INFLUENCE OF LOADING PULSE DURATION ON DYNAMIC LOAD-TRANSFER IN A SIMULATED GRANULAR MEDIUM

AN EXPERIMENTAL and numerical investigation was conducted to study the dynamic response of granular media when subjected to impact loadings with different periods or wavelengths. The granular medium was simulat ed by a one-dimensional assembly of circular disks arranged in a strai ght single chain. In the experimental study, the dynamic loading was p roduced using projectile impact from a gas gun onto one end of the gra nular assembly, and the measured wave signal was collected using strai n gages. The numerical simulations were conducted using the distinct e (element method. It was found from the experiments and numerical simul ations that input waves with a short period (tau approximate to 90 mu s) will propagate in this granular medium with little waveform change under steady amplitude attenuation; whereas longer waves (tau > 200 mu s) will propagate with significant waveform dispersion. For these lon ger wavelength signals, the smooth waveform will undergo separation in to a series of short oscillatory signals, and this rearrangement of en ergy allows a portion of the transmitted signal to increase in amplitu de during the initial phases of propagation. Thus the granular medium acts as a nonlinear wave guide, and local microstructure and contact n onlinearity will allow input signals of sufficiently long wavelength t o excite resonant sub-units of the medium to produce this observed rin ging separation. Following a modeling scheme originally proposed by NE STERENKO [J. Appl. Mech. Tech. Phys. 5, 733 (1983)], a nonlinear wave equation model was developed which is related to soliton dynamics and leads to travelling wave solutions of specific wavelength found in our experimental and numerical studies.