Upload
ramnadh803181
View
221
Download
0
Embed Size (px)
Citation preview
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 1/66
Fracture and Crack Propagation in Weldments.
Uwe Zerbst , BAM Berlin
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 2/66
Outline
Specific aspects of weldments
Determination of fracture toughness
Determination of the crack driving force
Shallow crack propagation and fatigue strength
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 3/66
Outline
Specific aspects of weldments
Determination of fracture toughness
Determination of the crack driving force
Shallow crack propagation and fatigue strength
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 4/66
Fracture mechanics of weldments: Specific aspects
Inhomogeneousmicrostructure
Residual stresses
Misalignment
Strength mismatchSusceptibility
to cracking
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 5/66
Fracture mechanics of weldments: Specific aspects
Susceptibilityto cracking
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 6/66
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 7/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 8/66
Fracture mechanics of weldments: Specific aspects
Inhomogeneousmicrostructure
Susceptibilityto Cracking
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 9/66
Material inhomogeneity
Reason: Inhomogeneous cooling & TTT behaviour
Figure according to Toyoda, 1998HAZ regions
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 10/66
Consequence
Toughness scatter
Specific requirementson toughness testing
identification ofspecific micro-structure
Figure according to Toyoda, 1998
number of testspecimens
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 11/66
Fracture mechanics of weldments: Specific aspects
Inhomogeneousmicrostructure
Strength mismatchSusceptibility
to cracking
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 12/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 13/66
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 ( σ YW / σ YB)
Global constraint interdependency (W-a)/HResidual stresses
F h i f ld S ifi
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 14/66
Fracture mechanics of weldments: Specific aspects
Inhomogeneousmicrostructure
Residual stresses
Strength mismatchSusceptibility
to cracking
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 15/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 16/66
Welding residual stresses
Scatter and uncertainty in simulation and measurement
Figures according toBouchard, 2008
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 17/66
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 18/66
Welding residual stresses
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
Fracture mechanics of weldments: Specific aspects
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 19/66
Fracture mechanics of weldments: Specific aspects
Inhomogeneousmicrostructure
Residual stresses
Misalignment
Strength mismatchSusceptibility
to cracking
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 20/66
Types of misalignment:
(a) Axial misalignment between flat plates
Welding residual stresses
(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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 21/66
Outline
Specific aspects of weldments
Determination of fracture toughness
Determination of the crack driving force
Shallow crack propagation and fatigue strength
F h d i i
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 22/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 23/66
Fracture toughness determination: Scheme
According to ISO 15653
F t t gh d t i ti
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 24/66
Fracture toughness determination
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.
Fracture mechanics of weldments: Specific aspects
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 25/66
Inhomogeneousmicrostructure
Susceptibilityto cracking
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 26/66
Fracture toughness determination
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
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 27/66
Randomly distributed small regions of low toughness (“weak links”) across the ligament;
Fracture toughness determination
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 28/66
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 29/66
Fracture toughness determination
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
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 30/66
Fracture toughness determination
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?
Fracture mechanics of weldments: Specific aspects
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 31/66
Inhomogeneousmicrostructure
Strength mismatchSusceptibility
to cracking
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 32/66
Fracture toughness determination
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 −
2
pl
K UJ E B W a
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 33/66
ηηηη pl function for strength mismatch (EFAM , Schwalbe et al.)
Some additional solutions in the literature
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 34/66
Fracture toughness determination
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.
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 35/66
Fracture toughness determination
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)
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 36/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 37/66
Micro tensile testse.g., Kocak et al., 1998
BS 7448: Estimation from hardness
p0.2B
p0.2W
Base plate : R 3.28 HV 221 for 160 < HV < 495Weld metal : R 3.15 HV 168 for 150 < HV < 300
= −= −
Fracture mechanics of weldments: Specific aspects
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 38/66
Inhomogeneousmicrostructure
Residual stresses
Strength mismatchSusceptibility
to cracking
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 39/66
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
Fracture mechanics of weldments: Specific aspects
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 40/66
Inhomogeneousmicrostructure
Residual stresses
Misalignment
Strength mismatchSusceptibility
to cracking
Fracture toughness determination
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 41/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 42/66
Specific aspects of weldments
Determination of fracture toughness
Determination of the crack driving force
Shallow crack propagation and fatigue strength
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 43/66
Crack driving force and
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 44/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 45/66
( ) -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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 46/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 47/66
Examples
UM OM
Fracture analyses including mismatch: Examples
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 48/66
Fc = 569 kN
Fc = 589 kN
M = 1.5
Fc (homogenous) = 550 kN
Fracture mechanics of weldments: Specific aspects
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 49/66
Inhomogeneousmicrostructure
Residual stresses
Strength mismatchSusceptibility
to cracking
P i t p
Primary and secondary stresses
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 50/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 51/66
and secondary stresses
Primary stresses only
na
Primary + secondary stresses
n nn T
= π σ
( )n
nn
xx
T
σ = σ
∑
Interaction factor V
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 52/66
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
rmat
VK KK
K+
=
( )
2p s
I I
r
K K1J E f L
V + = ′
FAD approach:
CDF approach:
p sK K KV= + = + = + = +
Plasticity corrected
Determination of V
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 53/66
spK
= = = =
y„K factor“ for se-
condary stresses
s
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=
Lr
00.01
0.02
0.03……
( ) 2s
effY
K a 3 plane strain1
a a =2 1 plane stress
= + β βπ σ
Fracture analyses including residual stressesExample: Residual stress profile
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 54/66
( ) 2 3
T *R Y
z z zz t 1 0.917 14.533 83.115
t t t
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 55/66
Reduction in critical load: ca. 25%
Fracture analyses including residual stressesExample: Fatigue crack propagation and residual lifetime
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 56/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 57/66
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
Fracture mechanics of weldments: Specific aspects
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 58/66
Inhomogeneousmicrostructure
Residual stresses
Misalignment
Strength mismatchSusceptibility
to cracking
Fracture analyses including residual stressesMisalignment
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 59/66
Example:
Angular distorsionButt weldclamped
( ) ( )s
m
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!)
t Eσ
β = ℓ
Outline
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 60/66
Specific aspects of weldments
Determination of fracture toughness
Determination of the crack driving force
Shallow crack propagation and fatigue strength
Initial defects in engineering alloys
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 61/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 62/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 63/66
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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 64/66
- Crack initiation N i
Contribution to overall lifetime Nt:
Polak (CSI, 2003):
- s
- 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
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 65/66
8/11/2019 Zerbst (1)
http://slidepdf.com/reader/full/zerbst-1 66/66
Fracture and Crack Propagation in Weldments.
Specific aspects of weldments
Determination of fracture toughness
Shallow crack propagation and fatigue strength