FATIGUE FRACTURE ANALYSIS INMEDIUM CARBON STRUCTURAL STEEL

andAUSTENITIC STAINLESS STEEL BY X-RAY

FRACTOGRAPHYlNlS-mf —1

Mr. N.N. Rao and Dr. Azmi Rah matSchool of Materials and Mineral Resources Engineering

Universiti Sains MalaysiaPerak Branch Campus

Sen Iskandar, 31750 TronohPerak Darul Ridzuan, MALAYSIA

ABSTRACT

• i•HI

i l l

A:v.:: from the reidual stresses present in the bulk material, a growing fatigue crack may

de\ j'.^p its own stress field ahead of the crack tip which in turn could influence the crack

p:\.>:\;i::itio-< behaviour. A fracture surface analysis through measurement of the residual

stress of a failed component may provide some additional usefull information to that

obi.'.ir.e through conventional metallurgical and fracture mechanics investigations. This

me: hod of fracture surface analysis using x-ray diffraction technique is known as "X-ray

Frac.ography". Residual stress (ff) and the full width at half maximum (FWHM) of the

x-ray diffraction profile of any reflection are determined at different crack lengths on the

fracturs surface. These are then corelated to the fracture toughness parameters such as

fr.:.••'.:re toughness Kj(\ the maximum stress intensity factor K m a x and the stress

in vn -.ky factor range AK.

The j-resent investigation aims at detailed x-ray analysis of the fatigue fractured surfaces

ct" ::w compact tension specimens prepared from ferritic and austenitic stainless steels.

The fe.-ritie steel has been subjected to various heat treatments to obtain different

microstruaures and mechanical properties. The overall observations are analyzed

th:o'.i h fatigue (cumulative) damage and material science concepts.

ANALIS1S PERMUKAAN PATAH LESU DlDALAM KELULi STRUKTUR BERKARBON

RENDAHdan

KELULI NIRKARAT AUSTEN1T DENGANGAMBARAJAH PATAH SMAR-X

Oleh

En. N.N. Rao & Dr. Azmi RahmatPusat Pengajian Kejuruteraan Bahan dan Sumber Mineral

Universiti Sains MalaysiaKampus Cawangan Perak

"•*" Seri Iskandar, 31750 TronohPerak Darul Ridzuan, MALAYSIA

ABSTRAK

Selain dari tegasan-tegasan bakian^yang wujud di dalam suatu bahan pukaE,

retak ]esu mpngk-in m^nj^n^ mfri^n icgasannya sendiri di hujung retakan yang akan

mempengaruhi sifat-sifat perambatan retakan. Analisis permukaan patah melalui

pengukuran tegasan bakian suatu komponen yang gagal mungkinmeirjberikanketerangan

tambahan yang penting bagi yang diperolehi melalui penyelidikan metaiurgi secara lazim

d;in mekanik patah. Kaedah analisis permukaan patah yang menggunakan teknik

pembelauan sinar-x dikenali sebagai "Gambarajah patah sinar-x". Tegasan bakian fcY)

dan lebar penuh pada separuh maksimum (FWHM) susuk pembelauan sinar-x sebarang

pcmantulan ditentukan pada panjang retakan yang berlainan pada permukaan patah.

Keputusan ini seterusnya dihubungkan dengan parameter-parameter keliatan patah seperti

kdiatan patah K ^ , faktor keamatan tegasan maksimum Kjn^ dan julat faktor keamatan

tegasan AK.

I'enyelidikan kini tertumpu pada analisis sinar-x permukaan patah lesu suatu spesimen

tcgangan mampatan yang disediakan daripada keluli nirkarat austentik dan feritik. Keluli

feritik dikenakan pelbagai rawatan haba untuk memperolehi mikrostruktur-mikrostruktur

dan sifat-sifat mekanikal yang berbeza. Pemcrhatian keseluruhan dianalisa melalui

kerosakan lesu dan konsep Sains Bahan.

INTRODUCTION

The traditional design approach for most structures is generally based on the use of safety

:".!. :>\-s limiting the maximum stress level to some percentage of either yield or ultimate

strength. Bi:t this approach was found not to give proper assurance of sateiy with

re :>-v: u> catastropic hnttle fracture occuring at extremely high speeds. This failure can

•x- characteir/ed by a tlat fracture surface icleavage) and without any sign of prior plastic

li-jvrm.stion i!). A majority of failures in various structures like storage tanks, pressure

Vessels, pipe lines, bridges, ships, turbine generator rotors, aircraft, rocket motors, etc.,

h.-.s e shown that cracks have initiated from sites of stress concentration and propogated

further to critical sizesr causing fast, fractures without any warning. Such sites include

cracks which are inherently present in the structures either due to microstructtiral

inhomogenities or fabrication defects. The. ovethelining, number Q£ instances of such,

caiastrophic failures in brittle fashion have led the research workers to develop different

n:e:hods such as impact test. Linear Elastic Fractures Mechnaics Concept (LHF.M.J,

F.ui-.io-plastic fracture mechanics (EPFM) principles involving J-intcgeral, crack opening

displacement (COD) and R-curve methods to evaluate the notch toughness of the

material.

During the last few years much research has been oriented towards analyzing the

mechanism of crack propogation in a fatigue process employing X-ray diffraction. In

this method residual stresses left behind on the fracture surface due to the plastic

deformation in the wake of tne fatigue crack are measured. The measured X-ray residual

stre^ (ff ji and the full width at half maximum (B) of the diffraction propile are condat-.-ii

to the fracture mechanic parameters such as fracture toughness K/£\ the maximum stress

intensity factor K m a x and stress intensity range AK. This method was found to be

, advantageous in addition to those obtained by the conventional surface analyses using a

: scanning electron microscope (SEM). By this method even a fracture surface damaged in

a cor.-itsi'. •„• environment can also be examined (2). So far, several investigators have: emp! '\ ed this method to analyse fracture mechanisms in specimens tested for evaluatingr:..v ;;.:cture toughness (3-6). fatigue crack growth rate (2, 7-10) and stress corrosioni

;crack::ig properties of some materials (11, 12). Some attempts were also made to apply

jhe ir.j'.hod io identify the failure mechanisms and causes rcsposible for failure of i!;•..•

•.comjvnenis in service (13-19).

THEORY

By the application of Liner Elastic Fracture Mechanics (LIEFM) concepts, fuciuie

3eha\ :• >.:r n{ structural materials can be analysed through the single most parameter ca!k\i;tress ::-:.•:,-,;:y !".-:. •:• reprevj-.te..! i > t;:e expression (20;.

K =• C VJ-a

Aa (StressRange )

TIME

Hg. ] : A schematic representation of constant ampplitudosinusoidal type of cyclic loading

h - l

.0 1 w

• Z5"«o CDS j ; «

/ 2 Holea

*

k— a —H- * • WlO-OOS f-

* ilSl.otW >• '

fg

i

•'•l\.y. : A sc.hfciniit.ic fc:prt:s;<:nOil.ion of <i Cottipnt:!' I 'nusion Spocinnsri (<''!'.'>)

Fur this specimen geometry the stress intensity factor, K, is given by the following

K = f (:i/w)

tisv) 0-5

uhere V is the applied load, 't' is the specimen thickness, V is the specimen width

and f (a/w) is a polynamial function of the crack-depth ratio a/w.

By using P m a x . Pmin o r ^ values in- place- of 'P in- the- above equation- it is possible to-

obtain the values of Km a x , Kmin or AK, respectively. As the crack length extends in

fatigue the respective 'K' values also increases.

Fatigue crack growth data for a given stress ratio are generally represented by a log-log

plot of the crack growth rate, da/dN and range of stress intensity factor AK. Such plots

(Fig. 3) generally consists of three regions corresponding to low, intermediate and high

AK regions. Region I is the low AK region in which the i'nti'ated crack grows at slow

rates.

3 -\ typical loi; (da-'dN) versus lnj; ( AK j plul"hiaincU m constant amplitude load laii.uu"' rack (jrovvih k'sts.

By perfonning experiments in this region, it is possible to-determine a minimuiv, AK,

v:ilue below which the crack does nor propogate. This minimum is known as (lie

threshold values ;:r.d is represented by 'AK,j,'. Region It resprc:K-ii(:i tlio majoncv of ;!:>.•

l::;e.'.: portion ;;:'.;! is represented by Paris law (2!) usint'. the Ibllowiiv.: reLitinii:

C and m are known as Paris constants. In region III the fatigue crack growth rates are

much higher than those obtained in region EL Towards end, a total failure of the

specimen occurs where the stress intensity approaches the KQ and Kic value.

Residual stresses may be defined as those stresses present in a material under no

applications of external forces or moments. The residual stresses are classified in to

three types known as first, second and third kinds, which are shown schematically in

Fig. 4. Residual stress of the first kind is known as macroresidual stress and are nearly

Grain boundaries

1 ':•'. i : A schematic representation of differentkinds of residual stresses

homogenous across large areas (several grains). Residual stresses of the second and-

third kind are known as micro residual stresses. In general the residual stresses are

!<;;i>v. n to develop due to any manufacturing process which produces a non-uniform

pbstic deformation in the material such as machining, casting welding, shot blasting,

he a treatment etc. Apart from these residual stresses present already in the bulk material,

>wing fatigue crack may develop its own residual field in the wake of the crack tip,

:;ch in turn could influence the sign and shape of the plastic zone in front of the crack.

O.i. lie deformation is known to produce forward and reverse plastic yielding at the crack

tip due to the increasing and decreasing part of each load cycle. The next result of this

cycling is a fatigue crack with a plastically deformed narrow layer along the flanks. This

1-jLives some residual stressures at the fracture surfaces. Because of this, each element on

tin' cr::ck edge is deformed from its precrackine positon by rotation (ie lilting the edge)

•••• i.x:; is produced when the element participates in the fatigue process as the crack tip

;•.!.- ^LN li (22), which is schematically shown in Fig, 5.

KoMJion '•^r.^i and rnlat.

Y

< " r u < " k l l a n k

X«.'r:j''k i; mwili

|.-,i;. =. : A iclK'ni-iu.- repr'-'seiUalion ul n-siJu.ilJi.-lormaliun in an oliMiicnl ;il llv- Ira.-inn-

surface oreaU-cl in a fatigui' process

This additional residual stress field generated ahead of the crack tip could influence the

crack, propagation. Foe example,, a comprcs.si.ve residual.stress was found, to increase

crack closure and retard growth (23). Detailed procedures to estimate the macro residual

stresses were given in (24) by measuring the line shift. Micro residual stresses broaden

the diffraction profile and one can estimate them by measuring the full width at half

maximum (FWHM).

MATERIALS AND EXPERIMENTAL DETAILS:

This research work was planned to investigate the fatigue and fracture mechanisms

tlirough performing fatigue crack growth tests on different steels and subsequent X-ray

analysis at the fracture surface. The material selected for investigation in this programme

was a medium carbon structural steel of. C45 grade (ASTM),. which, is, ammenabk. to.

changes in the mechanical properties when subjected to different heat treatments". X-ray

fractography analysis of fatigue fractured surface of specimens prepared from fully

austeniric stainless steel' (A.S.T.M1 GRADE SS 304) is also included in this research.

Actual chemical composition of the steels is given in Table I. Of the ferritic C45 grade

steel, blanks of suitable dimensions were cut from the stocfc material to prepare different

test, specimens. These were austenized for 1 hour at 85O°C for 1 h.,ur and subsequently

water quenched. There after, a few of them were subjected to tempering treatments at

200°C, 400°C and 600°C and a stress relief treatment at 720OQ In ail cases the

duration of treatment was 2 hours. In the discussion that follow, the steel in the as

where ' c~ ' is applied, stress to the structure (far away from the crack) and 'a' is length

of the crack pre-existing in the structure, C is a dimensionless geometry factor. Unstahle

fracture occurs when 'K' reaches a critical value designated as 'Kc' which depends on

thickness or constraint. Other limiting value 'Kc' for a specified test temperature and

slow loading rate is known as fracture toughness. It is designated as 'K\Q' under

plane-strain (tri-axial stress state) conditions. The subscript T stands for an opening

mode/iension of loading. Appropriate test procedures to determine the fracture toughness

have been prescribed in ASTM standards.

In general, the fatigue life of structural components may be considered to be composed of

three continous stages (i) fatigue - crack initiation (ii) fatigue -crack propagation (iii) final

fracture. Fatigue crack propagation characteristics of components are of primary interest

when the components contains stress concentrations or initial defects.

Fractures mechanics based studies on fatigue crack propagation involves tests in which a

fatigue cracked specimen is subjected to constant amplitude load (stress) fluctulaion

between a maximum "Pmax" (°"max) a n^ a minimum 'Pmin' (6inin) value. The most

generally employed sinusoidal stress fluctuation is shown in Fig. 1, consisting of a

steady mean stress m e a n and a fluctuating stress range A (= 2 A<J"a||). In terms ofa n d "iran- the 'Wan and A S can be expressed as:

mean - max + min

then

Aff =

ffalt =

-max

^max

2

' °~min

-^min

Tlie ratio between <Tmjn to C"max ' s known as the stress ration (R) and is expressed as

R = ffmin

max

A ie.-.t p ; \ V L \ h i r e to d e t e r m i n e f a t i g u e cr;n/k g r o w t h r a t e s is d--- .upi . -d us AS ' l M K-(>!7

s;..:,,:.::J. A s ^ h e n m i c repreventa tn .n i ol ui.v- v>t ihe reeiinnneiuli . - i! :eM sj.v. t u i i c n s k n i i w n

a> cvi-.-.'.p.'.ct l e n s i o n s p e c i m e n ( C l ' S ) is s h i ' w n in |-"ig. 2 .

l.ihlv 1 : Actual Chemical Composition of the SteelsInvestigated (Weight %)

Steel

C4S

SS304

C

0.44

0.06

Mn

0

1.

.64

65

Si

0.23

0.33

P

-

0.027

S Ni

0.032

0.005

-

8.82

Cr

-

17.

Mo

72

Cu

-

.040 0.2')

::.•::::L. the enure crack growth.

•.:•.•-• ^ . s r ^ - i i i . l\V-a) = max

V.

i-.e general procedure to determine fatigue crack growth rate, cla/dN, involved

•.•.•.:>;:rcmcnt of crack extensions at different elapsed number of load cycles. Prior to

,:...-.; crack growth data acqusition, the machine notches were extended to an acceptable

:;::•. u l v u t 1 to 1.5 mm) by fatigue pre-cracking. The number of load cycles were

••:;:- tor every 0.5 mm crack extension. The crack extensions were measured by a.c

•:.-:::;n..irop mehtod. da/dN variation with AK was computed based on the methods

. .•; ::: ASTM E-6-17. X-ray analysis of the fatigue fractured surfaces of different steels

• .• •::_.:.:ted was performed using the multiple expousre s i n ^ ^ method (25). A

.!(i.\KT STKAl.NTLEX MSF-2M type portable X-ray stress analyses was employed

•:• :h:s purpose. All X-ray studies were carried out using (Cv-K« ) radiation, with a

. t- -.(."itagsr of ?() kV and a current of 10 mA. For normal stress measurement the 20

• -^-r.in-eis 1-10° to 170°.

I.1 ..:.-•.:::.;;;vj ii'.e.isurements of austenite/manensiw fractions arc also possible, with t':.j hv!j)

.•: :: .• ;:•:.'.'oprn^essor and an attachment provided for this purpose. This additional

.:'.•...•• :•.:.••.•:. arm is designed in such a way that the normal 20 range can Ivj changed from

quenched condition. 21XPC, 400°C, 6(K)°C tempered and 72O°C annealed states will be

refered to as conditions 1, 2, 3, 4 and 5 respectively. The austenidc stainless steel was

investigated in the as received (rolled and heat tieatmem) condition.

SPECIMEN PREPARATION DETAILS:

The compact tension specimens (CTS) were made according to the ASTM 1:647 for

fatigue crack growth tests. Machine notches were made using efectro discharge

machining with notch length coinciding with the rolling direction. For a schematic of the

CT specimen see Fig. 2. The initial machine nothes were made to lengths which

correspond to crack length to width ratio (a/w) of about 0.3 for the fatigue crack growth

test specimens whose thickness is" 8 m.m.

Tensile specimens and Charpy V-Notch specimens were made according to ASTM A-370

and A.S.T.M E-23 standards respectively for C45 steels in different heat treated

conditons. No mechanical tests were carried out for austeniric stainless steel.

EXPERIMENTAL DETAILS:

On C45 steel which was subjected to different heat treatments, vickers hardness

mea.v rements were performed on sections obtained from blanks of dimension which are

equal to those of a CT specimen. Variation in hardness from surface to the interior vi the

blanks at different locations of the sections, both in width and thickness directions wit ,

observed.

In a similar way variation in microstructure as result of the heat treatment was observed

using conventional optional microscopy.

SlllMAD/.U UNIVERSAL TESTING MACHINE (UMII-50) of 500 kN capacity was

used for (ensile testing. Strains were measured over a gauge length with a high precision

liitlerential Hxtensomcicr. Charpy V-Notch samples were tested in 300 Joule TeMing-

Machine Inc. Impact Tester. The impact energies were recorded with an accuracy vl' !

Joule (torn the dial readings by means of the pointer attached to the hammer.

I-iitigue crack growth tests were conducted on a universal static/dynamic INSTKO.N

NI-.KVO liydr.iiihc test system. The te.sl .specimens wese subjected the constant aiiiplituiie

sinusoidal i-j:!..!::ii; in a tension mode (Fiji.I) following the procedure i!e:v;iU\! in

A S . I .\1 1-. (•'. ' \t.mdaid. The tests were peiion:icd :•.'. room leniperature on <!ilic;cni

sle.ek i:m;-..o_. .-,;• ;, siie:,s ratio of 0.1 ami a lest iics|ii.-;;cy of 20 H/.. 'J lie luaii lewis an:

selected ii; :..\ - .".es Mieh that the maximum Miev.^s i:. si::'.'e H crack grolh tiiii nut eiuecd

( • ' I ' i o ! i : . e i : . - ' \ v : e l d s i r e n i i i h . In t h i s w a y t h e ?' >1 i< >.-.: :i _• i e i | u i r e m c i i l w a s :;! o ; : u i

the standard set of 140° to 170° to 120° to 150°. The volume percentage of austenite of

transformed manensite were obtained by performing measurements usingir(220) and

« (211) reflection with C ? K« radiation and comparing the relative integerated

intensities of the diffraction profiles from both phases. In addition, a seperate automation

program was also developed to perform the stress measurements in the 20 range of 120°

to 150° using Cr Ko, radiation on the (220) austenitic reflection. For this purpose, an

optical RIGAKU imterface board was employed along wiih a personal computer coupled

to the system through a RS 232 interface.

In all the stress measurements X-ray irradiated area was restricted to a stri'pof Iffmm in

length and about 1 m.m width with the aid of a mask whose width coincided with the

direction of crack growth. The measurements were performed at different crack locations

on the fracture surface which correspond to different Km a x or AK values.

_ RESULTS AND DISCUSSIONS

INVESTIGATIONS ON C45 STEEL:

Miciostructures in each condition of the C45 steel were examined and are given in Fig

6-10. The material in the water quenched condition appear to be fully martensitic (Fig.

6). More or Jess similar observations were made in case of 2f)0°C tempered condition

(Fig. 7), as this temperatures is not high enough to cause significant microstructural

changes. Samples of 400°C and 600°C temper conditions, exhibited tempered

manensitic/beinitic microstructures (Fig. 8 and Fig. 9). The material in annealed

condition i.e 720°C tempered condition showed a pearlitic microstructure. (Fig. 10).

The room temperature tensile properties, Charpy V-strech impact energy values and the

hardness values of the C45 steel in different heat treated condition are given in Table 2.

Since the C.T specimen thickness is only 8 mm, unifonn hardness values were observed

through section thickness for each condition as a consequence of uniform microstructure

throughout. It may be seen as the tempering temperature is increased, surface hardness

values shows a decreasing trend.

The Charpy V-notch impact strengths were found not to show much variation as a

i unction of heat treatment. They were found to vary from a minimum value of 13 Joules

for water quenched and 200° tempered steels to a maximum of about 24 Joules for

6()()°C tempered steel.

Fig. 6 : Typical microstructures observed for the 01-5steel in water quenched from 850 °C surfaceregion

T.vpicnl i.'iicro.'ilruc.tiirof; obse rved for CMfisloi:l in V/Q ;ind .'iuh.s.-ijiicntly (.ompcrcil :\\. yo

'.. 8 : Typical microstructures observed for 045steel in WQ and subsequently tempered at 400°C.

' • ' ; : • •' •' T y p i r . i l i : n , ,;,:;{, u ; - « i

s l . < - ' . - l i n Vi't,- , i i i ' ) . M i !:;; ob.';'..-r vr;(l Cm

- i ' . . J

'• i;,r. 10: Typical rtiicrfist ivictiirGs obworved for C45 slfolin iVQ and subsequen t ly tomporud aiine;ili:d al 7^0

V.0 "

/4•4

k1 >:• , .

AK

V.

AK.(MPa-v/irT) AK.(Ml'aVm)

V. : I oi;( d.i - i!N ) vc rsu- . I m ; ( A K ) l . i U ^ u c cr.n-K.r a l e p l o t s o h L i i h r d I n r C-l S s i r c l • • |n t i - i n i fM\l i 'Mc t I in Oi l I<T i-u l . i m i l >l i o n \

300

210

200

ISO

100

10.

.. 0'i

-10!

-100

Q ConJl'Icr

A. CiKlt"0

luo t so1

cu :o MO w mu M U i ru

Fig. 12: Residual stress, <T > distribution as a functionof Km a x on the fracture surface of C45 steelin different conditions

6 i.:o!|

«_, zoo

_ I 7U

A ConditJo

O Cofwiltion 2tUQ t JCO'C tr*prr)

O Condition 3

(U(J L LOO": l r r ; . r

Cor-dttlon !,l«0 It 7.'0"l J

Fig. 13: Full Width at Half Maximum, B, distribution a:.- afunction of Kmax on the fracture surface of C45stool in different conditions

•. ,-r.T r:i-s • • / f . 'n*

d.T^ttr kloagation Surface Charpy V-f tct - 25mm) h:ir<Jne« nofrh Impactjh l%) tVUSi energy tj)

l,i'." f t O %«•() / ' •

IW(> .V ^ ' • (1 ' • • " ' " • • • " • • S- " '

f J

frmprrj

fciripcr

(IV'QA 770 C 3*0 5f.S /fl-O l'>7

It may be noted from table 2 that the tensile strength is higher for the steel when tempered

at 400°C in composition with that in other conditions. Similar results were observed by

other investigator (26) on plain carbon steels. The tensile properties of the material in

other conditions <than 400°C tempered ones) were found to be as expected i.e an increase

in ductility with increase in tempering temperature.

The fatigue crack growth rates were determined from tests performed on CT specimens

with a w = 4t configuration. The results are presented in the normal log (da/dN) versus

log (AK) plots. Typical plots obtained from such tests performed at room temperature

employing a 0.1R ratio are shown in Fig. 11. At any AK value the crack growth rate

(da/dN) was found lo be higher for conditions 4 and 5 while lower for conditions 1 and

2. Condition 3 has shown an intermediate value. The transition from stable crack

growth region to the onset of fast fracture, was found to occur at about 80 MPa Vm for

conditions 1 and 2, 60 MPaVni for conditions 3 and 4, and 40 MPa Vin for condition 5.

Paris constant 'rrr1 and 'c' for the stable crack growth region (stage II) of the plot arc

included in the Figures.

The stresses on the fracture surface of the fatigue crack growth specimens were examined

using a RIGAKU X-ray stress analyser. The residual stress (6 r ) value as measured in

the crack growth direction as well as the breadth (B) of the diffraction profile (at f = O

deg) as a function of Km a x are shown in Fig. 12 and 13, respectively. The reported

residual stress values in Ki^. 12. are the absolute values us obtained from the X-ray

stress measurements and do not involve any correction for ihe already present residual

stresses, for instance those due lo the preceeding heat treatment.

Residual stresses were measured on the unfatigued samples in different heat treated

conditions and were found to be about -78 MPa for condition 1, -40 MPa for condition 2,

-25 MPa for condition 3, -5 MPa for condition 4 and 0 MPa for condition 5. Generally

hardening results in comprassive stress which decreases as the tempering temperature

increases.

Residual stresses, €j , distribution as a function of K m a x on the fracture surface of

C45 steel in different condition is given in Fig. 12. It is noticed with increasing Km a x ,

the residual stress (6^ ) exhibits an increasing trend in the fatigue region and a decreasing

trend in the fast in fracture region. In all cases, the K m a x value at which 6^ is a

maximum is found to concide with the value at which the transition in crack growth from

stage II to III has beeb observed. In conditions 1,2 and 4, the maximum €T was found

to be about 250 MPa, while for the material in condition 3 (400°C temper) it was about

325 MPa. This can be attributed to the high yield strength and tensile strength of the

material in condition 3. Unlike the others, condition 5 (annealed) exhibits a small €^

increase in fatigue region (up to 50 MPa Vm) followed by a sharp decrease to a a level

almost equal to zero stress at a Km a x value of about 70 MPa Vim (fast fracture region).

It should be noted that the rate of increase of e, with K m a x is higher in conditions 1, 2

and 3 than in conditions 4 and 5.

It may be mentioned here that towards the final rupture locations, the 6^ was found to

decrease to a low value which is anout 50 MPa. This occured in all cases except in

condition 5 at very high Km a x levels where as in condition 5 it occured at a lower Km a x

which is related to the brittle nature of failure in conditon 5, which may be attributed to

the grain boundary effect. The observed decrease in 6y at high Km a x may be related to

the mode of deformation which is monotonic in stage III later parts, in comparision to

fatigue cyclining in stage II.

The variation of breadth B with Km a x as obtained from the X-ray stress measurements

performed on the same fracture surfaces on which the above mentioned 6? - K m a x

variations have been recorded, is given in Fig. 13. In all the cases the breadth B was

found to show an increasing trend with increasing K m a x , both in the fatigue and fast

fracture regions. The rate of increase was observed to be lower in condition 1, 2 and 3.

In contrast to other conditons, the material in condition 5 showed a sharp increase (about

20%) in B values (from 1.9 to 2.3 deg) over Km a x range of 50 to 80 MPa Vm (stage

III). During this stage of crack growth €y values were found to be decreasing to zero

level. This may be possibly due to brittle nature of fracture in condition 5.

Under the action of the applied stresses the material will be subjected to at and ahead of a

crack tip, to a tria.xial stress situation JIKI the acting stresses will be tensile. The

magnitute of stresses will be higher in the (.-rack growth difraciion (say, X-direction) and

in the loading direction (say, Y-direction) as compared with the thickness direction (i.e.,

Z - direction). In this situation a plane strain condition prevails in the material. Then

these stresses exceed the yield strength of the material, plastic deformation takes place.

This deformation being tensile, results in positive strains (elastic + plastic) which

elongates the material within the deformed zone in the X and Y directions. As crack

propogates and rupture takes place, the elastic strain tend to recover but is constrained by

the underlying underformed bulk of the material. Such a constraint will be larger for the

X-direction than for the Y-direction which can be attributed to the created fractured

surface which is perpendicular to the Y-direction. This leads to some relexation of the

stress in to Y-direction. This results in tensile residual stress in X-direction (crack

growth) in t]je,deformed material. The stress distribution in Y-direction can not be

determined with X-ray due to its limited penetration.

The magnitude of tensile stresses will increase with cumulative strain cycling effect,

which increase with crack length or AK or K m a x , which explains the increase of £y

with AK in stage II crack growth

In stage III, a slightly decreasing trend was noticed. This is explained by the fact that in

stage III the material ahead of the crack tip will experence cycling at large plastic

amplitudes but for a small number fatigue cycles and will also experience a plane stress

situation.

Changes in FWHM (B) of the X-ray diffraction profile are sensitive to microstructai

changes in a meterial caused by plastic deformation or recovery. This need not be the

case for the residual stress (6"T) as it can be influenced by several other factors like

geometry of the sample. On the other hand, the observed stress relations has no effect

on B. Inline with this, the FWHM values as a function of KmoX ' n all cases show a

continously increasing trend in stage II as well as in stage III. However in stage III the

rate of increase in higher. This indicate that the deformation was corunously increasing

with Km a x with the rate being higher in stage III.

INVESTIGATION ON SS 304 AUSTENITIC STAINLESS STEEL:

The chemical composition of the stainless sice I used is given in i.ihle I. Kitipie n a i lgrowth r:i!es were determined at room !cmj>L-I;JIUIL; by testing IT specimens :u\onling toASTM I--647 standard procedure.

A log-log plot of da/dN VS AK is givet? in Fig. 14

iin - ? 2i).-v(j ,- - 5.0-H'1 lii'1-"

- 10

HI"

10 I Oi i

SS304

l:ig. u : Log(itaAlN) versus lug <AK) fatigue crai:k.growthplots for SS3O4 Muinlrss sleds

5 so

= 40

' I ' « •

" 10 20 JO 40 50 00 70 SO 'JO 100

K,,.,. .(\ll\i Vi"")

Ucfornia i iunint iu i ••<.! m.irtcn:.ili: conu-n i v;iri.u'ionu lut^cnon nf K n l ; . ^ lur SS5O4 M.IUIII-SN SIIM-IS

The Plot is linear up to a AK value of 30 MPA Vin. This appear to be transition point

from stable crack growth (stage H) to the onset of fast fracture (stage III). A

significant necking (reduction in thickness) was noticed on sections of the- test samples

at locations where AK is more than about 40 MPa Vm. It may be noticed that 'm'

value are lowest than those of C45 steel.

Transformation of austenite to martensite as a consequence of fatigue cycling

deformation effects at the crack tip was observed. The martensite content was

estimated («<% = 100-V %)by comparing the integrated intensities of «(211) and

V (220) profile. The variation of its amount with Knnax is given in Ffg. T5"

700

600

LOO

<00

300 A | , ; l l I M . i r l . M M l i1 1 ( . ' . ' < ! ) • \ i i s t r n l i i -

(stablecrack (unsiaDlc lastgrowth) ITactiiro)

(0 20 30 <O SO "50 70 trn '.'') IfO

KMJ, (MPaVm

I i1.!.. ' j : Residual s t ress , 1 •' , dis tr ibutions as a lur.rlion olv iii.sN measured on tho Cracturo surlaces ol' SSS04

stainless steals.

This- variation is appraxirrrately linear up ta a. K m a x value of 30 MPa Vin and is

intresting to note the observed transition from stage IF to stage III in fatigue crack

growth coincidies with this value of Km a x

Coenistence of two phases (austenite and deformation induced mariensite) at the

fracture surface has made it necessary to perform stress measuremants. on both phases

in order to understand the c/ack growth mechanism in austenitic steel. This has achived

in the present investigation by means of using the (211} martensite (<*) and (220}

austenite (t) reflections and Cr K^ radiation. A vanadium filter was used to prevent

the influence of the KB radiation. The residual stress (ff|0 values as a funtion of K i nax

is shown in Fig. 16.

The corresponding breadth (B) variation with K m a x in both-phases- are given in Fig.

17.

( 2 1 I ) M a r l i - ' ;

( / .'. I I ,1 A I! -> i I •! •

.--0

-1 1

i !•• r-

' • i . •( Ml'.i \At\ )

I n ; : W u i H i HI I I . i l t m a M n i " 1 1 ! . l > .1-'.

l l l M i ' l r ' l l

, i ! S S . < ' ' i ' i

ii I ' " '

The profile breadths obtained at any K-max from the martenasite phase are much larger

than those from the austenitc. The B variations exhibit a gradual increase with Kmax .

CONCH SSIONS:

1. The residual stress G, distributions on the fracture surface of C45 steel in

different heat treated conditions were found to exhibit an increase followed by a

decrease with increasing Km a x values, while FWHM (B) distributions showed a

monotonic increase.

2. The position corresponding to fr maximum was observed to shift to a lower

K m a x value for materials with higher ductility imparted by tempering. This

position matches with the location of transition from stage n to III in crack

growth.

3. The observed Km a x value at which transition from stage II to stage III in fatigue

crack growth rate occur was found to coincide vary well with the values at which

a deviation from the linear trend of martensite transformation was observed in

austenitic stainless steel.

ACKNOWLEDGEMENTS

The authors wish to thank

(a) The authorities of University Sains Malaysia and Government of Malaysia for

providing funds to carry out this research project.

(b) Mead, Department of Mechanical Engineering, Institute Technology Mara, Shah

Alam, Selangor Darul Ehsan, Malaysia for carrying out some fatigue

experiments.

(c) Dr. B. Pathray, Foundation for Advanced Metal Science, HENGELO, THE

NETHERLANDS for conducting X-Ray studies.

(d) All the staff of School of Materials and Mineral Resouces Engineering, Bianch

Campus, University Sains Malaysia. Tronoh, Perak Darul Ridzuan.

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