The crankshaft in an engine is subject to cyclic, and therefore repetitive, loading. Repeated bending and torsional stress can cause fatigue, which leads to damage.
In vibration testing, Sine Resonance Track and Dwell (SRTD) tests can be used to evaluate fatigue associated with structural resonance. After identifying a crankshaft’s resonant frequencies, engineers can dwell at resonance as damage accumulates. The test may be run for a specified duration or number of cycles, or until failure, to characterize resonant behavior and estimate fatigue life.
Sine Resonance Track and Dwell (SRTD)
SRTD can be added to a fatigue testing program after a critical resonance has been identified. By dwelling at a resonance, the test can accelerate fatigue damage and help engineers evaluate how the crankshaft behaves under a known stress condition. The results can support broader activities such as model correlation with FEA, design changes, and durability tests.
A sine dwell test runs a single tone at the component’s resonant frequency rather than sweeping through the full frequency range. The controller can automatically track the resonance so that the excitation remains on target even if fatigue damage or temperature shifts the resonant frequency.
SRTD complements broader durability testing by isolating a resonance-driven failure mechanism rather than evaluating the full operating environment.
Crankshaft Modes
Crankshafts are typically evaluated for two primary vibration modes: bending and torsion. In service, both modes are driven by cyclic engine loading and can contribute to fatigue damage.
A typical vibration test setup aims to approximate the crankshaft’s in-engine behavior, reproducing the target modes under representative boundary conditions. For test purposes, bending and torsion are often addressed separately, but in the engine environment, the two can interact.
Bending
Bending fatigue damage is common in modern engine crankshafts. Pressure and inertia forces acting through the system can create alternating radial loads and bending moments. Repeated bending stress can initiate and propagate fatigue cracks at stress concentrations.
SRTD test setups designed to replicate bending often use a tuning-fork-shaped configuration where the crankshaft is mounted between two plates. The fixture and attached masses are tuned so that the crankshaft experiences alternating bending moments concentrated in critical areas.
Torsion
A particular concern for test engineers is torsional vibration, a back-and-forth twisting motion of the shaft. The periodic torque pulses from cylinder firing can excite the crankshaft’s torsional modes. If an engine order coincides with a torsional natural frequency at a particular RPM, the resulting resonance can amplify the vibration to damaging levels.
SRTD torsional tests use rotational excitation to apply cyclic twisting to the crankshaft and excite its torsional resonance.
Force Sensors
Often, a force input is desired for running these types of tests. A force or current shunt can convert the armature current to force output.
High-Cycle Crankshaft Fatigue Testing
Fatigue damage often develops over many load cycles. Tests may need to run for an extended period to evaluate high-cycle fatigue behavior.
High-cycle fatigue testing is common for engine crankshaft durability evaluations. The system applies cyclic loading to the component for a defined number of cycles or until fatigue damage is identified. Depending on the objective, the test profile may include varying loads and speeds. The resulting cycles to failure help identify the crankshaft’s fatigue limit or critical loading condition.
Engineers use predefined metrics to evaluate and optimize the crankshaft design. They may assess material fatigue strength, fatigue crack initiation, or the safety factor against fatigue failure.
Engineers can compare laboratory fatigue results to internal targets or industry requirements to determine whether the component requires design changes or additional testing. They can also generate an S-N curve with the number of cycles to failure, correlating stress levels and fatigue life.
SRTD in VibrationVIEW
Vibration Research’s SRTD software features advanced controls such as peak tracking, adaptive feedback, and tracking filters.
Peak tracking automatically shifts resonances by finding and maintaining the peak transmissibility between two channels. This feature minimizes the need for precise detection during the sine sweep so the user can run a faster sine sweep. Most importantly, it maintains peak transmissibility throughout the tracked dwell portion of the test.
Adaptive feedback limits the problems of high-Q resonances, providing better control. As the test sweeps through the resonance, the software automatically adjusts the response time and slew rate to the most appropriate values, if necessary.
