Topics in Ship Structures - SNU OPEN COURSEWARE LCF for... · 2018. 4. 16. · Tank Arrangement 4 DNV 30.7 Appendix I Low Cycle Fatigue Internal Turret Ballast Tanks Cargo Tanks

OPen INteractive Structural Lab

Topics in Ship Structures

04 Low Cycle Fatigue for Welded Joint – Design Rules

Reference : DNV30.7

2017. 9

by Jang, Beom Seon


Strain-Life Approach

Fundamentals of Metal Fatigue Analysis

A

PS

Nominal Stress range

∆S

Strain-life Curve

True Stress range ∆

True Strain range ∆ε

Nf : fatigue life

Applied Load P

Neuber’s rule

Cyclic Stress-Strain Curve

2


Stress-Life Approach

DNV 30.7 Appendix I Low Cycle Fatigue

Linear elastic stress

by FEA ∆elastic(=Kt∆S)

Stress-life Curve Stress redistribution

factor

Pseudo Stress range

∆pseudo(E∆εhs)

Nf : fatigue life

Neuber’s rule

True Stress range ∆hs

True Strain range ∆εhs

Cyclic Stress-Strain Curve

∆elastic

from FEA

∆hs

∆pseudo

∆εhs∆εelasticfrom FEA

3


Tank Arrangement 4


Internal

Turret

Ballast Tanks

Cargo Tanks


Midship Part


Deck

Bottom

Long bhd

2.5

Long BHD

Hopper

Long stiffener

Side shell

Long bhd stiff

Inner skin

Stringer

Deck trans Web

Long stiffener

Web plate

inner bottom

Web plate

5


Midship Part 6


Transverse

Bulkheads

Longitudinal

Bulkheads

Web sections


Midship Part 7


Transverse

BulkheadsLongitudinal

Bulkheads


Horizontal Stringers 8


Transverse

Bulkheads

Stringers


Midship Part 9


Stringers

Transverse

Bulkhead

Web section


Web Sections 10


Web Section Ordinary Section

Balla

st Tank

Balla

st Tank

Carg

o T

ank

Carg

o T

ank

Carg

o T

ank

Carg

o T

ank


Critical locations for low cycle fatigue

Heel and toe of horizontal stringer of transverse

bulkhead


Trans Bulkhead

Stringer

Stringer

Toe

Stringer Heel

11




Full load condition

Ballast condition

Alternate 2

Alternate 1

12




13



Web stiffener on top of inner bottom longitudinal.


LCF LCF

HCFHCF

Transverse Web

Longitudinal

stiffener

Inner BottomTypical Elevation

14




Full load condition

Ballast condition

LCF LCF

HCFHCF

Transverse Web

Longitudinal

stiffener

Inner Bottom

15



Inner bottom connection to transverse bulkhead


16


General


17


Load conditions for assessment

Low cycle fatigue is mainly due to mainly loading and unloading of

cargoes and ballast. Quasi-static load.

(HCF is caused by Wave loading nHCF=108, number of waves

encountered during 20 years, average period = 7 sec)

The number of design cycles may vary depending on the ship in

operation


Ship Type Recommended Cycle, nLCF

Tankers over 120,000 TDW 500

Tankers below 120,000 TDW 600

Chemical tankers 1,000

LNG carriers 800

LPG carriers 800

Shuttle tankers 1,200

18


Hot spot stress range for low cycle fatigue

Static elastic hot spot stress range for load combination

k for low cycle fatigue


19


Combined hot spot stress range

A peak to peak stress

= static stress induced by static hull girder and static

pressure loads during loading and unloading

+ elastic dynamic hot spot stress amplitude at 10-4

probability level due to wave actions


k

comb

20


General


21


Plasticity correction factor (Ke)

Method 1 : from 1) Cyclic stress-strain curve and 2) Neuber's rules


'/1

'

2 n

hshs

hshshs

KE

'/1

'

n

hshshs

KE

'/1

'

222 n

hshs

hsn

KEE

K

SeK t

2

E

KK n

nnelasticelastichshs

22

2

constant elasticelastichshs

elastic

=Kn

hs

pseudo

εhsεelastic

=K

n(S)

n(e)

①

②

③

'/1

'

n

hshshs

KE

elastic

pseudo

eK

hspseudo E

σhs can be obtained from an iterative way.

22

①

②

③


Plasticity correction factor

K(≈Kt) = theoretical stress concentration factor

σhs, εhs = actual hot spot stress and strain in hot spot

σelastic, εelastic= linear elastic hot spot stress and strain by FEA

σn (=S) = nominal stress

E = Young’s modulus

n', K' = material coefficients

σpseudo = Pseudo linear elastic hot spot stress=Ehs


23



Method 2 : from an empirical formula

Plasticity correction factor is proportional to Δσcomp. If Δσcomp is

below 2.0 σY, the factor =1 since it remains within elastic limit.


0.2for 10

0.1max

0.2for 1.0

factor correction plasticity : K

Y

comb

3

comb

Y

comb

e

ba stress yield : Y

Stress range ∆𝝈𝒄𝒐𝒎𝒃

𝝈𝒇>2.0

Mild steel a = 1.16

b = 0.524

NV-32 and NV-36

steels

a = 1.0

b = 0.53

24



Method 3 : a non-linear finite element analysis

εhs can be directly obtained form a non-linear FE analysis.

Cyclic stress-strain curve should be provided to non-linear FE

analysis.

σelastic can be linear elastic hot spot stress and strain by FEA.


elastic

pseudo

eK

hspseudo E

25


General


26


Effective pseudo stress range

Factor due to stress redistribution.

Stress redistribution factor is applied when Δσcomp goes beyond

elastic limit.

Effective pseudo stress range


eK factor correctionlinearity -non :

0.2 ifsteelNV36orNV32for 8.0

0.2 ifsteelmildfor .90

0.2 if 1.0

tionredistribu stress todueFactor :

Y

comb

Y

comb

Y

comb

k

comb

k

eff

NV : High tensile steel. NV32 : yield stress =315MPa NV32 : yield stress = 355 MPa

27


General


28


Fatigue damage calculations for LCF

One-slope S-N curve for low cycle fatigue strength

Nk : number of cycles to failure for low cycle fatigue stress range

σkeff : effective stress range for the k-th load combination

This design curve is applicable to both welded joints and base metal

for LCF region.


k

effk LogmaN loglog

Welded joint with cathodic protection

29


Fatigue damage calculations for LCF

The damage due to low cycle fatigue is calculated as

follows,

nLC : total number of design load condition

Lk : fraction of load combinations


k

LCF

n

k

k

LCF

n

kLCFN

nLDLD

LCLC

11

30


Low Cycle Fatigue Factors

Thickness effect : not accounted

Mean stress effect : not accounted

Environmental reduction factor : not considered

Weld Improvement : Benefit of weld improvement methods like

grinding, hammer peening and TIG-dressing should not be applied

Fabrication tolerance : are assumed applicable.


31


General


32


Combined fatigue damage due to HCF and LCF

Combined damage ratio due to high cycle fatigue and low cycle

fatigue shall be satisfied when DLCF ≥ 0.25.

For low cycle fatigue damage below

0.25, fatigue damage due to HCF

shall be satisfied.

DHCF≤1 for DLCF ≤0.25


DHCF = damage due to high cycle fatigue based on design life

DLCF = damage due to low cycle fatigue based on the design cycles

The combined fatigue criteria

0.125.00.1)75.0

25.0( 22

LCF

HCFHCFf Dfor

DDD

33


Example


Location to be checked

Hot spot stress components at HS1, N/mm2

Combined stress range for low cycle fatigue strength assessment, N/mm2

34


Example


Low cycle fatigue strength assessment

0.2for 10

0.1max

0.2for 1.0

factor correction plasticity : K

Y

comb

3

comb

Y

comb

e

ba

35

NV-32 and

NV-36 steels

a = 1.0

b = 0.53

0.2 ifsteelNV36orNV32for 8.0

0.2 ifsteelmildfor .90

0.2 if 1.0

tionredistribu stress todueFactor :

Y

comb

Y

comb

Y

comb

Documents

Topics in Ship Structures - SNU OPEN COURSEWARE LCF for... · 2018. 4. 16. · Tank Arrangement 4 DNV 30.7 Appendix I Low Cycle Fatigue Internal Turret Ballast Tanks Cargo Tanks