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8/18/2019 Fatigue History.pdf
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1 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Professor P J Haagensen
Norges teknisk-naturvitenskapelige universitet
Fakultet for ingeniørvitenskap og teknologi
Institutt for konstruksjonsteknikk
Trondheim
runnleggende utm ttingsberegninger
Innledning: Historikk, eksempler på
utmattingsbrudd.
Tirsdag 20 oktober 2009
kl. 9.00 - 10.00
Utmattingsberegninger for stålkonstruksjoner
iht gjeldende regelverk og Eurokode 3, del 1-9
2 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Skade- og havarityper
Utmattingsproblemet i praksis
Utmatting - definisjoner
Eksempel 1: Utmatting av
maskinkomponenter – NSB hjulaksler
Eksempel 2: Utmatting avsveistekonstruksjoner – Alexander L. Kielland
ulykken
Utmatting - grunnlag•
Temaer
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3 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Unstable fracture (brittle or ductile)
Plastic collapse
Elastic instability (buckling)
Stress corrosion cracking
Hydrogen induced fracture
Corrosion
Fatigue and corrosion fatigue
Wear
Failure modes - How things go wrong
Depending on the operating conditions and the type of environmenta component or st ructure may fail in many different modes:
“Erika” Dec. ‘99
SS“Schenectady”, 1943
Time dependentfailures
Aloha Airlines,Flight 243, 1988
“Alexander L. Kielland”,Mar. 1980
4 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
0
5
10
15
20
25
30
F a t i g
u e
V e s
s e l i
m p a
c t
D r o
p p e d
o b j
e c t s
I n s t a l l a t i
o n f a u l t
F a b r
i c a t i o
n f a
u l t
D e s i g
n u p
g r a d
e
C o r r o
s i o n
D e s i g
n f a u l t
O p e
r a t i n g
f a u l t
O t h
e r
24.7
%Damage
1/4 of all structural
damage requiring repairis caused by fatigue
Fatigue – how big is the problem?• Generally: 80-90% of all fractures are fatigue failures
• North Sea offshore structures:
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5 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Fatigue problems - aircraft structures
Causes of major damage in aircraftstructures
(Royal Aerospace Establishment, UK)
Almost 60 % of to tal damage is caused by fat igue and corros ion
fatigue
6 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Causes of failures incomponents and structures
Design
Operation
Fabrication
- Wrong materials- Poor fabrication quality- Inadequate inspection
- Wrong material properties- Wrong design l i fe- Wrong design method- Missed failure modes
- Unknown environment- Unknown fatigue loads- Improper use- Poor inspection
Failure
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7 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Fatigue problems – pressure equipment
UK 1998 - 2000: 3500 failures, about 25% caused by fatigue
Pressure
vessel
Unknown
Piping
Heat exchanger
Water tube
Shell
boiler
0Percent
10 3020 40 50 60
Data from:Pressure Equipment Directive (PED)
Types of equipment for which fatigue
damage or fai lure was found
8 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Why is fatigue assessment diff icult to perform?European Pressure Equipment Research Council survey in 2000
Main dif ficulties encountered in
applying fatigue assessment
N u m b e r o f r e p l i e s
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11 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Three stages in fatigue process
3. Final fracture
Total life:
N = Ni + Np
No. of cycles to
crack initiationCycles of crack
propagation to failure
1. Initiation of fatigue crack
2. Crack propagation
12 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Primary factors affecting fatigue
Note:
There are significant differences between
welded and unwelded com ponents regarding what
factors have the strongest influence on fatigue life
Primary factors influencing fatigue strength
• Material
• Type of loading tension, bending, shear,combinations
• Mean stress
• Geometry, notches, defects
• Size
• Surface condition roughness, material condition
• Residual stresses
• Environment temperature, corrosion
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15 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Example 1: Failure in mechnical components
NSB train axle failures
NSB Signature Trainaxle fracturesSummer 2002
Crackinitiation
Beachmarks
Finalfracture
16 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
NSB train axle failures
Load history: High loads in short radius turns
Presence of defects: Corrosion pits
Geometry of detail: Cracking in areas of high stress concentrations
Material: High st rength , notch sensitive material, UTS = 1000 MPa
Main contributing factors – DNV failure investigation
Corrosion and cracks in axle filletRubber band prevented moisture
from drying out in fi l let
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Fatigue design of axles
The endurance limit is used as a design criterionfor axles that endure a large number of cycles, e.g.
N = 2x108 during 500 000 km, i.e. the maximum
load cycle in the load spectrum must be lower than
the fatigue limit:
N, cycles to failure
E
max
106 107 108
max
E
18 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Effect of corrosion on the fatigue limit
The drop in fatigue
strength due to corrosionis higher for a high
strength steel than for amild steel
F a t i g u e l i m i t
Corroding specimens
Ultimate tensile strength, ksi
MPa
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19 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
NSB train axle failures
Corrosion damage caused early crack
initiation
High strength material resulted in large
loss of fatigue strength
High local stresses caused short crack
growth stage
Conclusions:
20 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
The Alexander L. Kielland accident
Place: Ekofisk field
Time: 27 March, 1980,18.30 hrs
Persons killed: 123Survivors: 89
10 similar platforms built ALK plat form del ivered in
1976
Time from first failure in
brace D6 to capsize: 20 min
Example 2 Failure of a welded structure
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The accident
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ALK s tructural arrangement
Pentagone design
1st fracture
D
D
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23 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Brace D6 and hydrophone support tube
Hydrophonesupport
ColumnD
Br ac e D6
24 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Fracture in Brace D6
Lamellar tear crack
Area of final
fracture
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25 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Crack initiation in D6 at support pipe
When the weld around thesupport pipe is uncracked, the
stress concentration factor at theweld is 1.6
Weld intact: SCF= 1.6 Weld fractured: SCF = 3.0
When the weld around the
support pipe is cracked, the
stress concentration factor at theweld is 3.0, i.e. stress is almost
doubled
Fatigue crack
Fatigue crack
26 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Lamellar tear cracking
D6
Support pipe
Lamellar tear crack
Small penetration
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Crack initiation in D6 at support pipe
Striations
• Crack growth
direction
10 m
10 mm
Beachmarks
•Beach marks are lines visible tothe naked eye, indicating changes
in loading or corrosion conditions.
•Striations indicate start-stoppositions of the crack tip.
•The presence of beach marks and
striations proves that fatigue
caused the fracture.
•
Crackinitiation
BeachmarksBeachmarks
28 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Materials
Structural steel used in platform:Standard C-Mn steel with C = 0.18% max,
YS = 350 MPa, UTS = 512 MPa, ductil ity 30%Steel in support pipe:Standard C-Mn steel with C = 0.18% max,
YS = 355 MPa, UTS = 500 MPa, ducti lity 4.8 % in thicknessdirection
Microstructure of support pipe:Fine grain banded ferrite and pearlite, indicating low strength in thicknessdirection
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29 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Stresses in brace D6
Max nominal st resses in brace D6 at accident:141 to 173 MPa or 40 to 50% of yield st ress, giving very high localstresses at the hydrophone support pipe
Miner-Palmgren summation:- Using the F2 design curve a life of 0.7 to 5 years was
calculated, assuming various uncertainties in loadspectrum
Fatigue life predictions, brace D6
30 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Conclusions, ALK accident
Main causes
1. Design fault 1: Lack of redundancy, i.e. all braces attached to
column D failed by overloading when brace D6 fractured
5. Poor fabrication: Too small penetration in weld joining support
pipe to brace D6
3. Poor materials: Low strength in thickness direction inhydrophone tube gave lamellar tearing, which in turn increased
local stresses in brace D6 at weld
2. Design fault 2: Too high operating stresses in D6;platform not designed against fatigue
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31 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
Summary
Fatigue of unwelded componentsFatigue strength is closely related to base
materials strength for parts with smooth
surfaces, but corrosion and surface damage
gives large reduction in fatigue strength
Fatigue strength depends on mean stress
32 Utmatting - grunnlag Oslo, 20 oktober 2009 P J Haagensen
• Fatigue strength is independent of base
material strength
• Fatigue strength is independent of applied
mean stress
• Fatigue strength is strongly reduced by
corrosion
SummaryFatigue of welded components