research motivation: effect of loading environment on ... · pdf filemulti‐sitecrack growth...

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The Effect of Loading Environment on Cracking in Structural Metals

James T. BurnsResearch Assistant Professor

Department of Materials Science and EngineeringUniversity of Virginia

MSE SeminarUniversity of Virginia

Department of Materials Science and Engineering

Aug 2014

Understand and predict the influence of environment on subcritical cracking (ie, fatigue, SCC, HEE) to enhance the design and

safe management engineering components

Research Motivation:

Airframe‐ Cyclic Loading(Fatigue)‐ Ground Based Corrosion‐ Benign Env



Nuclear‐Monotonic and Cyclic Loading‐ PWR Reactor Environment

Staehle, 2013



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Pipeline ‐ Carbon Steel 5LX – Anhydrous Ammonia (NTSB‐DCA05‐MP001)

‐Monotonic Loading ‐ Aggressive Environment



Ships‐Monotonic Loading ‐ Aggressive Environment



Bridges‐Monotonic Loading ‐ Pre‐charged H






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In each case material‐environment‐mechanics interactions play a critical role in determining the

cracking behavior that leads to failure



How are engineering components managed to ensure safe operation?

To what extend are (or should!!) environmental effects be considered?

1. Safe Life: Stress/Strain Life Empirical Relationships

How is FATIGUE cracking managed to ensure safe operation?

Empirical S‐N Data

Test Specimen



Empirical S‐N Data Establish Empirical Constants (b c σf’ ε f’ )

Test Specimen

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Coffin‐Manson/Basquin (SWT)

Engineering Component




Coffin‐Manson/Basquin (SWT)

Solve for life (Nf) = End of Component Life: For high performance “low‐cycle fatigue” application;

propagation is often considered negligible and Nf ≈ Initiation Life



High Strength Stainless Steel

Zhou/Turnbull, 1999Total Cycles to Failure

104 105 106 107

Max

imum

Stre

ss (M

Pa)

50

100

150

200

250

300

350

400

Pristine (600 Grit)EXCO 6h LTEXCO 6h LSANCIT 24h LT

H2O/N2 (RH > 95%)R = 0.1 f = 10 Hz

7075-T6511

- Data offset about 150 and 240 MPa for clarity

Pristine

Corroded

Burns, Kim, 2009

Pristine

Corroded

Corrosion damage will drastically alter the empirical

S‐N relationship

Empirical S‐N Data

Airframe Aluminum Alloy

2. Damage Tolerance: Assume an existing flaw, model crack growth via Fracture Mechanics to set inspection protocol


From KIC

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2. Damage Tolerance: Assume an existing flaw, model crack growth via Fracture Mechanics to set inspection protocol

Krishnamurthy, 1990

Speidel, 1990

Steel

Titanium

Burns

Aluminum


Gangloff, 1990

Steel

This material property is critically dependent on

loading/crack tip environment

How is SCC/HE cracking managed to ensure safe operation?

“Go or No‐Go” Criteria: Material either considered immune or susceptible in a given environment


Historical Approaches:“Go or No‐Go” Criteria: Material either considered immune or susceptible in a given

environment


1. Non‐Fracture Mechanics ASTM Standardized Testing: ‐ SSRT, U‐bend, Cantilever, Breaking Load, C‐ring, Bolt, etc

Historical Approaches:

Bovard

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“Go or No‐Go” Criteria: Material either considered immune or susceptible in a given environment


1. Non‐Fracture Mechanics ASTM Standardized Testing: ‐ SSRT, U‐bend, Cantilever, Breaking Load, C‐ring, Bolt, etc

2. Fracture Mechanics Based Approach: ‐ Continued Focus on Initiation (ie KTH or KISCC)

Historical Approaches:


State of the Art Approach:Enhanced damage tolerant materials (0.5h → 10,000h lives) and characterization

capabilities (<1μm crack advance detection) enable:


1. Quantification of crack growth kinetics as a function of K

State of the Art Approach:

UVa (Gangloff/Burns), GE (Andresen), VEXTEC (LEFM Software)

Enhanced damage tolerant materials (0.5h → 10,000h lives) and characterization capabilities (<1μm crack advance detection) enable:

‐ Participation in efforts to move technique towards ASTM Standardization


1. Quantification of crack growth kinetics as a function of K

2. LEFM Modeling of crack progression

UVa (Gangloff/Burns), GE (Andresen), VEXTEC (LEFM Software)

State of the Art Approach:Enhanced damage tolerant materials (0.5h → 10,000h lives) and characterization

capabilities (<1μm crack advance detection) enable:

‐ For a specified loading condition, environment, and initial flaw size

‐ Directly analogous to fatigue modeling

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Response Surface to Select Particles Most Likely to Crack

Fatigue Incubation

Critical Plane Criterion, Coffin‐Manson (MSF)Select Particles Most Likely to Nucleate

Fatigue Nucleation

MSC FASTRAN AFGROWMicrostructurally Small Crack Growth

Transition Rules

Multi‐Site Crack Growth (FASTRAN, AFGROW)Crack Link-up to Dominant Crack

Large Crack Growth

Geometry, Material & Fatigue Loading

SelectMaterial

Calculate Notch & Grain Scale ResponseStresses & Strains

OUTPUT

Crack Size, a

P(a)

Crack Size, a

P(a)

INPUT

Next Load Cycle

Microstructure Statistics Grain SizeGrain OrientationParticle Aspect RatioParticle Size & SpacingConstitutive Rules

Processed DataParticle Filters (I&II)Material Samples

a

b

c

10 μm

(μStructure Builder)

DARPA‐SIPSRollet, Ingraffea, Horstmeyer

Newman, Tryon

Next Generation Life Management: Microstructure‐Based Multi‐scale Models

Response Surface to Select Particles Most Likely to Crack

Fatigue Incubation

Critical Plane Criterion, Coffin‐Manson (MSF)Select Particles Most Likely to Nucleate

Fatigue Nucleation

MSC FASTRAN AFGROWMicrostructurally Small Crack Growth

Transition Rules

Multi‐Site Crack Growth (FASTRAN, AFGROW)Crack Link-up to Dominant Crack

Large Crack Growth

Geometry, Material & Fatigue Loading

SelectMaterial

Calculate Notch & Grain Scale ResponseStresses & Strains

OUTPUT

Crack Size, a

P(a)

Crack Size, a

P(a)

INPUT

Next Load Cycle

Microstructure Statistics Grain SizeGrain OrientationParticle Aspect RatioParticle Size & SpacingConstitutive Rules

Processed DataParticle Filters (I&II)Material Samples

a

b

c

10 μm

(μStructure Builder)

DARPA‐SIPSRollet, Ingraffea, Horstmeyer,

Newman, Tryon

Computational muscle available, but needs:‐ A detailed understanding of the crack tip environment and material

interaction

‐ Understanding of the relationship between the governing failure mechanism and the microstructure

‐ High fidelity experimental data on crack formation, MSC cracking, and long crack kinetics to validate modeling efforts

‐ A realistic and validated Failure Criteria!!!

Next Generation Life Management: Microstructure‐Based Multi‐scale Models

Use high fidelity experimental characterization to inform mechanism level understanding that feeds engineering level modeling: Two Foci

Environmental Cracking Research Focus:


Governing factors for crack formation and microstructure scale cracking (MSC) growth about corrosion damage


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Current Research Efforts:1. The effect of [Cl‐] concentration on pitting, crack formation and MSC in UHSSS

‐ ONR (Vasudevan): Burns2. Modeling crack formation life in air, chloride solutions and high temperatures in UHSSS and Ni‐alloys

‐ Rolls Royce (Mills): Burns3. Investigating the role of galvanic coupling parameters and inhibitors on the factors that govern crack formation from corrosion damage in aerospace Al

‐ ONR (Nickerson): Burns, Scully, Kelly4. LEFM modeling of remaining fatigue life of field exposed corroded components

‐ SAFE Inc.(Fawaz): Burns



Advised Effort: 1 PDRA 2 GRA 0 (1) Undergrad


Current Research Efforts:1. The effect of [Cl‐] concentration on pitting, crack formation and MSC in UHSSS

‐ ONR (Vasudevan): Burns2. Modeling crack formation life in air, chloride solutions and high temperatures in UHSSS and Ni‐alloys

‐ Rolls Royce (Mills): Burns3. Investigating the role of galvanic coupling parameters and inhibitors on the factors that govern crack formation from corrosion damage in aerospace Al

‐ ONR (Nickerson): Burns, Scully, Kelly4. LEFM modeling of remaining fatigue life of field exposed corroded components

‐ SAFE Inc.(Fawaz): Burns



The effect of bulk and crack tip environments on crack growth kinetics



Current Research Efforts:5. The effect of low temperature on the crack growth behavior of 7075/2199 Al alloys

‐ALCOA (Warner), SAFE Inc.(Fawaz): Burns6. The effect of lot‐to‐lot variation on the HEAC behavior of Monel K‐500 and its impact on LEFM modeling

‐ ONR (Perez): Scully, Burns7. The effect of grain orientation and composition the HEAC of Al‐Mg alloys

‐ ONR (Perez): Burns, Kelly8. The effect of loading rate on the HEAC behavior of two Ni‐based super‐alloys

‐ USAFA (Shoales): Scully, Burns9. Mechanistic studies of IG corrosion and stress corrosion cracking under atmospheric exposure conditions

‐ ONR (Perez): Kelly, Burns, Scully10. Mechanism‐based approach to development of corrosion and hydrogen resistant aircraft alloys

‐ NAVMAR (Waldman): Burns, Gangloff11. The effect of plate thickness on the environmental fatigue behavior of 7085 aluminum

‐ ALCOA (Boselli): Burns12. The effect of precipitate character on the crack tip damage character during HEAC of Monel K‐500

‐ ALCOA Fellowship: Burns


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Current Research Efforts:5. The effect of low temperature on the crack growth behavior of 7075/2199 Al alloys

‐ALCOA (Warner), SAFE Inc.(Fawaz): Burns6. The effect of lot‐to‐lot variation on the HEAC behavior of Monel K‐500 and its impact on LEFM modeling

‐ ONR (Perez): Scully, Burns7. The effect of grain orientation and composition the HEAC of Al‐Mg alloys

‐ ONR (Perez): Burns, Kelly8. The effect of loading rate on the HEAC behavior of two Ni‐based super‐alloys

‐ USAFA (Shoales): Scully, Burns9. Mechanistic studies of IG corrosion and stress corrosion cracking under atmospheric exposure conditions

‐ ONR (Perez): Kelly, Burns, Scully10. Mechanism‐based approach to development of corrosion and hydrogen resistant aircraft alloys

‐ NAVMAR (Waldman): Burns, Gangloff11. The effect of plate thickness on the environmental fatigue behavior of 7085 aluminum

‐ ALCOA (Boselli): Burns12. The effect of precipitate character on the crack tip damage character during HEAC of Monel K‐500

‐ ALCOA Fellowship: Burns



Advised Effort: 1 PDRA 5 GRA 2 (1) Undergrad


Environmental Cracking Interests

Metallurgy

Chemistry/ElectrochemistryMechanics

‐ Strength ‐ Strengthening mechanism‐ GB character (PPC, orientation, segregation)

‐ Slip character ‐ Grain size‐ Composition ‐ PPC ‐ Crack tip dislocations‐ H‐trapping behavior ‐Surface modification

‐ Additive manufacturing properties

‐ Echem Potential ‐ Electrolyte ‐ Halides ‐ pH ‐Moist Gas‐ H‐pressure ‐ Solution flow

‐ Corrosion damage morphology ‐ Coatings

‐ Crack tip occlusion ‐ Temp‐ Irradiated Materials

‐ Bio‐medical conditions

‐ ΔK/K ‐Mean Stress‐ Crack tip stress/strain/plasticity

‐ Frequency ‐Wave‐form‐ Grain specific constitutive laws

‐ Corrosion concentrated stress/strains

‐ Crack closure

Environmental Cracking

Metallurgy












‐ Crack closure


Environmental Cracking Interests

Metallurgy












‐ Crack closure


Environmental Cracking Interests in the Context of: CESE Expertise

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Metallurgy












‐ Crack closure ‐ Crack tip strain rate



Metallurgy















Metallurgy















Metallurgy















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Metallurgy














Environmental Cracking Interests in the Context of: MSE Expertise

Metallurgy















New ALCOA Research Scientist

Metallurgy















Metallurgy















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Metallurgy














Environmental Cracking Interests in the Context of: Current External Collaborators

Characterization:Robertson (UW), Goswami (NRL), Sofronis (UI), Burnett (Manchester)

Modeling:Microstructure: QuesTek (Olson),

VEXTEC (Tryon), GaTech(McDowell/Castelluccio)

Engineering: LUNA (Friedersdorf), SAFE (Fawaz)

ExperimentationWMTL (Plotner), NSWC (Gaies), NRL (Matzdorf), NRL (Bayles,

Knudsen), USNA (Schubbe), Sandia (Somerday), ALCOA (Bray, Warner)

Metallurgy














Environmental Cracking Interests in the Context of: Potential External Collaborators

Nuclear:Was (UM), Staehle, QuesTek/Bettis APL

Crystal Plasticity Modeling:McDowell/Neu (GT), Anderson (OSU), Solanki (ASU), Hochhalter

(NASA)

Bio‐Medical Materials/Environments:

Gilbert (SU)

Infrastructure:UVA‐CE, VDOT (Sharp),

Riddell (Rowan)

Characterization:Robertson (UW), Goswami (NRL), Sofronis (UI), Burnett (Manchester)

Modeling:Microstructure: QuesTek (Olson),

VEXTEC (Tryon), GaTech(McDowell/Castelluccio)

Engineering: LUNA (Friedersdorf), SAFE (Fawaz)

ExperimentationWMTL (Plotner), NSWC (Gaies), NRL (Matzdorf), NRL (Bayles,

Knudsen), USNA (Schubbe), Sandia (Somerday), ALCOA (Bray, Warner)

‐ High fidelity experimental capabilities (Cracking‐Burns; Echem‐Kelly/Scully)

‐ State of the art characterization techniques (Burns/UW/UI/NRL/UVa?)…

Field is poised for unprecedented advances in identifying the damage mechanism and failure criteria by coupling:



This mechanistic understanding will:

1. Inform next generation mechanism‐based multi‐scale computa on modeling (atoms → components)

(GaTech/QuesTek)


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(GaTech/QuesTek)

2. Enable near‐term incorporation of environment into engineering‐scale structural management/alloy selection

(Burns/LUNA/Fawaz/VEXTEC)






(GaTech/QuesTek)

2. Enable near‐term incorporation of environment into engineering‐scale structural management/alloy selection

(Burns/LUNA/Fawaz/VEXTEC)

3. Inform traditional and ICME‐based alloy development aimed at enhanced environmental cracking performance


The Effect of Water Vapor Pressure on the Threshold Behavior of Aerospace

Aluminum AlloysJames T. Burns

Research Assistant ProfessorDepartment of Materials Science and Engineering

University of Virginia

MSE SeminarUniversity of Virginia

Department of Materials Science and Engineering

Aug 2014

Fighter

Flight Environment…

Primary Loading• Aggressive Maneuvers• ≈30,000 ft = ‐44°C• f = 0.005‐0.2 Hz• Aicher, 1976; Aronstein, 1997

Jonge, 1979

40%

Transport

Wing Loads• Taxi/Take‐off/Landing• Wind Gusts• 40% >10,000 ft; Thus < ‐5°C• f = 0.1‐10 Hz• Jorge, 1979

Fuselage Loads• Pressurization• 8,000‐50,000 ft ‐5 to ‐57°C• f = 0.00003‐0.001 Hz• Hunt; Wanhill, 2001

Aerodynamic Loads• Fuselage/Control Surfaces• 0‐50,000ft; Thus 0‐60°C• f = 0.0003‐30 Hz• Fawaz

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Flight Environment…

Active loading at Low Temperature

and Low Water Vapor Pressure

Fatigue Resistance is Drastically Increased at Low T and PH2O

Fatigue Resistance is Drastically Increased at Low T and PH2O1. Can be accurately modeled using LEFM and env‐specific rates

2. Potential to significantly reduce the inspection burden

Temperature/PH2O Specific Growth Rate Data

LEFM Code (AFGROW)

23C

‐50C

‐90C

Temperature/PH2O Specific Growth Rate Data

Fatigue Resistance is Drastically Increased at Low T and PH2O1. Can be accurately modeled using LEFM and env‐specific rates

2. Potential to significantly reduce the inspection burden

LEFM Code (AFGROW)

23C

‐50C

‐90C

TAKE AWAY: Significant fatigue loading likely takes place at low T and PH2O

Airframe Prognosis (Safe‐Life and DTA) = Laboratory Room Temp Material Properties for Life Prediction

Motivates investigation of low T/PH2Ocracking behavior

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7075 ‐ Complete23C; Low PH2O

7075 ‐ OngoingLow T

2199 ‐ Complete23C; Low PH2O

2199 ‐ OngoingLow T

Extensive and unprecedented data‐base for 7075 and 2199 at pertinent T/PH2OGrowth rates systematically decrease with decreasing exposure

In collaboration with ALCOA

2199 is more fatigue resistant at both High RH and UHVWhy?

2199 is more fatigue resistant at both High RH and UHV

2199‐T86 – Al‐Cu‐Li‐ Shearable δ’‐phase (Al3Li); strong texture‐ Heterogeneous planar slip ‐ reversibility‐ Extrinsic toughening mechanisms

‐ Roughness induced closure‐ Crack deflection/branching‐ Mode II displacement

7075‐T651 – Al‐Zn‐Mg‐Cu‐ Non‐shearable ƞ (or ƞ’)‐phase‐ Dislocation looping – Homogenous slip‐ Limits slip reversibility and crack

roughness‐ *However, SBC at UHV and low ΔK

2 3 4 5 6 7 8 9 10 11121314

10-7

10-6

10-5

10-4

10-3

AA 7075-T651--L-T

f = 20 Hz, C = -0.07 mm-1

Kmax = 16.5 MPa√m

T=16oC, Vacuum:0.25~0.50 μPa

T= 26oC, Water Vapor: 0.006 Pa





T= 23oC, Water Vapor: 2.4 kPa


da/d

N, m

m/c

ycle

Applied ΔK, MPa√m

Constant Kmax of 16.5 MPa√mIncreasing R with decreasing ΔK

Constant R =0.5

Decreasing ΔK tests at various PH2O show a novel “apparent threshold” behavior At intermediate exposures a minima is followed by increasing da/dN for

both…

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Apparent Threshold behavior correlates with changes in fracture surface morphology

Transgranular features observed prior to the apparent threshold…

(1)

(1)

(1) ΔK=7 MPa√m

(1)

(1) ΔK=7 MPa√m

(2)

(1)(2)

(2) ΔK=6 MPa√m

Transgranular features observed prior to the apparent threshold…

(1)(2)

(1)(2)

(3)

(3)

(3) ΔK=5 MPa√m

A SBC morphology is observed in the “dip” region…

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(1)(2)

(1)(2)

(3)

(3)

(4)

(4)

(3) ΔK=5 MPa√m

(4) ΔK=3 MPa√m

Transitions back to transgranular as da/dN increases…

This behavior begs two questions:1. Why does the apparent threshold behavior initiate?

2. What causes the subsequent rise?

Understood via the Hydrogen Environment Embrittlement Process

23C HUMID

HighMouth PH2O

σ High: Plasticity, ┴

Hydrostatic stress

RAPID

Uptake:Interstitial H

Surface Reaction

Al

Atomic H

H2O

Process Zone

CrackPit

Mouth PH2O > Crack tip PH2O

Molecular Flow

23C HUMID

HighMouth PH2O

σ Molecular Flow High: Plasticity, ┴

Hydrostatic stress

RAPID

Uptake:Interstitial H

Surface Reaction

Al

Atomic H

H2O

Process Zone

CrackPit

Mouth PH2O > Crack tip PH2O

Literature models exist based on rate limitation by:Molecular flowSurface reaction

Diffusion

Understood via the Hydrogen Environment Embrittlement Process

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All “apparent threshold” behavior occurs in the regime where molecular flow governs the environmental cracking

1

( )Satcf

ysOH

cf dNda

MT

kTEd

RfNf

P

dNda

−

⎟⎠⎞

⎜⎝⎛

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛ 2

1

2

2

4362σ

αβ

Wei, et al.

Molecular transport can occur via either:

Advection (Turnbull)• Bulk fluid flow induced by

the cyclic displacement of the crack walls

Diffusion Based Flow (Wei)• Pressure gradient results from

flow impedance of water molecules interacting with crack walls

• Free molecular (Knudsen) flow for our test conditions

Molecular transport can occur via either:

Advection (Turnbull)• Bulk fluid flow induced by

the cyclic displacement of the crack walls

Diffusion Based Flow (Wei)• Pressure gradient results from

flow impedance of water molecules interacting with crack walls

• Free molecular (Knudsen) flow for our test conditions

A simple 1‐D flow criteria:

suggests that molecular transport will be dominated by:Diffusion Based Flow

How does this influence the da/dN?

lcrit ≈ (DH2O/f )1/2 / (1 – R1/2)Turnbull et al.

( )Satcf

ysOH

cf dNda

MT

kTEd

RfNf

P

dNda

−

⎟⎠⎞

⎜⎝⎛

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛ 2

1

2

2

4362σ

αβ

Wei, Ruiz

Solving coupled differential equations that account for tip surface reaction and impeded molecular (Knudsen) flow yielded a model for da/dNcf

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( )Satcf

ysOH

cf dNda

MT

kTEd

RfNf

P

dNda

−

⎟⎠⎞

⎜⎝⎛

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛ 2

1

2

2

4362σ

αβ

β is an empirical constant related to crack wake flow geometry

Wei, Ruiz


( )Satcf

ysOH

cf dNda

MT

kTEd

RfNf

P

dNda

−

⎟⎠⎞

⎜⎝⎛

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛ 2

1

2

2

4362σ

αβ

Increased roughness causes flow geometry to change…

β decreases

Thus da/dN to fall!!!


Wei, Ruiz

Decreasing β

( )Satcf

ysOH

cf dNda

MT

kTEd

RfNf

P

dNda

−

⎟⎠⎞

⎜⎝⎛

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛ 2

1

2

2

4362σ

αβ

Increased roughness causes flow geometry to change…

β decreases

Thus da/dN to fall!!!

Wei, Ruiz

Decreasing βWhy does roughness develop??


7075-T651C(T) - L-T Orientation

R=0.5; f =20 Hz

ΔK (MPa m1/2)

1 10

da/d

N (m

m/c

ycle

)

10-8

10-7

10-6

10-5

10-4

10-3

10-2

7075 - RH>95%7075 - UHV

Fracture surface morphology is strongly dependent on the environment and ΔK

TransgranularCleavage and

High Index Planes

TransgranularCleavage

Micro‐voidingSub‐boundary

TransgranularSome SBC

High ΔK enables cross‐slip

SBCShears ƞ (ƞ’)High Level ofRoughness

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7075-T651C(T) - L-T Orientation

R=0.5; f =20 Hz

ΔK (MPa m1/2)

1 10

da/d

N (m

m/c

ycle

)

10-8

10-7

10-6

10-5

10-4

10-3

10-2

7075 - RH>95%7075 - UHV

Fracture surface morphology is strongly dependent on the environment and ΔK

TransgranularCleavage and

High Index Planes

TransgranularCleavage

Micro‐voidingSub‐boundary

TransgranularSome SBC

High ΔK enables cross‐slip

SBCShears ƞ (ƞ’)High Level ofRoughness

Critically, the level of roughness increases with decreasing ΔK and exposure!!!!

What governs the morphology change?

Method: EBSD + Stereology

Identify the Crystallographic Character of the Crack Path Based on Fracture Surface Analysis

‐Classic Slip‐Band Cracking along {111}

‐ Observed for a wide range of planar slip Al‐alloys

‐ Also observed at low‐ΔK for wavy slip (7075‐T651)


Gangloff, Ro, Gupta, Agnew

Ultra High Vacuum

High Purity 2024‐T351

‐{001}, {011}, high index; Never {111}

‐ H‐Enhanced Decohesionthrough:

‐ Planes with lowest cohesive strength {001}, {011}

‐ Dynamically recovered sub‐grain structures/LEDS


Gangloff, Ro, Gupta, Agnew

High Humidity

High Purity 2024‐T351

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At low/intermediate exposures, as ∆K decreases the proportion of SBC features increases

1. Why does the apparent threshold behavior initiate?HYPOTHESIS:


This increases roughness associated with these SBC




This increased roughness leads to impeded flow (decreased β)


SAT2

2YSH2O

dNda

kEσf(R)

αNβ436

T1

fP

dNda

⎟⎠⎞

⎜⎝⎛

⎥⎦

⎤⎢⎣

⎡=

M




As predicted via Knudsen Flow models, this exacerbates the crack tip PH2O reduction


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As predicted via Knudsen Flow models, this exacerbates the crack tip PH2O reduction

Decreased crack tip PH2O results in less HEE and slower da/dN


Consistent with experimental findings:As PH2O Decreases

Consistent with experimental findings:As PH2O Decreases→ rougher crack wake

1.8 Pa

0.2 Pa

Consistent with experimental findings:As PH2O Decreases→ rougher crack wake → lower β

1.8 PaHigher β

0.2 PaLower β

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Consistent with experimental findings:As PH2O Decreases→ rougher crack wake → lower β→ lower minima

SAT2

2YSH2O

dNda

kEσf(R)

αNβ436

T1

fP

dNda

⎟⎠⎞

⎜⎝⎛

⎥⎦

⎤⎢⎣

⎡=

M

1.8 PaHigher β

0.2 PaLower β

2 3 4 5 6 7 8 9 10 11121314

10-7

10-6

10-5

10-4

10-3

AA 7075-T651--L-T

f = 20 Hz, C = -0.07 mm-1

Kmax = 16.5 MPa√m

T=16oC, Vacuum:0.25~0.50 μPa








da/d

N, m

m/c

ycle

Applied ΔK, MPa√m

Constant Kmax of 16.5 MPa√mR ≈ 0.75 at dip

Constant R =0.5

≈5

≈4

P

1.8 Pa

0.13 Pa

Consistent with experimental findings:Minima occurs at lower ΔK and lower exposures for Constant Kmax

More open crack associated with higher R…

A more open crack requires more roughness to achieve the same level of flow impedance…

Consistent with experimental findings:Different loading histories results in changes in da/dN behavior in

molecular transport controlled regime.

( )Satcf

ysOH

cf dNda

MT

kTEd

RfNf

P

dNda

−

⎟⎠⎞

⎜⎝⎛

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛ 2

1

2

2

4362σ

αβ

Consistent with experimental findings:Thinner specimens → Decreased molecular flow distance

Original Flow Distance

New Flow Distance

8/29/2014

24

Consistent with experimental findings:Thinner specimens → Decreased molecular flow distance

→ faster da/dN

Original Flow Distance

New Flow Distance

As expected, thickness dependence only observed in molecular flow controlled regimeNot observed for High RH and UHV



As ∆K continues to decrease, the area and proportion of SBC features in the crack wake increases



This roughness leads to surface asperity contact during cycling

At constant CMOD

8/29/2014

25




Transition from diffusion (Knudsen) controlled molecular flow to TURBULENT – GAS MIXING (Turnbull, Hartt)

Via Knudsen Flow Via Turbulent

Mixing




Transition from diffusion (Knudsen) controlled molecular flow to TURBULENT – GAS MIXING (Turnbull, Haartt)

Turbulent mixing may increase the crack tip PH2O to near‐bulk levels






Increased tip PH2O leads to increased HEE;consistent with morphology change from SBC to flat TG






Increased tip PH2O leads to increased HEE;consistent with morphology change from SBC to flat TG

Thus an increase in da/dN!!!

8/29/2014

26

Is asperity contact in the crack wake feasible? Analysis Ongoing Yes!! Asperity contact in the crack wake is reasonable1. Comparing 3D fracture surface roughness to crack wake opening calcs

• 3D Crack wake roughness profiling • Fracture mechanics based crack wake opening displacement calculations

00.020.040.060.080.1

0.120.14

0 5 10 15 20

total displacem

ent (mm)

x (mm)

Crack Wake Profile with a crack length a=20 mm

Crack wake asperity ≈ 5‐20 μmCrack opening (at ‐500 μm) ≈ 5 μm

Yes!! Asperity contact in the crack wake is reasonable2. Use far‐field compliance‐based closure metrics to estimate the degree

of crack wake contact

2199-T860.2 Pa-sR=0.5

10

da/d

N (m

m/c

ycle

)

10-7

10-6

10-5

10-4

ACR

Rat

io

0.8

1.0

1.2

1.4

1.6

Kshed - 1

ACR Ratio = ΔKeffective / ΔKnominal

ΔK (MPa√m)

Yes!! Asperity contact in the crack wake is reasonable2. Use far‐field compliance‐based closure metrics to estimate the degree

of crack wake contact

2199-T860.2 Pa-sR=0.5

10

da/d

N (m

m/c

ycle

)

10-7

10-6

10-5

10-4

ACR

Rat

io

0.8

1.0

1.2

1.4

1.6

Kshed - 1

ACR Ratio = ΔKeffective / ΔKnominal

ΔK (MPa√m)

So What?How does this data and mechanistic understanding impact

fracture mechanics modeling?

8/29/2014

27

Mechanistic understanding provides critical insights into the necessary loading protocol to characterize environmental fatigue


(a) Environmentally Induced False Threshold!!!

Common experimental stopping point



Failure to recognize the “apparent threshold” behavior

could lead to highly non‐conservative LEFM predictions




Failure to recognize the “apparent threshold” behavior

could lead to highly non‐conservative LEFM predictions

Options for mitigation:‐ K‐rise confirmation‐ Fractography‐ Testing to low da/dN


8/29/2014

28


(b) Component Geometry Influences Molecular Flow Path!!!

Similitude is compromised; Despite same ΔK

≠


(b) Component Geometry Influences Molecular Flow Path!!!

Options for mitigation:Test specimens should be representative (similar flow path) of the component to

be modeled

Similitude is compromised; Despite same ΔK

≠

Significant increases (5‐10X) in LEFM predicted fatigue performance at exposures relevant to cruise altitude


Format for LEFM Input

Mechanistic understanding of the dip informs the conservative assumption that the “dip” behavior

should be excluded from modeling

8/29/2014

29


50

100

150

200

250

300

350

400

450

500

100 1000 10000 100000 1000000 10000000

Max Stress (MPa

)

Cycles to 1.5mm

AFGROW PredictionsSingle Corner Crack at HoleR=0.5; a=250µm, c=250µm

1334 Pa‐s0.2 Pa‐s0.027 Pa‐sUHV

LEFM Code (AFGROW)

Cruise Altitude(40,000 –60,000 ft)

Format for LEFM Input

Next Steps: Research


Temp (oC) 23 ‐4 ‐15 ‐30 ‐37 ‐50 ‐57 ‐65 ‐73 ‐90Vacuum

PH2O/f (Pa‐s)133 17 8.25 1.9 0.9 0.2 0.09 0.027 0.009 UHV

Compare 23C‐Vacuum results with Low Temperature behavior at the same PH2O‐ICE (Burns/ALCOA)

1.9 Pa‐s23C, Vacuum

1.9 Pa‐s‐30C


Temp (oC) 23 ‐4 ‐15 ‐30 ‐37 ‐50 ‐57 ‐65 ‐73 ‐90Vacuum

PH2O/f (Pa‐s)133 17 8.25 1.9 0.9 0.2 0.09 0.027 0.009 UHV


1.9 Pa‐s23C, Vacuum

1.9 Pa‐s‐30C

Temperature effect on either:Surface Reaction, Diffusion,

Dislocation Dynamics

8/29/2014

30


Temp (oC) 23 ‐4 ‐15 ‐30 ‐37 ‐50 ‐57 ‐65 ‐73 ‐90Vacuum

PH2O/f (Pa‐s)133 17 8.25 1.9 0.9 0.2 0.09 0.027 0.009 UHV


FIB then TEM deformed structure, 20 to 200 nm under facet surface…Collaboration with I. Robertson

Al‐Cu‐Mg‐Mn(T351)

Moist Air

Gangloff, Ro

Dislocation Dynamics Behavior Studied via:

Next Steps: Application


Supporting transition efforts with the listed collaborators:‐ Develop coupled load‐environment spectra (USAFA)

US Coast Guard; C‐130H

temperature

pressure

humidity

Mechanical Loading Spectrum Environmental Loading Spectrum

Time (s)


Supporting transition efforts with the listed collaborators:‐ Develop coupled load‐environment spectra (USAFA)‐ Enhance LEFM software for Environmental‐Fatigue predictions

(SAFE/AFGROW)

+ Environmental Condition

8/29/2014

31



(SAFE/AFGROW)

‐ Investigate the effect of environmental and loading transients on the growth rate response (ALCOA/Airbus)

temperature

pressure

humidityTime (s)



(SAFE/AFGROW)

‐ Investigate the effect of environmental and loading transients on the growth rate response (ALCOA/Airbus)

‐ Support data generation for different alloys and flight conditions (ALCOA, Airbus, SAFE, A‐10 SPO)

Conclusions

1. Unprecedented data base developed for 7075 and 2199 showing increased fatigue resistance with decreasing exposure

Conclusions


2. At intermediate exposures and ΔK, a novel dip in fatigue crack growth rates was observed and understood via a molecular flow argument

8/29/2014

32

Conclusions



3. This work has outlined the considerations necessary to develop a testing protocol for environment assisted cracking (full ΔK ranges)• Test to below this threshold so predictions are conservative• The thickness is important due to it being the dominant flow distance

Conclusions



3. This work has outlined the considerations necessary to develop a testing protocol for environment assisted cracking (full ΔK ranges)• Test to below this threshold so predictions are conservative• The thickness is important due to it being the dominant flow distance

4. Incorporating the generated data into simple constant amplitude fracture mechanics modeling show a 5‐10x increase in lifetime

Collaborators –Students/PDRA: J. Jones, S. Winston, A. Lass, J. Ai

Colleagues: R. Bush, R. Gangloff, S. Agnew, A. Turnbull, S. Fawaz

Funding –USAFA‐TCC (Hayes/Dunmire); ALCOA (Warner/Bray)

Acknowledgments

Questions??

Collaborators –Students/PDRA: J. Jones, S. Winston, A. Lass, J. Ai

Colleagues: R. Bush, R. Gangloff, S. Agnew, A. Turnbull, S. Fawaz

Funding –USAFA‐TCC (Hayes/Dunmire); ALCOA (Warner/Bray)

Acknowledgments

8/29/2014

33

The Effect of Microstructure on the HEAC of Monel K‐500

James T. Burns (J. Dolph)John R. Scully (B. Rincon‐Troconis, H. Ha)

Department of Materials Science and EngineeringUniversity of Virginia

UVa‐MSE SeminarAug 2014

EXAMPLE

Monel K‐500 Long Life (10yr) Service Failures in Marine Cathodic

Polarization Conditions

State of the Art/High Fidelity Experimental Characterization

Cracking(Burns)

E‐chem (Scully)State of the Art/High Fidelity Experimental Characterization

Cracking(Burns)

E‐chem (Scully)

Generates Novel and Critical Data

Gangloff, Burns, Scully; 2014

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UVa:SEM, EBSD, EDS

UVa‐NMCF Characterization and Collaborations Provide Insights into How Metallurgy/Electrochemisty Influences Cracking Behavior

Burns, Scully

0 300 600 900 1200 1500

Arb

itrar

y un

it

Energy (eV)

Al

SCl

C

O

Ni

NiCu

Cu

Al

CuNi

CuNi

UVa‐NMCF Characterization and Collaborations Provide Insights into How Metallurgy/Electrochemisty Influences Cracking Behavior

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

Nor

mal

ized

%

Depth (nm)

S Ni Cu C

UVa:SEM, EBSD, EDS

Collaborators:FIB, TEM, STEM, Auger

Case Western

U Wisconsin (Robertson), U Manchester (Burnett)

Burns, Scully

Data are coupled to 1. Inform/evaluate micro‐mechanical models

0 200 400 600 800 1000 1200 1400

0

20

40

60

80

100

120

140

160

0

20

40

60

80

100

120

140

160

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

Stress Enhanced Crack Tip Diffusible H Concentration, CHσ (wppm)

Threshold Stress In

tensity

(M

Pa√m

)

Threshold Stress In

tensity

(MPa

√m)

Crack Tip Diffusible H Concentration, CH‐DFF (wppm)

ATI Allvac

Special Metals

Model Predicted

Low alpha (6.0)

High alpha (12.0)

Monel K‐5003.5% NaClTDS H Uptake (ATI Allvac)α = 7.3 MPa√m (at frac H)‐1

kIG = 0.65 MPa√m

⎥⎦

⎤⎢⎣

⎡ ⋅−=

YS

2HσIG

TH σα")Cα(kexp

β'1K

Gangloff, Burns, Scully; 2014

Data are coupled to 1. Inform/evaluate micro‐mechanical models

2. Provide insights into the controlling failure mechanisms0 200 400 600 800 1000 1200 1400

0

20

40

60

80

100

120

140

160

0

20

40

60

80

100

120

140

160

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

Stress Enhanced Crack Tip Diffusible H Concentration, CHσ (wppm)

Threshold Stress In

tensity

(M

Pa√m

)

Threshold Stress In

tensity

(MPa

√m)

Crack Tip Diffusible H Concentration, CH‐DFF (wppm)

ATI Allvac

Special Metals

Model Predicted

Low alpha (6.0)

High alpha (12.0)

Monel K‐5003.5% NaClTDS H Uptake (ATI Allvac)α = 7.3 MPa√m (at frac H)‐1

kIG = 0.65 MPa√m

⎥⎦

⎤⎢⎣

⎡ ⋅−=

YS

2HσIG

TH σα")Cα(kexp

β'1K

Robertson, et al (2012)

FIBSEM

TEM

Combined HELP/HEDE Mechanism

• Crack tip dislocation cell structure favors H trapping proximate to grain surface

• Cell structure promotes locally high work hardening to support high normal stress

• H decohesion is enabled

8/29/2014

35

‐850 mVSCE versus ‐1000 mVSCE

Mechanistic understanding is then used to inform engineering level alloy selection and structural management decisions…

SCCrack (LEFM Code)

0

10

20

30

40

50

60

1.E+00 1.E+02 1.E+04 1.E+06 1.E+08

Inial K (M

Pa√m

)

Time to Failure (h)

Monel K‐500 (DCT); Initial K vs. TTF

‐850 mVsce

‐1000 mVsce

Gangloff/VEXTEC

‐850 mVSCE versus ‐1000 mVSCE

Mechanistic understanding is then used to inform engineering level alloy selection and structural management decisions…

SCCrack (LEFM Code)

0

10

20

30

40

50

60

1.E+00 1.E+02 1.E+04 1.E+06 1.E+08

Inial K (M

Pa√m

)

Time to Failure (h)

Monel K‐500 (DCT); Initial K vs. TTF

‐850 mVsce

‐1000 mVsce

Has this concept worked??‐ LEFM‐based simulation used to help justify modification of the USN cathodicprotection system protocol

‐Burns, Bayles (NRL), Knudsen (NRL)

‐ Evaluation of next generation replacement alloy MP‐98T ‐Burns, Scully, Horton (NRL)

‐ Transition testing and modeling techniques to industry and DoD labs ‐Burns, Gangloff ‐ DoD ‐Waldman (NAVMAR), Knudsen/Lee (NRL), Frazier (NAVAIR), Graham (USN)‐ Industry/Academia ‐ Plotner (WMTL), Webler (CMU)

research motivation: effect of loading environment on ... · pdf filemulti‐sitecrack growth...

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