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91
Fracture Toughness Evaluation of S355 Steel Using CNRB 2019 47 2
Abstract
In engineering applications, steels are commonly used
in various areas. The mechanical members are exposed
to different loading conditions and this subject can be
investigated in fracture mechanics. Fracture toughness (KIC)
is the important material property for fracture mechanics.
Determination of this properties is possible using a compact
tension specimen, a single edge notched bend or three-point
loaded bend specimen, which are standardized by different
institutions. Researchers underline that these standardized
methods are complex, the manufacturing process is difcult,
they require special xtures for loading during the experiment
and the test procedures are time consuming. Alternative
methods are always being sought by researchers. In this work,
two different approaches are investigated for S355 steels. In
the rst method, a circumferentially cracked round bar was
loaded in tensile mode and pulled till failure. Using suitable
equations, fracture toughness can be calculated. In the second
method, a circumferentially notched bar specimen without
fatigue pre-cracking was loaded in a tensile machine. By means
of fracture load values, fracture toughness was determined by
the proposed equations. It can be stated that these two different
approaches for calculating fracture toughness are simple, fast
and economical.
Keywords
fracture toughness, notched round bars, pre-cracked specimen
1 Introduction
Fracture is a problem that society has faced for as long as
the man – made structures existed. Fracture mechanics is the
branch of solid mechanics. This branch explains behaviour
of bodies having cracks under different loading conditions.
The reasons of most structural failures are negligence during
design, construction of the structure, application of new
design or material which produces an unexpected result. If
the structural member contains a crack, then the component
becomes weak and nally fracture occurs. Fracture toughness
is the measure of resistance to crack propagation. Machines
and structural, components are oversized in order to avoid
failure. This situation leads to the consumption of more
material than designed and thus a high price. The main reason
for these problems is the non-availability of fracture toughness
data. In this respect, the value of fracture toughness is useful in
designing machine or structural components which are strong
but not oversized and overly heavy.
Fracture toughness is measured in terms of KIC (plane strain
fracture toughness), where K stands for the stress intensity
factor at the crack tip, “I” denotes the fracture toughness test
is performed in tensile mode and “C” denotes that value K is
critical. When K attains a critical value crack propagation then
becomes unstable and results in fracture of the components
(Dieter, 1988). Generally, KIC is determined by different meth-
ods such as using a compact tension specimen, a single edge
notched bend or three-point loaded bend specimens. Some
institutions have proposed several fracture toughness measure-
ment methods, for example, the American Society for Testing
and Materials (ASTM Designation). But the proposed meth-
ods are difcult and also time consuming. Fast and reliable test
methods are always being sought by researchers. In the liter-
ature, there are two different approaches to determining frac-
ture toughness of metallic materials. The rst approach uses a
notched round bar that is allowed to rotate under fatigue load
in an R.R. Moore fatigue testing machine, then a pre-cracked
specimen is loaded in a tensile testing machine and pulled till
failure. After that, crack lengths are measured with optical
measuring devices and fracture toughness calculated using the
1 Department of Mechanics, Materials and Machine Parts,
Jan Perner Transport Faculty, University of Pardubice,
Pardubice, Studentská 95, Czech Republic
* Corresponding author, e-mail: st43852@student.upce.cz
47(2), pp. 91-95, 2019
https://doi.org/10.3311/PPtr.11560
Creative Commons Attribution b
P
P
Periodica Polytechnica
Transportation Engineering
Fatih Bozkurt1*, Eva Schmidová1
This article was originally published with an error.
This version has been corrected/amended. Please see
Corrigendum (https://doi.org/10.3311/PPtr.12579)
92 Period. Polytech. Transp. Eng. F. Bozkurt, E. Schmidová
proposed equations (Londe et al., 2010; Londe et al., 2015).
The second approach uses a notched round bar that is directly
loaded in a tensile testing machine, and fracture toughness is cal-
culated using suitable equations (Bayram et al., 2002;Bayram
and Uguz, 1999; Bayram et al., 1999; Alaneme, 2011) and the
advantages of using circumferentially notched bars for fracture
toughness testing can be summarized as follows:
• The plane strain condition can be obtained;
• Because of radial symmetry of heat transfer, the micro-
structure of material along the circumferential area is
completely uniform;
• Machining and preparation of the specimens are easy;
• Performing of the fracture toughness test is simple;
• Does not require any special xtures to mount to speci-
men and costly instrumentation like clip gauge (Bayram
et al., 2002 and Londe et al., 2015)
In this work, two different approaches were examined for S355
steels and the results between the two methods were compared.
2 Experimental Procedure
S355 steels are structural steels that are used extensively
in general engineering applications. They are particularly
useful because they offer a unique combination of good
welding properties with guaranteed strengths. The chemical
composition of the steel used for the tests is shown in Table 1.
No heat treatment was applied.
Table 1 Chemcial Composition of S355 (wt. %)
CMn Si P S
0.2 1.82 0.27 0.012 0.003
Microstructure of S355 steel is shown in Fig. 1. It consists
equiaxed grains ferrite – pearlite microstructure. Lines shaped
micro volumes of pearlite is observed as a primary heterogene-
ity of carbon content.
Fig. 1 Microstructure of S355 steel
The round bar specimens were machined for the fatigue and
tensile tests. The dimensions of the specimens were: gauge
length 220mm (L0), diameter of notched section 10mm (d),
diameter of unnotched section 12mm (D), V-notch angle (α)
60° as shown in Fig. 2.
Fig. 2 Dimensions of round bar specimens
For the pre-cracking procedure, the samples were subjected
to cyclic tensile – compressive loads of equal amplitude were
applied with the stress ratio R equal to minus one (R = –1).
Pre-cracking (shown in Fig. 3) was done at a suitable bending
load (M) using a four-point R.R. Moore rotating beam fatigue
testing machine (shown in Fig. 4).
Fig. 3 Fatigue crack at notch tip
Fig. 4 R.R. Moore rotating beam fatigue testing machine
The limit load selected was such that the maximum stress
intensity factor (Kmax) should not exceed 60% of the minimum
expected fracture toughness KIC of the test material. In the
present work, a 40kg mass was hung from the fatigue testing
machine. Three samples were subjected to the fatigue tests with
a differing number of cycles and then pre-cracked samples were
loaded monotically in tension with a crosshead displacement
rate of 0.5 mm/min until failure. The displacement was
measured using extensometer (Fig. 5).
93
Fracture Toughness Evaluation of S355 Steel Using CNRB 2019 47 2
Fig. 5 Mounted round bar specimen and extensometer
For calculation of the fracture toughness value, crack lengths
of the fractured surface were examined by scanning electron
microscopy (SEM). The effective diameter (deff ) was calculated
by the sum of the machined notch depth (am) and the length of
the fatigue pre-crack (af) as in Eq. (1)
dD
aa
ef
fm
f
=−
()
+2
Equation (1) is used for calculation of the fracture toughness
in Eq. (2) (Londe et al., 2015) where Pf is the fracture load,
KP
D
D
d
IC
f
eff
=×
−
32 17
21
27
/
..
The valid range for the use of Eq. (2) is 0.46 < deff / D < 0.86,
where deff is the effective ligament diameter.
The second approach uses a notched round bar without
fatigue test that is directly loaded in a universal tension testing
machine. The term of the notched tensile strength (σNTS) is cal-
culated by Eq. (3) where Pf is the fracture load
σ
π
NTS
f
P
d
=
×
×
4
2
Eq. (3) is used for calculation of the fracture toughness in
Eq. (4) (Bayram et al., 2002) where Pf is the fracture load,
KD
IC NTS
=××0 454
12
./
σ
Some researchers suggest using Eq. (5), which is same as
Eq. (2) (Dieter, 1998), but in this formula the notched section
diameter is used instead of (deff ) for calculation of the fracture
toughness,
K
P
D
D
d
IC
f
=×
−
32 17
21
27
/
..
3 Results
During the experiments, an electro-hydraulic test stand was
used for tensile testing in both approaches (Fig. 6).
Fig. 6 Electro – hydraulic test stand
Fracture load which is maximum force can be read from data
acquisition for every specimen. The representative force – dis-
placement graph of the tensile test is shown in Fig 7. The most
important parameters for this approach are the dimensions of
fractured surface of specimen after tensile test. The samples are
cut and prepared for scanning electron microscopy. The machined
notch depth (am) and the length of the fatigue pre-crack (af) are
measured circumferential direction of fractured surface (Fig. 8).
At least 4 points are measured and average values are calculated.
The fracture toughness values calculated using Eq. (2) from the
data of the tensile test on the circumferentially cracked round bar
(CCRB) specimens and also the dimensions of the fractured sur-
face (with SEM observations), fracture load, notched and unno-
tched dimension, are tabulated in Table 2.
Fig. 7 Representative force – displacement graph of the tensile test
Table 2 Summary of Fracture Toughness Values and Dimensions
for CCRB specimen
Sample
No Pf kN D
mm am mm af mm deff
mm deff/D KIC
MPa m
S-1 46.6 11.9 0.99 0.67 8.58 0.72 40.4
S-2 47.6 11.8 0.98 0.5 8.84 0.74 38.13
S-3 51.4 11.72 0.99 0.188 9.36 0.79 35.78
(1)
(2)
(3)
(4)
(5)
94 Period. Polytech. Transp. Eng. F. Bozkurt, E. Schmidová
Fig. 8 Representative force – displacement graph of the tensile
The fracture toughness (KIC) of S355 steel varies from 35.78
MPa
m
to 40.4 MPa
m
. The average fracture toughness
experimentally obtained is 38.1 MPa
m
.
According to the second approach, which uses notched bar
specimens without fatigue pre-cracking, the calculated fracture
toughness (KIC) varies from 39.4 MPa
m
to 39.9 MPa
m
and the average value is 39.6 MPa
m
for Eq. (4). Because
the fracture toughness values were calculated by using the
fracture loads of the notched specimens, an increase in fracture
toughness with the increase in notch tensile strength is evident.
Fracture toughness was also calculated using Eq. (5) and in
this approach the values vary from 37.78 MPa
m
to 38.20
MPa
m
and the average value is 37.94 MPa
m
. Fracture
toughness, notch tensile strength and also dimension are
tabulated in Table 3. According to the results without pre-
cracked methodology, the calculated fracture toughness values
using Eq. (4) and Eq. (5) are in good agreement.
Table 3 Summary of Fracture Toughness Values and Dimensions
for Notched Bar Specimens
Sample
No Pf kN D mm d mm σNTS
MPa
KIC
MPa m
Eq. (4)
KIC
MPa m
Eq. (5)
S-4 58.19 11.58 9.58 807.3 39.4 37.78
S-5 59.88 11.68 9.68 813.7 39.9 38.20
S-6 58.71 11.62 9.62 807.7 39.5 37.85
The fracture surfaces of un-precracked S355 sample is
shown in Fig. 9 in loading direction and perpendicular to the
loading direction (side view of the samples). The surface of
S355 steel has moderate amount of necking and it is almost
cup and cone fracture characteristics. In central region has an
irregular and brous appearance, which signies plastic defor-
mation. In outer side of the fracture surface can be seen 45°
shear lips. This angle represents the direction of maximum
shear stress that causes shear lip in nal stage.
The value of the fracture toughness difference between
Eq. (2) and Eq. (4) is about 3% and the difference between
Eq. (2) and Eq. (5) is about 4%. It can be stated that the results
of the method which uses notched bar specimens are more
consistent than the rst method. In the literature, the fracture
toughness of structural steels (including low, medium and
high carbon steels) varies from 12 MPa
m
to 92 MPa
m
. These values are calculated using standardized test methods
while some of them also concern heat treated steels, which
means that higher fracture toughness values can be achieved
(Materials Data Book, 2003).
Fig. 9 The fractured surface of un – precracked of the sample
95
Fracture Toughness Evaluation of S355 Steel Using CNRB 2019 47 2
4 Conclusions
In this research, a method which uses a circumferentially
cracked round bar (CCRB) specimen and another approach
which uses a circumferentially notched bar specimen not
fatigue pre-cracked, can be used to determine the fracture
toughness values of metallic materials and they are observed
to be reliable procedure. The difference between two different
suggested approaches is remarkable and it is investigated from
fracture mechanics aspect. The SEM and optical observations
of tensile fractured surface shows two different regions which
are pre-cracked regions and sudden crack growth regions. The
fatigue fractured surfaces smoother than the tensile fractured
surfaces. The methodology of these experiments is simple, and
saves time regarding specimen preparation and the test pro-
cedures. They use simple instrumentation and do not require
costly measuring devices and equipment. The obtained values
are found to be in good agreement with the literature but in
future experiments standardized test methods should be per-
formed on samples of S355 steel, the methods could be com-
pared and the suggested equations should also be investigated.
Acknowledgement
This work was made possible with the support of the student
grant system of the University of Pardubice, project number
SGS_2017_009.
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