Investigating the micro mechanics of cemented sand using DEM

The discrete element method has been used to investigate the micro mechanics of cemented sand. High pressure drained triaxial tests are modelled in 3D using a flexible membrane which allows the correct deformation to develop. Simulations with up to 12 MPa confining pressure are presented, which are compared with laboratory experiments on a sand with a range of cement contents. Cementation is modelled using ‘parallel bonds’, and various parameters and strength distributions are investigated. Varying levels of cementation are successfully modelled, with the correct qualitative behaviour observed, and the separate effects of cementation and confining pressures demonstrated. The triaxial behaviour is found to be highly influenced by the distribution of bond strengths

Dance as an anti-aging activity

Dance can help preserve mobility, prevent injury, and increase communal connections as we age—why isn’t it everywhere?&nbsp

Prediction of strength and deformability of an interlocked blocky rock mass using UDEC

The accurate prediction of strength and deformability characteristics of a rock mass is very challenging. In practice, properties of a rock mass are often estimated from available empirical relationships based on the uniaxial compressive strength (UCS). However, UCS does not always give a good indication of in-situ rock mass strength and deformability. The aim of this paper is to present a methodology to predict the strength and deformability of a jointed rock mass using UDEC (universal distinct element code). In the study, the rock mass is modelled as an assemblage of deformable blocks that can yield as an intact material and/or slide along pre-defined joints within the rock mass. A range of numerical simulations of UCS and triaxial tests were conducted on rock mass samples in order to predict the equivalent mechanical properties for the rock mass under different loading directions. Results are compared against the deformability parameters obtained by analytical methods

Making and Preserving the Nation’s Public Housing

Humanities: 1st Place (The Ohio State University Edward F. Hayes Graduate Research Forum)This is an extended version of the paper presentation I presented at the 2015 Hayes Graduate Research Forum, humanities division. It draws upon some of my dissertation research, including time spent with local preservationist activists. The paper examines the issue of historic preservation of public housing projects in the United States at both the national and local levels. It presents some mapping visualizations of the first wave of American public housing as well as the locations which have been officially listed on the National Register of Historic Places. The case study focuses on Poindexter Village in Columbus, Ohio, including some oral-history findings to analyze who and what the American nation chooses to value.A five-year embargo was granted for this item

DEM of triaxial tests on crushable cemented sand

Using the discrete element method, triaxial simulations of cemented sand consisting of crushable particles are presented. The triaxial model used features a flexible membrane, allowing realistic deformation to occur, and cementation is modelled using inter-particle bonds. The effects of particle crushing are explored, as is the influence of cementation on the behaviour of the soil. An insight to the effects that cementation has on the degree of crushing is presented

3D continuum-discrete coupled modelling of triaxial Hopkinson bar tests on rock under multiaxial static-dynamic loads

Rock engineering projects at depth are frequently subjected to dynamic loadings under in-situ stress state, and the studies should be conducted to decipher the coupled effect of confining pressure and strain rate on the behaviour of rocks. The triaxial Hopkinson bar system has been applied to investigate the responses of materials to the coupled multiaxial static-dynamic loads. In this study, a three-dimensional (3D) continuum-discrete coupled method is employed to establish a numerical-based triaxial Hopkinson bar system, and the steel bars and a cubic specimen are modelled by continuum zones and bonded-particle material, respectively. Firstly, the detailed numerical modelling is performed to verify some prerequisites and uncertainties in the experiments, including stress wave propagation and attenuation in three directions, dynamic stress equilibrium, boundary effects, interfacial frictions, and controversial methodologies for applying confining pressure, by using the flat-joint model and parallel bond model. Then, both experimental tests and numerical modelling are carried out on sandstone under multiaxial pre-stress conditions (i.e., uniaxial, biaxial and triaxial compression) followed by dynamic loads. The dynamic responses of rock, including stress-strain curves, dynamic strength, energy evolutions, and damage patterns, exhibit confinement dependence, which is in good agreement with experimental observations. Under uniaxial compression, the specimens are broken into fragments by multiple fractures; while under biaxial compression, two symmetrically distributed V-shaped damage zones form near the free surfaces. Under triaxial compression, the degree of damage is substantially reduced, and microcrack localisation zones are initiated from the surface, propagate to the interior and eventually form macroscopic fractures. Moreover, a series of numerical simulations is conducted to investigate the strain rate dependence of sandstone under multiaxial load conditions. Both dynamic strength and peak lateral dynamic stresses increase with increasing strain rate. The increase of dynamic strength and failure strain becomes obvious at high strain rates with the enhancement of lateral confinement.</p

Discrete crack growth analysis methodology for through cracks in pressurized fuselage structures

A methodology for simulating the growth of long through cracks in the skin of pressurized aircraft fuselage structures is described. Crack trajectories are allowed to be arbitrary and are computed as part of the simulation. The interaction between the mechanical loads acting on the superstructure and the local structural response near the crack tips is accounted for by employing a hierarchical modeling strategy. The structural response for each cracked configuration is obtained using a geometrically nonlinear shell finite element analysis procedure. Four stress intensity factors, two for membrane behavior and two for bending using Kirchhoff plate theory, are computed using an extension of the modified crack closure integral method. Crack trajectories are determined by applying the maximum tangential stress criterion. Crack growth results in localized mesh deletion, and the deletion regions are remeshed automatically using a newly developed all-quadrilateral meshing algorithm. The effectiveness of the methodology and its applicability to performing practical analyses of realistic structures is demonstrated by simulating curvilinear crack growth in a fuselage panel that is representative of a typical narrow-body aircraft. The predicted crack trajectory and fatigue life compare well with measurements of these same quantities from a full-scale pressurized panel test

Grain-based modelling of dynamic shear rupture of heterogeneous rock using a coupled continuum-discrete model

Dynamic shear rupture of rocks plays a significant role in the formation of earthquake faults and the stability of underground engineering structures at depth. This paper aims to explore shear strength, progressive fracturing and seismicity, and underlying shear failure mechanisms of heterogeneous rock under dynamic loads. A lab-scale direct shear test is conducted on a cubic granite sample by using a Triaxial Hopkinson bar (Tri-HB) system, and a coupled continuum-discrete element method is applied to simulate the functionality of the full-scale Tri-HB system for dynamic direct shear tests. The evolution of shear stress and strain distribution shows obvious concentration around the shear rupture band, which verifies the feasibility of the designed testing and modelling configuration for characterising dynamic shear stress and deformation. A grain-based discrete element method (GB-DEM) is adopted to represent the mineralogical heterogeneity of granite and to reveal multi-scale fracturing and seismic activities under dynamic shear loads. The microcracking process shows a transition of the dominant failure mode from intergranular crack to transgranular crack during the shear process. The shear rupture zone is typically initiated from the discrete fractures inclined at certain angles, which are gradually coalesced to form a main shear fracture that breaks the rock into two main blocks. The seismicity of the shear rupture process shows a drop in the b-value before the peak shear stress, followed by a certain degree of recovery at the post-failure stage. We found that shear mechanical response and rupture behaviour show a strong dependency on both strain rate and initial normal stress. The number of transgranular cracks is increased with increasing strain rate, which consequently results in intensively crushed mineral grains. Inclined fractures become steeper under higher initial normal stresses which further widen the shear rupture band. The normal-shear stress curves show a common trend of two linear increase stages followed by nonlinear damage and failure processes. The shear strength can be approximated by the Mohr-Coulomb strength envelope under various initial normal stresses and strain rates.</p