Investigation of 3D Grain Shape Characteristics of Lunar Soil Retrieved in Apollo 16 Using Image-Based Discrete-Element Modeling

摘要

This paper describes three-dimensional (3D) grain shape characteristics of returned lunar soil (No. 60501) and its numerical simulation by using the image-based discrete-element method (DEM). First, the lunar soil sample was investigated by X-ray computed tomography (CT) at the SPring-8 facility. Next, the obtained grain shapes were modeled by an original technique based on a clumped sphere method. The CT images were processed by an originally developed image analysis, and 74 grains were identified. Based on their 3D shapes and intraparticle voids and cracks, the grains were classified into four categories: (1) agglutinate (ag), (2) breccia type A (brA), (3) breccia type B (brB), and (4) plagioclase (pl). The content ratio of each grain category favorably agreed with those reported in previous studies. The 3D shape indices, namely, the aspect, flatness, and elongation ratios; the Krumbein's sphericity; and the newly proposed volume ratio to ellipsoid were computed for 74 grains. Evaluating the differences in grain shape among the four categories, it was found that ag grains were considerably more angular (irregular) than grains belonging to the other categories. The volumes of the internal voids and cracks in the four categories were also quantitatively evaluated. There is no production of ag grains on Earth because the atmosphere prevents micrometeorite impacts. Although the effect of ag grains on the bulk properties of lunar soil should be evaluated, mechanical experiments may damage the lunar soil grains. Thus, the authors used computer simulations via image-based DEM. The grain shapes were modeled by clumping 10 spheres in the image-based DEM simulations. The effect of ag content on the dynamic (angle of repose) and quasistatic (simple shear) behaviors of the grain assembly were evaluated in a series of image-based DEM simulations. In simulations of ag = 0%, ag = 27%, and ag = 100%, the ag = 100% specimens developed the largest angle of repose. In addition, it was found that the ag content primarily affects the bulk density compared with the shear strength. This work is important for understanding the mechanical properties of lunar soils because simulation methods offer a suitable alternative to mechanical experiments, which may damage the specimens. (C) 2014 American Society of Civil Engineers.