Structure of hexapod 3D packings: understanding the global stability from the local organization

Abstract

Abstract.Aggregates of non-convex particles have shown to be particularly stable which makes them good candidates to design new lightweight and reversible structures. However, few is known about the fundamental reason of their stability. In this paper we presents a novel experimental method to investigate the local structure of piles made of hexapod particles. This method is based on X-ray scanning and on an accurate homemade particle detection code. It permits to get the position and orientation of each particle as well as to detect their contact points. Measurement of the coordination numbers, statistics of the contact positions and local density evaluation for different packing configurations show a good agreement with the previous studies carried out at the global scale and permits to explain the main local mechanisms leading to stable structures.

Yielding, rigidity, and tensile stress in sheared columns of hexapod granules

“…To extract packing structures from the 3D image, we use codes developed previously [16]. Each two-dimensional (2D) slice of the 16-bit image is binarized using Otsu's method [29], producing a 3D density in which voxels occupied by the material are set to 1.…”

“…The resulting connected regions of 1's correspond to individual particles, allowing for an estimate of the center of mass of each particle and the Euler angles specifying its orientation. We then use a template-matching technique on the original binarized image [16,30] to refine these estimates. The template is taken to be an ideal hexapod with the appropriate dimensions.…”

“…The subtracted term accounts for any preexisting distortions, which are usually less than 0.3 • . Finally, we detect interparticle contacts and particle-tube contacts using a previously developed technique [16]. We first estimate contact locations from the skeletons.…”

“…The source of tensile stress on a single particle can be understood as follows. In α = 10 packings, contacts between particle arms tend to occur far from the particle centers [16], and the angle φ between the contact force f and the vector pointing from particle center to the contact r is expected to be nearly 90 • , which implies that the contact force exerts a substantial torque on each particle [24]. To see that such forces can give rise to tensile stresses, consider a cross-shaped rigid particle subjected to four equal magnitude contact forces with φ = 90 • , as shown in Fig.…”

“…For noncohesive particles, spherical or nearly spherical shapes form packings that deform plastically under shear [6][7][8][9][10]. However, packings of highly elongated and/or strongly nonconvex particles show stiffening behavior under shear [11][12][13][14][15][16][17]. A dramatic illustration of this effect is the formation of freestanding walls and columns consisting of slender rods, staples, granular chains, or star-shaped particles [18][19][20][21][22].…”