Micromechanics of granular material response during load reversals: Combined DE and experimental study Catherine O'Sullivan a, ⁎, Liang Cui b a Department of Civil and Environmental Engineering, Skempton Building, Imperial College London, London SW7 2AZ, United Kingdom b Mechanical Engineering, School of Electrical, Electronic & Mechanical Engineering, University College Dublin, Dublin 4,Ireland a b s t r a c t a r t i c l e i n f o Available online 9 March 2009 Keywords: DEM Load–unload cyles Granular materials Triaxial test The use of ideal granular materials with regular, simple geometries (e.g. steel spheres) allows an accurate geometrical representation of physical test specimens to be made in DEM simulations. Physical tests on the materials can then be used to validate DEM models and these DEM models can be con fidently used to develop insight into the micro-scale interactions driving the macro-scale response observed in the laboratory. A novel approach to simulating triaxial tests with DEM using circumferential periodic boundaries has been developed by the authors. In a previous study this approach was validated analytically and by considering a series of laboratory monotonic triaxial tests on specimens of uniform and non-uniform steel spheres.The current paper extends the earlier research ofthe authors by simulating the response of specimens of about 15,000 steel spheres subject to load–unload cycles in quasi-static triaxial tests. In gene good agreement was attained between the physical tests and the DEM simulations. Following a description the simulation and testing approach adopted, the results ofthe DEM simulation are used to explore the particle-scale mechanics during the load reversals. The micro-scale analyses considered both the magnitude and orientation of the contact forces as well as the motion of the particles during the load –unload cycles. These micro-scale analyses revealed that the relatively stiff, almost elastic macro-scale response observed the load–unload cycles is underlain by a particle-scale response involving a substantial redistribution of the contact forces without a significant disturbance to the contact force network. © 2009 Elsevier B.V. All rights reserved. 1. Introduction As a consequence of rapid increases in computer processing speeds, discrete element modelling (DEM) is gaining popularity across a wide range of disciplines. If DEM is to be used with confidence in engineering analysis and design, quantitative validation ofDEM codes is essential to develop confidence amongst both researchers and practising engineers in its reliability. This paper initially includes a discussion on the available approaches to validate DEM codes, prior to a description of a coupled experimental-DEM study. This study ex- tends the earlier research of Cui et al. [6] by demonstrating that DEM codes can accurately capture the response of a granular materials ubject to non-monotonic loading.The experiments considered are strain controlled quasi - static triaxial tests including 2 pre-peak load reversals. The benefits of simulating element tests using DEM are then illustrated via a micro-mechanical analysis of the material response during the tests. Particular emphasis is placed on understanding the macro–micro scale relationships during the load reversals. 2. Validation of DEM codes As proposed by Cundall [8], amongst others, one approach to using DEM in geotechnicalengineering is to calibrate DEM results (using idealized particle geometries) against the results of laboratory tests on real soils. In this calibration approach, the rheological model parameters are varied until the macro-scale response observed in the DEM model matches the field response. Examples of such an approach to calibration include Barla and Barla [1] and Dolezalova et al. [10]. The use of DEM in this manner should be approached with caution.Analysts should consider,for their application,whether it is valid to vary the inter- particle coefficient of friction between the DEM particles to compensate for the differences in geometry between a real soil particle and a sphere Care should also be taken when using two-dimensional particles to represent real soil grains. A real soil will develop contacts in the out of plane direction,consequently the micro-mechanics will be different. There is merit in carrying out two-dimensional DEM simulations where mechanisms can be more easily visualized, however assemblies of two- dimensional DEM particles must be considered as analogue soils and the validity of calibrating a 2D DEM model against experimental data on physical materials should be carefully considered. While well designed calibration exercises can undoubtedly advance understanding of granular material response (e.g. [2]), the Powder Technology 193 (2009) 289–302 ⁎ Corresponding author. E-mail addresses: cath.osullivan@imperial.ac.uk (C. O'Sullivan), liang.cui@ucd.ie (L. Cui). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.03.003 Contents lists available at ScienceDirect Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c