Fatigue is the progressive, localized and permanent structural change that occurs in a material subjected to repeated, or fluctuating, strains at nominal stresses with maximum values less than the static yield strength of the material. Fatigue may culminate into cracks and cause fracture after a sufficient number of cycles. Fatigue damage is caused by the simultaneous action of cyclic stress, tensile stress and plastic strain. Plastic strain resulting from cyclic stress initiates the crack and tensile stress promotes crack growth (propagation). Although compressive stresses will not cause fatigue, compressive loads can result in local tensile stresses, and a fatigue crack may form even in a flow-free metal having a highly polished surface and no stress and no stress concentrators.
Microcracks may be initially present due to heat treatment. They can be generated during induction hardening of crankshafts if quenching is not controlled properly, and their presence will adversely affect the fatigue life of the crankshaft.
Fatigue strength determination
The fatigue strength of crankshaft material is determined using a fully reversed bending load applied to a single throw cut from the crankshaft. Data are recorded using a strain gage in a fillet, so the results are in the form of material strength, including effects of process variables. Material and process variables include surface finish from grinding and lapping, hardness, microstructure and residual stresses (from induction hardening, grinding).
The crankshaft material is tested using the correct state of stress, and the predominant engine-failure mode is duplicated exactly. Therefore, failure criteria can be ignored. The maximum principal stress is used for convenience, and results are analyzed using statistical methods to determine the mean strength and the standard deviation.
Crankshaft bending fatigue test procedure
Inertial weights are attached to a crank specimen to create a "tuning fork" like dynamic system. The system is then excited at resonance so minimal input energy is required to create alternating bending stresses in the pin and main fillets.
The test is modeled after the energy loading. In an engine, the pin fillets experience peak tensile bending stress a few degrees after TDC (top dead center) during the start of 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 involves:
- Set-up: weights are suspended from the load frame and set-up shaker
- Specimen preparation: the specimen is cut, marked, gaged and installed into the specimen fixture
- Test: run a calibration curve, calculate test strain levels, set control parameters, run test and inspect the specimen surface visually
- Analysis: run SAFL and CRANK FLR
- Inspection: metallurgical and dimensional
- Documentation: record and format results
Set-up is required only before the first specimen is tested, after which the preparation and test stages are conducted for all specimens. The quality of results is influenced by test set-up.
The test system consists of the crank specimen, attached weights and suspension arrangement. The stiffness of the test system has a direct effect on calibration curves, which are run later in the process. Two areas that may have a significant influence on the system stiffness are the weight suspension technique and the clamping procedure. When either suspension or clamping is incorrect, the shape of the weights can change, which could produce a change in the gravity level-to-strain relationship.
Specimens are cut from the test crankshaft so that a full main is on the either side of a pin (Fig. 1). Three specimens are cut from a single crank using every other main-pin-main combination. Either odd or even pins from a single crankshaft are used. A source approval test contains a maximum of 18 specimens, and a production audit typically contains nine specimens.
Before cutting the crank, pin number and the direction toward the front of the crank on a counterweight by each pin are marked. Specimens are cut to allow the maximum clamping area on the mains, and the cut is made perpendicular to the main axis of the crank. An equal mix of odd and even pins is used so processing issues can be identified during testing. Sample and corresponding crank are clearly identified, and samples are deburred to prevent fretting in the fixtures. A line is marked between the centers of the main and pin using a matched pair of gage blocks (Fig. 2).
It is important to make sure that all scribe marks are light and metal removal is kept to a minimum so that a stress riser is not created, which could affect the test results. The centerline height is scribed only on the journal diameters, and care is taken so the scribe line does not extend into the fillet radius. Lines are scribed next to the gage location and on the cut ends of the mains. The lines on the end of the mains are used to ensure the specimen is mounted vertically in the fixtures. The lines on the inside of the pin and top of the main journals are used as a guide when mounting strain gages.
Historically, a crankshaft develops a maximum stress level near 45 degrees through the pin fillet radius. The crankshaft has a larger pin journal, which produces a greater overlap between the main and pin journals. The stresses are now nearly equal in both the main and pin fillets. Because of this, the fillet can fail.
The first three or four specimens in each test have both the pin fillets and main fillets gaged. If good correlation exists between the pin and main fillet strains, the remaining specimens only have the pins gaged.
Strain gages are 0.125 in. (32 mm) long and cover about 37 degrees of the fillet. Centering the gage length at 45 degrees in the fillet with the gage length in the axial direction allows the gage to average over the maximum stress point in the fillet.
The strain gage is mounted using adhesive one half of the way through the fillet radius (Fig. 2) using the journal scribe line for alignment. The gage is attached to the fillet that is initially toward the front of the crankshaft, and the gage length is oriented along the crankshaft axis. Either a two or three wire hookup can be used. Tapered split lock rings are first mounted on the specimen and then installed into matching tapered collars on the suspended inertial weights.
The stringer rod, which is connected to the fixture plates (Fig. 3), vibrates when the shaker is switched on, and the crankshaft sample functions as tuning fork. If there is any failure in the specimen, the system goes out of phase and cycle stops automatically. The number of cycles the test has run can be seen from the controller.
The following test results illustrate a representative analysis of crankshafts using the test procedure described above. A total twelve specimens from 6 TCL cranks were subjected to fatigue testing. Fatigue data for hard-turned crank specimens was compared with specimens from con cast specimens. Hard-turned specimens were identified as numbers 1 to 6 corresponding to the data plotted on the SN graph and to the information in Table 1. Figure 4 shows an SN plot with test data for both specimen types. The specimens have an endurance limit of 818 MPa (118.6 ksi). As indicated in Table 1, data show that one unexpected failure occurred in a main bearing fillet (specimen No. 9) at 0.92 million cycles tested at 862 MPa (125 ksi).