Full text of DOI:10.1002/1521-4052(200106)32:6<562::AID-MAWE562>3.0.CO;2-K

Measurement techniques for determination of
ductile crack initiation on Charpy specimens
Messmethoden zur Bestimmung der duktilen Rissinitiierung in Charpy-Proben
Gy. B. Lenkey, L. To th
In the engineering practice it is of importance to know the effect
In der Ingenieurpraxis ist es wichtig, den Einfluss der Bela-
of loading rate on the material behaviour including the fracture me- stungsgeschwindigkeit auf das Werkstoffverhalten einschlieûlich
chanics properties. Depending on the material behaviour under a der Bruchmechanik-Parameter zu kennen. AbhaÈngig vom Werk-
given loading condition, different fracture mechanics parameters stoffverhalten unter den jeweiligen Beanspruchungsbedingungen
should be determined. The critical values of these parameters muÈssen unterschiedliche bruchmechanische Kennwerte verwendet
are usually related to the onset of crack initiation which can be de- werden. Kritische Kennwerte, mit denen gewoÈhnlich die Rissini-
termined easier in the case of brittle fracture. But in the case of tiierung festgelegt wird, lassen sich aber nur beim sproÈden Bruch
ductile fracture additional measurement techniques are required. einfach bestimmen.
This paper presents some possibilities to characterise the fracture
Dagegen sind beim duktilen Bruch zusaÈtzliche Messtechniken
resistance against ductile fracture using instrumented impact test- erforderlich. In diesem Beitrag werden einige MoÈglichkeiten zur
ing. The magnetic emission technique will be introduced as a po- Charakterisierung des Bruchwiderstandes beim duktilen Bruch
tential measurement method for determining the onset of ductile mit Hilfe des instrumentierten Kerbschlagbiegeversuch vorgestellt.
crack propagation in magnetizable metals.
Die magnetische Emission kann als eine potentielle Methode zum
Nachweis des Beginns der duktilen Rissausbreitung in magnetisier-
baren Werkstoffen eingesetzt werden.
Introduction
In the present paper some methods for characterising the
ductile crack initiation resistance will be demonstrated and
compared.
The collection of metals behaviour under impact loading
conditions has been started at the end of the last century.
One of the milestone in the testing procedures was created
in 1901 by G. Charpy [1]. From this period of time the spe-
cialists almost in all countries published their overviews [2±
10] and the present state of the testing procedures and results.
Implementing of the instrumented impact testing [3±10] has
started a better knowledge of the materials behaviour under
impact loading condition.
In fracture mechanics testing determining the instant of
crack initiation is the basic task for the measurement of critical
material parameters. This task can be solved easier in the case
of brittle fracture since a sudden drop in the force signal usual-
ly accompanies brittle crack initiation. But in the case of duc-
tile fracture or if stable crack propagation occurs before un-
stable one, the instant of crack initiation cannot be determined
directly from the force signal. In these cases additional meas-
urement techniques should be applied.
A number of techniques are available for quasi-static appli-
cations, but only some of them can be used when higher load-
ing rate is applied. Due to the special arrangement of the im-
pact pendulum the following direct measurement techniques
can be used for crack initiation detection:
± laser COD-measurement ± only with ªreversedº pendulum
[11],
± strain gauge measurement [12],
Simple method for determining ductile
crack initiation resistance
At the beginning of the application of instrumented impact
testing, when there was not available measurement techniques
for detecting the ductile crack initiation, initiation was thought
to occur at the maximum load. On the basis of this approxi-
mation the fracture energy can be divided into two parts: crack
initiation energy (Wi) and crack propagation energy (Wp)
(Fig 1).
Both partial energy values can be determined by integration
of the load-displacement curve:
sm
Z
Wi ˆ
Fꢀs†ds; and
ꢀ1†
ꢀ2†
sˆ0
st
Z
Wp ˆ Wt Wi ˆ
Fꢀs†ds;
sˆsm
where sm ± displacement at maximum load, mm
st ± displacement at final fracture, mm
Wt ± total absorbed energy for fracture, J.
± acoustic emission measurement [13],
± potential drop measurement [14],
± magnetic emission measurement [15, 16].
Besides these methods there are two indirect methods:
± compliance method [17],
± stretch zone measurement [18].
On the basis of large number of experiments on different
structural steel it was proved that the crack initiation energy
does not depend on the temperature when the fracture is duc-
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0933-5137/01/0606-0562$17.50 ‡ .50/0
Mat.-wiss. u. Werkstofftech. 32, 562±567 (2001)
Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001

Fig. 3. The principle of magnetic emission technique
surfaces, thereby changing the external magnetic field.
These field variations can be observed locally by a mag-
netic transducer which basically consists of a coil. The
transducer's output voltage is the magnetic emission
(ME) signal which is proportional to the derivative of
the magnetic field (MF) (Fig. 3).
Fig. 1. Separation of impact energy into initiation and propagation
energy
The emission probe was installed into an instrumented im-
pact testing system with computer aided data-acquisition
which is shown in Fig. 4. The force measurement is carried
out by strain gauges glued on both sides of the tup (3). The
magnetic emission probe (5) is located in a metal box close
to the notch root of the specimen. The instrumented tup and
the emission probe can be seen in Fig. 5.
An external optical trigger device (6) makes the data acqui-
sition start. When the two pins flag on the hammer (7) goes
through the optical trigger device, two pulses are produced
and the time interval between these pulses is measured by
a clock (9). From this time the impact velocity can be deter-
mined. The strain gauges and the emission probes are con-
nected to the voltage supplies and the amplifiers (8) and their
signals are recorded by a TEKTRONIX TDS 420A digital os-
cilloscope. The data could be transferred to the PC either
through a GPIB interface or with diskettes. Then the data eva-
luation procedure is usually done with spreadsheet programs.
Fig. 2. Total, initiation and propagation energies of Charpy-V
specimens as a function of temperature (St 52-3 steel)
tile or mixed (in the upper shelf and in the transition region)
[19]. One example is shown in Fig. 2.
When some new measurement techniques had been devel-
oped for detecting ductile crack initiation during impact test, it
was proved that the ductile initiation usually occurs much be-
fore the maximum load. One of these techniques was the mag-
netic emission measurement. Many investigations have been
done during the last decade to study the applicability and lim-
itations of this technique.
Application of magnetic emission
technique for detecting ductile crack
initiation
Establishment of the ªfield methodº
Stable crack initiation can usually not be determined di-
rectly from ME signals. This is demonstrated by an experi-
ment using a ductile behaving pre-cracked steel specimen
(BS4360-50E), see Fig. 6a [22]. For these applications a so
called ªfield methodº was developed [21] which uses the in-
tegrated ME signal, i. e., the magnetic field history, MF(t):
Magnetic emission measurement
technique
The magnetic emission technique has been developed espe-
cially for impact testing [15, 16], but can be applied for in-
vestigating fast fracture processes as well, like brittle fracture.
The principle of this measurement technique is demonstrated
in Fig. 3.
t
Z
MFꢀt† ˆ
MEꢀs†ds
ꢀ3†
Two physical phenomena contribute to the magnetic emis-
sion sigsnal:
rˆ0
(a) mechanically induced Barkhausen signals appear when
the internal magnetic structure changes during loading,
and
(b) a propagating crack causes the internal magnetic field to
emerge from the solid into the gap between the two crack
It was observed that ME signals originated by crack propa-
gation can be distinguished from those originated by Barkhau-
sen noise in the field curve by a change of the slope. This is
demonstrated in Fig. 6b [22]. A continuous increase of the
field can be seen from the beginning of the impact event
Mat.-wiss. u. Werkstofftech. 32, 562±567 (2001)
Charpy specimens
563

Fig. 4. Instrumented impact pen-
dulum and the data acquisition sys-
tem
with increasing force. The rupture event is indicated by a dis-
continuity in the slope of the field curve. This becomes even
more obvious when this result is compared with a field curve
of a low-blow experiment with no crack extension (Fig. 7). In
both cases first the MF curve is increasing continuously. After
the elastic part of the load curve a stabilisation can be seen in
the MF curve. The magnetic field starts to change again when
crack propagation occurs.
Application for V-notched specimens
This method was applied for determining initiation energy
of Charpy-V specimens of a heat resistance steel. In some
cases a significant change was observed in the slope of the
Fig. 5. The instrumented tup with the magnetic emission probe
Fig. 6. Force, ME and MF signals of a low-blow experiment on a pre-cracked BS4360-50E steel sample (v0 ˆ 1,57 m/s)
564 Gy. B. Lenkey, L. ToÂth Mat.-wiss. u. Werkstofftech. 32, 562±567 (2001)

Fig. 7. Force and MF signals of pre-cracked BS4360-50E steel Fig. 9. Average Wm values and real initiation energies (zones of the
sample without crack extension, (low-blow experiment: welded joint of 10 Cr Mo 9±10 forged and GS 18 Cr Mo 9 10 cast
v0 ˆ 0,8 m/s)
pipe)
field signal (MF) before the maximum point ± as is shown in
Fig. 8 ± which can be correlated to the initiation of stable
crack propagation [23]. The energy absorbed up to this initia-
tion point (Wi) has been calculated for two specimens of the
cast base material and is plotted in Fig. 9. For these specimens
the initiation energy was 56% and 65% of the Wm (energy
absorbed up to the maximum load) values.
Application for pre-cracked specimens
Other methods have also proved [11, 12, 14] that in the case
of pre-cracked specimens the ductile crack initiation also oc-
curs before the maximum load. Fig. 10 shows an example how
the magnetic emission measurement delivers the initiation
point applying the field method [24]. The ductile crack exten-
sion preceded the brittle fracture can be observed on the SEM
picture of the fracture surface as well (Fig. 11).
Knowing the onset of initiation the critical J-integral value
related to the ductile initiation can be then derived with eq. 4
[25]:
Fig. 8. Force and magnetic field curves of a Charpy-V specimen of
GS 18 Cr Mo 9 10 cast base material (T ˆ 0 8C; v0 ˆ 5,5 m/s)
Fig. 10. Force, ME and MF signals of pre-cracked sample of E420-C steel (v0 ˆ 2,75 m/s; T ˆ 40 8C)
Mat.-wiss. u. Werkstofftech. 32, 562±567 (2001)
Charpy specimens
565

2 Á Ui
Jid
ˆ
ꢀ4†
B Á ꢀW a0†
where Ui is the energy absorbed by the specimen up to the
initiation and is calculated by integrating the force-displace-
ment (F-f) curve using eq. 5:
fi
Z
Ui ˆ
Fꢀf†df
ꢀ5†
fˆ0
Then the initiation toughness (KId) can be determined for
plain stress condition using eq. 6:
p
KId
ˆ
E Á Jid
ꢀ6†
where E is the Young modulus of the investigated material.
In this way the real material resistance against ductile crack
initiation can be characterised. Experiments on pre-cracked
specimens of E420-C steel showed that while the transition
Fig. 13. Dynamic fracture toughness vs. temperature for
v0 ˆ 5,5 m/s impact velocity (E420-C steel)
temperature can depend on impact velocity (Fig. 12, 13),
the ductile initiation toughness does not depend on it.
Comparison with other methods
In order to compare the magnetic emission measurement
results with other methods, different measurement and evalua-
tion procedures were applied to determine the critical dynamic
J-integral value for 15H2MFA steel at T ˆ 200 8C (Table 1)
[21]. The critical J-integral values determined on the basis
of the magnetic emission signal are in a good agreement to
those obtained with stretch zone measurements [18]. Both
methods detect the onset of physical crack growth. There
is, however, a considerable difference to the Jd0:2 values based
on the dynamic R-curve. This method uses the artificial 0.2
point on the J-R curve, consequently these values are usually
much larger.
Fig. 11. SEM picture of pre-cracked sample of E420-C steel with
ductile crack extension followed by brittle fracture (v0 ˆ 2,75 m/s;
T ˆ 40 8C)
Summary and conclusions
In the present paper some possible methods for determining
ductile crack initiation toughness of metals have been intro-
duced and compared. On the basis of the presented results the
following conclusions can be drawn:
Table 1. Critical dynamic J-integral values determined with differ-
ent methods for 15H2MFA reactor pressure vessel steel
(T ˆ 200 8C)
Evaluation method
Critical dynamic
J-integral, [kJ/m2]
on the basis of dynamic R-curve according Jd0:2 ˆ 350
to ASTM E 813-89 [26]
stretch zone measurements according to
DVM 002 [18]
Jdis ˆ 123 Æ 20
magnetic emission detection of crack
initiation
Jdim ˆ 103 Æ 16
Fig. 12. Dynamic fracture toughness vs. temperature for
v0 ˆ 2,75 m/s impact velocity (E420-C steel)
566
Gy. B. Lenkey, L. ToÂth
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magnetic emission signal showed good agreement to those
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Â
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Anschrift: Mrs Gy. B. Lenkey, Bay ZoltaÂn Foundation for Applied
Research, Institute of Logistics and Production Systems, IgloÂi u. 2.,
H-3519 Miskolc-Tapolca, Ungarn
Received: 3/26/01
[T 398]
Mat.-wiss. u. Werkstofftech. 32, 562±567 (2001)
Charpy specimens
567