Modelling the transmission of ultrasound pulses through grain-to-grain contacts

Abstract

Wave propagation through packed spherical particles is characterized by two distinct mechanisms, depending on the frequency of the wave content. Coherent wave pulses occur when the Hertzian contact model can be used, i.e., for frequencies low enough so that the granules behave as rigid bodies. Above a certain frequency, a chaotic time signal is the result of diffusive energy transmission through the grain contacts. This so-called coda wave is important for applications tracking the microstructure of materials, e.g., in nondestructive testing. This work looks for parallels between the two approaches by investigating the transmission of short ultrasound pulses in the unit cell of a granular material: two spheres in contact. Laser measurements on two identical large steel spheres show that the energy transmission happens in discrete steps, due to guided surface waves. The measured and modeled energy distribution evolution are similar to the one predicted by the diffusive theory. However, we show that the Her...