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Fracture and Crack Propagation in Weldments.

Uwe Zerbst , BAM Berlin

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Outline

Specific aspects of weldments

Determination of fracture toughness

Determination of the crack driving force

Shallow crack propagation and fatigue strength

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Outline

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Fracture mechanics of weldments: Specific aspects

Inhomogeneousmicrostructure

Residual stresses

Misalignment

Strength mismatchSusceptibility

to cracking

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Susceptibilityto cracking

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ISO 5817: Arc welded joints in steel - Guidance on quality levels

for imperfections

26 different types of weld imperfections

(a) Cracks and crack-like imperfectionshave to be avoided or – if they occur – are immediately subject to

fracture mechanics analysis

(b) Material imperfections which act as crack initiation sites

(c) Geometric discontinuitiesincrease the local stresses, affect crack initiation, propagation and final failure

(d) Imperfections which probably are of no effect on fracture or fatigue life

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Susceptibilityto Cracking

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Material inhomogeneity

Reason: Inhomogeneous cooling & TTT behaviour

Figure according to Toyoda, 1998HAZ regions

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Consequence

Toughness scatter

Specific requirementson toughness testing

identification ofspecific micro-structure

Figure according to Toyoda, 1998

number of testspecimens

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to cracking

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Unintended and intendedmismatch

Usually in steel:

Strength mismatch

Cases of undermatching:aluminium, high strength steels

Pronounced mismatching:laser & electron beam welding

YW YBM = σ σ

W = Weld metalB = Base plate

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Strength mismatch

Effect on crack driving force

Effect on crack path deviation

Figures: Dos Santos et al., Koçak

Factors affecting the mismatch effect

Crack location (weld metal, fusion line etc.) Mismatch ratio ( σ YW / σ YB)

Global constraint interdependency (W-a)/HResidual stresses

F h i f ld S ifi

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Residual stresses

to cracking

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Welding residual stresses

Reason: inhomogeneous cooling

constrained shrinkingsolid state phase transformations

External restraint

macro-residual stresses (residual stresses of thefirst kind); vary within the cross section over adistance much larger than grain size

Internal forces and moments are in equilibrium withrespect to any cross section and axis respectively

Figure according toLeggatt, 2008

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Scatter and uncertainty in simulation and measurement

Figures according toBouchard, 2008

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Residual stress profiles

Individual determination

Compendia (upper bound curvesto literature data

Membrane stress (as-welded:max. value: yield strength)

p r Yσ + σ ≥ σPost weld treatment:

Membrane stress (yield strength atannealing temperature + correctionfor ratio of E modules at room &annealing temperatures

Mechanical post weld treatment

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Residual stresses

Misalignment

to cracking

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Types of misalignment:

(a) Axial misalignment between flat plates

(c) Angular misalignment in a fillet welded joint

Consequence:

Notch effect/local bending stress

Strong effect of fatigue life andshallow crack propagation

Effect on long crack fatiguepropagation and (sometimes)on failure load

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Outline

F h d i i

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Specimen geometries most appropriate

Fracture toughness determination

Modifications compared to testing of non-welded material

for weldments, e.g., shallow crackedbend specimens

Weldment specific aspects of specimenpreparation such as the introduction of

the notch, minimisation of residualstresses and misalignment

Generation of a straight crack front

Validity criteria

Required number of test specimens

Strength mismatch effects for testingin the net section yielding range

ISO 15653

F t t h d t i ti S h

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Fracture toughness determination: Scheme

According to ISO 15653

F t t gh d t i ti

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Adapted testing

Perform test as much as possible representative with respect to the componentin service. Relevant factors and parameters are:

Welding process including filler material

Base plate compositionJoint thickness

Preheat and interpass temperatures

Heat inputDetailed welding procedure

Joint configuration

Restraint

Postweld treatment

Time between welding and testing

Environment

Test temperature

Hydrogen release heat treatmentprior to testing can be necessarywhen the time between weldingand the beginning of service is

much longer than those betweenwelding and testing.

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Susceptibilityto cracking

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Specific features because of inhomogeneous

microstructure, metallographyHAZ testing: Pre and post testmetallographic examination

In steel: crack tip no more distantthan 0.5 mm from target microstructure

Crack front should sample either 15%or at least 7 mm of the HAZ microstructure

Both within the central 75% of the specimen thicknessISO 15653

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Randomly distributed small regions of low toughness (“weak links”) across the ligament;

Specific features due to inhomogeneous

microstructure: Weakest link approach (1)

in weldments: HAZ brittle zonesDuring load increase, when stress peak is shifted into the ligament to the location ofthe nearest “weak link” the whole specimen (or component) fails

Due to the random distribution of the “weak links”in the ligament area the distance of thefirst one from the crack tip varies fromspecimen to specimen and so does thewor necessary to s t t e stress pea

to the “right” position

fracture toughness scatter

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Specific features due to inhomogeneous

microstructure: Weakest link approach (3)

BS 7910: Minimum of 12 valid HAZ tests for ductile-to-brittle transition

Figures according to Toyoda, 1998

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Pop-in behaviour

Pop-in: Discontinuity in the load versus displacement curve in the fracture mechanics testdisplacement suddenly increases andload decreases

Different reasons:

Limited cleavage fracture propagation + arrestOut-of-plane slits

Other reasons

Fig.: Dos Santoset al., 2001

Load drop more than x %Increase in compliance

Problem: When is a pop-in eventcomponent relevant?

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to cracking

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Specific features because of strength mismatch

ISO 15653: Error in J integral or CTOD (standard equations) due to mismatchless than 10% as long as

Weld metal testing:CTOD tests:

J integral tests:

M > 1.5 or 1.25: overestimation of J or CTODM < 0.5 underestimation

< <0.5 M 1.5

< <0.5 M 1.25

HAZ testing: Error for J and -20% to +10% for CTOD as long as

Else mismatch specific η pl function in

< <0.7 M 2.5± 5%

( )= + η −

K UJ E B W a

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ηηηη pl function for strength mismatch (EFAM , Schwalbe et al.)

Some additional solutions in the literature

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Definition of weld width H for other than prismatic welds

Proposals:

(a) H = average of 2H 1 and 2H 2

(b) equivalent H, H eq , on the basis ofthe shortest distance between the

crack tip and the fusion line alongthe slip lines emanating from thecrack tip

However: Systematic investigationstill missing.

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Effect of strength mismatch on constraint and toughness

According to Toyoda, 2002

According to Kim (Schwalbe et al., 1996)

Complex issue: Various constraint parameters

Damage mechanics simulation (e.g. GTN)

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Effect of strength mismatch on toughness

and crack path deviation

Electron beam weld, steelKocak et al., 1999

Probability of crack path deviationdecreases with longer crack front Laser beam weld, steel

Heerens & Hellmann, 2003

Stress-strain curves

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Micro tensile testse.g., Kocak et al., 1998

BS 7448: Estimation from hardness

Base plate : R 3.28 HV 221 for 160 < HV < 495Weld metal : R 3.15 HV 168 for 150 < HV < 300

= −= −

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Residual stresses

to cracking

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Specific features because of residual stresses

Considered at applied side(crack driving force in component)

pec men poss e res uastress free (but not realistic)

Specimen preparationin order to generate

straight crack front

From left to right:

- oca compress on

- (Reversebending)

- High R ratiotest

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Residual stresses

Misalignment

to cracking

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Specific features because of misalignment

Deformation of s ecimen win s in order to avoid bendin

However, no plastic deformation within a distance B from weld

Outline

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Crack driving force and

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Crack path simulation by damagemechanics methods, e.g., GTN model

fracture assessment

weld metal and HAZ

Negre et al., 2004

Conventional fracture mechanics(finite element based and analytical)

Lower bound toughness or R curveor probabilistic analysis

Effect of mismatch and residual stresses

Mismatch corrected limit loadon R curve or toughness scatter!

(crack path deviation)

Again: When are pop-in events componentrelevant?

Crack driving force: R6 type assessment

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( ) -2

e rJ J f L = ( )r rK f L=

FAD approach CDF approach

2eJ K E ′=r matK K K=

( ) ( )-1 22 6

r r rf L 1 0.5 L 0.3 0.7 exp L = + + −µ r0 L 1≤ ≤

( ) ( ) ( )N 1 2Nr r rf L f L 1 L −= = max

r r1 L L≤ ≤

maxL 0.5 R R R = +

Example. Option 1B analysis (no Lüders‘ plateau)

r Y ref YL F F= = σ σ

( )p0.20.001 E Rmin

0.6µ =

( )p0.2 mN 0.3 1 R R = −

Replace F Y by F YM

Mismatch corrected limit load F YM Example

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Example

Conservative option:

FYM determined as F Y based on the lower yieldstrength of base plate and weld metal

Individual determinationFYM solutions as functions of global geometry,mismatch ratio M and (W-a)/H

Limit states:

long crack a and/or wide weld (large H) short crack and/or narrow weld (small H)

plastic zone mainly in weld metal plastic zone mainly in base plate

FY based on σ YW gives good estimate F Y based on σ YB gives good estimate

(e.g. laser or electron beam weld)

Mismatch corrected limit load F YM

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Examples

Fracture analyses including mismatch: Examples

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Fc = 569 kN

Fc = 589 kN

M = 1.5

Fc (homogenous) = 550 kN

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Residual stresses

to cracking

P i t p

Primary and secondary stresses

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Primary stresses σσσσ p :

Arise from the applied mechanical

contribute toload, including dead weight or plastic collapse

inertia effects

Secondary stresses σσσσ s :

Result from suppressed local do not contribute

distortions, e.g., during the to plastic collapse

welding process, or are due

to thermal gradients

Self-equilibrating across theK factor determination is based

on both primary and secondarystructure, .e., net orce an

bending moment are zero

However: Secondary stresses can act like primary stresses in the crack carrying section

Treatment as primary conservativ

stresses but only the primary

stresses are taken into accountfor the limit load F Y,

Crack driving force due to primaryd d t

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and secondary stresses

Primary stresses only

Primary + secondary stresses

n nn T

= π σ

σ = σ

Interaction factor V

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Small scale yielding:K = Kp + Ks

However: because of rather highσσσσ in as-welded structures

K > Kp + Ks Lr < 1

and because of stress relief

K < Kp + Ks Lr > 1

Although secondary stresses don‘tcontribute to plastic collapse theycontribute to li ament ieldin

p sI I

K K1J E f L

V + = ′

FAD approach:

CDF approach:

p sK K KV= + = + = + = +

Plasticity corrected

Determination of V

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= = = =

y„K factor“ for se-

condary stresses

KK factor forsecondary

stresses

Fit function to finiteelement results

Different options for determining spK ( )s p

p rK K L 0 0.02 0.04 …

e.g., plastic zone corrected K:

( ) ( )s sp effK a a K a=

0.03……

( ) 2s

K a 3 plane strain1

a a =2 1 plane stress

= + β βπ σ

Fracture analyses including residual stressesExample: Residual stress profile

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( ) 2 3

T *R Y

z z zz t 1 0.917 14.533 83.115

σ σ = − − + 4 5 6

z z z215.45 244.16 93.36t t t

− + −

Transverse residual stresses (compendium)

Fracture analyses including residual stressesExample: Critical load for stable crack initiation

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Reduction in critical load: ca. 25%

Fracture analyses including residual stressesExample: Fatigue crack propagation and residual lifetime

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No effect on ∆K

But on R = K min /Kmax

Reduction in

residual lifetime:ca. 25%

Simplified assumption:

R > 0.5 (BS 7910)

Fracture analyses including residual stresses

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Ongoing discussion on

less conservative deter-mination of V factor

This workshop

Including solutions

-Large elastic follow-up

for application to short crack propagation problems

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Residual stresses

Misalignment

to cracking

Fracture analyses including residual stressesMisalignment

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Example:

Angular distorsionButt weldclamped

( ) ( )s

tanh tanhyt t

β βσ α = = σ β β

ℓ2 23 32 2 2

Solution for bending stress σ s

refered to membrane stress σ m

Alternativ: Finite element stress distribution1 2

m32 (rad!)

β = ℓ

Outline

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Initial defects in engineering alloys

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Frequently: Inclusions at orclose to surface arecrack initiaton sites

Further crack initiation sites:

Primary phases

Pores/cavities

Corrosion pits

Crack initiation at inclusions in steel (42CrMoS4)(Figs. Pyttel)

Surface roughness(scratches)

Welding defects

Weld discontinuities and defects

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Distinguish between geometrical dis-continuities (considered at appliedside and material defects

Applied side Material

- Misalignment - Slag lines- weldment geometry - Pores- Undercuts - Lack of fusion- Overla - Cracks

Initial crack size and

geometry (multiple cracks)

Usually excluded

Specified byweldment

qualitysystem

Steel 350WTCrack initiation in WAZ

0.3 mm deep surfacerdefect(Josi, 2010)

Example: Weldment quality grades: VOLVOGroup Weld Quality Standard 181-0004, 2008

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Discontinuity VD (normal quality) VC (high quality) VB (post weld treated)

Overlap < 0,5 mm < 0,1 mm not permissable

ac o us on not perm ssa e not perm ssa e not perm ssa e

Transition > 0,25 mm > 1 mm > 4 mmradius

Undecut < 0,05 t (max 1 mm) < 0,025 t (max 0,5 mm) not permissable

inadequate < - 0,2a (max 2 mm) smaller not permissable smaller not permissableweld thickness

Misalignment < 0,1 t (max 2 mm) not permissable not permissable

Single Pore 0,4 t (max 4) 0,3 t (max 4) 0,2 t (max 2)0,3 t (max 3) 0,2 t (max 2) 0,1 t (max 1)

Pores cluster 6% / 3% 4% / 2% 2% / 1%

Contributions to fatigue life

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- Crack initiation N i

Contribution to overall lifetime Nt:

Polak (CSI, 2003):

- long crack growth N l

t i s lN N N N= + +

Crack initiation stage N i at smooth, nominally defect-free surfaces:

- less than 5-20% of overall lifetime N t

- even less for existing initial defects

Allows to treat defects as initial cracks in a fracture mechanics model

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Fracture and Crack Propagation in Weldments.

Uwe.zerbst@bam.de

Zerbst (1)

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