Destructive testing methods of welded joints - IDC-Online · 2019. 2. 4. · Keywords: Impact test,...
Transcript of Destructive testing methods of welded joints - IDC-Online · 2019. 2. 4. · Keywords: Impact test,...
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
durin
Sr.
No.
1
2
t exhibit b
rmation und
cimens to i
ure as mos
mpact force
tudy the be
ng) toughne
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
Held
cantil
Held
as sim
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
nder dynam
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
Holding
Cantilever
the pendulu
Simply sup
is opposit
impact (not
o resist cr
entionally in
o increase
ss raisers. T
test.
nditions (at
two metho
the common
nergy absor
owever, the
e and shap
lum that hit
type and
um
pported type
te side of
t facing to pe
racking an
n impact tes
tendency t
To withstan
high rate o
ods used fo
n principle o
rbed (kg m o
ere are som
pe, method o
s the sampl
notch faces
e and notch
f pendulum
endulum)
nd
st
to
nd
of
or
of
or
me
of
le
s
h
m
Stand
mach
hits th
Fig. 3
Since
raise
notch
with n
the re
Resu
(Nm)
samp
purpo
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
xpressed acc
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
Hence, tou
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
ce and then g
e, near or s
gnetic powd
surface or n
fect. Format
ails of size a
agnetic meta
of dye penet
ssing the su
simple princ
hrough meta
ed using ele
performed u
. Presence
the path of f
poles. The m
sprinkled ove
migrate towa
get piled up
shallow sub-
der particles
near surface
tion of very t
and location
al only.
trant test
urface and
ciple of the
al from so
ectrical energ
using suitab
of any dis-
flow of these
magnetic pow
er the surfa
ard the locat
(Fig. 32.9).
-surface disc
s suggest th
e region. Haz
thin line of p
n of crack. H
near surfac
flow of mag
outh to nor
gy or suitab
ble yoke whi
-continuity in
e lines resu
wder particle
ace of comp
tion whereve
The particle
continuities.
he location,
zy pile of po
powder partic
However, th
ce defects i
gnetic line o
rth-pole. Th
le permanen
ich is applie
n the form o
lts in leakag
es (in dry form
ponents to b
er leakage o
es align alon
The locatio
size, type o
owder particl
cles suggest
his method o
in
of
he
nt
ed
of
ge
m
be
of
ng
on
of
le
ts
of
32.4.
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not o
ultras
and a
to m
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prese
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medi
meta
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All th
Trans
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3 Ultrasoni
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only indicate
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tions from th
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um from air
l to air. The
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ese method
smission ap
transmission
mitter of vi
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mission app
ts from tran
l system wit
op surface a
Fig. 3
c testing
e most popu
s the prese
ons. Ultraso
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sence of dis
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r to metal an
ultrasonic v
ibrations to
s are very e
pproach
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ibration and
tions and r
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32.9 Principle
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ows the two
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hange of med
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nce of discon
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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