A typical complete acceleration event lasts for less than
0.01 seconds in our airplane skin coin-tap inspection. To
safely avoid losing useful information we took 512 points
for the following analysis.
The data file begins with a quiet lead (36 points, or 0.75
milliseconds). As described in the last section, we first
manually collected about impacts with similar contact
duration values, calculated the mean and standard
deviation of the duration distribution. Then we did the
Fourier analysis and calculated the 1/3 power accumulation
ratio factor s. The result is shown inTable 1
As the above data are sampled at 48 kHz and 512 points
were used to do the FFT analysis, the frequency granularity
in spectrum analysis (calculating ) is about 94 Hz.
In contradiction to the single-spring model, where in
the unsupported case k should be smaller and so the contact
time should be longer, we observe a shorter contact
time in the unsupported case. We speculate that this happens
because in the supported case the impact is coupled
to high frequency modes of the stiff under-structure.