Acoustic Emission Analysis of Experimental Impact Processes in Comparison to Ultrasound Measurements and Numerical Modeling

In this paper we present acoustic emission (AE) data recorded in hypervelocity impact experiments. In particular we focus on the presentation of the experimental setup and the data analysis. The AE data are used to localize the impact point, to analyze the propagation of the compressive wave in the target, and to calibrate specific material properties under dynamic conditions required in numerical simulation of impact cratering and the propagation of shock waves. We present a detailed comparison between experimentally determined data and numerical models. Additionally, we measured the wave velocity of the material with ultrasound tomography before the impact and compare the expected travel time of the compressional wave at each sensor with the arrival time of the compressional wave recorded by AE technique during an experiment under dynamic conditions. We recorded the stress signal in numerical models at gauges that were located at exactly the same positions as the AE sensors. A good agreement has been found between experimentally and numerically determined wave speeds. The impact experiments provide information about wave propagation that may contribute to a better understanding of the generation of earthquake-like seismic waves during the hypervelocity impact of a meteorite on earth. The calibration of numerical models is of particular importance for the up-scaling of the experimental results. 1. Introduction and Motivation The generation and growth of fractures during a hypervelocity impact and the propagation of elastic-plastic waves are not yet sufficiently investigated. Hypervelocity impact experiments are limited in terms of the data that can be recorded and the characterization of processes during impact. Several non-destructive testing methods have been used to characterize ongoing processes such as the generation, propagation of shock waves, their attenuation into elastic waves, and permanent modifications in the target material caused by an impact. During an impact event a shock wave is generated that propagates through a target material. Due to compression of the material a great part of the initial impact energy is converted into plastic work. Thus, the shock wave attenuates with distance and eventually turns into elastic waves. Here we present complex measurements of the resulting elastic waves using AE during impact experiments to determine material parameters, the compressional wave propagation in the target, to quantify fracturing in the target, as well as the localization of the impact. Additionally, measurements with elastic waves (ultrasound) were conducted before the impact experiments to calculate the expected wave propagation in the target.

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