understanding of twi j-r curves

8/3/2019 Understanding of TWI J-R Curves

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Understanding of TWI R CurvesA method to measure fracture toughness

Shell contribution, Jan 7, 2009


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Fracture Mechanics vs. Strength

Applied Stress Yield or Tensile Strength

Applied Stress

Flaw Size Fracture Toughness

The strength of materials approach

The fracture mechanics approach


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Fracture Analysis

Two alternative approaches

- Energy criterion approach

Gf= (2EWf/ a )

When Gf GfC, failure occurs

- Stress intensity approach

KI = (a)1/2

When KI KIC, failure occurs

- The two approaches are essentially equivalent forlinear elastic materials


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Materials Fracture Mechanics

Linear Elastic

Fracture Mechanics

Elastic-Plastic

Fracture Mechanics

Dynamic

FractureMechanics

Viscoelasti

c FractureMechanics

Vicoplastic

FractureMechanics

Linear

Time-

Independent KIC

Nonlinear

Time-

IndependentCritical J/CTOD

Time-

Dependent


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Effect of Material

Properties on Fracture

Material Typical Fracture Behavior High strength steel

Low- and medium-strength steel

Austenitic stainless steelPrecipitation-hardened

aluminum

Metals at high temperature

Metals at high strain rates

Linear elastic

Elastic-plastic/Fully plastic

Fully plasticLinear elastic

Viscoplastic

Dynamic-viscoplastic


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Driving Force Material

ResistanceCrack grows


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Instability and the R Curve

R curve

- Material resistance vs. crack extension

- Flat R curve

- Material resistance remains constant

- Rising R curve- Material resistance varies with crack size

Crack growth may be stable or unstable

- Depends on how the driving force and resistancevary with crack size


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R-Curves: Flat vs. Rising Curves

Crack size

G, R

a0Crack size

G, R

a0

R

R

G1

G3

G2

G1

G2

Gc

Stable

Unstable

Gc

G4

Instability

ac

R: Resistance; G: Driving force

Conditions forstable crack growth: G=R and dG/dadR/da

Conditions forunstable crack growth: dG/dadR/da


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Reasons for the R Curve Shape

For an ideally brittle material: flat

- Surface energy is an invariant material property

For a ductile material: rising

- A plastic zone at the tip of the crack increases in

size as the crack grows For materials with cleavage fracture: falling

- The material resistance is provided by the surface

energy and local plastic dissipation.


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Industry Standards

BS 7448: Fracture mechanics toughness tests

- Part 1: Method for determination of KIC, critical

CTOD and critical J values

- Part 4: Method for determination of fracture

resistance curves and initiation values for stable

crack extension in metallic materials;

ASTM E 1820: Standard test method for

measurement of fracture toughness.


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Construction of R-Curves-Blunting and Exclusionlines

Calculate the crack extension limit amax- Geometry dependent

- amax=0.25(w-a0) for CTOD, or- amax=0.10(w-a0) for J

Determine the slope of the blunting line

- Material dependent (Yield & Tensile strength)- CTOD = 1.87x (tensile/yield0.2) x a

- J = 3.75 x Tensile x a

Construct crack extension limit exclusion line- Minimum: 0.1mm

- Maximum: parallel to the blunting line at an offset ofamax

Same

amax for agiven geometry

Higher strength

Steeper slope for J


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Construction of R-Curves-Data Spacing

Multiple specimen

method

- A minimum of 6specimens

- Each crack sector shall

contain at least one data

point

Single specimen method

- Unloading compliance

technique- Potential drop technique

for crack extension

monitoring


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0

50

100

150

200

250

300

350

0 0.5 1 1.5 2 2.5 3

Crack Extension, a, mm

F22V/625, square (14403-W04)

F22/625 , square (14403-W05)

8630M/625, square (14403-W02)

F22/625, square (14403-W06)

F22/625, square (14403-W05P)

4130/ 625, square

(14403-W07)

8630M/ AK10, bevel led (17860-W01)

410/ 625, closure weld (17860-W03)

410/625, square (17860-W02)

F6NM/410NiMo, square (18403-W04)

F6NM/625, square ( Bodycot e)

8630/625 BS1, square (B odycot e)

8630/625 BS3, square (Bodyc ote)

Recent TWI testing data

Bodycote data

Early TWI testing dat

Limits


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TWI R-Curves

Limits

Crack extension, amaxCTOD J

BS7448-4 1.5 0.6

amax to be used 1.5 2??

0

10

20

30

40

5060

70

80

90

100

110

120130

140

150

160

170

180

190

200

210

220

230

240

250

0 0.5 1 1.5 2 2.5 3Crack extension, Da, mm

J,

N/mm

8630M / 625, square

Predominantly

cleavage + tiny patches

of blue and green

disbonding

Blunting line

Minimum exclusion line

Maximum exclusion line

F6NM / 625, square


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So, what do we know?

Interpretation of R-curves

- Can be used for ranking material resistance to fracture in a

given environment- The rising R curve is more ductile than the flat R curve

- The higher end R curves are more resistant to fracture than the

lower end R curves

Testing

- TWI has partially followed BS7448

- The crack extension range has been shifted to the left

- Bodycote uses a wider crack extension range

- Validity??


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So, what do we not know?

Interpretation of R-curves

- How good is enough?

- What is the driving force?

Testing

- What is the valid crack extension range?- How do we curve fit the data?


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Tim-dependent Crack Growth and

Damage Tolerance

Plays a role in life prediction of components

- Fatigue

- Stress corrosion cracking

For instance

da/dN = C (K)m

Fla

ws

ize

Time

Failure

Useful service life


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Specimen Dimensions Required for a Valid

KIC Crack length (a)

Thickness (B)

Uncracked ligament (W-a)

Each of the above not less than


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understanding of twi j-r curves

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