enhancing reliability of hydrogen-assisted cracking ... reliability of hydrogen-assisted cracking...

Enhancing Reliability of Hydrogen-Assisted Cracking Properties Measured in H2 Gas

Brian SomerdaySandia National Laboratories, Livermore CA, USAInternational Institute for Carbon-Neutral Energy

Research (I2CNER), Fukuoka, Japan

Hydrogen embrittlement - Multi-scale modelling and measurement:What is the impact?

National Physical Laboratory, Teddington, UKOctober 7, 2014

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000

SAND2014-18591 C

Steel pipelines and pressure vessels are central components for delivery and storage of H2 fuel

• ASME developed Article KD-10 in Section VIII-3 of Boiler and Pressure Vessel code for high-pressure H2 vessel design– “Special Requirements for Vessels in High Pressure Gaseous

Hydrogen Service”– Mandatory for seamless vessels with H2 pressure > 41 MPa and

welded vessels with H2 pressure > 17 MPa (upper limit 100 MPa)– Design method also considered for H2 pipelines (ASME B31.12)

Codes require fracture mechanics measurements on containment materials in high-pressure H2 gas

Design life analysis in ASME Article KD-10 employs damage-tolerance principles

Need reliable measurements of subcritical cracking thresholds and fatigue crack growth relationships in H2 gas

measured in laboratory H2 gas

p

RiRo

at

H2

H2

H2H2

K = p[f(a, t ,Ro, Ri)]

“Reliable”: measurement integrity and transferability

• Are the measurements accurate?– Best practices for materials testing in H2 gas: ANSI/CSA CHMC1-2014, “Test methods for evaluating material compatibility

in compressed hydrogen applications – Metals” Advancing Materials Testing in Hydrogen Gas workshop, SNL, April 2013

(www.sandia.gov/matlsTechRef/advmat.html)

• Does the measurement represent non-conservative behavior?– Example: effect of load-cycle frequency on H2-assisted fatigue

A3750 steel4 MPa H2

A.H. Priest, British Steel, EHC-(1)42-012-81UK(H), 1983

4 MPa H2

H2-assisted cracking thresholds depend on test method

Key difference in test methods: propagating crack vs. stationary crack

H2H2

H2H2

H2

load

HH

H2H2

H2H2

H2

load

HH

H2H2H2

H2

loadHH

H2H2H2

H2

loadHH

constant displacement method:“arrest” threshold, KTHa

rising displacement method:“initiation” threshold, KTHi

K.A. Nibur et al., Metall Mater Trans A, 2013

KTHi

time

load

KTHa

crack extension

Kapp

H2-assisted cracking thresholds measured for martensitic pressure vessel steels

• Material– Cr-Mo(-Ni) pressure vessel steels– Yield strengths = 600 to 1050 MPa

• Mechanical loading– Constant displacement– Rising displacement = 0.05 mm/min

(~3 MPa m1/2/min)

• Environment– Supply gases: 99.9999% H2– Pressure = 103 MPa– Room temperature

H2H2

H2H2

H2

load

HH

H2H2

H2H2

H2

load

HH

H2H2H2

H2

loadHH

H2H2H2

H2

loadHH


Crack arrest thresholds (KTHa) higher than crack initiation thresholds (KTHi) at lower YS

Can observed trends be rationalized based on crack-tip mechanics?

• Divergent thresholds not attributed to difference in fracture mechanism

• Assume fracture mechanism is strain-controlled, i.e., governed by local– Consistent with plasticity-related

hydrogen-induced cracking (PRHIC)Takeda and McMahon, Metall Trans A, 1981

KTHa

KTHi

pressure vessel steels 4130X pressure vessel steelKTHa KTHi


Mechanics of propagating vs. stationary cracks illuminated by crack growth resistance curves

Crack-tip strain field weaker for propagating crack vs. stationary crack

steady state propagation

blunting

rKss

local

2

ln~

rK

local

2

~

a

Hutchinson; Rice and Rosengren, J Mech Phys Solids, 1968

Rice et al., ASTM STP 700, 1980

Weaker crack-tip strain field for propagating crack can account for higher crack-arrest thresholds

Strain-controlled fracture criterion (local) coupled with strain field amplitudes leads to KTHa > KTHi

rK

local

2

~

rK

local

2

ln~

r*

*local

local = *localK = KTH

H2H2

H2H2

H2

load

HH

H2H2

H2H2

H2

load

HH KTHi

H2H2H2

H2

loadHH

H2H2H2

H2

loadHH

KTHa

Crack arrest thresholds (KTHa) depend on initial applied stress-intensity factor (Kapp)

Can observed trends be attributed to intrinsic behavior of propagating cracks?

K.A. Nibur et al., Sandia Report SAND2010-4633, 2010

C110 steelYS = 793 MPaDCB specimens

S.K. Saha and X. Long, SteelyHydrogen2014 Conference Proceedings, 2014

Crack growth resistance curves for lower-strength martensitic steels suggest KTH depends on a

• Crack growth resistance curve represents locus of KTH points– Bounding KTH values (KTH

stationary and KTH

SS) reflect limits of crack-tip strain fields

– For strain-controlled fracture, KTHdepends on crack extension for either test method

• For Kapp > KTH, crack at “instability” condition relative to KTH locus– As crack extends, K declines

following method-dependent K vs. a trajectory

– Crack arrests at KTH locus (e.g., KTHa), arrest point depends on strain field evolution as crack grows

Crack extension-dependent KTH may account for observed KTHa vs. Kapp behavior

H2H2H2

H2

loadHH

H2H2H2

H2

loadHHKTHaKapp

KTHa

Kapp

KTHstationary

KTHSS

rK

local

2

ln~

rK

local

2

~H2

H2

H2H2

H2

load

HH

H2H2

H2H2

H2

load

HH

What is the impact?

• Crack-arrest thresholds measured under constant displacement can be non-conservative

– Micromechanics interpretation illustrates that trend may be universal for strain-controlled, H2-assisted cracking

– Standardized test methods may be non-conservative ASTM E1681 NACE TM0177 - Method D

– Reliable KTHa measurements expected for stress-controlled, H2-assisted cracking

H2-accelerated fatigue crack growth rates may unexpectedly decline at lower load-cycle frequency

A.T. Stewart, Mechanisms of Environment Sensitive Cracking of Materials, 1977

A. Macadre et al., Engineering Fracture Mechanics, 2011

• Do oxygen impurities in H2 gas lead to inhibition at low frequency?

• Does intrinsic H-material interaction account for reduced da/dN at low frequency?

Systematically measured fatigue crack growth rates in high-pressure H2 with controlled O2 levels

• Material– X52 ERW pipeline steel

• Instrumentation– Internal load cell in feedback loop– Crack-opening displacement measured

internally using LVDT– Crack length calculated from compliance

• Mechanical loading– Triangular load-cycle waveform– Constant load amplitude or constant K

• Environment– Supply gases: 99.9999% H2

H2 with 10-1000 vppm O2– Pressure = 21 MPa– Room temperature

B.P. Somerday et al., Acta Mater, 2013

Fatigue crack growth measurements performed on X52 line pipe steel

• API 5L X52 steel composition

• Tensile properties– Yield strength: 428 MPa– Ultimate tensile strength: 483 MPa

C Mn P S Si Cu Ni Cr V Nb Al CE

0.06 0.87 0.011 0.006 0.12 0.03 0.02 0.03 0.002 0.03 0.034 0.11

ERW seam

324 mm OD x 12.7 mm WT

ferrite + 8 vol% pearlite


Measurements demonstrate pronounced inhibition at lower frequency and higher O2 content

Can interplay between mechanical and environmental variables be quantified through physics-based model?


• Initial inert-environment crack growth modeled by blunting-resharpening

• Oxygen out-competes hydrogen for adsorption sites on freshly exposed crack-tip surface

• Extent of oxygen adsorption depends on crack-tip area, proportional to crack-growth increment (da)– when da < dacrit, crack tip fully

passivated by oxygen– when da > dacrit, crack tip not fully

passivated H uptake

• Develop model that quantitatively relates adsorbed oxygen (H uptake) to mechanical and environmental variables

O2

O2

K = Kmin

H2

a

H2O2H2

H2

H2 H2H2OO

O

a

H2

H2 H2O2

O2

O2

H2 H2H2

H2

H2

O2 H2

O2

O2

O

O

O

O

O O2

H2

O2

H2

H2

O2

H2

O

O

O

H

H

HHH

H

H2

H2

H2H2

O2

H2H2H2H2

H2H2 H2

H2H2

H2

H2

da > dacrit

K = Kmax2 > Kmax1

a

O2

H2

H2H2O2

H2

O2

H2H2

H2

H2

H2

O2 H2

H2

H2

O O

O

OOO

O

O2H2

O2

H2

H2

H2

H2

da < dacrit

K = Kmax1

Central physical premise: oxygen adsorption at crack tip can inhibit H uptake


Model developed based on idealized crack geometry and diffusion-limited oxygen adsorption

a

O2

O2 O2

O2

O2O2

O2h ~ Kmax

O2HH

Ha*

OO

O

a*distance from crack tip, x

ptot

pO2

• Goal: quantify amount of adsorbed oxygen (n) during load-cycle time (t)

• Key assumption: adsorption rate-limited by O2 diffusion in crack channel– constant crack-channel height (h)

during diffusion– steady state pO2 profile

• Model foundation: oxygen delivered to crack tip (Jht) = oxygen adsorbed on crack tip (Sa)

*flux

TaRpDJg

tot

2

0

02

)1(160height channel

RσK

Eσ)ν(.h

coverageoxygen

ft /1

2

0

2

)1(*)1(3.0)(

Ra

KTERS

Dpfagcrit

totcrit

H uptake and accelerated crack growth when = crit

density site surfaceS


Model formulation quantifies interplay between f, O2 concentration, da/dN, and R ratio on onset of H2-accelerated crack growth

Model predictions consistent with da/dN vs. frequency data measured in H2+O2 gas

2

0

2

)1(*)/()1(3.0

Ra

KTERSdNda

Dpfgcrit

totcrit

Scrit from measured da/dN vs. K data in H2+100 vppm O2


Enhanced inhibition at higher R ratio associated with effect of crack channel height on O2 transport

Model identifies variables that can distinguish effects of O2 inhibition → example: R ratio

2

0

2

)1(*)1(3.0

Ra

KTERSf

DpdNda

gcrit

tot

crit

What is the impact?

• Measured fatigue crack growth rates in H2 gas with O2impurities may be non-conservative

– Measurements and modeling demonstrate that O2 inhibition depends on interplay between da/dN, load-cycle frequency, R ratio, and O2 concentration

– Inhibition can be pronounced in H2 gas with ppm-level O2concentrations, particularly at low frequency

– Physics-based model of O2 inhibition can aid in distinguishing mechanism for reduced H2-assisted crack growth rates at low frequency

Enhancing Reliability of Hydrogen-Assisted Cracking Properties Measured in H2 Gas

• Comprehensive experiments coupled with mechanistic interpretations enhanced reliability of subcritical cracking threshold and fatigue crack growth rate measurements– Impacts life prediction of pipelines and pressure vessels

for H2 gas delivery and storage

Acknowledgments

• Sandia National Laboratories– Kevin Nibur (currently at Hy-Performance Materials Testing)– Chris San Marchi– Jay Foulk– Joe Ronevich– Zach Harris– Ken Lee– Jeff Campbell– Mark Zimmerman

• International Institute for Carbon Neutral Energy Research– Prof. Petros Sofronis (University of Illinois)– Prof. Reiner Kirchheim (University of Göttingen)– Prof. Alex Staykov (Kyushu University)– Dr. Mohsen Dadfarnia (University of Illinois)

Back-up Slides

SNL core capability in hydrogen embrittlementfeatures Hydrogen Effects on Materials Lab

• Static-loading crack-growth system– Wedge opening load (WOL) and double

cantilever beam (DCB) specimens– H2 pressure up to 200 MPa– Temperature -70 to 170 oC

• Dynamic-loading crack-growth system– Compact tension (CT) and single edge

notch (SEN) specimens– H2 pressure up to 138 MPa– New pressure vessel design with

target temperatures -100 to 200 oC

Materials testing in H2 supports technology development in several mission areas

Yield strength and alloy composition of martensitic pressure vessel steels

KTH vs. yield strength trends can be rationalized based on crack-tip mechanics framework

Assumption that local decreases as yield strength increases can explain converging KTHa and KTHi

*local

*local

local = *localK = KTHlow-strength steel

local = *localK = KTHhigh-strength steel


KTHi

KTHa

decreasing *local

KTHi reaches lower bound as loading rate decreases

KTHa

KTHi

Clark and Landes, ASTM STP 610, 1976 Nibur et al., SAND2010-4633, 2010

Higher KTHa compared to KTHi not attributed to crack arrest near back face in conventional WOL

• Chevron-notched WOL specimen– Reduced compliance leads to lower initial loads– Lower initial loads allow shorter precracks, which

limit final crack length at KTHa

Conventional WOL specimen

Chevron-notched WOL specimen

Plasticity-related hydrogen-induced cracking assumed for martensitic pressure vessel steels

4130X100 MPa H2 gas

slip lines at notch root plasticity-related hydrogen-induced cracking (PRHIC): glide plane decohesion

HY 130-type0.2 MPa H2 gas

Takeda and McMahon, Metall Trans A, 1981

Elastic unloading behind crack tip leads to distinct strain field for propagating crack vs. stationary crack

Plane strain slip-line representation of crack tip stress states of Prandtl field for stationary crack and propagating crack

Ritchie and Thompson, Metall Trans A, 1985

unloading

stationary crack

propagating crack

Stress fields near crack tip similar for stationary and propagating cracks

Local stresses ahead of crack tip as a function of angle for stationary crack and propagating crack

Ritchie and Thompson, Metall Trans A, 1985

Gas constituents such as O2 can inhibit H2-accelerated fatigue cracking in steels

R. Wei and R. Gangloff, ASTM STP 1020, 1989

H. Johnson, Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Based Alloys, 1977

4340 steel

How is O2 inhibition affected by variables such as K, frequency, R ratio, and O2 content?

Density functional theory (DFT) simulations suggest mechanism for O2 inhibition of H uptake

Preadsorbed oxygen on iron surface raises activation energy for H2 dissociation

Ene

rgy

(eV

)

Distance from surface (Å)

transition stateEA = 0.6 eV

Potential energy surface scan for H2 approaching Fe(100) surface

H2 molecule approaches directly on top Fe atom


Ene

rgy

(eV

)

dissociationtransition stateEA = 0.1 eV

dissociation

Potential energy surface scan for H2 approaching Fe(100) surface with preadsorbed O atoms

H2 molecule approaches directly on top Fe atom

Staykov et al., Int J Quantum Chemistry, 2014

Electron density difference method provides insight into dissociation inhibition mechanism

• Oxygen atoms on surface localize e- density, reducing ability of neighboring Fe atoms to transfer e- to H2

• Less e- density available for H2 activation: dissociation hindered

H2 approaching Fe(100) surface H2 approaching Fe(100) surface with preadsorbed O atoms

FeO

H2

e- e-

e- e-

enhanced e- density

depleted e- density

DFT simulations provide rationale for preferential adsorption of O2 in mixed H2+O2 gas

Strong attractive force and absence of activation barrier allow O2 to out-compete H2

F dEdR

Potential energy surface scan for H2 approaching Fe(100) surface

• H2 is detected at 2.6 Å• Weak attractive force• Activation barrier: not all H2 molecules dissociate

Potential energy surface scan for O2 approaching Fe(100) surface

• O2 is detected at 3.6 Å• Strong attractive force• No activation barrier: all O2 molecules dissociate

EA = 0.1 eV

Ene

rgy

(eV

)

Distance from surface, R (Å) Distance from surface, R (Å)

Ene

rgy

(eV

)

2.6 Å 3.6 ÅF dE

dR

DFT simulations define potential scenario for O2 inhibition of H uptake

Key elements in O2inhibition scenario:

• O2 detected deeper in gas volume compared to H2

• Force on O2 >> force on H2

– O2 can out-compete H2for adsorption sites

•Adsorbed O leads to repulsive force on H2

Density Functional Theory (DFT): Methods of Calculation

Theory: plane wave DFT

Program: VASPFunctional: optB86b-vdW(includes long-range interactions)Results verified with GGA-PBE

Spin polarized calculations(accurate spin guess applied)

Energy cut-off: 400 eVK-points: 8x8x8 bulk; 3x3x1 surface

Transition state search:climbing nudged elastic band method8 images per band

Analysis:Bader population analysisElectron density difference method

BCC Fe latticeFerromagnetic coupling

2x2 surface supercell6 layers Fe atoms12 A Vacuum slab

Top 3 layers are relaxedBottom 3 layers are fixed

H2 starting geometries on Fe surface

On topside-on

On topend-on

Bridge Bridgecross

H2 dissociation on Fe surface

On topside-on

chemisorptionstate

transitionstate

trajectory

Single H2 moleculeapproaches the surfacein parallel orientation

Bridge cross


trajectory

Single H2 moleculeapproaches the surfacein parallel orientationacross a bridge site

Ene

rgy

(eV

)

H2-accelerated fatigue crack growth not absolutely inhibited by O2: depends on da/dN and R ratio

• At lower K, crack growth rates in H2 environments same as rates in air

• At R=0.1, hydrogen-accelerated crack growth observed at higher K– da/dN at onset of hydrogen-

accelerated crack growth depends on O2 concentration

• At R=0.5, hydrogen-accelerated crack growth not observed

• Why is da/dN at onset of H2-accelerated cracking a function of O2 concentration?

• Why does higher R promote inhibition?


Favorable correlation between measurements and model gives credence to physical assumptions

Model predictions consistent with da/dN vs. K data measured in H2+O2 gas at R = 0.1 and 0.5

2

0

2

)1(*)1(3.0

Ra

KTERSf

Dpagcrit

totcrit

Scrit calculated from measured da/dN at onset of accelerated crack growth for H2+100 vppm O2

assume a ~ da/dN


Why H2-induced intergranular cracking under cycling loading for lower-strength ferritic steels?

X52 steel21 MPa H2

Rationale: onset of accelerated crack growth dictated by threshold values of both da/dN and Kmax

Model does not predict onset of accelerated crack growth for high-purity H2 case

Scrit from measured da/dN vs. K data in H2+100 vppm O2

2

0

2

)1(*)1(3.0

Ra

KTERSf

DpdNda

gcrit

tot

crit

Suresh and Ritchie, Metal Science, 1982


• Question: inhibition governed by O2 concentration or absolute O2partial pressure?

• Results at constant system pressure inconclusive both concentration and absolute partial pressure vary

• Can isolate partial pressure by varying system pressure at constant O2 concentration– Results show no effect on critical

da/dN for accelerated crack growth– Conclusion: inhibition dictated by

O2 concentration

• Model predicts governing role of O2 concentration– Dptot term is constant

Model critically tested by performing fatigue crack growth experiments at lower pressure

2

0

2

)1(*)1(3.0

Ra

KTERSf

DpdNda

gcrit

tot

crit

Establishing physical description allows rationalization of O2 concentration and R ratio effects

2

0

2

)1(*)1(3.0

Ra

KTERSf

Dpagcrit

totcrit

• O2 concentration () → affects gradient in crack channel• R ratio → affects height of crack channel

More advanced model accounts for varying O2 profile in “breathing” crack

haTERfdNda

DpSg

tot

*)/()1(3.0

0

2

Model based on “breathing” crack retains dependence on O2 concentration () and frequency (f)

crack length, a*crack length, a*

gray-scale shading → oxygen concentration, cO


Does load-cycle waveform have intrinsic effect on H2-accelerated fatigue crack growth rates?

Does load-cycle waveform affect H-material interaction during fatigue crack growth?

f = 1 Hz

A.T. Stewart, Mechanisms of Environment Sensitive Cracking of Materials, 1977

B.P. Somerday and M. Barney, J Pressure Vessel Technology, 2014

triangular wave

trapezoidal wave

enhancing reliability of hydrogen-assisted cracking ... reliability of hydrogen-assisted cracking...

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