Accession Number : ADA313837

Title :   Evidence of Higher Pochhammer-Chree Propagation Modes in an Unsplit Hopkinson Bar.

Descriptive Note : Technical rept. 1 Jun 92-30 Jun 94,

Corporate Author : LOGICON R AND D ASSOCIATES LOS ANGELES CA

Personal Author(s) : Binky Lee, C. K. ; Crawford, Richard C. ; Mann, Ken A.

PDF Url : ADA313837

Report Date : 01 SEP 1996

Pagination or Media Count : 79

Abstract : A discovery is made in the propagation modes of stress waves in a Hopkinson bar. Signatures of the first three modes of propagation, given by the Pochhammer-Chree solution a century ago. are derived from a set of recent experiments. The data suggests that at frequencies where multiple theoretical modes are possible, the mode with the faster group velocity dominates the propagation. The Gaussian Windowed Fourier Transform technique is used to obtain time-dependent 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. These plots show clear evidence of the theoretical modes, which have never been conclusively observed from measured data prior to this work. A new dispersion curve for stress wave propagation in bars is proposed based on the above results. The new dispersion curve, used in DISBAS code (A bar gage deconvolution technique that uses dispersed basis functions), gives a highly resolved stress wave; in particular, it reveals the detailed structure of the peak, which in this case consists of several 'peaklets'. These peaklets depict the distortions of the stress wave introduced by the water jacket and the mounting structure for the bar. The proposed dispersion relation, in conjunction with the DISBAS code, can be used to distinguish differences in the measured stress wave structure due to water jackets and mounting devices. Such differences are not discernible by previous deconvolution techniques due to inherent inaccuracies in those techniques as well as the lack of an accurately measured dispersion relation.

Descriptors :   *STRAIN GAGES, FOURIER TRANSFORMATION, PEAK VALUES, DISPERSING, TIME DEPENDENCE, BLAST WAVES, POWER SPECTRA, GROUP VELOCITY, WAVE PROPAGATION, COEFFICIENTS, STRESS WAVES.

Subject Categories : Test Facilities, Equipment and Methods
      Mechanics

Distribution Statement : APPROVED FOR PUBLIC RELEASE