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he cylinder block is among the largest components in
internal-combustion engines. Whether a one-cylinder lawn mower
or large multi-cylinder diesel engine, the cylinder block is one
of the most criti-cally loaded components, and it experi-ences
cyclic loads in the form of bending and torsion during service
life. A typical automotive cylinder block is produced by hot
impression-die forging of microalloyed steel of SAE 1548
medium-carbon (0.4-0.44%), 1% Mn steel. Fatigue is a major
consideration in the design and performance evaluation of
materials, components and structures since 90% of all mechanical
failures are attributed to fatigue fractures. This is especially
true for motor vehicles and parts. The investigation emphasized
that this cost could be signi cantly reduced by using proper and ef
cient design and manufacturing. Such studies are neces-sary to
enhance the competitiveness of the vehicle components and their
appli-cation in the automotive industry. This helps in increasing
performance together with more ef cient working of the cyl-inder
block to the required higher and more damaging fatigue cycles per
hour and focusing on weight reduction due to the need for higher
payloads and reduced emissions.
The following areas are important for the design of
fatigue-loaded vehicle com-ponents in general and for cylinder
blocks in particular: Loading conditions Stress analysis Fatigue
testing Material quality and defects In uence from the
manufacturing process Fatigue assessments
Fatigue is the progressive, localized and permanent structural
change that occurs in a material subjected to repeated or uctuating
strains at nominal stresses that have maximum values less than the
static yield strength of the material. Fa-tigue may culminate into
cracks and cause fracture after a suf cient number of uc-tuations.
Fatigue damage is caused by the simultaneous action of cyclic
stress, ten-sile stress and plastic strain. The plastic strain
resulting from cyclic stress initiates the crack, and the tensile
stress promotes crack growth (propagation). Although compressive
stresses will not cause fatigue, compressive loads may re-sult in
local tensile stresses. Microcracks may be initially present due to
heat treat-ment. Even in a ow-free metal with high-ly polished
surface and no stress with no stress concentrators, a fatigue crack
may
form. The llets of cylinder-block pins are the critical
locations of the cylinder block that endure the highest level of
stress un-der service loading. Microcracks may be generated during
induction hardening if quenching is not controlled properly, which
will affect the fatigue life of the cylinder block adversely. The
material fatigue strength is deter-mined using a fully reversed
bending load applied to a single throw cut from a cylin-der block.
Data are recorded using a strain gauge
in a llet, so the results are in the form of material strength,
including effects of process variables.
hlcaTT
This paper deals with fatigue assessment of cylinder blocks in
the automobile industry. The topic was chosen because of increasing
interest in higher payloads, lower weight, higher ef ciency and
shorter load cycles in cylinder-block equipment. Fatigue results of
induction-hardened and case-hardened cylinder blocks were
investigated in this experiment.
FEATURE | Materials Characterization & Testing
Fatigue Testing of Six-Cylinder Diesel Engine HeadM.K. Paswan
and A.K. Goel National Institute of Technology, Jamshedpur,
INDIA
IndustrialHeating.com - August 2009 55
Fig. 1. Sectioned cylinder block
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56 August 2009 - IndustrialHeating.com
FEATURE | Materials Characterization & Testing
Material and process variables - Surface nish (grinding,
lapping) - Hardness - Microstructure - Residual stresses (induction
harden- ing, grinding)
The cylinder-block material is tested with the correct state of
stress, and the predominant engine-failure mode is du-plicated
exactly. Therefore, the failure criteria can be ignored. Maximum
prin-cipal stress is used for convenience.
Results are analyzed using statisti-cal methods to determine the
mean strength and the standard deviation.
Bending Fatigue-Test ProceduresInertial weights are attached to
a cylinder-block specimen to create a tuning fork-like dynamic
system. The system is then excited at resonance so that minimal
input energy is required to create alternating bending stresses in
the pin and main llets. The test was modeled after the energy
loading. In an engine, the pin llets ex-perience peak tensile
bending stress a few degrees after TDC during the start of the
power stroke. Likewise, the main llets achieve peak tensile bending
stress at TDC during the start of the intake stroke due to the
inertial loading of the rod and piston. The test process is as
follows:
Setup: Suspend weights from load frame Setup shaker
Preparation: Cut and mark specimen Gauge specimen Install
specimen into xtures
Test: Run calibration curve Calculate test strain levels Set
control parameters Run test Visual surface inspection
Analysis: Run SAFL Run cylinder block
Inspection: Metallurgical Geometric
Documentation: Records result
Test SetupThe setup only needs to be performed be-fore the rst
specimen is tested. Then, cy-cle through the preparation and test
stages until all specimens have been tested. The test setup in
uences the quality of the results. The test system consists of the
cylinder-block specimen, attached weights and suspension
arrangement. The stiffness of the test system has a direct effect
on the calibration curves, which are run later in the process. Two
areas that are believed to have a signi cant in uence on the
system stiffness are the weight-suspension technique and the
clamping procedure. When the suspension or clamping is incorrect,
the shape of the weights can change, which could produce a change
in the g-level-to-strain relationship.
Weight CalibrationThe inertial weights are suspended from a load
frame with adjustable threaded rods and elastic bungee cords. The
weights are adjusted until they are level, parallel and the
centerlines of the cylinder-block holes are aligned. Specimen
PreparationSpecimens are cut from the test cylinder block so that a
full main is on either side of a pin (Fig. 1). Three specimens can
be cut from a single rank using every other main-pin-main
combination. Either the odd or even pins will be used from a single
cylinder block. A source approval test will
Fig. 2. Different components of the cylinder block
Fig. 3. Engine for fatigue testing Fig. 4. Strain-gauged
cylinder-block sample
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IndustrialHeating.com - August 2009 57
contain a maximum of 18 specimens, and a production audit will
typically contain nine specimens. Before cutting the cylinder
block, mark the pin number and the direction toward the front of
the cylinder block on a counter-weight by each pin. The specimens
should be cut to allow the maximum clamping area on the mains, and
the cut should be made perpendicular to the main axis of the
cylinder block. An even mix of odd and even pins should be used so
that processing issues might be identi ed during testing. After the
cylinder block is cut, steel stamp the serial number, pin number
and forging supplier initials on the end of the main that
originally faced the front of the cylinder block. To prevent
fretting in the xtures, be sure to grind off the burrs on the end
of the mains, which were created from cutting the specimens. For
more details of the test process or speci c calculations, contact
the author.
Results and Discussion Induction-hardened cylinder blocks
usu-ally have longer fatigue life than the alter-native. Fatigue
results of induction-hard-ened and case-hardened cylinder blocks
were investigated in this experiment. The good fatigue properties
of induction-hardened components mainly depend on high surface
hardness and high compres-sive residual stresses at the surface.
The compressive stress at the surface is caused by the volumetric
expansion from the martensite transformation and the plastic
strains caused by fast cooling. However, high hardness does not
mean higher fatigue limit. To utilize high hardness, it is
therefore important to use material with high purity to avoid crack
and surface roughness. The transition zone between the hardened and
unhard-ened areas must be placed in a region with relatively low
stress. Straighten-ing of the induction-hardened cylinder block is
necessary. This is because the hardening process is not completed
axi-symmetric. IH
Conclusion1. Using low induction-hardening power
and frequency, it appears to be possible to reduce the tensile
stress at the core in the investigated cylinder block.
2. In spite of this, the transition zone be-tween the hardened
and unhardened zones must be placed in a region with relatively low
stress.
3. Quenching for the induction-hardening process must be
optimized for a given setup to prevent microcracks.
4. Reduction in cutting/testing frequency saved 17 cylinder
blocks per month re-sulted in a $3,000/month total savings.
For more information: Contact Dr. Mani-kant Paswan, professor,
Dept. of Mechanical Engineering, National Institue of Technology,
Jamshedpur, INDIA; tel: 09931185530; e-mail:
[email protected]
References (online only)
Additional related information may be found by searching for
these (and other) key words/terms via BNP Media SEARCH at
www.industrialheating.com: fatigue, bending, torsion, tensile
stress, plastic strain, microcracks, induction
Fig. 5. Cylinder-block sample mounted on xture
Fig. 6. Close-up of tested cylinder block
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