Session V02: Mechanics of Geological Materials

Material Point Methods Applied to Grain Compaction

The material point method is applied to investigate compaction and crushing of rock sugar spheres undergoing large deformation and failure. The method is chosen because of its capability of simulating complex geometry and of avoiding mesh tangling and distortion often encountered in a Lagrangian mesh-based methods. The application, however, introduces two new challenges about pre-processing and post-processing. The first challenge is the generation of a suitable initial material point configuration from a complex experimental geometry to describe initial geometry of spheres. A second challenge is how to identify surfaces from material point data, which is important in identifying cracks of the grains. In this work, a fast method to generate initial material point locations given a stereolithography file (STL) that is scalable to generate billions of material points using a single computer node is introduced to address the first challenge. To address the second challenge, a method to generate isosurfaces from the material point data is developed by extending the Marching Cubes 33 algorithm (Chernyaev, CERN Technical report CN/95-17, 1995). These tools allow for better comparison between experiments and simulations. The techniques are applied to investigate the large deformation and breakup of grains under compaction. Using these tools, numerical results are compared with X-ray tomographic 3D imaging experiments during early compaction.   

Variable Strain Rate Properties of Rammed Earth

Rammed Earth (RE) is a low-cost alternative building material which can be used in construction. Like concrete, RE is composed of fine aggregate, coarse aggregate, cement, and water. RE and concrete differ in the type of aggregates used. In RE, the soil that is readily available is used as both the coarse and fine aggregate, mixed with cement and water, then compressed. The aim of this project was to research RE as a potential building material as a cheaper alternative to conventional concrete masonry units and concrete. While typical use case for structures typically involves low strain rate loading, the effectiveness of structures at high strain rate are of additional importance. To find low strain rate properties of RE, unconfined compression and Brazilian splitting tests were performed. Higher strain rate tests utilized Hopkinson bar and penetration with characterized spherical and dart penetrators. Compressive strengths of 2620 ± 855 psi were roughly equivalent to traditional concrete masonry units and specimens were also shown to harden based on strain rate.   

Temporal and spatially resolved compaction in granular materials using synchrotron radiation: Initial Results

Each year approximately 99% by mass of energetic material use takes place in the quarrying, mining, construction and petrochemical industries. The coupling of explosively driven shock waves to heterogeneous geological materials is of scientific and industrial relevance.

Here we report the first experiments conducted with energetic materials at the ID19 beamline at the European Synchotron Research Facility, Grenoble, France. The experiments used commercial detonators to explosively load sand samples and glass spheres. Time resolved images showing the compaction wave produced by the explosive loading of both coarse- and fine-grained sand as well as large and small diameter glass spheres. The bulk shock properties of these materials have been reported previously at SCCM 2011 by Neal et al.

The captured images clearly show the range and variability of the compaction wave within a ‘mono-modal’ size range, under the same loading conditions. The results indicate that while previous reports by the authors, e.g. Proud et al. SCCM 2017 implied averaged compaction and shock behaviour through the use of embedded shock gauges and velocity interferometry, the use of synchrotron-radiation methods greatly increases our spatial and temporal knowledge of these ‘simple’ systems.   

The effect of 2D Particle Morphology on Shock-compacted Granular Materials

Granular materials are crucial in a wide range of industrial applications, including civil engineering, mining technology and the energy sector. A challenge concerning particulate materials is the effect of particle morphologies (e.g., size and shape) on the rearrangement of microstructures leading to the variation in macroscopic responses. In this study, the evolution of the fabric (e.g., void volume) is explored with a wide range of particle geometry of granular materials subjected to shock compaction. The shock compression simulation is conducted via the continuum hydrodynamics code, FLAG (Free-Lagrange) enabling the dynamic compaction modeling with various conditions of packing assemblies in particle size and shape. Furthermore, this study compares the quantities of evolving mesostructure between two- and three-dimensional simulations to address the difference entailed by the dimensionality which is especially important to the spatial-induced features such as fabric in granular media. By comprehending the importance of particle morphological on the effects on dynamic response of granular materials, this study contributes to  the understanding of dependence of shock compaction response on the underlying mesostructure.   

Dynamic Relaxation and Recovery of Shear Strength Granular Compaction

Under quasistatic loading, the shear strength of granular materials is pressure dependent due to frictional resistance at contact points, and the strength increases with pressure and with the solid volume fraction until at high pressure, plasticity in the solid phase limits the strength in the material. For an initially porous material that has been compacted to zero porosity, the high-pressure shear strength is often assumed to equal that of the solid phase, in the absence of additional damage or thermal softening.  New methods for modeling comminution and compaction for shock-loaded brittle materials were developed to numerically investigate the validity of this assumption.

Mesoscale simulations of shock compression of both ductile and brittle powders reveal that the shear stress in a shocked material may be far below the quasistatic strength or the porosity-scaled shear strength of the solid phase – even in the absence of damage or thermal effects.  A dynamic relaxation mechanism is identified that results from heterogenous deformation and locking of misaligned shear stresses, but we show that strength may be recovered under re-shock or sustained shear deformation until thermal softening occurs.  

These results imply that the apparent strength that should be used when computing pressure in a 1-D shock Hugoniot experiment may be much lower than the shear strength needed to match the material response in high-shear applications such as wellbore completion, ballistic penetration, or asteroid deflection. 

We present mesoscale the modeling results and proposed a new continuum strength model form to describe this relaxation and recovery.  We also provide numerical simulations of various shock-shear experimental platforms to assess the extent to which the observables in these experiments are sensitive to the relaxed vs. the recovered shear strength and provide a path towards validation of the new strength models.