2004-01-0628
Fatigue Life Comparisons of Competing Manufacturing
Processes: A Study of Steering Knuckle
Mehrdad Zoroufi and Ali Fatemi
The University of Toledo
Copyright © 2003 SAE International
The steering knuckle, being a part of the vehicle’s
suspension system, has alternatives of forging and
casting as its base manufacturing process. Since it is
connected to the steering parts and strut assembly from
one side and the wheel hub assembly from the other, it
has complex restraint and constraint conditions and
tolerates a combination of loads. In addition, parameters
such as internal defects, stress concentrations and
gradients, surface finish, and residual stresses can have
considerable influence while designing for fatigue.
ABSTRACT
A vehicle steering knuckle undergoes time-varying
loadings during its service life. Fatigue behavior is,
therefore, a key consideration in its design and
performance evaluation. This research program aimed
to assess fatigue life and compare fatigue performance
of steering knuckles made from three materials of
different manufacturing processes. These include forged
steel, cast aluminum, and cast iron knuckles. In light of
the high volume of forged steel vehicle components, the
forging process was considered as base for
investigation. Monotonic and strain-controlled fatigue
tests of specimens machined from the three knuckles
were conducted. Static as well as baseline cyclic
deformation and fatigue properties were obtained and
compared. In addition, a number of load-controlled
fatigue component tests were conducted for the forged
steel and cast aluminum knuckles. Finite element
models of the steering knuckles were also analyzed to
obtain stress distributions in each component. Based on
the results of component testing and finite element
analysis, fatigue behaviors of the three materials and
manufacturing processes are then compared. It is
concluded that the forged steel knuckle exhibits superior
fatigue behavior, compared to the cast iron and cast
aluminum knuckles.
A common practice of fatigue design consists of a
combination of analysis and testing. A problem that
arises at the fatigue design stage of components is the
transferability of data from smooth specimens to the
component. The component geometry and surface
specifications often deviate from that of the specimen
investigated and neither a nominal stress nor a notch
factor can be defined in most cases. An advantage of
component testing is that the effects of material,
manufacturing process parameters, and geometry are
inherently accounted for, even though synergistically.
Gunnarson et al.(1) investigated replacing conventional
forged quenched and tempered steel with precipitation
hardened pearlitic-ferritic cast steels. They also
compared toughness and machinability characteristics of
forging versus casting components. They observed
insufficient toughness but higher machinability for cast
components. Lee(2) evaluated fatigue strength for truck
axle housing, crankshaft, leaf spring, torque-rod
assembly, and steering knuckle. For the case of the
steering knuckle, two sets of tests at constant load
amplitude were conducted. The load was applied to the
wheel stud (spindle) and carrier tube (body), and the
acceptance criteria were no crack initiation and no
permanent deformation until 2x105 cycles. Among five
total tests, none of the knuckles failed. Lee et al.(3)
developed a methodology to quantitatively assess
fatigue lives of automotive structures and to identify
critical
and
non-damaging
areas
for
design
enhancement and weight reduction. An MS-3760A cast
INTRODUCTION
There has been a strong trend towards the adoption of
optimum materials and components in automotive
industry. Automotive designers have a wide range of
materials and processes to select from. Steel forgings
are in competition with aluminum forgings and castings,
cast iron, and sintered powder forgings. The competition
is particularly acute in the chassis, and it is not unusual
to find a range of different materials and manufacturing
technologies employed within modern chassis
components.
1
iron steering knuckle was the example component of this
study. The methodology combines load-time history file
with results from elastic FEA to estimate fatigue lives.
Knuckle strain gage measurements were made for
elastic as well as inelastic load ranges. The correlations
of maximum principal strains between the FEA and the
experimental results showed average errors of 23% and
27% for lateral and fore/aft loads in elastic range,
respectively. The differences between observed and
predicted lives in the inelastic range were found to be
factors of 3.9 and 1.4 at the R50C50 life (the fatigue life
with reliability of 50% and confidence level of 50%) for
fore/aft and lateral loading tests, respectively.
for both the specimens and the components, were
preconditions for the transferability of material data
obtained from specimens to the component. The
maximum stressed/strained material volume appeared
to be suitable for taking into account the statistical and
mechanical size effects in a relatively simple manner.
The objectives of the current study were to compare
fatigue performance and assess fatigue life for steering
knuckle, a fatigue critical part, made from three materials
of different manufacturing processes. Knuckles of three
vehicles were selected. These included forged steel
SAE Grade 11V37 knuckle of the rear suspension of a
4-cylinder sedan weighing 2.5 kg, cast aluminum ASTM
A356-T6 knuckle of front suspension of a 6-cylinder
minivan weighing 2.4 kg, and cast iron ASTM A536
Grade 65-45-12 knuckle of the front suspension of a 4cylinder sedan weighing 4.7 kg. Only the forged steel
knuckle included the spindle portion. Figure 1 shows the
digitized models of the three components.
Beranger et al.(4) assessed fatigue behavior of a forged
suspension arm and investigated the fatigue strength
reduction of the "as-forged" surface resulting from the
surface roughness in the presence of residual stresses.
FEA using shell elements and LEFM calculations were
used. For component testing, constant-amplitude loadcontrolled tests at 10 Hz frequency and with R = -0.5
(typical braking/acceleration cycles measured on
vehicle) were considered. Endurance limit was defined
at 2x106 cycles. A multiaxial fatigue model based on a
critical plane approach was implemented. The minimum
value of the safety factor on the part was located in the
area where failure occurred on the test rig. They
reported very good correlation between experimental
and fatigue life predictions.
To perform fatigue analysis and implement the local
stress-strain approach in complex vehicular structures,
Conle and Chu(5) used strain-life results, simulated 3-D
stress-strain models and multiaxial deformation paths to
assess fatigue damage. After the complex load history
was reduced to a uniaxial (elastic) stress history for each
critical element, a Neuber plasticity correction method
was used to correct for plastic behavior. Elastic unit load
analysis, using strength of material and an elastic FEA
model combined with a superposition procedure of each
load point's service history was proposed. Savaidis(6)
verified the local strain approach for durability evaluation
of forged bus axle steering arm. It was concluded that
the local strain approach, in conjunction with the SmithWatson-Topper and J-Integral parameters, are able to
represent and estimate many influencing factors
explicitly. These include mean stress effects, load
sequence effects above and below the endurance limit,
and manufacturing process effects such as surface
roughness and residual stresses.
Figure 1: From left to right the digitized models of the
forged steel, cast aluminum and cast iron steering
knuckles.
Sonsino et al.(7) discussed the procedure of specimen
fatigue data transferability to real components using the
example of a randomly loaded truck stub axle. S-N
curves under constant amplitude loading and strain-life
curves under variable amplitude loading for unnotched
and notched specimens and components were
compared. It was concluded that the same failure
criterion (i.e. first detectable crack), accurate
determination of the local equivalent stress or strain, and
the same maximum stressed/strained material volume
Identical flat plate specimens with square cross section
and uniform gage section length, as shown in Figure 2,
were machined from the steering knuckles. The
relatively short gage section length was chosen to
prevent buckling in compression. Specimens in three
geometrical orientations were taken from the forged
steel knuckle to investigate the effect of directionality.
For cast aluminum and cast iron knuckles, since the
material properties are less dependent on geometrical
orientation, the specimens were taken from the hub and
In this paper, first the results of specimen testing of the
three materials are presented. Monotonic, cyclic and
fatigue properties are compared. Then the procedure of
finite element analysis is detailed, the methods to verify
the models are described, and the critical location of the
components with respect to the selected boundary
conditions are identified. Details of component testing of
the forged steel and cast aluminum knuckles are
described, and fatigue lives for the three components
based on S-N and strain-life predictions are compared.
MATERIAL FATIGUE BEHAVIOR AND
COMPARISONS
EXPERIMENTAL PROGRAM
2
one of the arms, respectively. The degree to which
anisotropic behavior may exist depends on the specific
casting practice.
triangular waveform. Test data were automatically
recorded at regular intervals throughout each test.
The monotonic and cyclic specimen tests were
performed by a 50 kN closed-loop servo-hydraulic
uniaxial testing machine with computer control and
hydraulic-wedge grips. Total strain was controlled using
an extensometer rated as class B1, according to ASTM
classification. All tests were conducted at room
temperature. Significant effort was put forth to align the
load train and minimize bending. According to ASTM
Standard E606(8), the maximum bending strains should
not exceed 5% of the minimum axial strain range
imposed during any test program, which was fulfilled. All
monotonic tension and constant amplitude fatigue tests
in this study were performed using test methods
specified by ASTM Standard E8(9) and ASTM Standard
E606, respectively.
Figure 2: Specimen geometry used for all specimen
tests. All dimensions are in mm.
EXPERIMENTAL RESULTS AND COMPARISONS
Specimens of the three materials were tested at strain
amplitudes ranging from 0.7% to 0.125%, resulting in
fatigue lives between about 102 and 107 (run-out)
reversals. Strain amplitudes larger than 0.7% were not
possible due to specimen buckling limitation. Strain
control was used in all tests, except for some long-life
and run-out tests, which were conducted in load-control
mode. For the strain-controlled tests, the applied
frequencies ranged from 0.1 Hz to 2 Hz. For the loadcontrolled tests including run-out tests, the frequency
was increased to up to 30 Hz in order to shorten the
overall test duration. All tests were conducted using a
A summary of the monotonic properties for the three
materials is provided in Table 1, including the ratios of
each property with respect to that of the forged steel.
From Table 1 it can be seen that cast aluminum and
cast iron reach 42% and 54% of the forged steel yield
strength, respectively, and 37% and 57% of forged steel
ultimate tensile strength, respectively. The percent
elongation, as a measure of ductility, for the cast
aluminum and cast iron are 24% and 48% of the forged
steel, respectively.
Table 1: Summary of mechanical properties and their comparative ratios (forged steel is taken as
the base for ratio calculations).
Forged
Steel
11V37
Cast
Aluminum
A356-T6
ratio
Cast
Iron
65-45-12
ratio
Monotonic Properties
Modulus of elasticity, E (GPa)
201
78
0.39
193
0.96
Yield strength (0.2% offset), YS (MPa)
556
232
0.42
300
0.54
Ultimate strength, Su (MPa)
821
302
0.37
471
0.57
Percent elongation, %EL (%)
21
5
0.24
10
0.48
37
10
0.27
25
0.68
Strength coefficient, K (MPa)
Percent reduction in area, %RA (%)
1,347
418
0.31
796
0.59
Strain hardening exponent, n
0.157
0.095
0.6
0.187
1.19
True fracture strength, σf (MPa)
496
301
0.6
219
0.44
True fracture ductility, εf (%)
47
10
0.23
28
0.59
Cyclic and Fatigue Properties
Cyclic modulus of elasticity, E’ (GPa)
195
73
0.38
169
0.87
Cyclic strength coefficient, K’ (MPa)
1,269
430
0.34
649
0.51
Cyclic strain hardening exponent, n’
0.137
0.063
0.46
0.075
0.55
541
291
0.54
407
0.75
1,157
666
0.58
761
0.66
-0.082
-0.117
1.42
-0.076
0.92
3.032
0.094
0.03
0.864
0.28
-0.791
-0.61
0.77
-0.771
0.97
352
122
0.35
253
0.72
Cyclic yield strength, YS’ (MPa)
Fatigue strength coefficient, σf’ (MPa)
Fatigue strength exponent, b
Fatigue ductility coefficient, εf’
Fatigue ductility exponent, c
6
Fatigue strength, Sf @ 10 cycles (MPa)
3
Figure 3 represents the superimposed plots of
monotonic and cyclic curves for all three materials. As
can be seen in this figure, the forged steel has mixedmode cyclic behavior. This material cyclically softens at
low amplitude, but slightly hardens at higher strain
amplitudes larger than 0.5%. The cyclic deformation
curve of the forged steel was found to be independent of
the geometrical direction (i.e. isotropic behavior) based
on the results of fatigue tests in three geometrical
directions. Cast aluminum and cast iron show cyclic
hardening behavior by about 20% and 30%,
respectively. In addition, strain hardening is more
prominent for forged steel. The cyclic yield strengths of
cast aluminum and cast iron were found to be 54% and
75% of the forged steel, while the cyclic strain hardening
exponent of cast aluminum and cast iron were 46% and
55% of the forged steel, respectively. These indicate
higher resistance of the forged steel to plastic
deformation.
direction coincides with the primary stressing direction of
the forged knuckle, it was selected as the basis for
comparisons in Figure 4 and subsequent figures. The
fatigue data presented in Table 1 are also for this
direction.
700
600
True Stress (MPa)
500
Monotonic
Cyclic
Figure 4: Superimposed true stress amplitude versus
reversals to failure for forged steel 11V37, cast
aluminum A356-T6, and cast iron 65-45-12.
Cyclic
400
Monotonic
300
In automotive design, cyclic ductility is a major
consideration when designing components subjected to
occasional
overloads,
particularly
for
notched
components, where plastic deformation can occur. This
is typical of suspension components, such as steering
knuckle. Figure 5 presents a comparison of the plastic
strain amplitude versus fatigue life behavior for the three
materials. Forged steel was found to be superior to cast
aluminum and cast iron with respect to low-cycle fatigue
(i.e. cyclic ductility), as could be seen in Figure 5.
Cyclic
Monotonic
200
Forged Steel 11V37
Cast Iron 65-45-12
Cast Aluminum A356-T6
100
0
0.0%
0.2%
0.4%
0.6%
0.8%
1.0%
True Strain (%)
Figure 3: Superimposed plots of cyclic and monotonic
stress-strain curves for forged steel 11V37, cast
aluminum A356-T6, and cast iron 65-45-12.
1.000%
True Plastic Strain Amplitude (%)
Figure 4 shows a direct comparison of the three
materials with respect to S-N behavior. Comparison of
long-life fatigue strength, Sf , which is defined as the
fatigue strength at 106 cycles, shows that fatigue
strength of cast aluminum and cast iron are 35% and
72% of the forged steel, respectively. In addition, while
the fatigue strength of forged steel at 106 cycles is
expected to remain about constant at longer lives,
fatigue strength of cast aluminum and cast iron
continuously drops with longer lives (i.e. beyond 106
cycles). Figure 4 indicates that at a given stress
amplitude forged steel results in about two orders of
magnitude longer life than cast iron, and more than four
orders of magnitude longer life than cast aluminum. With
regards to anisotropy influence of the forged steel,
fatigue test results indicated that both the long-life as
well as the short-life fatigue in the spindle centerline
direction were longer by about a factor of two, as
compared with the other two directions. Since this
0.100%
0.010%
Forged Steel 11V37
Cast Iron 65-45-12
Cast Aluminum A356-T6
0.001%
1E+2
1E+3
1E+4
1E+5
1E+6
Reversals to Failure, 2Nf
Figure 5: Superimposed true plastic strain amplitude
versus reversals to failure for forged steel 11V37, cast
aluminum A356-T6, and cast iron 65-45-12.
4
Comparisons of strain-life fatigue behavior of the three
materials, as shown in Figure 6, demonstrates that the
forged steel provides about a factor of 20 and 4 longer
lives in the short-life regime compared to the cast
aluminum and cast iron, respectively. In the high-cycle
regime, forged steel results in about an order of
magnitude longer life than the cast iron, and about a
factor of 3 longer life than the cast aluminum.
1.00%
FINITE ELEMENT ANALYSIS
Linear and nonlinear static finite element analyses
employing IDEAS-8 software were conducted on each
knuckle. Nonlinear analysis was necessary due to local
yielding in most cases, as well as gross yielding in some
cases, as mentioned previously. In order to generate
precise geometries of the three steering knuckles, a
Coordinate Measuring Machine (CMM) was used, with
the resulting digitized models as presented in Figure 1.
Material cyclic properties were used for the analysis. von
Mises yield criterion and a kinematic hardening rule that
used a bilinear stress-strain curve, adequately
representing the material cyclic deformation behavior,
were assumed for the nonlinear analysis.
Forged Steel 11V37
True Strain Amplitude (%)
Cast Aluminum A356
Cast Iron 65-45-12
0.10%
1E+2
1E+3
1E+4
1E+5
1E+6
1E+7
The boundary conditions and loading were selected to
represent the component service and testing conditions.
For the forged steel knuckle, the primary loading was
applied to the spindle, and the four suspension and strut
holes were restrained. The analysis showed that
changing the location in the spindle length at which the
load is applied does not affect the location and
magnitude of the stresses at the critical point. To verify
the model, other alternatives were analyzed by switching
the loading and boundary conditions, and also by
releasing any one of the fixed points, to ensure the
critical locations remained the same. For the cast
aluminum knuckle in service, while the loading is applied
to the strut joints through struts, the four hub bolt holes
are connected to the wheel assembly. Several trials for
boundary conditions were analyzed, including fixing the
whole area of the four hub bolt holes, fixing the
centerline of the hub bolt holes, only fixing the pair of
bolt holes away from the load application point, and
fixing two points at the middle area of the hub. It was
found that except for the case of fixing the bolt holes, for
which the value of stress was lower and the critical
location was different, all the other three cases provided
approximately similar results. Therefore, the choice of
fixing the hub bolt hole-centerlines was selected. For the
cast iron knuckle, where the geometry and service
conditions are close to the cast aluminum knuckle,
similar loading and boundary conditions were applied.
1E+8
Reversals to Failure, 2Nf
Figure 6: Superimposed true strain amplitude versus
reversals to failure for forged steel 11V37, cast
aluminum A356-T6, and cast iron 65-45-12.
The product of strain and stress amplitudes versus life,
known as Neuber plot, is shown in Figure 7. This plot is
useful when analyzing component geometries with
stress concentrations, where the notch root fatigue
behavior is a function of both local stress and strain.
Therefore, rather than considering the individual effects
of stress amplitude (Figure 4) or strain amplitude (Figure
6), a Neuber plot considers the combined effects of both
stress and strain amplitudes.
10.0
ε aσ a (MPa)
Forged Steel 11V37
Cast Iron 65-45-12
Cast Aluminum A356-T6
While defining a solid mesh for the components, free
meshing feature of the software was employed since it
has no geometry restrictions and it could be defined on
complicated volumes. The free mesh generator used an
algorithm that minimizes element distortion. 3-D linear
tetrahedral solid elements, with global element sizes of
3.81 mm for the forged steel and cast iron knuckles and
5.08 mm for the cast aluminum knuckle were used.
Local element sizes of 0.254 mm for the forged steel
and 0.635 mm for the cast aluminum and cast iron
knuckles were considered at the critical locations (i.e.
spindle fillets for forged steel knuckle, and hub bolt holes
for cast aluminum and cast iron knuckles). These mesh
sizes were obtained based on the convergence of stress
and strain energy at certain geometry locations.
1.0
0.1
1E+2
1E+3
1E+4
1E+5
1E+6
1E+7
1E+8
Reversals to Failure, 2Nf
Figure 7: Neuber curves for forged steel 11V37, cast
aluminum A356-T6, and cast iron 65-45-12.
5
A potential source of significant error in fatigue analysis
is inaccuracy of stress and strain predictions. Therefore
validating the FEA models was critical to this study. To
validate the models, values of strains as measured by
strain gages in component testing and as predicted
using the finite element analysis were compared and are
listed in Table 2. The strain gages for the forged steel
knuckle were positioned at the vicinity of the spindle root
and the first step fillets, and for the cast aluminum
knuckle two gages were positioned at the goose neck of
the strut arm and two at the hub bolt holes where the
crack initiation was observed during component testing.
These locations are identified in Table 2. Depending on
the location of the gage, the proper component of the
strain obtained from the FE analysis was selected for
comparison. The component testing was only conducted
for forged steel and cast aluminum knuckles; therefore
no data is presented for the cast iron knuckle. The
differences between measured and predicted strains
obtained for the two knuckles were considered
reasonable for the complex knuckle geometries. For the
forged steel knuckle, which has a relatively simpler
geometry, results of strain calculations from analytical
mechanics of materials equation are also listed in Table
2. As can be seen, these results are mostly in between
the measured and FEA-predicted strains. The results of
the finite element analysis were also checked with
regards to symmetry and linearity of the loading in the
elastic range. It should be noted that the position of the
strain gages and the magnitudes of the applied loads
were such that all measured strains were in the elastic
range.
gross (in the spindle) and locally (at the fillet), whereas
for the cast aluminum knuckle at the highest
experimental load only local yielding (at the hub bolt
hole) occurred.
Table 2: Measured and predicted strain values at 2.2 kN
static load. Locations of the gages are also shown.
Gage
Number
Measured
Strain
(µstrain)
P Mc
+
A
I
(µstrain)
Predicted
Strain from
FEA
(µstrain)
Diff.
Meas.
and FEA
(%)
583
8
Forged Steel
1
542
575
2
-527
-557
-546
4
3
1561
1571
1716
10
4
-1489
-1536
-1590
7
1
455
-
434
5
2
534
-
470
12
3
228
-
268
18
4
289
-
320
11
Cast Aluminum
The equivalent von Mises stress contours and critical
locations for a typical load value are presented for the
three models in Figure 8. The spindle 1st step fillet area
for the forged steel knuckle, the hub bolt holes for the
cast aluminum knuckle, and the strut arm root and hub
bolt hole for the cast iron knuckle, were found to be the
areas of high stresses. von Mises equivalent stresses
and strains are used for subsequent fatigue life analysis
and comparisons. For the forged steel knuckle, at the
highest experimental load level yielding occurred both
Figure 8: Contours of von Mises stress showing the critical locations for the forged steel (left), cast aluminum (middle),
and cast iron (right) knuckles. The darker areas (from left to right, spindle 1st step for the forged steel, hub bolt hole for the
cast aluminum, and strut arm root and hub bolt hole for cast iron knuckle) are the highest stressed locations.
6
COMPONENT FATIGUE TESTS
To obtain stress-life behavior of the components and to
be able to compare the fatigue behavior of the knuckles,
constant-amplitude load-control fatigue tests were
performed for forged steel and cast aluminum knuckles.
EXPERIMENTAL PROGRAM
The suspension system of each vehicle that the
component belongs to was identified and the loading
and attachment conditions of the knuckle in each vehicle
were investigated. A typical drawing for the suspension
system of the vehicle with the forged steel knuckle is
shown in Figure 9. The strut mounts on one side of this
component and on the other side the spindle attaches to
the wheel hub assembly. These attachments were
considered as the primary restraint and loading
conditions of the test, respectively. Similar procedure
was followed for the aluminum steering knuckle. The
critical points of highest stress in the component were
obtained from the stress analysis, as described
previously. Accordingly, specific test fixtures for each
one of the two knuckles were designed and machined.
Figure 10: Schematic drawing for forged steel knuckle
test arrangement.
.
Figure 9: The forged steel knuckle within the suspension
system. This arrangement drawing was used to
determine loading and boundary conditions.
As shown in Figure 10 for the forged steel knuckle, the
spindle was fixed by a 2-piece block where threaded
rods tightened the block to the spindle. A pair of Lshaped moment arms transferred the load from the
testing machine loading actuator to the spindle blocks in
the form of a bending load. The strut and suspension
connections on the knuckle body were fixed to the bench
using round and square blocks. For the cast aluminum
knuckle, a two-strut-attachment test was conducted, as
shown in Figure 11. In this arrangement, the strut
attachment of the arm was connected from both sides to
Figure 11: Cast aluminum knuckle test arrangement
showing details of fixturing, schematic (top), and arm
fixturing close up (bottom).
7
a pair of moment arms. The moment arms transferred
the bending load from the loading actuator to the
knuckle. The four hub bolt holes were fixed to the bench.
linear trend versus number of cycles. The lives to failure
used in latter comparisons for the cast aluminum
knuckle were considered to be those of macro-crack
nucleation.
A closed-loop servo-controlled hydraulic 100 kN axial
load frame was used to conduct the tests. The
calibration of the system was verified prior to the
beginning of the tests. A rod end bearing joint was used
to apply the load from the actuator to the moment arms,
in order to avoid any out of plane bending. Due to
relative rigidity of the fixtures, the effect of horizontal
friction force was found to be significant at the jointfixture contact point. Therefore, a needle roller bearing
was installed on each side of the pin of the bearing,
allowing the moment arm to roll horizontally to minimize
friction force. Care was taken to ensure symmetry of the
bending load transferred from the two moment arms. All
fixture bolts and nuts were tightened with identical
torque values to maintain consistency.
1.5
Displacement Amplitude (mm)
Forged Steel Knuckle
Cast Aluminum Knuckle
crack nucleates
0.5
The load levels were determined based on stress
analysis results and the true stress-true strain curve of
the materials. A minimum load of 220 N was used in all
tests, corresponding to an R-ratio (Pmin/Pmax) of less than
0.07. A total of 7 tests at four load levels with amplitudes
between 1100 N and 2350 N for the forged steel
knuckle, and a total of 6 tests at four load levels with
amplitudes between 1550 N and 3000 N for the cast
aluminum knuckle were conducted. The frequency of the
tests ranged from 0.5 Hz for higher load levels, to 5 Hz
for lower load levels. The load levels chosen resulted in
fatigue lives between 104 and 2x106 cycles.
0
0.2
0.4
0.6
0.8
1
Normalized Number of Cycles, N/Nf
Figure 12: Displacement amplitude versus normalized
cycles for typical forged steel and cast aluminum
knuckles.
EXPERIMENTAL RESULTS
Displacement amplitude versus cycle data of the
component during each test was monitored in order to
record macro-crack nucleation (i.e. a crack on the order
of several mm), growth, and fracture stages. Due to the
nature of the loading and restraints on both knuckles,
the locations of crack initiation could not be reached to
enable detecting crack nucleation. Therefore, a marked
displacement amplitude increase during the test was
considered as the crack nucleation point, and a sudden
increase as the final fracture.
Variations of displacement amplitude versus cycles for
two typical tests of forged steel and cast aluminum
knuckles are shown in Figure 12. As can be observed
from this figure, for the forged steel knuckle the
displacement amplitude was nearly constant until about
the end of the test. This indicates that the time lag
between macro-crack nucleation and fracture was a
small fraction of the total life. On the other hand, for the
cast aluminum knuckle, the crack growth portion of the
life was significant. The crack lengths of the cast
aluminum knuckles were also visually observed and
recorded. For the typical cast aluminum knuckle data in
Figure 12, the crack lengths were 8 mm, 13 mm, 20 mm
and 27 mm at N/Nf equal to 0.3, 0.5, 0.7 and 0.9,
respectively, where crack grew with an approximately
Figure 13: Superimposed stress amplitude versus life
curves for forged steel and cast aluminum knuckles.
The stress amplitude versus life curves of the two
knuckles are superimposed in Figure 13. For the cast
aluminum knuckle S-N lines based on failure defined as
either macro-crack nucleation or fracture are shown. As
can be seen, on the average, about 50% of the cast
aluminum knuckle life is spent on macro-crack growth.
This figure also shows that the forged steel knuckle
8
σ max ε a E = (σ ′f ) 2 (2 N f ) 2b + σ ′f ε ′f E (2 N f ) b +c
results in about two orders of magnitude longer life than
the cast aluminum knuckle, for the same stress
amplitude level. This occurs at both short as well as long
lives. Note that that the difference can be even larger at
long lives, due to the run-out data points for the forged
steel knuckle. It could also be seen from this figure that
the highest load levels provided life in the range of 104 to
5x104 cycles. Load levels corresponding to this life
range are considered to be representative of overload
conditions for suspension components, such as a
steering knuckle in service.
where σmax is the maximum stress (σmax = σa + σm) and
εa is the strain amplitude. The strain-life properties in this
equation are defined and their values for each material
are listed in Table 1.
Superimposed stress amplitude versus life curves based
on stress-life approach, and SWT parameter versus life
based on the strain-life approach for the three knuckles
are presented in Figures 14 and 15, respectively. To
obtain stress unit in Figure 15, square root of the left
side of Equation 3 is plotted as the SWT parameter.
Component test data for the forged steel and cast
aluminum knuckles are also superimposed in these
figures for comparison with predictions. Figure 14
indicates that predictions based on the S-N approach
are overly conservative for both the forged steel and
cast aluminum knuckles. This is partly due to the
conservative nature of the modified Goodman equation.
The predictions based on the SWT parameter are closer
to the experimental results, as shown in Figure 15.
Comparison of the forged steel and cast iron knuckle
prediction curves in Figure 15 demonstrates that the
forged steel knuckle offers more than an order of
magnitude longer life than the cast iron knuckle, at both
short as well as long lives. As compared with the cast
aluminum knuckle, the predicted lives for the forged
steel knuckle are longer by about three orders of
magnitude.
COMPARISONS OF COMPONENT FATIGUE
BEHAVIORS AND LIFE PREDICTIONS
Manufacturing process, such as forging or casting,
generally determines the strength level and scatter of
mechanical properties, but the geometry can suppress
the influence of the material, as indicated by Berger et
al.(10). According to Sonsino et al.(7), for a complex
component geometry where no notch factors could be
defined, transferability of material test data could be
performed only through local equivalent stresses or
strains in the failure critical areas. In this study, the local
equivalent stresses and strains corresponding to the
experimental loading conditions were obtained by
applying equivalent loads to the simulated finite element
models. Since the tests were conducted with a mean
load, the modified Goodman equation was used to
account for the effect of mean stress:
σa σm
+
=1
σ Nf S u
(1)
1000
Forged Steel Knuckle - Test Data
Cast Aluminum Knuckle -Test Data
Forged Steel Knuckle - Prediction
Cast Iron Knuckle - Prediction
Cast Aluminum Knuckle - Prediction
Stress Amplitude (MPa)
where σa, σNf, σm and Su are alternating stress in the
presence of mean stress, alternating stress for
equivalent completely reversed loading, mean stress,
and ultimate tensile strength, respectively. The Basquin
equation was then used to obtain the fatigue life using
the material properties listed in Table 1:
σ Nf = σ ′f (2 N f ) b
(3)
(2)
Normally, a surface finish reduction factor is applied to
the fatigue strength of a component. However, the fillet
of the forged steel knuckle was machined and polished
and, therefore, no surface finish factor was applied. For
the two cast knuckles, due to the nature of the casting
materials and the fact that the defects of a casting
material is uniform internally and externally, no surface
finish factor was implemented either.
100
1E+3
1E+4
1E+5
1E+6
1E+7
Cycles to Failure, Nf
Figure 14: Superimposed stress amplitude versus life
curves based on the stress-life approach for the forged
steel, cast aluminum and cast iron knuckles.
In the strain-life approach, the local values of stress and
strain at the critical location were used to find fatigue life,
according to the Smith-Watson-Topper (SWT)
parameter that considers the mean stress effect:
In the forging process, hot working refines grain pattern
and imparts high strength and ductility, therefore forged
9
components have lower possibility of internal defects,
whereas castings are weaker in this respect. In addition,
lower ductility of castings limits their capacity for cyclic
plastic deformation which often occurs at stress
concentrations and at overloads, and therefore
shortening their fatigue lives. Residual stresses at the
critical locations of the component generated during the
manufacturing process could be a significant source of
strengthening (if compressive) or weakening (if tensile),
in terms of fatigue life.
indicate higher resistance of the forged steel to
cyclic plastic deformation.
3. Better S-N fatigue resistance of the forged steel was
observed, as compared with the two cast materials.
Long-life fatigue strengths of cast aluminum and
cast iron are only 35% and 72% of the forged steel,
respectively.
4. Comparison of the plastic strain amplitude versus
fatigue life behavior for the three materials showed
higher capacity of the forged steel for cyclic plastic
deformation, and therefore better low cycle fatigue
behavior, as compared with cast aluminum and cast
iron.
5. Comparisons of strain-life fatigue behavior of the
three materials indicated that the forged steel
provides about a factor of 20 and 4 longer lives in
the short-life regime compared to the cast aluminum
and cast iron, respectively. In the high-cycle regime,
forged steel resulted in about an order of magnitude
longer life than the cast iron, and about a factor of 3
longer life, compared to the cast aluminum.
6. The differences between measured and FEApredicted strains obtained for the forged steel and
cast aluminum knuckles were found to be
reasonable for the complex knuckle geometries
considered.
7. Based on the component testing observations, crack
growth life was found to be a significant portion of
the cast aluminum knuckle fatigue life, while crack
growth life was an insignificant portion of the forged
steel knuckle fatigue life.
Figure 15: Superimposed SWT parameter versus life
curves based on the strain-life approach for the forged
steel, cast aluminum and cast iron knuckles.
8. Component testing results showed the forged steel
knuckle to have about two orders of magnitude
longer life than the cast aluminum knuckle, for the
same stress amplitude level. This occurred at both
short as well as long lives.
CONCLUSIONS
Specimen tests, finite element analyses, and component
tests were conducted in this study to assess and
compare fatigue behavior of forged steel, cast
aluminum, and cast iron steering knuckles. Based on the
experimental results and analyses presented, the
following conclusions can be made:
9. The S-N predictions were overly conservative,
whereas strain-life predictions were relatively close
to component experimental results. Comparison of
the strain-life prediction curves of the components
demonstrated that the forged steel knuckle offers
more than an order of magnitude longer life than the
cast iron knuckle.
1. From tensile tests and monotonic deformation
curves it was concluded that cast aluminum and cast
iron reached 37% and 57% of forged steel ultimate
tensile strength, respectively. The percent
elongation of cast aluminum and cast iron were
found to be 24% and 48% of the forged steel,
respectively.
REFERENCES
1. Gunnarson, S., Ravenshorst, H., and Bergstorm, C.
M.,
“Experience
with
Forged
Automotive
Components in Precipitation Hardened PearliticFerritic Steels,” Fundamentals of Microalloying
Forging Steels, Proceedings, Metallurgical Society
of AIME, 1987, pp. 325-338.
2. The cyclic yield strength of cast aluminum and cast
iron were found to be 54% and 75% of forged steel,
respectively, while the cyclic strain hardening
exponent of cast aluminum and cast iron were 46%
and 55% of the forged steel, respectively. These
10
2. Lee, S. B., “Structural Fatigue Tests of Automobile
Components under Constant Amplitude Loadings,”
Fatigue Life Analysis and Prediction, Proceedings,
International Conference and Exposition on Fatigue,
Goel, V. S., Ed., American Society of Metals, 1986,
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3. Lee, Y. L., Raymond, M. N., and Villaire, M. A.,
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Assessment
Methodology
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8. ASTM Standard E606-92, "Standard Practice for
Strain-Controlled Fatigue Testing," Annual Book of
ASTM Standards, Vol. 03.01, 1998.
9. ASTM Standard E8-03, "Standard Test Methods for
Tension Testing of Metallic Materials," Annual Book
of ASTM Standards, Vol. 03.01, 2003.
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Naundorf, H., Schuetz, W., Sonsino, C. M., Wimmer
A., and Zenner, H., “Betriebsfestigkeit in Germany
— An Overview,” International Journal of Fatigue,
Vol. 24, 2002, pp. 603-625.
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