Fracture and Crack Propagation in Weldments. A Fracture ...

Fracture and Crack Propagation in Weldments.

A Fracture Mechanics PerspectiveA Fracture Mechanics Perspective

Uwe Zerbst, BAM Berlin

Outline

� Specific aspects of weldments

� Determination of fracture toughness

� Determination of the crack driving force

� Shallow crack propagation and fatigue strength� Shallow crack propagation and fatigue strength

Outline

�� Specific aspects of weldments




Fracture mechanics of weldments: Specific aspects

Inhomogeneous

microstructure

Residual stresses

Misalignment

Strength mismatch

Susceptibility

to cracking

Misalignment


Susceptibility

to cracking

Weld imperfections

Figure according to Gagg, 2005

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

for imperfections

� 26 different types of weld imperfections

� Can be assigned to distinct groups from the perspective of mechanical integrity� Can be assigned to distinct groups from the perspective of mechanical integrity

(a) Cracks and crack-like imperfections

have to be avoided or – if they occur – are immediately subject to

fracture mechanics analysis

(b) Material imperfections which act as crack initiation sites

of paramount importance for fatigue strength and fatigue life analysesof paramount importance for fatigue strength and fatigue life analyses

(c) Geometric discontinuities

increase the local stresses, affect crack initiation, propagation and final failure

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


Inhomogeneous

microstructure

Susceptibility

to Cracking

Material inhomogeneity

Reason: Inhomogeneous cooling & TTT behaviour

Figure according to Toyoda, 1998HAZ regions

Consequence

Toughness scatter

Specific requirements

on toughness testing

� identification of

specific micro-

structure

Figure according to Toyoda, 1998

� number of test

specimens


Inhomogeneous

microstructure

Strength mismatch

Susceptibility

to cracking

� Unintended and intended

mismatch

� Usually in steel:

moderate overmatching

Strength mismatch

moderate overmatching

� Cases of undermatching:

aluminium, high strength steels

� Pronounced mismatching:

laser & electron beam welding

YW YBM = σ σ

W = Weld metal

B = Base plate

Strength mismatch

Effect on crack driving force

Effect on crack path deviation

Figures: Dos Santos et al., Koçak

UMOM

Factors affecting the mismatch effect

� Crack location (weld metal, fusion line etc.) � Mismatch ratio (σYW/σYB)

� Global constraint interdependency � (W-a)/H

� Residual stresses


Inhomogeneous

microstructure

Residual stresses

Strength mismatch

Susceptibility

to cracking

Welding residual stresses

Reason: � inhomogeneous cooling

� constrained shrinking

� solid state phase transformations

External restraint

macro-residual stresses (residual stresses of the

first kind); vary within the cross section over a

distance much larger than grain size

Internal forces and moments are in equilibrium with

respect to any cross section and axis respectively

Figure according to

Leggatt, 2008


Scatter and uncertainty in simulation and measurement

Figures according to

Bouchard, 2008


Dependency on location along the weld

Further effect: Stop-start features

Figures according to Hosseinzadeh and Bouchard, 2011; (b) Bouchard, 2008


Residual stress profiles

�� Individual determination

� Compendia (upper bound curves

to literature data)to literature data)

� Membrane stress (as-welded:

max. value: yield strength)

p r

Yσ + σ ≥ σ� Post weld treatment:

Membrane stress (yield strength at

annealing temperature + correction

for ratio of E modules at room &

annealing temperatures

�� Mechanical post weld treatment


Inhomogeneous

microstructure

Residual stresses

Misalignment

Strength mismatch

Susceptibility

to cracking

Misalignment

Types of misalignment:

(a) Axial misalignment between flat plates

(b) Angular misalignment between flat plates


(b) Angular misalignment between flat plates

(c) Angular misalignment in a fillet welded joint

Consequence:

Notch effect/local bending stress

� Strong effect of fatigue life and

shallow crack propagation

� Effect on long crack fatigue

propagation and (sometimes)

on failure load

Outline


�� Determination of fracture toughness



� Specimen geometries most appropriate

Fracture toughness determination

Modifications compared to testing of non-welded material

for weldments, e.g., shallow cracked

bend specimens

� Weldment specific aspects of specimen

preparation such as the introduction of

the notch, minimisation of residual

stresses and misalignment

� Generation of a straight crack front

ISO 15653� Validity criteria

� Required number of test specimens

� Strength mismatch effects for testing

in the net section yielding range

ISO 15653

Fracture toughness determination: Scheme

According to ISO 15653


Adapted testing

Perform test as much as possible representative with respect to the component

in service. Relevant factors and parameters are:

� Welding process including filler material

� Base plate composition

� Joint thickness

� Preheat and interpass temperatures

� Heat input

� Detailed welding procedure

� Joint configuration

� Restraint

� Postweld treatment

� Time between welding and testing

� Environment

� Test temperature

Hydrogen release heat treatment

prior to testing can be necessary

when the time between welding

and the beginning of service is

much longer than those between

welding and testing.


Inhomogeneous

microstructure

Susceptibility

to cracking


Specific features because of inhomogeneous

microstructure, metallography

HAZ testing: Pre and post test

metallographic examination

� In steel: crack tip no more distant

than 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 thickness

ISO 15653

� 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 zones

� During load increase, when stress peak is shifted into the ligament to the location of

the 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 the

first one from the crack tip varies from

specimen to specimen and so does the

work necessary to shift the stress peak work necessary to shift the stress peak

to the “right” position

fracture toughness scatter

� The longer the crack front the higher the

probability of a “weak link” next to it




probability of a “weak link” next to it

� Toughness scatter becomes smaller

for longer crack fronts but lower bound

remains constant

� Same lower bound toughness can be

determined by using few specimens

with large crack fronts or by using

many specimens with short crack fronts

� Usually: 3-Parameter Weibull distribution; e.g., Stage 2 and 3 Options of SINTAP Master

Curve approach




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

Figures according to Toyoda, 1998


Pop-in behaviour

Pop-in: Discontinuity in the load versus displacement curve in the fracture mechanics test

displacement suddenly increases and

load decreases load decreases

Different reasons:

� Limited cleavage fracture propagation + arrest

� Out-of-plane slits

� Other reasons

� Criteria: > 4 (2) % of (W-a) crack propagartion

Fig.: Dos Santos

et al., 2001

� Criteria: > 4 (2) % of (W-a) crack propagartion

� Load drop more than x %

� Increase in compliance

Problem: When is a pop-in event

component relevant?


Inhomogeneous

microstructure

Strength mismatch

Susceptibility

to cracking


Specific features because of strength mismatch

ISO 15653: Error in J integral or CTOD (standard equations) due to mismatch

less than 10% as long as

Weld metal testing:

CTOD tests:

J integral tests:

M > 1.5 or 1.25: overestimation of J or CTOD

M < 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 ηpl function in

< <0.7 M 2.5

± 5%

( )= + η

−

2

pl

K UJ

E B W a


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

Some additional solutions in the literature


Definition of weld width H for other than prismatic welds

Proposals:

(a) H = average of 2H1 and 2H2

(b) equivalent H, Heq, on the basis of

the shortest distance between the

crack tip and the fusion line along

the slip lines emanating from the

crack tip

However: Systematic investigation

still missing.


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)


Effect of strength mismatch on toughness

and crack path deviation

Electron beam weld, steel

Kocak et al., 1999

Probability of crack path deviation

decreases with longer crack front Laser beam weld, steel

Heerens & Hellmann, 2003

Stress-strain curves

Micro tensile tests

e.g., Kocak et al., 1998

BS 7448: Estimation from hardness

p0.2B

p0.2W

Base plate : R 3.28 HV 221 for 160 < HV < 495

Weld metal : R 3.15 HV 168 for 150 < HV < 300

= −

= −


Inhomogeneous

microstructure

Residual stresses

Strength mismatch

Susceptibility

to cracking


Specific features because of residual stresses

� Considered at applied side

(crack driving force in component)

� Specimen if possible residual � Specimen if possible residual

stress free (but not realistic)

� Specimen preparation

in order to generate

straight crack front

From left to right:

- Local compression- Local compression

- (Reverse

bending)

- High R ratio

test


Inhomogeneous

microstructure

Residual stresses

Misalignment

Strength mismatch

Susceptibility

to cracking

Misalignment


Specific features because of misalignment

� Deformation of specimen wings in order to avoid bending� Deformation of specimen wings in order to avoid bending

� However, no plastic deformation within a distance B from weld

Outline



�� Determination of the crack driving force



Inhomogeneous

microstructure

Strength mismatch

Susceptibility

to cracking

� Crack path simulation by damage

mechanics methods, e.g., GTN model

Local parameters for at least base plate,

Crack driving force and

fracture assessment

Local parameters for at least base plate,

weld metal and HAZ

Negre et al., 2004

}

� Conventional fracture mechanics

(finite element based and analytical)

Lower bound toughness or R curve

or 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 component

relevant?

( ) -2

e rJ J f L = ⋅ ( )r rK f L=

Crack driving force: R6 type assessment

FAD approach CDF approach

2

eJ K E′=r matK K K=

( ) ( ) -1 2

2 6

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

( ) ( ) ( )N 1 2N

r r rf L f L 1 L−

= = ⋅max

r r1 L L≤ ≤

( )max

r p0.2 m eLL 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

0.6

µ =

( )p0.2 mN 0.3 1 R R = ⋅ −

( )r p0.2 m eLL 0.5 R R R = ⋅ +

Replace FY by FYM

Mismatch corrected limit load FYMExample

� Conservative option:

FYM determined as FY based on the lower yield

strength of base plate and weld metal

� Individual determination

FYM 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 σYW gives good estimate FY based on σYB gives good estimate

(e.g. laser or electron beam weld)

Mismatch corrected limit load FYM

Examples

UM OM

Fracture analyses including mismatch: Examples

Fc = 569 kN

Fc = 589 kN

M = 1.5

Fc (homogenous) = 550 kN


Inhomogeneous

microstructure

Residual stresses

Strength mismatch

Susceptibility

to cracking

Primary stresses σσσσp:

� Arise from the applied mechanical � contribute to

load, including dead weight or plastic collapse

inertia effects

Primary and secondary stresses

Secondary stresses σσσσs:

� 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 the

structure, i.e., net force and

K factor determination is based

on both primary and secondary

stresses but only the primarystructure, i.e., net force and

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 account

for the limit load FY,

Crack driving force due to primary

and secondary stresses

Primary stresses only

na

K a f

= π ⋅ σ ⋅ ⋅ ∑ }

Primary + secondary stresses

n nn

aK a f

T

= π ⋅ σ ⋅ ⋅

∑

( )n

nn

xx

T

σ = σ ⋅

∑

}

Interaction factor V

Small scale yielding:

K = Kp

+ Ks

However: because of rather high

σσσσs

in as-welded structuresσσσσs

in as-welded structures

K > Kp

+ Ks

Lr < 1

and because of stress relief

K < Kp

+ Ks

Lr > 1

Although secondary stresses don‘t

contribute to plastic collapse they

contribute to ligament yielding

∼

∼

contribute to ligament yielding

p s

I Ir

mat

VK KK

K

+ ⋅=

( )

2p s

I I

r

K K1J

E f L

V + ⋅= ⋅

′

FAD approach:

CDF approach:

p sK K KV= + ⋅= + ⋅= + ⋅= + ⋅

s

p

s

KV

K= ⋅ ξ= ⋅ ξ= ⋅ ξ= ⋅ ξ

Plasticity corrected

„K factor“ for se-

condary stresses

Determination of V

sV

K= ⋅ ξ= ⋅ ξ= ⋅ ξ= ⋅ ξ

K factor for

secondary

stresses

Fit function to finite

element results

Different options for determining s

pK ( )s p

p rK K L 0 0.02 0.04 …

e.g., plastic zone corrected K:

p

( ) ( )s s

p eff K a a K a= ⋅

Lr

0

0.01

0.02

0.03……

( ) 2

s

eff

Y

K a 3 plane strain 1a a =

2 1 plane stress

= + ⋅ β

βπ σ

Fracture analyses including residual stressesExample: Residual stress profile

( ) 2 3

T *

R Y

z z zz t 1 0.917 14.533 83.115

t t t

σ σ = − ⋅ − ⋅ + ⋅

4 5 6z z z

215.45 244.16 93.36t t t

− ⋅ + ⋅ − ⋅

Transverse residual stresses (compendium)

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

Reduction in critical load: ca. 25%

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

� No effect on ∆K

� But on R = Kmin/Kmax

� Effect on crack closure behaviour

Reduction in

� Effect on crack closure behaviour

Reduction in

residual lifetime:

ca. 25%

Simplified assumption:

R > 0.5 (BS 7910)

Fracture analyses including residual stresses

Ongoing discussion on

less conservative deter-

mination of V factor

This workshop

Including solutions

� Without elastic follow-up� Without elastic follow-up

� Large elastic follow-up

� for application to short crack propagation problems


Inhomogeneous

microstructure

Residual stresses

Misalignment

Strength mismatch

Susceptibility

to cracking

Misalignment

Example:

�� Angular distorsion

�� Butt weld

�� clamped

Fracture analyses including residual stressesMisalignment

( ) ( )s

m

tanh tanhy

t t

β βσ α ⋅ = = ⋅

σ β β

ℓ2 23 3

2 2 2

Solution for bending stress σs

refered to membrane stress σm

Alternativ: Finite element stress distribution 1 2

m32 (rad!)

t E

σ⋅ β =

ℓ

Outline




�� Shallow crack propagation and fatigue strength�� Shallow crack propagation and fatigue strength

Frequently: Inclusions at or

close to surface are

crack initiaton sites

Initial defects in engineering alloys

crack 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

Distinguish between geometrical dis-

continuities (considered at applied

side) and material defects

Weld discontinuities and defects

side) and material defects

Applied side Material

- Misalignment - Slag lines

- weldment geometry - Pores

- Undercuts - Lack of fusion

- Overlap - Cracks

Initial crack size and

geometry (multiple cracks)

Usually excluded- Overlap - Cracks

Specified by

weldment

quality

systemSteel 350WT

Crack initiation in WAZ

0.3 mm deep surfacerdefect

(Josi, 2010)

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

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

Lack of fusion not permissable not permissable not permissable

Example: Weldment quality grades: VOLVO

Group Weld Quality Standard 181-0004, 2008

Lack of fusion not permissable not permissable not permissable

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

radius

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 permissable

weld 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%

- Crack initiation Ni

- short crack growth N

Contributions to fatigue life

Contribution to overall lifetime Nt:

Polak (CSI, 2003):

- short crack growth Ns

- long crack growth Nl

t i s lN N N N= + +

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

- less than 5-20% of overall lifetime Nt

- even less for existing initial defects

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

Long crack growth(a > 0,5 mm, 2c > 1 mm)

Short crack growth

∆∆∆∆K concept not applicable

Alternatives:

�� „plasticity corrected“ K

Specifica of mechanically short cracks

�� „plasticity corrected“ K

(e.g., plastic zone size corrected)

�� ∆∆∆∆J-Integral

�� ∆∆∆∆CTOD

Gradual built-up of plasticity-induced

crack closure effect:

Fracture and Crack Propagation in Weldments.

A Fracture Mechanics PerspectiveA Fracture Mechanics Perspective

�� Specific aspects of weldments

�� Determination of fracture toughness


[email protected]


�� Shallow crack propagation and fatigue strength

Fracture and Crack Propagation in Weldments. A Fracture ...

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