I. YAVUZ: FAILURE ANALYSIS OF A TRACTOR FRONT AXLE
163–167
FAILURE ANALYSIS OF A TRACTOR FRONT AXLE
ANALIZA PO[KODBE PREDNJE TRAKTORSKE OSI
Ibrahim Yavuz
Faculty of Technology, Afyon Kocatepe University, Afyon, Turkey
Received: 2022-12-12 - Accepted for publication: 2023-02-06
doi:10.17222/mit.2022.711
Tractors, high-power, low-speed traction vehicles and power units are similar to trucks or automobiles but designed for use
off-road. They are defined as motor vehicles with wheels that allow them to hold on to loose terrain and be fitted with trailers.
Tractors operate in challenging operating conditions. Therefore, it is possible that they become damaged. In some cases the
damage is due to disrupted engineering, but more frequently, it is due to failures in material processing and maintenance, raw
material errors, design and manufacturing errors, and user-related errors. This study examined the fractured front axle of a trac-
tor. Spectroscopic, metallographic and hardness measurements of the axle parts were made. Stress analyses were also performed
using finite elements to determine the stress conditions in the renewed section. The finite element analysis showed that the bro-
ken region was exposed to maximum stresses. Stress analyses using finite elements were also carried out to determine the stress
conditions in the repeated section. With the fracture surface analysis, mild fatigue was observed, and it was concluded that the
fracture occurred suddenly.
Keywords: fatigue failure, finite element analysis, front axle, tractor
Traktorji oziroma tra~na vozila z veliko mo~jo in majhno hitrostjo so mobilne naprave podobne tovornjakom ali avtomobilom,
vendar so oblikovana za vo`njo izven urejenih cesti{~. Definirana so kot motorna vozila s kolesi, ki se dr`ijo terena in so
prirejena tudi za pritrditev prikolic. Traktorji obratujejo oziroma delajo v zelo te`kih pogojih. Zato je mo`no, da na njih pride do
nekaterih po{kodb ali okvar. Nekatere okvare so in`enirske narave toda nekateri bolj pogosti vzroki so slaba izdelava ali izbira
materiala, napake nastale zaradi slabega vzdr`evanja, napake osnovnih surovin, oblikovanja in izdelave ter napake povezane
z uporabnikom vozila. V {tudiji so raziskovali vzroke za po{kodbo oziroma zlom prednje traktorske osi. Izvedli so spektro-
skopske, metalografske in mehanske preiskave delov traktorske osi. Opravili so napetostne analize s pomo~jo programskega
orodja na osnovi metode kon~nih elementov (MKE) in dolo~ili obremenitve obnovljenega dela. Rezultati analize dobljeni s
pomo~jo MKE so pokazali, da nastopajo maksimalne obremenitve na mestih, blizu katerih se je dejansko zlomila traktorska os.
Napetostno analizo MKE so izvedli tudi za pogoje dinami~nih (izmeni~nih) obremenitev. Povr{inska analiza preloma je
pokazala, da je pri{lo do rahlega utrujanja materiala, zato so zaklju~ili, da je verjetno pri{lo do nenadnega (trenutnega) zloma
preiskovane traktorske osi zaradi utrujanja materiala.
Klju~ne besede: dinami~na po{kodba, utrujanje materiala, metoda kon~nih elementov, prednja traktorska os
1 INTRODUCTION
Today, the tractor is one of the most critical machines
in agriculture. The effect of tractors on agriculture is in-
disputable.
1
In recent years, the rapid development of
technology has led to the modernization of agriculture.
Agricultural machines and tractors are important exam-
ples of this modern technology.
2–4
The quality of me-
chanical inputs, land, and labour productivity can vary
considerably.
5–7
Tractors carry loads with trailers at-
tached to their backs; they are used for ploughing, plant-
ing, and so on. They operate in very harsh conditions.
Tractors are affected by various loads including the
above-mentioned land operation and the complex dy-
namic loads caused by the changing surface of the
ground during their use on the farm. The dynamic stress
spectrum is a history of loading time that reflects the
state of the load of the entire machine structure.
8,9
The braking and steering systems of tractors are simi-
lar to those of the other vehicles. The tractor front axle
shaft is the part in a tractor that enables rotation and
carries the load. The tractor front axle is one of the most
important components. It requires a perfect design so
that the entire tractor can operate in extreme conditions.
Axle shafts are subjected to axial loads at different an-
gles depending on the site conditions. The road rough-
ness depending on the terrain increases these difficulties
and causes axle damage. A schematic view of the front
axle system of a 2WD tractor is shown in Figure 1.
Materiali in tehnologije / Materials and technology 57 (2023) 2, 163–167 163
UDK 623.437.42:620.1/.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(2)163(2023)
*Corresponding author's e-mail:
iyavuz@aku.edu.tr (Ibrahim Yavuz) Figure 1: Tractor front axle

A damage analysis and prevention are of interest to
all engineering branches. A damage analysis is an engi-
neering approach that determines how and why a part or
device becomes unusable. When performing a damage
analysis, safety, performance and economic use are taken
into account. A damage analysis is necessary to prevent
damage and understand the progress of the system and
product quality. Overall, the goal is to achieve the de-
signs and products that are produced in line with the
highest standards and best meet the expectations of users
and consumers.
Fracture damage is the most unwanted kind of dam-
age. Therefore, several studies were conducted on bro-
ken parts for a damage analysis. In the studies of trans-
mission, axle shafts and joints were examined and it was
found that the axle shafts were damaged by heat. Some
researchers concluded that the damage was due to some
defects in designs.
10–12
Many studies of the rear axle shafts were conducted.
Chemical analyses, examinations of mechanical proper-
ties, microstructure and breaking surface (fractography),
as well as EDX analyses were carried out as part of dam-
age-analysis examinations. Many researchers also used
this method to study models. The failure or fracture of a
rear axle shaft can cause death and injuries in transit and
significant financial losses. An improper design or other
metallurgical causes usually lead to a rear axle shaft frac-
ture.
13–16
Other researchers conducted some simulation
studies to estimate the damage. The simulations allowed
them to conclude that the damage and stress zones were
similar.
17–20
This study investigated an early failure of an (operat-
ing) tractor front axle with similar methods of inspec-
tion. Stress analyses were also performed using the finite
element method. Figure 2 shows the tractor front axle
and the broken surface analysis.
2 EXPERIMENTAL PART
In this study, a chemical analysis, examinations of
mechanical properties, microstructure and fracture sur-
face (fractography), SEM and EDX analyses, and finite
element (stress) analysis were used for an in-depth fail-
ure analysis.
Samples cut from the front axle were prepared for a
chemical analysis. A GNR device was used to obtain the
chemical structure of the axle.
The samples were taken from the damaged tractor
front axle for a micro-structure analysis. They were pol-
ished with 60, 120, 240, 400, 600, 800, 1000, and 1200
sandpaper, and a Forcipol sandpaper machine was used.
An alumina-pure water solution was poured and polished
on a diaper. At the end of the polishing process, abrasion
was carried out and images were taken at 100× and 200×
magnifications using an Olympus optical microscope.
The specimens were examined under a scanning elec-
tron microscope (SEM) (LEO 1430VP, Carl Zeiss, Jena,
Germany) with an energy-dispersive X-ray analysis de-
vice (EDS, Oxford Instrument Link ISIS, Oxford, UK)
operating at 10–20 kV with a working distance of 10
mm.
A SHIMADZU device was used for the hardness test.
The average hardness value determined using the Vickers
hardness measurement method under a 1,961 N load was
HV 0.02.
3 RESULTS AND DISCUSSION
3.1 Chemical analysis
The average values of the tests performed in three
different regions were used for the chemical analysis of
the axle shaft. As a result of the chemical analysis, the
material of the axle shaft was determined as 50CrMo4
(AISI 4150) steel with a nominal chemical composition,
as shown in Table 1. This is a low-alloy steel with a me-
dium carbon content, often used in automotive applica-
tions and heat-treated to improve the surface mechanical
properties.
21
50CrMo4 steels are also called chromium-molybde-
num steels. These steels contain 0.80–1.10 % of chro-
mium and 0.15–0.25 % of molybdenum. Molybdenum
inhibits the growth of grains and increases the hard-
I. YAVUZ: FAILURE ANALYSIS OF A TRACTOR FRONT AXLE
164 Materiali in tehnologije / Materials and technology 57 (2023) 2, 163–167
Figure 2: Tractor front axle and a broken surface analysis
Table 1: Comparison between nominal and chemical-analysis compositions
Composition, w/%
Fe Cr Mg C Si Mo S P
Axle 96.13 1.19 0.9 0.5 0.16 0.16 0.018 0.014
50CrMo4 96.6–97.8 0.9–1.2 0.7–1.1 0.48–0.53 0.15–0.3 0.15–0.25 < 0.04 < 0.035

enability of steel. Chromium increases the capability of
the material to retain stiffness and strength. Tempered
steels are used for the production of automobile and air-
plane parts with a high foam content such as crankshafts,
axle shafts and sleeves. A comparison between the nomi-
nal composition and chemical analysis is presented in
Table 1.
3.2 Micro-Structure Analysis
The martensitic and ferritic structures stand out in the
microstructure examinations (Figure 3). This is one of
the steel alloys suitable for tempered steel hardening. As
can be seen from the rigidity analysis, the axle was sub-
jected to surface hardening. Therefore, the outer surface
is hard, and the inner parts are ductile.
3.3 SEM (Scanning electron microscope) and EDX
analysis
The device can perform qualitative and semi-quanti-
tative elementary analyses of images with the point, line,
area and mapping methods.
In Figure 4a, signs of a brittle fracture of the struc-
ture are observed. The fracture surface gradually ob-
tained micro-scale pit characteristics as shown in Fig-
ure 4b. The reason for this is the martensitic structure.
After SEM images of the damaged-axle sample sur-
face were taken, an EDX analysis was performed. The
elemental analysis of the fractured surface is presented in
I. YAVUZ: FAILURE ANALYSIS OF A TRACTOR FRONT AXLE
Materiali in tehnologije / Materials and technology 57 (2023) 2, 163–167 165
Figure 3: Optical-microscope photographs: a) 50 μm magnification
image and b) 100 μm magnification image
Figure 5: Tractor-axle EDX analysis Figure 4: SEM images of the fracture surface: a) 1000× and b) 5000×

Figure 5. In the EDX analysis results, Cr and Mn stand
out. Cr makes alloy steels more durable and more rigid
than the standard carbon steel. Mn increases the
hardenability of steel. The most crucial feature of Mn is
that it makes a MnS compound with sulphur and pre-
vents the formation of a FeS compound. FeS causes hot
brittleness.
The semi-quantitative chemical analysis with EDX,
added to SEM, was performed to qualitatively identify
the damaged tractor axle shaft alloy and confirm the
presence of any other components. No components that
could cause damage were detected with the EDX analy-
sis.
3.4 Hardness Analysis
Measurements were made between the outer side and
the centre and the results are given in Figure 6.
Figure 6 shows that the hardening takes place on the
outermost part and the axis reaches the proper hardness
towards the centre. It is noteworthy that the hardness is
highest on the outer surface, with a value of 900 HV, de-
creasing towards the centre of the axis and reaching a
core hardness of 323 HV. In the literature review con-
ducted, it was determined that the outer surface hardness
is around 700 HV and the inner surface hardness is
around 250 HV at the end of hardening.
21
The outer surface becomes too hard, making the ma-
terial brittle. Inside, the material is softer but relatively
rigid when compared to the rest of the material. This
makes the material extremely brittle. Brittleness, on the
other hand, causes fractures due to impacts on the mate-
rial.
3.5 Numerical modelling and finite elements analysis
In Figure 7, the distribution of the von-Mises stress
values for the loading and boundary conditions, the finite
element network, and the tractor axle end are shown for
the original design condition.
A variable element mesh model was applied to the
mathematical model. Thus, critical regions were divided
into more frequent elements. The total numbers of nodes
and elements were 25345 and 14410, respectively. The
vertical part of the axle was anchored, and the forces un-
der the operating conditions were applied in the wheel
hub area. The bearing load applied to the tractor front
hub seating surface was calculated by considering the av-
erage weight of the tractor. The force per front wheel of
the tractor was calculated as 5.9 kN. However, loads
have a highly variable character for real-road and terrain
conditions. These load values need to be taken into ac-
count by providing the extreme values encountered by
the analyzed component. Afterwards, a static stress anal-
ysis was performed for the tractor front axle under dam-
aged design conditions.
Figure 7 shows the distribution of the von-Mises
stress values for the tractor front axle for the damaged
design condition. As shown in Figures 7c and 7d,itwas
determined that the maximum von-Mises stress at the
critical fracture zone of the tractor front axle end for the
original design condition and damaged design condition
is  = 115 MPa. However, the variable character of the
applied stress and possible effect of the forces from
road-surface irregularities can decrease the fatigue limit.
Consequently, the tractor front axle end should have
an infinite life at this loading and the original design
condition. However, from the above observations, the
fact that the fracture-damaged area and the high-stress
concentration region of the axle shaft end occur in the
same parts as the damaged crack origin proves that it
contributed to the failure (Figure 8). The fracture region
acts as a stress enhancer and may be an additional factor
in the present situation. A fatigue fracture almost always
I. YAVUZ: FAILURE ANALYSIS OF A TRACTOR FRONT AXLE
166 Materiali in tehnologije / Materials and technology 57 (2023) 2, 163–167
Figure 7: Von-Mises stress distribution for the tractor front axle dam-
aged by fracture
Figure 6: Micro-hardness analysis results Figure 8: Comparison of the damaged area and stress concentration

occurs in parts such as notches, cracks or other stress
concentrations. In the analysis, it was seen that the maxi-
mum stresses occur in the critical section of the transi-
tion region. It should be considered that this condition
causes a high-stress concentration and may lead to the
initiation of fatigue cracking during operation.
In brittle fractures, a macroscopic division is ob-
served. It does not matter if the materials are metallic or
non-metallic as long as they are brittle. Separation is de-
fined as the breaking of a body on a single surface.
22
As
the cleavage will show slight variations at the plane lev-
els of multiple fractures, the surface will show fine crev-
ices called the fatigue pattern that gives the fractures
their characteristic aspects with a macroscopic cleavage,
as illustrated with the example in Figure 8. The propaga-
tion exhibits a greater roughness as the crack progresses
towards the final split point.
4 DISCUSSION AND CONCLUSIONS
In this study, a damage analysis of a tractor main axle
shaft was carried out. The chemical analysis, scanning
electron microscopy (SEM), and mechanical test results
determined the damaged axle-shaft material. As a result
of the analyses, the material was determined as hardened
and tempered 50CrMo4 alloy steel. It was believed that
there were some defects in the manufacture of the mate-
rial. In particular, it was observed that the hardness of the
material was much higher than average. It was concluded
that this increased the brittleness of the material.
Visual inspection showed a typical fatigue fracture
with traces of damage. According to the finite element
analysis, the most stress occurred on the surface close to
the mechanically damaged area. As a result of the finite
element numerical analysis, it was concluded that the fa-
tigue fracture of the axle shaft was consistent with the
actual fracture. Although the fatigue zone was tiny due to
excessive hardening of the material, it was concluded
that the axle was damaged due to fatigue. It is thought to
have broken suddenly due to excessive hardening.
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