高力ボルトの遅れ破:壊に関する実験および解析的研究

NII-Electronic Library Service

[ntUDC:Nl624.014.2:621.791.019

_JournalH"eee\ftMisthktsAWEM ee430e ･1991_ny12A

ef Struct,Censtr.Engng, AIJ,No.430,Dec.,1991 .

EXPERIMENTAL

AND

ANALYTICAL

STUDY

OF

DELAYED

FRACTURE

IN

HIGH

STRENGTH

BOLTS

fi-

7]

;S"JV

F

cD

ff

ti.ueee

_C:

ee

-d-

₆

scva

_Zi

_.k

_U

_neMRYMYE

Hdegui

SHIMOMURA"

and

Ftt

tt

g,

es7'kekeshi

SLUNODA**

an

mall

Dela.yed

iragture

testswith cathodically _pre-charged specimens were _performed measuring acoustic _emission

{AE).

Experimental_{parameters were ratio of axial} force_{to designbolt axial} force

, cathodicplly hydrogen charged time and effect of heattreatment.It was observed that time

tofracturewas somewhat affected bycharged time and axial forceratio.

Analytical_{process of} diffusiblehydrogen around a thread root and a tip of notch was compared with experimental one by AE signal and branching_phenomenon near crack tib,and resulted ina

good agreement. '

Keymonts :deLayedfinctare,high-streugthbolt,

_lyirzlgen

induced crack,

finite

element methadl

sion

menueve,

rti"rt

£JV _F,

rk#thec:iJh,

_awaerMtz,

_tutw

1.

Introduction

The presenttrend

for

tallerbuildingsand _greaterspans hasledto590

MPa

tensile_{strength class or} higherclasses of

high

strength steels

being

considered

for

use

in

building

construction.

Unlike

inthe case of mild steel, a shortage of the ability of a stress re-distribution

due

toa

higher

_yieldratio won't show thesufficient

load-carrying

capacity of

frames

after reaching mechanism, Horizontal

load-carrying

capacity of

frames

depends

much on cennections such as beam tocolumn eonnection or

bracings

which are

jointed

by

welding or

bolt,

Inthecase of

bolted

joints

boltholesreduce the sectional area of a base metal so that a stress concentration becomes

higher

around

bolt

holes,

and consequently _{the yielding} aRd

deterioration

occur much earlier than they are expected _to.

The lossofsectional area of basemetal

due

_to

bolt

holesmust

be

smaller as

far

as _{possibletopromote} a souRd stress re-distribution around boltholes,and itisnecessary that_the strength of a

high-strength

bolt

must become

higher

toreduce the lossof section, and also toinsurethe

load-carrying

_{capacity and}

deformability

_.of

bolted

_joints.

But

strengthening of high strength

bolts

has

much _{possibilityof a}

delayed

fTacture

as reported.

Many

studies about

hydrogen

inducedcracking

(HIC)

in

high

strength weld metais or

high

_tension steels _{under stress} have

been

widely reported

[3-10]

.Mathematical efforts considering _the

distribution

of stress

field

were

done

toexplain the _phenomenon of

hydrogen

embrittlement ahead of cracks where stress concentrates

[2].

A

delayed

failure

of the

high

tension

bolts

has

been

reported since they

had

been

widely

introduced

into

steel structures and

bridges.

Most of these failures

have

beenthought to

be

induced by

diffusible

hydrogen

because

these failuresoften occurred under theenvironmental circumstances seaside or industrialareas that_promote corrosion and

hydrogen

charging

[11]

.

Although

many studies about

HIC

* _{kM]t#AggeH4VK enptff･Ire}

*' _{zthes)t[\z\gmarv7ou t xl\IwmemJIff}

Assoc.Prof., _Dept.ofArchitectuTe GifuNational Col-legeof Technology, Dr.Eng.

Lecturer, _{Faculty of EngineeringNagoya Univ.,}

Dr.Eng.

-Architectural Institute of Japan

ArchitecturalInstitute of Japan

have

been

performed,

behavior

of

diffusible

hydrogen inhightension boltswhich

have

a geometrical

notch, i.e.,a root of thread,

has

not _yet

been

selved satisfactorily

due

tothe cemplexity _{of many} problems about

HIC.

The analytical solution needs to acceunt fornot only 'thegradientof

hydrogen

concentration

but

also the stress

distribution

inorder tosolve the

diffusional

_processof

hydrogen

in

high

tension boltsunder stress.

Nomenclature

D:hydrogen

diffusivity

a:hydrostatic component of stress tensor or applied stress on the axial section

a. : standard

bolt

stress of F13T

bolt

J

:hydrogen

flux

r:activity; chemical potentialinthe standard state

C

:

hydrogen

concentration

T

:absolute temperature ' '

R:gas

constant.. .. , , ' '

t:tlme

.

VH

:specific molar volume of hydrogen atoms

2.

Mathematical

Development and

Discretization

DifiusiveProcess of

Hydrogen

Dependent

on Stress

Gradinet

and DiscretizationtorNumeric

AnalysisFiniteElement Method

(FEM)

Consider

a cubic element

dy

in

arbitrary

location

(x,

_y,z) as shown inFig.1 and time t

in

some

body

;hydrogen concentration can

be

defined

as

C

(x,

y,_z,

t).

Volume of

hydrogen

accumulated in

its

cubic element can

be

expressed

by

Fourier's

law

and thevariation of concentration with time

t

can

be

wntten as, .

O,c,

-ig}

(D.･

gg

)+

,O,

(D,

eis)+

,a.

(D.

gg

)-Dvzc

････-･････+--･-･･･----･-･･-･-････-･･･

(

i

)

In

the case of

diffusion

of

hyclroge'n

atoms in steel subjected tostress, from Fick's

first

law,

the ,flow .t

vectorJ of the atoms

diffusing

through unit area _perunit time at any point

in

the material can

be

expressed as follows

J=-Dc

(

VcC + V77-

_RVHT

va)･-･･---･---･････---･･-･･-･-･--･-･･---

(

2

)

Also,

from

Fick's

second

law

'

ac

=,-VJ

at

;Dv'c+v

(e

cv

r)-

DRVTH

(v

cv

a+ _{cv!a)･---･--･-･---･}

(

3

)

Assuming

the 'activity₇

is

constant allover the object and thestress isa single-valued and continu.ous

function

under analysis, V _7, V2o and inthe second and thirdterms of equation

<

3

)

respectively can

be

considered to

be

zero so that the variation of

hydrogen

concentration with time can thus

be

independentof the activity _r and expressed as a

function

of thehydrogen concentration and the stress gradient,

z

x

Y

dx

''

Fig.1 Hydrogen fluxincubic element

-60-Let us next consider the application of numerical analysis tothisresult using FEM, The object to

be

analyzed is

divided

into

finite,

triangular elements, and, using the nodal concentration _¢, thehydrogen

concentration C within an element at any time t

is

NII-Electronic Library Service

C(x,

y,2,

t)=[N(x,

y,z)･_¢

(t)]

---･･･-･-･････--･･--･-･-･･･････-･---･･･---･-･･･-･-･-･----

₍

₄

₎

Here,

[N]

is

the

interpolation

function

matrix.

When

[N]

itselfis

_{taken as the weighting}

_function,

the

following

equation

is

obtained using

Galerkin's

method which is one of weighted residual methods _:

J[1

[iv]'I-

gf

+Dv2c-

DRVTH

vcval

dv=o---･---･---･

₍

s

₎

where

Vle

expresses the elemental volume.

'

Using

Green's

theorem and some

boundary

conditions, considered _asa two-dimensional_problem, equation

(5)

_yields for_{a single triangular element:}

[k]iipl+[m](ge1-ifl･･J･･･-･･･-････----･･--･･-･----"."..H..".-"".""."..,..,."H..."....(6)

where _'

[k]=[kd]+[hs]+[kc]

[hd]

:

hydrogen

diffusion

matrix of an element

[ks]

:stress

dependent

hydrogen

conductivity matrix

[hc]

:concentration

dependent

hydrogen _{conductivity matrix}

[m]:hydrogen

capacity matrix

lfl:hydrogen

flux

vector

For the entire _object under analysis,

[K]

le(t)i+[M]

I

aadi

(tt)

l-iFl

･---･---･---･---･･-･･-･･"-.k."".,...,.H...-.".,,..,

(

7

)

Using

this equation, the object under analysis can

be

expressed

in

a

ferrn

of

discretization

_of 2-dimensionalspace regions. And

discretization

_{of time regions also}

_becomes

possiblewith _this

diffusion

equation

by

employing

Crank-Nicolson's

finite

difference

_{method. Therefore,using these} two

discrete

techniques,

it

becomes

_{possibleto solve the ttnsteady state}

problem involvinghydrogen

diffusion

insteel of a _{given geometric} shape.

3.

Experiments･

3.1

Delayed

Fracture Test Specimen

F

10

T

high-strength

bolt

(F

10T

bolt)

is

widely used

in

steel structures and

bridges

as a

friction

bolt.

F13T

high-strength

bolt

_(F13T

bolt),

which is higher-strength

bolt

_than F10T

bolt

_{and whose} standard tensilestrength isover 1275

MPa,

had

been

_used

for

_{a while,}

_but

_now

prohibit.edby

Japan

IndustrialStandard

because

of the occurrence of delayed

fracture

as ithad

been

reported

[

8

]

_.An _actual

F

10

T

bolt

was only quenchecl and tempered in usual manner _toget

F

13

T

bolt.

Its

features

_{are that}

Boron

is

contained toimprove the

hardenability

and thread isrolled

hot,

Its

chemical composition and mechanical _properties are shown

in

Table

1.

The

mechanical saw notch, whose width and depthwas _O.2and 1.0mm respectively, was inserted_to promote the fractureat the imperfectroot of the _thread where screw cutting starts and also a stress concentration

is

higher

_{than the axial}

part. 3.2

Pre-charging

Condition

These specimens were _pre-chargedcathodically in2

%

H2SO,

_electrolyte

fer

_{1or 2}

_hours

with about o.5A.

_IcmZ

current

density

at room temperature,They _{were shielded with a sealing material to}

_be

charged

locally

_{around the notch and}

imperfect

_{root of a thread.}

The charging system isillustratedinFig,2. While charging,

bubbles

_generatedactively on both '

Table1 Chemicai composition and mechanical propertiesof F13T high-strengthbolt steel

che"iealCemposttien(WTX) ･ He6hanieal Preperties _(Hpa) '

'

C Si Mn P S Cu Ni Cr _{B Yieldstrength Tensile Strensth}

O.su O,22 1,2T O.024 O.OleO,Ol O,02 0.03 0,OOII F13TBolt 12sa.5 1363.1

..

'

-61-Architectural Institute of Japan ArchitecturalInstitute of Japan preampLifier AE-912 et) 9502U-PLot

ng

E :E

Fig.2 Schematicdiagram ef the hydrogen Fig.3 Diagrarnof the loadingsystem with AE

charging $et-up measunng equipment

surfaces of electrodes,

but

direct

current was

kept

constant by a regulated power supply.

3.3 Loading and Measuring

System

of

Acoustic

Emission ,

Pre-chargedspecimens were set up intothe

loading

system as soon as _possibletoprevent

diffusible

hydrogen

from

being

dismissedintothe atmosphere. The

loading

system consists of a

50

tonoil

jack,

reaction

beams

and equipments of acoustic ernission

(AE)

system 'asitisillustrated

in

Fig.3.

AE sensdr' was fixed

by

spTing at thissupporting

jig,

AE signal was measured through this

jig

indirectly,and thenstored inthe micro-computer at regular intervals.Specimens were tightened and stressed after

hydrogen

charging with each aimed constant

load

less

than astandard

design

stress

level

'

that isabout 65

%

of each yieldstrength,

3.4

Process

tillFailureand

Test

Results

The

initiation

and _propagation of a crack was indirectlyobserved through

frequency

of

AE

events, AE sensor was

kept

on the same _place,and

AE

signals over

90

deci

Belwere counted until the specimen was

fTactured

or

AE

signals were not observed any more.

When

stress

level

was

higher,

AE signal occurred as soon as

it

was tightened, and was continuously counted until

fracture.

On

the other

hand,

when short charged or

lower

stressed,

AE

events didnitoccur soon and they were obseTved intermittently

dntil

fracture.

Figure

4 shows examples of series of deciBel, lapsetime after loading,and

AE

events.

Specimen

No.

13-16

and No.13-18 were

in

the same stress

level

with pre-charged

2

and 1hours respectively.

Specimen

No,13-16, long-chargedspecimen, showed continuous and

intense

AE events

from

the

beginning,

and then

fractured

much earlier than the short-charged specimen No.13-18,

Specimen

Ne.

13-19was, at

first,

tightened

in

such a

low

stress level

(50

%

)

thatany AE event

didn't

occur

for

about 2hours.

After

stressed up to62.5

%

level,AE event then appeared intenselyand continuously

for

about lo

hours,

and after all this

bplt

fractured

inabout 17 hours.

Testresults were _plotted

in

Fig.5.

Delayed

fractureoccurred only in

F

13

T

belts.

This

figure

shows the relationship between applied stress leveland time to

fracture.

This

stress levelisnormalized by standard design

bolt

stress demanded forthisF 13

T

bolt.

Most of specimens were

fractured

even inthe case of a 6o

%

stress levelagainst thestandard

design

stress

level.

Inthis testprogram fracturewas promoted by notch and hydrogen charging so that testspecimens which took over about 12o hours to

fracture

were intentionally

finished

loadingeven

if

therehad some possibilitYof

fracture

inholdingthe

load.

These

unloaded

bolts

are _plottedwith arrow mark inthis same

figure.

Itcan be seen

from

this

figure

thatthereseems to havesome tendeney that the time to fractureis

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× `i`' chargedl hr. e. charged 2hrs. 1 10 100 hour 90

Fracture Time after Leadlng

4 e e2 i6 2o heur _{Fig.5 Fractttretime against stress level:stressis}

Fig.4 Recordsof AE event until

fracture

normalized by standard designboltstress

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Generalview of fracturesurface and SEM observation :_photo A shows intergranularand transgranular brittlefracture, and quasi-cleavage or ductilefracture can be seen inphoto B and C

somewhat related to

both

charging time and applied stress.

These

facts

reveal _{that the quantity} of a

diffusible

hydrogen

is

in

proportionto charging time and also this charged hydrogen is_{effective and} influential

in

assisting the initiationand _propagationof crack inthishigh-strength

bolt

whose strength

is

over 1300

MPa.

On

the contrary,

both

notched F13T specimen without hydrogen charging and notched

F10T

specimen with 2

hours

hydrogen charging were also triedunder standard

design

bolt

stress

Ievel,

but

neither

bolt

showed any AE events while

loading.

These

results show that

F13T

bolts

used

in

this experiment

have

sufficient resistant ability

for

crack initiationifitwere not

for

any _assistance ef

hydrogen,

and also show thatductility or lowcrack sensitivity, as F10

T

bolt

showed, could arrest the

initiationand _propagation of crack caused

by

the

hydrogen

embrittlement. 3.5

Fractography

General views of

fracture

surface and SEM photos are shown

in

Fig.

6.These figures

indicate

that cracks

initiated

at the mechanical saw notch ineach specimen and _propagatedina

brittle

manner in a plane strain field,and that

intergranular

and tTansgranular

brittle

fracture

can be seen ahead of the

notch. Getting apart fromthenotch root, _{quasi-cleavage}fracturealso can

be

observed accompanied with the _plasticdeformation.

View

of

Micro

section perpendicular to

fracture

surface around notch root, Fig.7,also shows that

fracture_{pathpropagates}along or across the_grain

boundary.

This

figure

also shows the branching of the crack. Itisconsidered that the crack mitiated ahead of notch

happened

to

branch

intwo

directions

-63-Architectural Institute of Japan

ArchitecturalInstitute ofJapan

Fig.7 Macro-sectionof

fracture

surface aroilnd notch root

where stTess

is

higher,

these cracks _propagated,

dismissing

hydrogen

into air, at the same rate

in

each

direction

for

awhile,･ and then only one of cracks ledtofracture.Thisfigure shows a typical

branching

phenomenon at the tipof saw notch.

Some

branchings

appeared soon likethis

figure,

and others

occurred after _propagating

in

thesame

direction

of notch. But mest of all specimens, teok a longertime to

fracture

or not

led

,to

fracture,

indicated

these

branching

_phenomena.

4. AnalyticalResults

Numerical analysis of the unsteady state _processof

diffusible

hydrogen inmetal was executed with FEM

based

on the

development

of

Fick's

laws

and takingaccount of the stress

field.

Inthisanalysis a

bolt

could

be

treated as a two-dimensional problem that isa planeplate with a unit thickness

(1

cm).

The

proportionof width tothickness inthe analytical'section

{,s

almost thesame so thatthe analytical results solved

by

FEM

as a _planestrain _problemwould _give alrnost same results as obtained

in

an actual bolt.Based on thismechanical assumption, and･considered a restraint

by

the nut,

division

around

thread intotriangular elements

is

shown

in

Fig.8.

It may

be

natural that

hydrogen

diffusivity

is･

affected

by

the micro structure.

In

the case of this

high-strength

bolt,

the micro structure and _grain size LZ!'1[!-,M is changing, showing

layers,

from surface to center

due

to manufacturing process

like

a hot rolling and

:: r･.t.."

Mk

ttttttttir'.J

Fig.8 Divisionof a bolt-nutunder stress into

triangular elements:shaded area in

sion isused foiillustrationef diffusional

proeess of hydrogen in the next figure - ₆₄

---heat

treatment.

But

these layersare very thin and

graduallychanged so thatitcan be considered that

hydrogen

diffusivity

is

uniform under analytical object.

Assuming a standard design

bolt

stress and an

initial

permeation 'of

hydrogen

by

this hydrogen charging system, equivalent nodal

loads

and nodal hydrogen concentration were introduced.

After

elernental stresses were solved under these

con-ditions, matrix

[ks]

could

be

obtained as

it･is

done

in matrix

[kd]

or

[hc],

assuming thatthe stress･gradient

between

elements could

be

expressed as nodal stress. This nodal stress is

defined

by taking ayerage of

NII-Electronic Library Service

1.0

_O.O

N

pt

ee

ee

D

NormaEized

Density

vdi92vL=e veots=UeL=N :nitiatState t=20 t:-li.,.,.ll･1, t;50 t=300 t=500 min tldi//,?･･"'･ 'fi'x'-::',,tt',di･:'Iil'?,,･s EmE'

il.'

･lliilllilllilll'11111111111111111;,,il,.-.･.,. ･'1111/li,1$!,

_;.f

t.

Fig.9 Analyticaldiffusionalprocess of hydrogen near imperfectroot of thread with notch

elemental stresses

in

each element where the node isheldincommon.

The

initial

stage of hydrogen diffusional_process, i.e.',initial_permeation, was illustratedinthe

initial

State

of Fig.9,

Values

for

computational analysis used hereare _;

hydrogen

diffusivity

D=4

_×10"S

(cm21sec),

_VH=2

(cmSlmol),

absolute _temperature T==300

(K),

_gasconstant

_R'=1.98

(callmol

K)

_and

7isneglected.

The process of

hydrogen

diffusien

from

initial

stage onwards isshown inFig.9.Shading _represents

hydrogen

concentration

level

which isnormalized

by

the initialone charged

2

hours.

With _passageof

time, those figures

indicate

thatthese _processes

have

a clear tendency

for

diffusive

hydrogen

_togather

toward thetipof notch atthe imperfect root of thread where the stress concentration ishigherthan other '

'

' reglons.

5.

Comparison

of Experiment _and

FE

_Analysis

It

was observed that

AE

events occurred more frequentlyin

long

charged specimens, This observation isin_good correspondence with the

hydrogen

accumulation

b6havior

_predicted

by

_FE analysis. In other words, when

hydrogen

content is

low

by short charging, _quantityof

hydrogen

dismissing

intothe air

is

considered tobeso much thathydrogen content

becomes

relatively

lower,

and results in taking long_time

for

diffusible

hydTogen

_{toreach around the}

higher

_{stressed region.}

_This

may

be

one _{of reasons} why the

low

charged specirnen needed the time lag to occur AE signal.

It

can

be

_{considered thatphenomenon of}

_branching

_{isclo'sely related to}

_local

_hydrogen

_{concentration} near crack tip. This assumption isdtte_totheanalytical results indicated

in

Fig.

9,where

diffusible

hydrogen

after rather

long

lapse

time accumulates not

just

in

front

of notch

but

inboth slant

fields

against the directionof notch.

The

direction

of _propagationof crack could

be

determined

not only

by

the stressed field

but

also

by

thedistributionof

hydrogen

because

initiation

of crack occurs much easily

in

the regien of

hydrogen

embrittlement, which

is

caused

by

supplying the

diffusible

hydrogen

foTm

hydrogen

rich regions.

In

this_point,analytical result explains well this

branching

_{phenomenon. After} area of

free

surface duetobranchingwould nearly

double,

dismissing

of hydrogen

into

the air would

be

promoted,

hydrogen

content would

be

lower,

and consequently itwould take a

longer

time for

belts

to

be

fractured.

It

can

be

considered that the tim6 to

fracture

must

be

affected bythisincreasing rate of

free

surface, and thatwhether therewas branchingor not isthereason why thetestresults were scattered in

Fig.5

The

above instances_prove these analytical results _to

be

effective inexplaining the experimental

-65-Architectural Institute of Japan

ArchitecturalInstitute of Japan

ones.

6.

Conclusion

The

hydrogen

induced

fracture.in

high-strength

bolts

under stress was inyestigatedexperimentally, and also thediffusional_process of

hydrogen

in

high-strength

bolt

was analyzed

byiFEM.

This

FE

analysis,

howevgr,

can't simulate the

behavior

of crack propagationprocesscaused

by

hydrogen

embrittlement

because

of the

fixed

division

of

finite

elements.

But

the experirnental and analytical results support the

following

conclusions :

(a)

Cathodically

charged

F13T

high strength

bolts

showed the _phenomenon of

delayed

fracture

induced

by

hydrogen,

and thisfractureis somewhat affected

by

hydrogen charged time and stressed '

level.

(b)

Crack

sensitivity

depends

much on hardenability

due

to

heat

treatment. Sufficient

ductility

and

low

crack sensitivity could resist the

initiation

of crack under

hydrogen

attacking.

(c)

Mathematical

development

based

on Fick's

laws

taking account of stress _gradientand

its

discretization

for

numeric computational analysis were done. Hydrogen was

diffused

and accumulated

in

the slant

fields

against'the

direction

ol themechanical saw notch where stress intensityishigh, and its_general

behavior

seems to agree with experimental results inview of

branching

phenomenon.

Acknowledgments, ,

This

fesearchwas

funded

by

a

Grant-in-Aid

for

Developmental

Scientific

Research

from

theMinist'ry of

Education

(1989-1991,

Head investigatorH. Shimomura). The authors. are very gratefultoDr. B.

Kato

_<emeritus

_prof.of Univ. of

Tokyo),

T.

Haze

(S-S

engineering

Co.

_, Ltd.

),

Y. Sugiharaand M.

Kaga

_(Takigami

Seiki

Steel

Cons'truction

Co.

, Ltd.

)

for

their

helpful

cooperation, many valtiable

'

discussions

and supplying the materials.

These

supports are _gratefullyacknowledged.

./. . .t

Referenc6's

1> O.C. Zienkiekricz:"The _FiniteElement Method

in

EllgineeTing

Science",McGRAW-HiLL, London (lg71)

2) H,J.Maier,W. Peppand H.Kaesche:"A MethodtoEvaluatetheCriticalHydrogenConcentTationfoTHydrogen-Induced

CrackPiopagation",ActaMetal.,Vol.35, No.4, 1987

3) W.F. Savageet al. :"HydrogenInducedCold CrackinginHY-80 SteelWeldments",WeldingJeuTnal.Vol,55 (lg76),

368s-376 _,s. 4) N, Y,urioka:`tStudy

en diffusionand accurnu]ati6n oi hydrogeninsteel and itsapplication forcolclcracking inweld metal" Degree thesisinKyotoUmiv. (March1981,inJapanese)

5) T. Araki:"Study on hydrogenembrittlement in steel" DegreethesisinOsakaUniy, (Aug.1969,m Japanese)

6) C.E.PriceandR.G. Norman :"ACornparison ofHydtogen andMercuryErnbrittlernent inAISI4142Steel",ActaMetall. '

VoL35. No,7,pp,1639-164& 1987 ' '

7) M, Arpaia,P. Perniceand A, Constantini:`'A method toevaluate hydrogenembrittlement susceptibility of high-strength

steel wiTes" Journalof MaterialsScienceLetters,9.(1990)pp.268-269 .

g) K. Hoshino, R. Yamashita,T. Shingdaand H. Shimomura:"Study on hydrogenInduced,CrackPropagationinHigh

'

StrengthSteelWelds",HYDROGEN and MATERIALS, 4thInternationalConfeienceProceedmgs,pp.224-231, 1988.

Beijing

9) H. ShgmomuTa, T.Shineda,K. Hoshino andR. Yamashita:Studyon Delayed CiackInducedbyHydrogeninHighStrength

. SteelWelds", _JouTnal of ArchitecturalInstitute of Japan,Voi.4eO{1989)No.6 pp.85-89,

(in

Japanbse) 10) H. Shimomura, T,Shinoda,K.Hoshino andR. Yamashita:"Apalytical Studyon HydrogenInduce.dCrackinginHT-Steel

Welds", Transactionef the JapanWelding Society,Vol.21. No,2,pp.44-50, Oet,+ l99e ,

11> _JSSCworking _group foidelayedfailureofhigh-strength bolt:`LAtmospheric Weathering Test of High-StrengthBoltedJoints

'

(Continued

RepoTt1,Z,'3,4}",JSSCjournal,Vol.70971),Vol.9 _(1973),Vol.iO(1974),Vol.11

(1975)

(in

Japanese) 12) H, Shimomura, T.Shinedaand T.llaze:"Hydrogen _{InducedFracture}

ofHighTEnsiofiBolts",9thIntemationalConference on OffshereMechanics and ArcticEngineering,_proceedings Vol.3-B,pp.611-615,1990. Houston, USA

'

(Manuscriptreceived May 9, 1991;Paper Accepted December 21. 1991)

--NII-Electronic Library Service 和文 要 約 1．序 　構造物の_{高層 化} ・大スパ ン_化に_伴い _{， 建築}の_{分 野}で も SM　50_{ク ラ}ス を越え る高 性 能 鋼の使用 も 現在 検 討さ れ つ つ ある状 況にある。一般に強 度 が 高 くな ると 降 伏 比 が 高く な る傾 向が あ り_， 高 力ボル ト接 合によるボル ト穴 欠 損 部 周 辺で は応 力 集 中により早 期に降 伏し，降 伏 比に よっ て は応力の再 分 配が十 分 行わ れず，ボル ト接 合 部に 期 待さ れ る耐 力を一．卜分 発 揮す ること がで き ない こと が予 期さ れ る。 これ を防ぐ た めには，ボル ト穴による断 面 欠 損をで き る だ け少な くす ること が肝 要で，そのた めに は 被接 合 部 材の _{強 度}に合わ せ高 力ボル トの高 強 度 化が必 要 と な る。 し か し，そ れ は過 去に F13T ク ラス の高 力 ボ ル トを使用し た ほ ぼ全 部の構 造 物で遅れ破 壊を経 験し， その結 果 使 用が禁止 された経 緯が あり_， 多 大な調 査 研 究 が な さ れ たにもか か わ らず 満 足い く結 果は得ら れなかっ た。 そ れ は，高 力ボル ト は幾 何 学 的な切 欠 （ネジ部 ）に よ る 複雑な 応 力分 布と 応力集 中を 生 じ て い る こと， また 腐食環境に よ る拡散性水素の_侵入 と応 力にか か わ る拡 散 の メカニ ズムが明ら か に さ れ ていな かっ た事にも起因し て い る。し た がっ て，本 報 告 では．応 力勾配 を考慮 し た水 素の拡 散 ・集積過程を調 査す るこ と を目的と す る。 2．水 素の応 力に依存する拡散 過 程と FEM への適 用 　 鋼 中にお け る水素の_{拡 散 過 程}を次の ように定 式 化 す る。物体内の_任意な位置に図一1の_微小6面 体を考え る。 拡 散 性 水素の 入 出に よっ て微 小6面 体の体 積 dvに蓄積 され る_水素量 は_， Fourierの_法則を用いる と，微 小 時 間 dtで拡 散 係 数が等方性であ る と き濃 度の時 間変化は 2） 式で表せ得る。応力状 態下に ある鋼 中 を 水素原 子 が 拡 散 する場 合， 任 意の点に お い て単位時 間に単位面積を通過 する拡 散 原 子の流 量ベ ク トル 」は_，Fickの_第一法則か ら 3）式の ご と く表せ得る。また，Fickの第2法 則 か ら， 水 素 濃 度の 時 間 変 化を水 素 濃 度お よ び応 力の勾配で 4） 式の ご と く表 すこ とが で きる。次にt こ の結 果 をFE 解 析に適 用 する こと を考え る。対 象 を有 限な 三角 形一次 要 素に分 割 し，ある任 意な時 刻 tで要素内の濃度C を 節 点 濃 度 φで 5）式の よ うに定める。こ の _［胡 自 身 を 重み関 数とし て Galerkin法 を用い る と 6）式と なる。6 ） 式 を Green の定 理 を 用 い て展 開し，さ らに それは解 析 対 象 全 体で は7）式と な る。これ に よ り，空間 的に解 析 対 象 を 離 散 化し，またCrank−Nicolsonの差 分 式によ り 時 間 的にも離 散 化で き任 意 形 状に も対 応できる非 定 常問 題 を解くこと が可 能 となる。 3．遅れ破 壊の実 験 的 検 証 3．₁遅れ破壊試 験体 　実 際に使 用さ れ てい る F10T 高 力 ボル トを通常用い ら れ てい る熱 処 理のみによりF13T 高 力 ボル ト試 験体 を作 成した。こ のボル トの特 徴は_， 焼入性 向上のた め ボ ロ ンが添 加され て い る こと と熱 間 転 造され てい ることで ある。ま た，不 完 全 ネジ部には破 壊 を促 進 するた めに切 欠を挿入 し て い る。 3．2　 水 素チ ャージ法 　拡散性水素を 図一2に示す硫酸 溶液に よ る陰 極 チャー ジ 法に よ りボル トに導入 し た。 どの試 験 体 も 同一の電 流 密 度を保つ た め定 電 流 電 源に よ り制 御し た。 3．3　加 力お よ び AE 測 定 法 法 　図一3に_{加 力 装 置}お よ び AE 解析 装置を示す。 試 験 体 を水素チャージ後 速や か に加 力装 置に組み 込 み，所定の 軸 力 を導入 した。AE センサーを常に 同一箇所に設 置し， 治具を介して_計測した。 3．4　破 壊 まで の過 程 と実 験 結 果 　 加 力 装 置に組み込 まれ た試 験 体におけ る亀 裂の発 生と 伝播は_，AE _解析装置に よ り間 接 的に知る こと がで き た。 こ の AE の発 生 状 況 （図一4）は_， 導入軸 力が高い ほ ど， また，水 素 チャージ時 間の長い ほど加 力 直 後か ら活発に 発生し早く破 断する傾 向があっ た。 　破断までの時 間 と導 入 軸 力 比そ し て水素チャージ時間 との関 係 を 図一5に示す。こ の図か ら，破断に至る時間 が導入軸力と水素チャージ時 間に_深く関与して い ること が分か る。また， 水 素チャ ー_{ジ し てい ない F13T と 2} 時 間チ ャージし た FlOT とを載 荷 し た が，共に AE の 発生もな く破 断に は至ら な かっ た。こ の こと は，F13T で は こ の水 素 チャージによ り水 素 脆 化 を生じ亀裂の_{発 生} と伝 播 を促 したこと，および， FユOT では水 素の 存在 にもか か わ らず亀 裂が発 生せず， 割れ感受性が十 分 低い 事 を示し，同一素 材でも熱 処 理によ り遅れ破 壊特 性が異 な ること を示し た。 3．5 破 面 観 察 　 SEM に より破 断 面 を観 察 した。図一6か ら は， 切欠 近 傍で破 壊は粒 内 ・粒 界を脆性 的に進 行し たこと が 分 か り，内部に行く に し たがっ て，擬 脆 性 を呈 す る よ う に な り，さ らには延 性 破 面 を呈 するよ うに なっ て い る ことが 観 察さ れ る。 ま た，破 面に垂 直な断 面の マクロ写 真 （図 一_{7）か ら} ， 破 面 は 粒 界 ・粒 内 を 進 展 して い る 様子が 分 か り， さ らに亀裂先 端 部か ら亀裂が 分岐し ているこ と も 示して い る。こ の分 岐に よ り自 由表 面が大 幅に増 加す る こと とな る た め，拡 散 性 水 素の空 中へ の逸 散は顕 著に な

67

一 N工 工一Eleotronio _Library

Architectural Institute of Japan

Arohiteotural エnstitute 　of 　Japan

り，水素濃度が減 少し，そ の結 果， 破 壊 発 生までの時間 が 遅 れ るこ と が予 想さ れ，図一5で結 果 がバ _ラつ いてい たの もこ の_{分 岐}の発生 に関 係して いる と思わ れ るD 4．解析結果 　2．で展開し た結果を高力ボル トに適用 し解 析 し た結 果が図一9で，水素の拡 散状況 を時 間経過で示して いる。 時 間の経 過と と もに_{水 素}が亀裂先 端付近に_{集 積} して い る 様 子 が 分か る。チャ ージ時 間に よ り水素量 が少ない と外 部に拡 散す る 量の方が相対 的に多く な るの で，経 過と と もに減少の_度合い が_高く な ること を示し て お り，切 欠 近 傍に集 積す る た めには あ る一定量の_水素が 必要であ るこ と が 分 か る。 5．実験結果と解 析結 果と の比 較 　実 験 で，水素チャージの _少ない試 験体が活 発な AE の発生に_時間を要す るこ と，破 断まで に長い時 間を要 し てい る結果と が得ら れ た。 この事柄は，解析結果で，切 欠近傍ま でに水素が 拡 散 ・集積 す るの には 時間を要し， ま た その_{時 間}は初期濃 度に_依存しているこ と と対 応して いる。また，長 時 間経 過 後の水 素 集 積の状 態で，亀 裂の 先端よ り離れ た位置に集 積し た結果を得た。こ の領 域で は_{水 素脆化}する の で亀裂の_伝播がその方向に生じ や す く な る こ と が予想され る が_， 亀裂が分岐しその方 向へ と進 んでい ること が実 験で観 察さ れ た。 6．結　論 　 陰 極チャージ を 施 し た F13T 高 力 ボル トの遅れ破 壊 試 験と，応 力 勾 配 を考 慮した非 定 常 拡 散 式 をFE 解 析 に適 応し解 析し た結 果より次の事がいえる。 　 （a_{） 陰 極}チャージの Fl3T ボル トは遅れ破 壊 し，破 壊は水素チャージ時 間と導入軸 力に依 存す る。 　（b＞ 割れ 感受 性は熱処 理に よ る焼入性に依 存し，十 分な延 性が あ れ ば水 素が存 在していて も亀 裂の_{発 生}に抵 抗す る。 　 （c_） 応 力勾 配を考慮 し た式の展 開をし，有 限 要 素法 に適 応 を試み た結 果，応 力 集 中して いる部 位に拡 散 性 水 素は集 積 し，そ の解 析 結 果は実 験 結 果と対 応して い る。 謝 　 辞 　 本 研 究は文 部 省 科 学 研 究 費 （試 験 研 究 （B） （1）課 題 番号 01850134代表 下村 波基 ）の助成を受け た。 遂行に 当 た り，加 藤　勉 博 士 〔東 京 大 学 名 誉 教 授 ）には 貴 重 な 示唆を， ま た， 櫨　忠夫 氏 （SS エ ン ジニ ヤ リング ）， 杉原義 則氏，加賀　稔 氏 （瀧 上 精 機 工 業 〉に は多 大な援 助を得たこ と に 謝意を表す る。

68