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

[email protected]

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


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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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


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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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


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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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


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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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


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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Example 1: Failure in mechnical components

NSB train axle failures

NSB Signature Trainaxle fracturesSummer 2002

Crackinitiation

Beachmarks

Finalfracture



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


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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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:


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


ALK s tructural arrangement

Pentagone design

1st fracture

D

D


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Brace D6 and hydrophone support tube

Hydrophonesupport

ColumnD

Br ac e D6


Fracture in Brace D6

Lamellar tear crack

Area of final

fracture


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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


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


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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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


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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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


• 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

fatigue history.pdf

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