EVIDENCE OF HIGHER POCHHAMMER-CHREE MODES IN AN UNSPLIT HOPKINSON BAR

Signatures of the first three modes of stress wave propagation in a Ho pkinson bar, given by the Pochhammer-Chree solution a century ago, are derived from a set of recent experiments. The data suggest that, at f requencies at which multiple theoretical modes are possible, the mode with the fastest group velocity dominates the propagation. The Gaussia n-windowed Fourier transform technique is used to obtain time-dependen t Fourier coefficients from a measured signal. The transformed signal is displayed in a gray scale plot of the power spectra of the Fourier coefficients as a function of time and frequency. This plot shows clea r evidence of the theoretical modes. A new dispersion curve for stress wave propagation in bars is derived from the transformed data. The ne w dispersion curve, used in the DISBAS code (a bar gauge deconvolution technique that uses dispersed basis functions), gives a highly resolv ed stress wave.