Destructive testing methods of welded joints

This chapter describes three important destructive testing methods of welded joints

namely toughness test, fatigue test and fracture toughness testing. Additionally, concept

of fracture toughness and conditions required for fracture toughness test for different

stress conditions has also been presented. Further, non-destructive testing methods

have also been presented.

Keywords: Impact test, Izod and Charpy test, fatigue test, endurance limit,

fracture toughness, plain strain condition, CT specimen, three point bending

specimen, Dye penetrant test, magnetic particle test, eddy current test and

ultrasonic test

32.1 Toughness testing

In actual practice, engineering components during service are invariably

subjected to various kinds of loads namely static and dynamic loads which are

classified on the basis of the rate of change in magnitude of load and direction.

Dynamic loads are characterized by high rate of change in load magnitude and

direction. Reverse happens in case of static loads. In the hardness test and

tensile tests, load is increased very slowly that corresponds to the behaviour of

material under more or less static loading condition. Moreover, very wide range

rate of loading (0.0001 to 1000mm/min) can be used in tensile test. Rate of

loading governs the strain rate and so rate of hardening which can affect

mechanical behavior of material. For example, material at low rate of loading

showing the ductile behaviour can exhibit brittle behaviour under high rate of

loading conditions.

The toughness test simulates service conditions often encountered by

components of the system used in transportation, agricultural, and construction

equipment. A material high impact resistance is said to be a tough material.

Toughness is the ability of a material to resist both fracture and deformation.

Toughness is the combination of strength and ductility. To be tough, a material

must

defor

spec

fractu

an im

To s

loadin

tough

apply

Joule

differ

holdin

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2

t exhibit b

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ure as mos

mpact force

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hness testin

ying the load

e) in breakin

rences also

ng of the sa

g the test.

Toughne

test

Izod

Charpy

both fairly

der impact

ncrease th

st of the me

, a notched

Fig. 32.1

ehaviour of

ess test is

ng namely I

d at high rat

ng the sam

in these tw

ample and m

ess Samp

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cantil

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good stre

loading. N

e stress co

echanical co

d material m

1 Principle d

material un

frequently

zod and Ch

te and meas

mple due to

wo methods

maximum en

ple

vertically o

ever

horizontally

mply suppor

ngth and

otches are

oncentratio

omponents

must be tou

diagram of

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conducted.

harpy test,

suring the a

impact (Fig

in terms of

nergy conte

n anvil as

y on anvil

ted beam

ductility to

e made inte

on so as to

have stres

gh.

toughness

mic load con

There are

based on t

amount of en

g. 32.1). Ho

sample siz

nt of pendu

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the pendulu

Simply sup

is opposit

impact (not

o resist cr

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o increase

ss raisers. T

test.

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the common

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um

pported type

te side of

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racking an

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tendency t

To withstan

high rate o

ods used fo

n principle o

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ere are som

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e and notch

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nd

st

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or

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m

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Fig. 3

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(Nm)

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shock

comp

differ

dard sample

hine in speci

he sample fr

32.2 Standar

e most of the

rs therefore

h under imp

notch. More

esults are ex

ults of impac

or amount

ple. It may

ose but the

k/impact loa

paring the re

rent process

e for both

ific ways i.e

rom back of

rd specimen

e engineerin

e, it become

act loading.

over, un-not

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ct tests are

of energy ab

be noted th

ese only in

ad i.e. load

esistance to

sing conditio

testing met

. notch face

the notch in

ns for a) izod

ng componen

es important

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tched samp

cordingly.

expressed

bsorbed per

hat values

ndicate the

d applied a

impact load

ons such a

thods having

s to pendulu

n Charpy tes

d and b) cha

nts are invar

t to know a

ughness test

les can also

in terms of

r unit cross s

of toughnes

ability of

at very high

ing of differe

as heat trea

g a notch

um in case

st (Fig. 32.2)

rpy impact t

riably design

bout the be

t is usually

o be used fo

f either amo

sectional are

ss are not

the materia

h rate. The

ent materials

atment, pro

and is mou

Izod test wh

.

test

ned with notc

ehaviour of

conducted u

or the toughn

ount of ener

ea (Nm/cm2

directly use

al to withs

ese tests ar

s or the sam

ocedure and

unted on th

hile pendulum

ch and stres

material wit

using sampl

ness test an

rgy absorbe

) by standar

ed for desig

tand agains

re useful fo

me material

d mechanica

he

m

ss

th

le

nd

ed

rd

gn

st

or

in

al

worki

surro

cond

Fig. 3

32.2

The

ways

range

cycle

samp

Redu

locati

the v

tensio

range

temp

used

repor

ing etc. Res

ounding temp

ucted must b

32.3 Schema

Fatigue b

fatigue perf

s a) enduran

e for infinite

e a joint can

ples are gen

uced radius

ion of intere

arious varia

on, reverse

e, loading

erature, cor

for the fat

rting. The fat

sistance to th

perature (Fig

be reported

atic diagram

behaviour o

formance of

nce limit i.e.

life (typically

be withstan

nerally prepa

sample ge

st (Fig. 32.5

ble related w

bending, te

frequency

rosion, vacu

igue test m

tigue test res

he impact lo

g. 32.3). Th

with results

m showing inf

of weld joint

f the metalli

indicating t

y more than

nd for a set

ared for fati

enerally ens

5 a, b). The f

with fatigue

ension-comp

and surr

uum, tribolog

must be care

sults should

oading of a m

erefore, tem

.

fluence of te

t

c componen

the maximum

20 million o

of loading

gue studies

sures fractu

fatigue perfo

test namely

pression, ze

ounding en

gical conditio

efully select

include follo

material app

mperature at

est temperat

nts in gene

m stress, st

of load cycle

conditions a

s as per AST

re from we

ormance is a

y stress ratio

ro-tension),

nvironmenta

ons. Each an

ted and rec

owing.

preciably de

t which toug

ure on tough

ral is determ

tress amplitu

es) and b) nu

as desired.

TM 466 (Fig

eld joint or

appreciably

o, type of str

maximum s

al condition

nd every par

corded with

pends on th

ghness test

hness

mined in tw

ude or stres

umber of loa

Two types o

g. 32.4 a, b

any specif

influenced b

ress (tension

stress, stres

ns such a

rameter to b

results whil

he

is

wo

ss

ad

of

b).

fic

by

n-

ss

as

be

le

Fig. 3

To co

estab

yield

test i

yield

for fra

yield

(Fig.

Test cond

Type of lo

Maximum

Stress rat

Temperat

Frequenc

Type of s

32.4 Standar

onducting fa

blishing the

strength of

s first condu

strength of

acture and t

strength of

32.6). Typic

Continuo

Width (W

Thicknes

Gripping

ducted accor

oading: axial

m stress:

tio (ratio of m

ture: ambien

cy of pulsatin

ample

a)

rd specimen

atigue test,

yielding stre

material. F

ucted with m

weld joint u

then in the s

weld joint u

cal dimensio

ous radius (

W): 10.3mm

ss *T): 11m

length: 50m

rding to AST

l pulsating/re

minimum stre

nt/vacuum/co

ng load: load

n for fatigue t

first step is

ength of me

For plotting

maximum ap

under study

same way te

until enduran

ons of a stan

(R): 100mm

m (as rece

mm

TM E466 sta

everse bend

ess to maxim

orrosion

d cycles per

testing

s conduct th

etal as max

the stress-n

pplied tensile

to determin

est is repeate

nce limits or

ndard specim

m

ived)

andard

ding/tension-

mum stress)

min

he tensile te

ximum stres

number of c

e load corre

e the numb

ed at 0.85, 0

r desired fat

men as per A

-compressio

)

b)

est on the w

ss becomes

ycle (S-N) c

esponding to

ber of load c

0.8, 0.75, 0.7

tigue life is

ASTM 466 a

on

weld joint fo

0.9 times o

curve, fatigu

o 0.9 times o

cycle require

7 …. times o

not achieve

are as-under

or

of

ue

of

ed

of

ed

r.

Fig. 3

contin

Fig. 3

cycle

32.3

The

tough

32.5 Fatigue

nuous radius

32.6 Typical

e relationship

Fracture

resistance t

hness and is

0

0.4

0.4

0.5

0.5

0

Peak

str

ess/

Ulti

mat

e st

ress

e test sample

s between e

data on fatig

p for structur

toughness

o fracture c

s measured

0.4

44

48

52

56

0.6

100000

e a) Schema

ends and b) p

gue test sho

re steel

conversely re

using variou

a)

b)

atic diagram

photograph

owing peak s

esistance to

us paramete

1000000

No. of cycles

m of standard

of typical sp

stress/ultima

o crack grow

ers such as a

s

d fatigue tes

pecimen

ate stress vs

wth is know

a) stress inte

1000000

t sample wit

s. number of

wn as fractur

ensity aroun

00

th

re

nd

the crack tip (K), opening of crack mouth also called crack tip opening displacement

(CTOD) and energy requirement for growth of crack (J or G). The mechanical properties

namely yield strength and ductility and thickness of the weld joint under study primarily

dictate the suitable parameter to be used for determining the fracture toughness. The

fracture toughness parameter namely stress intensity factor (K) is commonly used for

weld joint of heavy sections of high strength and low ductility material developing plain

strain conditions, while crack tip opening tip displacement and energy based methods (G

and J integral) are used for comparatively thinner sections made of low strength and

high ductility material and those develop plain stress condition under external loading.

Measurement of fracture toughness using any of above parameters is performed using

two types of samples a) compact tension specimen (CT) and b) three point bending

specimen (TPB). Schematics of two type of specimen are shown in Fig. 32.7. In general,

in these tests, applied external load is increased until strain/crack opening

displacement/energy vs. load relationship becomes non-linear. This critical value of load

(P) is used for calculations of fracture toughness using relevant formulas.

W

aW-a

0.3 B

a)

4 W

W

P/2P/2

a

P

b)

W=2B, a=B, W-a=B and radius of hole r = 0.25B where B is plate thickness

Fig. 32.7 Schematic of fracture toughness specimens using a) compact tension and b)

three point bending approaches

Although different standards have historically been published for determining K, CTOD

and J-integral, the tests are very similar, and generally all three values can be

established from one type of test.

In general, stress intensity factor (K) decreases with increase in specimen

thickness. This trend continues up to a limit of thickness thereafter K becomes

independent of the plate thickness. The corresponding value of K is called critical

stress intensity factor (Kc) and occurs in plane strain condition. KIC is used for

the estimation of the critical stress need to apply to a specimen with a given

crack length for catastrophic fracture to take place.

σC ≤KIC /(Y(π a)½)

Where KIC is the stress-intensity factor, measured in MPa*m½, σC is the critical

stress applied to the specimen, a is the crack length for edge crack or half crack

length for internal crack and Y is a geometry factor

32.4 Non-destructive testing (NDT)

To determine the presence of surface and surface imperfections, non-destructive testing

of weld joints can be carried out using variety of techniques as per needs. Apart from the

visual inspection, many non-destructive testing methods including dye penetrant test

(DPT), magnetic particle test (MPT), eddy current test (ECT), ultrasonic test (UT),

radiographic test (RT) etc. are used in manufacturing industry for assessing the

soundness of weld joints. In following section, principle and capability of some non-

destructive testing methods have been described.

32.4.1 Dye penetrant test

This is one of the simplest non-destructive testing methods primarily used for detecting

the presence of surface defects only. In this method surface to be tested a thin low

viscosity and low surface tension liquid containing suitable dye is applied (Fig. 32.8). The

thin liquid penetrates (by capillary action) into fine cavities, pores and cracks, if any,

present on the surface. Excess liquid present at surface is wiped out. Then suitable

developer like talc or chalk powder is sprinkled over the surface. Developer sucks out

thin liquid with dye wherever it is present inside the surface discontinuities present on

the weld joints. Dye with liquid changes colour of developer and indicates location, and

size of surface defects.

Fig. 3

32.4.

This

magn

force

comp

magn

acros

crack

of ma

or su

teste

magn

disco

and p

disco

indica

prese

testin

32.8 Schema

2 Magnetic

method is

netic materia

s. Magnetic

ponent to be

netic. The e

ss the locat

k, porosity, n

agnetic flux f

uspension fo

d. The mag

netic flux had

ontinuities on

pattern of p

ontinuity pres

ates the sub

ence of crac

ng is found fi

atic showing

particle tes

mainly use

al. It is bas

c flux flow

e evaluated

lectro-magn

ion / area t

near surface

forming two

orm in thin l

netic particle

d taken plac

n the surfac

piled up mag

sent on the

b-surface def

ck with deta

it for ferroma

g four steps o

sting

d for asses

ed on the s

s easily th

is magnetize

etization is

o be tested

e defects in t

additional p

iquid) are s

es tend to m

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e, near or s

gnetic powd

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fect. Format

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agnetic meta

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tion of very t

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al only.

trant test

urface and

ciple of the

al from so

ectrical energ

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of any dis-

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magnetic pow

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ard the locat

(Fig. 32.9).

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near surfac

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Fig. 32.12 Schematic of pitch catch method of ultrasonic testing

Coupler

For effective transmission of ultrasonic vibrations from the transmitting / source probe to

the metal surface, generally a fluid mostly in the form of water or low viscosity liquid

called coupler is used. The coupler ensures proper contact and transmission of vibration

from source probe to metal surface with minimum losses. Water is considered as the

best coupling media because it is readily available, low viscosity, and relatively safe to

use with most construction materials. In the pitch-catch method, a water-based gel has

proven to be the most practical coupling agent.

References and books for further reading

Inspection and testing of weld joints Welding handbook, American

Welding Society, 1983, 7th edition, USA.

Sindo Kou, Welding metallurgy, John Willey, 2003, 2nd edition, USA.

J F Lancaster, Metallurgy of Welding, Abington Publishing, 1999, 6th

edition, England.

Metals Handbook-Welding, Brazing and Soldering, American Society for

Metals, 1993, 10th edition, Volume 6, USA.

R S Parmar, Welding engineering & technology, Khanna Publisher, 2002,

2nd edition, New Delhi.

Richard Little, Welding and Welding Technology, McGraw Hill, 2001, 1st

edition.

H Cary, Welding Technology, Prentice Hall, 1988, 2nd edition.

S V Nadkarni, Modern Arc Welding Technology, Ador Welding Limited,

2010, New Delhi.

R S Parmar, Welding process and engineering, Khanna Publisher, New

Delhi