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 efficiency and shorter load cycles in cylinder-block equipment. Fatigue results of induction- hardened and case-hardened cylinder blocks were investigated in this experiment.



Fig. 1. Sectioned cylinder block

The 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 critically loaded components, and it experiences 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 significantly reduced by using proper and efficient design and manufacturing. Such studies are necessary to enhance the competitiveness of the vehicle components and their application in the automotive industry. This helps in increasing performance together with more efficient working of the cylinder 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 components in general and for cylinder blocks in particular:
  • Loading conditions
  • Stress analysis
  • Fatigue testing
  • Material quality and defects
  • Influence from the manufacturing process
  • Fatigue assessments


Fig. 2. Different components of the cylinder block

Fatigue is the progressive, localized and permanent structural change that occurs in a material subjected to repeated or fluctuating strains at nominal stresses that have maximum values less than the static yield strength of the material. Fatigue may culminate into cracks and cause fracture after a sufficient number of fluctuations. Fatigue damage is caused by the simultaneous action of cyclic stress, tensile 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 result in local tensile stresses. Microcracks may be initially present due to heat treatment. Even in a flow-free metal with highly polished surface and no stress with no stress concentrators, a fatigue crack may form. The fillets of cylinder-block pins are the critical locations of the cylinder block that endure the highest level of stress under 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 determined using a fully reversed bending load applied to a single throw cut from a cylinder block.
  • Data are recorded using a strain gauge in a fillet, so the results are in the form of material strength, including effects of process variables.
  • Material and process variables: surface finish (grinding, lapping), hardness, microstructure, residual stresses (induction hardening, grinding)
  • The cylinder-block material is tested with the correct state of stress, and the predominant engine-failure mode is duplicated exactly. Therefore, the failure criteria can be ignored. Maximum principal stress is used for convenience.
  • Results are analyzed using statistical methods to determine the mean strength and the standard deviation.


Fig. 3. Engine for fatigue testing

Bending Fatigue-Test Procedures

Inertial 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 fillets.

The test was modeled after the energy loading. In an engine, the pin fillets experience peak tensile bending stress a few degrees after TDC during the start of the power stroke. Likewise, the main fillets 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 fixtures

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

Fig. 4. Strain-gauged cylinder-block sample

Test Setup
The setup only needs to be performed before the first specimen is tested. Then, cycle through the preparation and test stages until all specimens have been tested.

The test setup influences 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 significant influence 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 Calibration
The 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.

Fig. 5. Cylinder-block sample mounted on fixture

Specimen Preparation
Specimens 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 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 counterweight 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 identified 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 fixtures, 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 specific calculations, contact the author.

Fig. 6. Close-up of tested cylinder block

Results and Discussion

Induction-hardened cylinder blocks usually have longer fatigue life than the alternative. Fatigue results of induction-hardened 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 compressive 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 unhardened areas must be placed in a region with relatively low stress. Straightening of the induction-hardened cylinder block is necessary. This is because the hardening process is not completed axisymmetric.

Conclusion

1. 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 between 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 resulted in a $3,000/month total savings.

For more information: Contact Dr. Manikant Paswan, professor, Dept. of Mechanical Engineering, National Institue of Technology, Jamshedpur, INDIA; tel: 09931185530; e-mail: mkpaswan_1@rediffmail.com

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