Experiments Confirm Peak Tracking Value
Sine Resonance Track & Dwell (SRTD) seems like a straightforward test. Run a Sine sweep, find a resonance frequency, then dwell on that frequency for a pre-determined time or until the DUT fails.
Sadly, things are not that simple in the real world. Resonances shift as material fatigues and, for the test to be valid, the test frequency also needs to shift.
The most common way to adjust SRTD testing for shifting resonance frequencies is Phase Tracking, implemented by the vibration test controller software. At the end of the Sine sweep, the phase difference between the control channel and the response channel at the resonance frequency is measured; in the real world, the phase difference will usually be close to, but not exactly, 90°. As the material fatigues and the response’s resonant frequency shifts, the software tracks it by adjusting the drive output to maintain a constant phase relationship between the two channels.
This approach is clearly better than tests that don’t adjust for resonance shifts, but it has limitations, largely because the phase difference at peak transmissibility will often change as fatigue effects progress.
Peak Tracking is a relatively new option for finding and maintaining the peak transmissibility between two channels. It works by constantly oscillating the phase between the two channels and observing if the peak transmissibility increases or decreases. As more data is collected, the amount of phase change becomes narrower in frequency and slower in time, limiting the amount of oscillation while still accurately tracking the peak.
Theoretically, Peak Tracking should do a better job than Phase Tracking. However, test engineers generally prefer data to theory. In that spirit, VR ran a series of SRTD tests, comparing the two tracking methods.
Fifty tests were conducted on notched aluminum beams, twenty-five using Peak Tracking and twenty-five using Phase Tracking. One end of the aluminum beam was attached to a shaker head and a mass hung from the cantilevered end of the beam.
As you might expect from this post’s title, our tests confirmed the theory. Peak Tracking did, in fact, cause the aluminum beams to fail faster than Phase Tracking.
See the new paper, ‘Improving SRTD Testing – Experimental Comparison of Tracking Methods’ for a full description of both resonance tracking methods and the detailed results of the tests.