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.

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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

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Fig. 1. Sectioned cylinder block

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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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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: mkpaswan_1@rediffmail.com

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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