

ress

(M

Pa)

Tem

per

atu

re (

°C)

Time (Hours)

Temperature(°C)

Load - Notch root stress (MPa)

Pre-cycled the material to draw hydrogen to the notch root and to form hydrides

Two levels of pre-conditioning applied – 10 and 30 cycles

Constant notch root load ~150 MPa

HH

3

Thermo-Mechanical Cycling


Fatigue Loading

4- point bending

Load controlled – targeted notch root stress range

Sinusoidal waveform

Crack detection via direct current potential drop (DCPD)

First sign of cracking taken to be initiation

Test stopped after ~75 ±25 μmdeep crack forms

DCPD System

Tests stopped v/v0 1.01

~75 micron average crack depth


Baseline Testing Results


Results – Influence of Temperature

0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)

Cycles to First Indication of Cracking

30°C Un-hydrided

150°C Hydrided

260°C Un-hydrided

260°C 0.1 Hz Un-hydrided

DNF

106105104103

Stress values are elastic notch root stresses – not realistic but consistent basis for comparison



0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided

150°C Hydrided

260°C Un-hydrided


DNF

106105104103

Data suggest fatigue limit, in agreement with other Zrstudies



0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided

150°C Hydrided

260°C Un-hydrided


DNF

106105104103


0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided

150°C Hydrided

260°C Un-hydrided


DNF

106105104103

Results – Test Frequency

Apparent drop in endurance limit at lower test frequency

10 Hz tests (not plotted) didn’t show an obvious increase


Endurance Variation vs. Yield Behaviour

0

100

200

300

400

500

600

700

800

900

1000

0 50 100 150 200 250 300

Stre

ss (

MP

a)

Temperature (°C)

Test Material Proof (yield) Strength

0.1 Hz (ε ̇= 0.002 /s) Peak Stress at Endurance Limit

1 Hz (ε ̇= 0.02 /s) Peak Stress at Endurance Limit

Plotting fatigue limit (as max cycle stress) against yield behaviour

Similar sensitivity to temperature

Sensitivity of endurance limit to frequency aligns reasonably with strain rate sensitivity of yield strength (m=0.02-0.04)


Hydrided Testing Results


Results – Hydrided Testing

0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided

30°C Hydrided 10 Pre-con Cycles

260°C Un-hydrided

260°C Hydrided 10 pre-con cycles

260°C Hydrided 30 Pre-con cycles

DNF

106105104103



0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided


260°C Un-hydrided



DNF

106105104103



0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided


260°C Un-hydrided



DNF

106105104103



0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided


260°C Un-hydrided



DNF

106105104103



0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided


260°C Un-hydrided



DNF

106105104103



0

200

400

600

800

1000

1200

Δσ

ela

sti

c(M

Pa)


30°C Un-hydrided


260°C Un-hydrided



DNF

106105104103

5-10x reduction in fatigue life at loads just above fatigue endurance

No effect of further pre-conditioning above 10 cycles


Interpretation

▪ Hydrides do no appear to alter fatigue limit – suggests underlying cause of limit (slip activation) is not influenced by hydrides

▪ Supported by bulk observations of insensitivity of yield to [H]

▪ Above fatigue limit – significant reduction in fatigue life▪ Suggests hydrides interact with plasticity in some way

▪ Possible reasons for reduction:▪ Hydride may enable earlier initiation – plasticity induces hydride

fracture or hydride causes intensification of plastic damage evolution

▪ Hydrides may enable more rapid propagation of embryonic cracks…Wanhill et al., Journal of Nuclear Materials 43 (1972)


End of test

Results – Comparing Early(ish) Crack Growth

Looking at number of cycles to achieve a given crack size (approximately)

First sign of initiation


Results – Comparing Early Crack Growth

600

620

640

660

680

700

720

740

760

780

800

0 10000 20000 30000 40000 50000 60000 70000

Max

Ela

stic

Str

ess

(M

Pa)

Cycles from Initiation to DCPD Threshold

Unhydrided 1 Hz Hydrided 1 Hz

Unhydrided 0.1 Hz Hydrided 0.1 Hz


30 10Pre-conditioning

Cycles

1 HzAt given load hydridedspecimens show more rapid crack development

Pre-conditioning may have an effect

1 Hz

1 Hz



600

620

640

660

680

700

720

740

760

780

800

0 10000 20000 30000 40000 50000 60000 70000

Max

Ela

stic

Str

ess

(M

Pa)






Cycles

0.1 HzDespite lower applied load, crack develops in fewer cycles in both hydrided and unhydrided specimens

1 Hz

1 Hz

0.1 Hz



600

620

640

660

680

700

720

740

760

780

800

0 10000 20000 30000 40000 50000 60000 70000

Max

Ela

stic

Str

ess

(M

Pa)






Cycles

Cycles for cracks to reach threshold size influenced by:

• Frequency• Hydriding• Pre-conditioning

1 Hz

1 Hz

0.1 Hz


Early Crack Development - Pre-Conditioned Specimens

10 Cycle pre-conditioning 30 Cycle pre-conditioning

Denser more continuous hydrides –facilitating crack growth?

Cycles for cracks to reach threshold size influenced by:

• Frequency• Hydriding• Pre-conditioning


Conclusions

▪ Hydrides influence the fatigue performance of irradiated Zircaloy-4 but fatigue limit not affected

▪ Test frequency influences fatigue limit and crack propagation rate

▪ Presence of aligned hydrides accelerates rate of crack propagation in notched tests

▪ The degree of hydride alignment appears to result in more rapid cracking propagation

Documents

Influence of Hydrides upon the Fatigue Initiation Behavior ......Suggests hydrides interact with plasticity in some way Possible reasons for reduction: Hydride may enable earlier initiation