N. VU^ETI] et al.: INTEGRITY ASSESSMENT OF AN AIRCRAFT CYLINDER ASSEMBLY WITH A CRACK
389–396
INTEGRITY ASSESSMENT OF AN AIRCRAFT CYLINDER
ASSEMBLY WITH A CRACK
OCENA INTEGRITETE GLAVE VALJA Z RAZPOKO V
LETALSKEM MOTORJU
Nikola Vu~eti}
1*
, Gordana Jovi~i}
2
, Ranko Antunovi}
1
, Sandra Sovilj-Niki}
3
,
Aleksandar Ko{arac
1
, Dejan Jeremi}
1
1
University of East Sarajevo, Faculty of Mechanical Engineering East Sarajevo, Vuka Karadzica 30, 71123 East Sarajevo,
Bosnia and Herzegovina
2
University of Kragujevac, Faculty of Engineering, Sestre Janjic 6, 34000 Kragujevac, Serbia
3
University of Novi Sad, Faculty of Education, Podgoricka 4, 25000 Sombor, Serbia
Prejem rokopisa – received: 2022-03-01; sprejem za objavo – accepted for publication: 2022-06-17
doi:10.17222/mit.2022.430
The repeatability of the air-cooled piston engine cylinder assembly failure due to a crack in the cylinder head, as well as its se-
verity from the aspect of crew and passenger safety were the main motives for our research. In this paper an integrity assessment
of a cylinder assembly with a crack was performed. By modeling cracks of different lengths in the cylinder head and consider-
ing the values of stress intensity factors and J-integral values at a given crack length on the one hand and determining the critical
values of these fracture mechanics parameters on the other hand, the stability of the crack was examined. As part of the re-
search, the dependence of the crack length on the stress intensity factor was established. The methodology proposed in this pa-
per can be adapted to assess the integrity of other similar structural elements.
Keywords: crack, aircraft cylinder head, stress intensity factor, integrity assessment
V ~lanku je opisana raziskava ponavljajo~ih se po{kodb na zra~no hlajenih letalskih motorjih zaradi pojava razpok v glavi
motorja. Varnosti letalske posadke in potnikov je bil motiv za izvedbo preiskave nastalih razpok. Avtorji opisujejo oceno
integritete letalskega motorja s prisotno razpoko na cilindru. Z modeliranjem razpok razli~nih dol`in na glavi cilindra so dolo~ili
vrednosti faktorja intenzitete napetosti in J-integrala za dano dol`ino razpoke. Po drugi strani so na osnovi poznavanja kriti~nih
vrednosti lomno mehanskih parametrov ocenili stabilnost razpoke oziroma nevarnost za njeno nenadzorovano napredovanje. V
raziskavi so dolo~ili tudi vpliv dol`ine razpoke na faktor intenzitete napetosti. Avtorji v raziskavi ugotavljajo, da se predlagana
metodologija lahko prilagojena uporabi za oceno integritete drugih podobnih strojnih oziroma strukturnih elementov.
Klju~ne besede: razpoka, glava valja v letalskem motorju, faktor intenzitete napetosti, ocena integritete
1 INTRODUCTION
The research in this paper includes an integrity as-
sessment of a Lycoming-IO-360-B1F aircraft air-cooled
piston-engine cylinder assembly which failed after
1389 h due to a crack occurrence in the cylinder head
1–4
(Figure 1).
Based on the reports from the competent aviation au-
thorities of countries around the world, there have been
as many as 47 such failures of conventional air-cooled
engines.
5,6
In this paper cracks of different lengths in the
cylinder head were modeled. The values of the stress in-
tensity factor and J-integral were obtained and compared
with the critical values of the mentioned parameters.
Based on this, the crack stability was examined and the
integrity of the cylinder assembly was estimated.
7–13
The
influence of the crack length in the cylinder head on the
value of the stress intensity factor was determined. A
cylinder head is made of aluminum casting alloy of
grade 242.0. In previous research, necessary mechanical
properties of this material were experimentally deter-
mined at room and elevated temperature
14
and a struc-
tural analysis of the cylinder assembly exposed to a com-
bined thermomechanical load was performed.
15
This re-
search is of great importance, given that the literature is
very scarce on the data related to aluminum casting alloy
of grade 242.0 and the analysis of its elements. The re-
sults obtained can be used for further research in the
field of fracture mechanics and fatigue related to the
problem of crack occurrence and failure of the structural
elements made of the above material, and also of other
materials.
2 EXPERIMENTAL PART
This section presents a numerical simulation of a
compact-tension (CT) specimen tensile test at elevated
temperature using Ansys Workbench. Fracture toughness
can be defined as the ability of a structure with a crack to
withstand the load to which it is exposed without failure,
i.e., fracture toughness is a measure of the resistance of a
material to crack propagation.
16
Fracture occurs when the
stress intensity factor exceeds the critical value, that is
the fracture toughness, K
C
. In order to determine the crit-
Materiali in tehnologije / Materials and technology 56 (2022) 4, 389–396 389
UDK 621.452:620.1/.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 56(4)389(2022)
*Corresponding author's e-mail:
nikola.vucetic@ues.rs.ba (Nikola Vu~eti})

ical value of the stress intensity factor, the tensile
strength of the aluminum casting alloy of grade 242.0 at
room and elevated temperature was first defined.
17
2.1 Tensile test
Determination of the tensile strength of the material
was performed on a series of six specimens at room tem-
perature. The dimensions and shape of the specimens
were in accordance with standards B 557M
18
and ASTM
E646-00.
19
The testing of the mentioned specimens was
performed at the Center for Engineering Software and
Dynamic Testing of the Faculty of Engineering, Univer-
sity of Kragujevac, on a SHIMADZU servohydraulic
pulsator/shredder, type EHF-EV101K3-070-0. The test
was performed at room temperature, including the con-
trol of displacement increment corresponding to the
static load conditions for fracture and in accordance with
standards B 557M and ASTM E646-00 (Figure 2).
Table 1 shows the experimentally determined values
of the tensile strength stress for aluminum casting alloy
of grade 242.0 at room temperature.
17
Table 1: Experimentally determined values of the tensile strength for
aluminum casting alloy of grade 242.0 at room temperature
Specimen designation Tensile strength R
m
(MPa)
1 205.7
2 198.4
3 192.5
4 186.7
5 201.3
6 190.8
The testing of static properties at elevated tempera-
ture was performed in order to determine the tensile
strength of aluminum casting alloy of grade 242.0 at a
temperature of 200 °C, corresponding to the operating
temperature of the cylinder assembly, i.e., the cylinder
head. The mentioned examination was performed at the
Kemal Kapetanovi} Institute in Zenica. The tensile
strength of aluminum casting alloy of grade 242.0 was
determined on a series of three test specimens. The di-
mensions and shape of the specimens were designed to
be in accordance with standard B 557M.
The testing of the tensile strength was performed on a
universal hydraulic machine for static testing, Amsler,
type 20 SZBDA (Figure 3). First, a test specimen was
placed in the heating chamber of the above machine with
the temperature set to 200 °C. After reaching the desired
chamber temperature the test specimen was, according to
N. VU^ETI] et al.: INTEGRITY ASSESSMENT OF AN AIRCRAFT CYLINDER ASSEMBLY WITH A CRACK
390 Materiali in tehnologije / Materials and technology 56 (2022) 4, 389–396
Figure 1: Crack in the cylinder head
2
Figure 2: Determination of the tensile strength at room temperature

the instructions, heated for another 15 min in order to
equalize the temperature field inside the specimen mate-
rial.
Table 2 shows the experimentally determined values
of the tensile strength for aluminum casting alloy of
grade 242.0 at elevated temperature.
17
Table 2: Experimentally determined values of the tensile strength for
aluminum casting alloy of grade 242.0 at elevated temperature
Specimen designation Tensile strength R
m
(MPa)
1-E 143
2-E 115
3-E 131
Based on the obtained tensile-strength values for alu-
minum casting alloy of grade 242.0 at elevated tempera-
ture, which were in a range of 115–143 MPa, it can be
seen that with the increasing temperature the tensile
strength of the material is decreased compared to the val-
ues obtained with the test at room temperature.
2.2 Numerical determination of the critical value of
the aluminum casting alloy 242.0 stress intensity
factor
A CT specimen model was created using Design
Modeler in Ansys Workbench. Specimen dimensions
were defined by the ASTM E399-12 standard.
20
Based
on the 3D model of the CT specimen, a finite-element
mesh of the CT specimen, with tetrahedral finite ele-
ments having internodes, was created and then the mesh
was chopped in the cracked area with the average size of
the elements being 0.3 mm (Figure 4). The finite-ele-
ment mesh thus generated consisted of 28293 elements
and 47536 nodes.
Using the fracture-tool module in Ansys Workbench,
the initial crack was defined. Also, the local coordinate
system related to the crack tip was generated. In order to
obtain the value of the stress intensity factor at elevated
temperature, thermal and structural analyses were
combined
17
, i.e., the results of the thermal analysis were
imported into the structural analysis. During the thermal
analysis, a temperature field of 200 °C was applied to the
CT specimen, corresponding to the cylinder-head operat-
ing temperature.
15
For the numerical simulation, the back surface of the
CT specimen was fixed. The upper and lower surfaces of
the specimen were subjected to a load in the vertical di-
rection so that the specimen was tension loaded to such
an extent that the safety factor at the crack front had a
value of approximately 1 (Figure 5). This was the mo-
ment when the measure of stress concentration in the vi-
cinity of the crack tip had such a value that it was on the
verge of causing its propagation, i.e., the obtained stress
N. VU^ETI] et al.: INTEGRITY ASSESSMENT OF AN AIRCRAFT CYLINDER ASSEMBLY WITH A CRACK
Materiali in tehnologije / Materials and technology 56 (2022) 4, 389–396 391
Figure 3: Determination of the tensile strength at elevated tempera-
ture
Figure 4: Finite-element mesh of the CT specimen

intensity factor in this case represented the critical value
of the stress intensity factor.
The volumetric J-integral was used to define the
stress intensity factor. This type of J-integral is suitable
for application in the analysis of structures in which, in
addition to mechanical, a thermal load also occurs and,
unlike the contour J-integral, it gives more accurate re-
sults.
21
After the numerical simulation, the critical values
of the stress intensity factor of the aluminum casting al-
loy of grade 242.0 for the six volumes of integration
were obtained, as well as a value of the J-integral of
12234.7 J/m
2
. Figure 6 shows the results of the stress in-
tensity factor for the first integration volume for Mode I
of the crack opening.
Figure 7 shows a diagram of the critical values of the
stress intensity factor for the CT specimen for six vol-
umes of J-integral integration for Mode I of the crack
opening.
For the analysis of crack stability determination dis-
cussed in the following section, the mean value of all in-
tegration volumes from Figure 7 was adopted so that for
Mode I of the crack opening, it is 16.25 MPa·m
1/2
repre-
senting the critical value of stress intensity factor of the
aluminum casting alloy of grade 242.0.
3 DETERMINING CRACK STABILITY FROM
THE ASPECT OF ITS FURTHER PROPAGATION
3D cracks were modeled in the cylinder head at the
location where they appeared in practice. The crack
lengths varied from 1 mm to 5 mm and, for each case,
the stress intensity factor was determined. The obtained
values of the stress intensity factor were compared with
the critical value of the stress intensity factor, i.e., the
stability of cracks was determined from the point of view
of their further propagation. A number of papers related
to the issue of structural analysis of crack propagation
can be found in the literature.
22,23
Within the 3D model of
the cylinder assembly, a local plane was formed, in
which cracks of different lengths were modeled (Fig-
ure 8). This plane corresponded to the plane with the
crack of the cylinder assembly that failed during flight.
When considering the stability of 3D crack propaga-
tion, the nodes along the crack front were observed. In
order to avoid an error when calculating the J-integral,
several integration volumes around the crack tip were de-
fined.
23
In the 3D model of the cylinder head, a 1-mm-long
crack was modeled. After that, the finite-element mesh
was defined. In order to analyze the stress state in the vi-
cinity of the crack tip in more detail, the local coordinate
system was set at the crack front and a radius of the
sphere of integration of the J-integral of 3 mm was de-
fined. Within this sphere, the finite-element mesh was
N. VU^ETI] et al.: INTEGRITY ASSESSMENT OF AN AIRCRAFT CYLINDER ASSEMBLY WITH A CRACK
392 Materiali in tehnologije / Materials and technology 56 (2022) 4, 389–396
Figure 7: Diagram of critical values of stress intensity factor for Mode I of the crack opening
Figure 6: Values of the stress intensity factor for the first integration
volume for Mode I of the crack opening
Figure 5: Safety factor at the crack front

chopped so that the average size of the tetrahedral finite
elements with internodes was 0.1 mm (Figure 9).
The testing of the stability of the defined crack was
performed under a thermomechanical load and boundary
conditions defined in previous research.
15
Figure 10
shows the lifetime of the cylinder head with a
1-mm-long crack.
It is clear that the minimum number of cycles to fail-
ure applies to the vicinity of the crack tip. The lifetime of
the rest of the cylinder head ranges from 2.80 × 10
8
to
2.98 × 10
8
cycles, which corresponds to the lifetime of
the cylinder head estimated at 3600 h
24
, i.e., the number
of cycles of 2.92 × 10
8
taking into account that one cycle
implies one operating stroke of the engine, that is two
full crankshaft revolutions which, based on the given
nominal number of revolutions, last for 0.044 s.
17
Based
on the analysis of the material fracture by the fracture
tool module in Ansys Workbench, the stress intensity
factor was determined, representing the measure of stress
concentration in the vicinity of the crack tip. The stress
intensity factor was obtained for the crack opening
mode, i.e., Mode I of the crack opening. Figure 11
shows the values of the stress intensity factor for the
N. VU^ETI] et al.: INTEGRITY ASSESSMENT OF AN AIRCRAFT CYLINDER ASSEMBLY WITH A CRACK
Materiali in tehnologije / Materials and technology 56 (2022) 4, 389–396 393
Figure 8: Local plane of the 3D crack on the cylinder assembly
Figure 10: Lifetime of the cylinder head with a 1-mm-long crack Figure 9: Finite-element mesh in the vicinity of a 1-mm-long crack

1-mm-long crack for the second of the six integration
volumes.
Figure 12 shows a diagram of the values of the stress
intensity factor for six analyzed volumes of integration.
For further crack stability analysis, a mean value of all
integration volumes of 3.34 MPa·m
1/2
was adopted.
Using the fracture tool module, the values of the J-in-
tegral in the case of a 1-mm-long crack were obtained.
All six volumes of integration were analyzed. A mean
value of the J-integral of 94.76 J/m
2
was adopted for fur-
ther analysis of the considered crack stability. The same
procedure was performed for (2, 3 and 5) mm-long
cracks.
In order to test the resistance to breakage of the cylin-
der head, the values of stress intensity factors for cracks
of different lengths were investigated. The dependence
of the stress intensity factor on the crack length is shown
in Figure 13.
From the above diagram, it can be seen that with the
crack growth from 2 mm onwards the value of the stress
intensity factor increases. The values of the stress inten-
sity factor in the presence of 1-mm and 2-mm-long crack
in the cylinder head are 3.34 MPa·m
1/2
and
12.01 MPa·m
1/2
, respectively. Taking into account that
the critical value of the stress intensity factor is
16.25 MPa·m
1/2
it is concluded that a crack length of
2.25 mm is considered the critical crack length so that
the cylinder head is a stable structure in terms of further
crack propagation during operation in the presence of a
crack length below 2.25 mm.
4 RESULTS AND DISCUSSION
The numerically obtained value of the stress intensity
factor for the 1-mm-long crack is 3.34 MPa·m
1/2
, while
the value of the J-integral is 94.76 J/m
2
. Taking into ac-
count the critical value of the stress intensity factor of
16.25 MPa·m
1/2
, and the critical value of the J-integral of
12234.7 J/m
2
, it is concluded that a 1-mm-long crack in
the cylinder head can be considered stable with regard to
further propagation. Based on the numerical analysis of
the cylinder assembly with a crack in the cylinder head,
it was found that the value of the stress intensity factor
for a 2-mm-long crack is 12.01 MPa·m
1/2
, while the value
of the J-integral is 1200.5 J/m
2
. If we take into account
the critical value of the stress intensity factor of
16.25 MPa·m
1/2
, and the critical value of J-integral of
12234.7 J/m
2
, it can be concluded that in the case of a
crack length of 2 mm the values of stress intensity factor
and J-integral are below the critical values so that the
crack of the stated length in the considered construction
can still be considered stable.
The value of the stress intensity factor in the case of a
3-mm-long crack is 28.89 MPa·m
1/2
where the value of
the J-integral is 3142.1 J/m
2
. Comparing the obtained
N. VU^ETI] et al.: INTEGRITY ASSESSMENT OF AN AIRCRAFT CYLINDER ASSEMBLY WITH A CRACK
394 Materiali in tehnologije / Materials and technology 56 (2022) 4, 389–396
Figure 13: Dependence of the stress intensity factor on the crack
length
Figure 11: Stress intensity factor values for a 1-mm-long crack
Figure 12: Diagram of values of the stress intensity factor for a 1-mm-long crack

values with the critical values of the stress intensity fac-
tor of 16.25 MPa·m
1/2
and the J-integral of 12234.7 J/m
2
,
it is concluded that in the case of a 3-mm-long crack the
value of the stress intensity factor is higher than the criti-
cal value. Based on the above, a 3-mm-long crack in the
considered construction cannot be considered stable.
The numerical analysis of the cylinder assembly with
a 5-mm-long crack in the cylinder head showed a value
of the stress intensity factor of 47.05 MPa·m
1/2
and a
value of the J-integral of 17284.3 J/m
2
. Compared with
the critical values of the stress intensity factor of
16.25 MPa·m
1/2
and the J-integral of 12234.7 J/m
2
,itis
concluded that in the case of a 5-mm-long crack the val-
ues of the stress intensity factor and J-integral are much
higher than the critical values, indicating an uncontrolled
crack growth.
5 CONCLUSIONS
The research reported in this paper refers to determi-
nation of the critical value of the stress intensity factor
for aluminum casting alloy of grade 242.0 as the constit-
uent material of the cylinder head of an aircraft piston
engine that failed due to a crack in the cylinder head. For
cracks of different lengths, values of the stress intensity
factor were obtained and compared with the critical
value; the crack stability, i.e., the integrity of the cylinder
assembly was assessed. Within the simulation, the stress
intensity factor for Mode I of the crack opening (the
opening mode) was considered. This crack mode is char-
acterized by a force that separates the crack surface by
acting perpendicularly to the crack plane. The research
in the paper shows that the critical crack length is
2.25 mm and that the analyzed cylinder assembly can be
considered a stable structure in the presence of a crack
length less than 2.25 mm. The obtained results are of
great importance for further research in the field of frac-
ture and fatigue mechanics related to the problem of
cracking and failure of structural elements made of alu-
minum casting alloy of grade 242.0, and also of other
materials. As the issues discussed in this paper are very
complex with respect to the nature of the load where me-
chanical and thermal loads are present, it is important to
emphasize that this methodology is comprehensive and
adaptable when assessing the integrity of any structural
element.
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