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Frequency Analysis of Stress Waves in Testing 3-D Angle-interlock Woven Composite at High Strain Rates

This article presents Fourier transform and wavelet packet analysis of stress waves in a split Hopkinson pressure bar apparatus where the compressive behaviors of 3-D angle-interlock woven composite were tested at strain rate of 800/s, 1600/s and 2100/s. The stress waves in incident and transmission bar are transformed into frequency domain to analyze the impact energy spectrum under different lengths and impact velocities of the strike bar. It was found that the energy-frequency distribution is irrelevant to impact velocity and will be left-shifted when the length of the striker bar (i.e., width of incident stress wave) increases. The impact energy is mainly concentrated on the low-frequency region. The wavelet packet analysis of the incident and transmission stress waves shows that the transform coefficients decrease from the low-frequency region to the high-frequency region. Furthermore, the coefficients in the high-frequency region only occur at the ascending and descending part of stress waves. Based on the time-frequency analysis results, a unique start point of the stress wave can be determined for the stress-time and strain-time history calculation. This method can improve the calculation precision of stress—strain curves of the 3-D woven composite under different strain rate compression. The calculating results indicate that the stress—strain curves of the 3-D angle-interlock woven composites are sensitive to strain rate. The compressive modulus, maximum compressive stress linearly increases with strain rate. The failure strain decreases when the strain rate increases. This start point determination method can also be extended to high strain rate tests (including compression, tension, etc.) of other materials with split Hopkinson bar apparatus.