C. Nataraj, Ph.D., Professor & Chair, Department of
Mechanical Engineering; Director, Center for Nonlinear
Dynamics & Control, Villanova University
This report describes the experimental and analytical evaluation of Sting-Free material Kevlar mounted on several paddings. These tests were carried out in the Dynamic Systems Laboratory at Villanova University.
Identical flat Aluminium plates were used with the different padding material pasted onto them. Nine impact locations were marked on the top. One end of the plate was firmly fixed to a work table with an overhang of about 75%.
Accelerometer mounts were fabricated from Aluminum and mounted on the bottom of the plate near the middle. Uniaxial accelerometers from Bruel & Kjaer were used in the experiment. They are high precision sensors capable of measuring high level accelerations. These were connected to a charged amplifier type 2635 which was in turn connected to a data acquisition front end (Module type 3109) which has a 25 KHz LAN interface module (type 7533) that was connected to the LAN port of a PC. The software used for data acquisition was Pulse Labshop version 10.2. There were three test runs for each case. The tests were run for impacts at nine locations (Fig. 1).
The accelerometer data was collected using the Pulse software. The data was gathered every 0.24 milliseconds for a total period of four seconds. This amounts to a sampling rate of 4096 Hz. There was one run for each impact location and specimen. Since there were five specimens, this amounted to 45 data files, each with 16,384 data points. The experiments were performed on five different paddings as listed in Table 1. In particular, we are interested in the performance of Sting Free Kevlar (#4 and #5) with the high density foam (#3) and the high density padding (#2).
After the raw data was collected computer programs written in MATLABTM were used to perform analysis on the effectiveness of the paddings. Also determined was the Fourier spectrum (FFT) using 4096 points to help us determine the magnitudes of the vibration at specific frequencies as the time response itself is extremely complicated and is difficult to assess. The Fourier power spectra helps us make sense of the vibration pattern we observe in this complex nonlinear system. Peaks at specific frequencies tell us that the vibration signal consists of predominantly pure sine waves at those frequencies. Hence we used the top peak magnitude as the performance criterion.
This section tabulates some important performance measures. The detailed time and FFT plots for all the runs along with some critical performance data are listed in the next section. (availale upon request.)
Most of the significant vibration is in the range up to 1000 Hz or so, and the FFT plots have been truncated to 1500 Hz for clarity. Clearly as is obvious from the acceleration plots the acceleration magnitude is reduced by use of the Sting Free Kevlar material. The highest frequency peak is also in general reduced. It is interesting to note that the primary resonant frequencies change a little for the various paddings, but not by very much; for peak reduction calculations we used the peaks closest to the fundamental frequency peak. In summary, Sting Free Kevlar (#5) provides a reduction of 80% compared to the high density foam (#3) and the Sting Free Kevlar (#4) maintains a reduction of 80% compared to the high density padding (#2).
Conclusions & Recommendations
The Sting Free Kevlar material clearly presents an advantage over the current foam paddings. The amplitude of vibration as measured by the accelerations is reduced in general. It is also clear that the peak frequency amplitudes - especially at resonant peaks - are in general quite reduced by the use of the Sting Free Kevlar paddings. Reductions in peak amplitudes can be as much as 80% at the resonant frequencies.