受限颗粒介质中冲击和振动产生的弹性波
Elastic waves generated by impact and vibration in confined granular media
摘要：
小行星的观测数据可以解释为将它们视为颗粒状物质的聚集体。了解这些物体的机械特性与许多科学原因有关：空间飞行任务设计、评估对地球的撞击威胁、了解小行星的性质及其在太阳系起源中的意义。机械性能的现场测量需要复杂和昂贵的空间任务。在这里，一个实验室规模的表征波传播在粒状介质中提出了使用一种新的实验装置，以及数值模拟。小行星内部的压力仍然是一个有争议的问题，但它肯定会向内部呈现压力梯度。这就是为什么冲击特性需要作为围压的函数来进行。我们的实验装置允许同时测量的外部围压，内部压力，总应变，和加速度在一个50厘米的方盒子充满了十亿粒。我们研究了在500 Hz范围内的冲击产生的地震体波的传播。通过随后的压缩-松弛循环，观察到颗粒介质在每次压缩期间平均表现得像具有恒定弹性模量的固体。颗粒介质的有效介质理论（EMT）解释了低压下的数据。在每个压缩-松弛循环之后，弹性模量增加，并且观察到高滞后：松弛显示出比压缩更复杂的行为。我们表明，由冲击和振动产生的地震波以压力波速传播。由于数值模型，我们测量了一个强大的波衰减$α\sim3.4$ Np/m。我们发现，波速增加与围压与$p^{1/2}$。
英文摘要：
Observational data of asteroids can be explained by considering them as an agglomerate of granular material. Understanding the mechanical properties of these objects is relevant for many scientific reasons: space missions design, evaluation of impact threats to our planet, and understanding the nature of asteroids and their implication in the origin of the solar system. In-situ measurements of mechanical properties require complex and costly space missions. Here a laboratory-scale characterization of wave propagation in granular media is presented using a novel experimental setup as well as numerical simulations. The pressure inside an asteroid is still a matter of debate, but it definitely presents a pressure gradient towards the interior. This is why impact characterization needs to be performed as a function of the confining pressure. Our experimental setup allows for the simultaneous measurement of the external confining pressure, internal pressure, total strain, and acceleration in a 50 cm side squared box filled up with a billion grains. We study the propagation of impact-generated and shaker-born seismic body waves in the 500 Hz range. Through subsequent compression-relaxation cycles, it was observed that the granular media behaves on average like a solid with a constant elastic modulus during each compression. Effective medium theory (EMT) for granular media explains the data at low pressure. After each compression-relaxation cycle, the elastic modulus increases, and a high hysteresis is observed: relaxation shows a more complex behavior than compression. We show that seismic waves generated by both impact and vibration travels at the pressure wave speed. Thanks to a numerical model, we measure a strong wave attenuation $α\sim3.4$ Np/m. We found that the wave speed increases with the confining pressure with a $p^{1/2}$.
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