In vibration-based fatigue testing a base-excited cantilevered plate is driven at one of its resonant modes until failure. For high frequency fatigue tests exceeding 1000 Hz, a resonant mode colloquially known as the “two-stripe” mode is often used, so named for the two node lines which extend from the clamped edge to the opposite free edge. The defining characteristics of this resonant mode are its existence at high frequencies and the presence of a large uniaxial bending stress concentrated along the free edge opposite the clamped edge. In the past, fatigue tests applying the two-stripe mode have utilized a large square plate specimen. However, due to crack initiation being concentrated in a small region near the free edge, this represented an excessive use of material to generate even a few data points. This design was later improved by splitting the plate into a two-piece assembly consisting of a U-shaped carrier plate and a replaceable insert specimen at the free edge. By ensuring the stress is sufficiently concentrated within the insert specimen, the carrier plate can be reused for many tests.

The purpose of the present work is to increase the throughput of vibration-based fatigue testing by extending the carrier plate to accommodate 3 insert specimens. All 3 inserts are shaken in tandem until one of them fails, but in the meantime the other two will have accumulated damage, thus reducing the total remaining time to failure for those two specimens. The geometry of the carrier plate is optimized in ANSYS to maximize the stress in the insert specimen while minimizing the stress in the plate. This will ensure the plate has a longer fatigue life than the specimens and can be used for many tests. Additional optimization criteria include ensuring that the resonant frequency of the two stripe mode is sufficiently isolated from other neighboring modes such that they cannot interfere with the oscillation; and tailoring the strain levels of each specimen so that they each target distinct datapoints on an S-N curve, but that the fatigue lives are of comparable magnitude such that significant damage accumulates simultaneously in all 3 specimens. The 3 insert carrier plate will then be tested on an electrodynamic shaker to ensure it produces results consistent with the ANSYS model. Failure is detected when the resonant frequency of the assembly drops by a significant percentage, indicating a softening of one or more specimens due to the presence of a crack. The cracks are then verified with fluorescent dye penetrant.


Modal Analysis of a High-Throughput Vibration-Based Fatigue Testing Assembly

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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