The role of microstructure in the incremental behavior of granular materials V. Magnanimo, N. Kumar and S. Luding MSM - University of Twente (NL)
We observe the (often disastrous) MACROSCOPIC behavior and we want to model it …. ….how is this related with the MICROSCOPIC structure?
Evolution of microstructure [Behringer (Duke)] Evolution of the microstructure
Inhomogeneity & anisotropy, instabilities & structures, rotations [Luding (UT)] Evolution of microstructure
Material idealization
Experimental background
Yang, Li and Yang (2008) Geotechnique 58(4) Triaxial compression samples prepared with: dry deposition moist tamping
ISOTROPIC SYSTEM
Numerical protocol
Coordination number: HISTORY of the sample Magnanimo et al. (2008) EPL
Elastic moduli We apply ( = inf) an incremental strain ij and measure the variation in stress ij after relaxation Calvetti et al. (2003) Rivista Italiana di Geotecnica 3 Magnanimo et al. (2008) EPL Sibille et al. (2009) EJECE 13(2)
Amplitude of the increment
Elastic moduli dependence on microstructure G/B Magnanimo et al. (2008) EPL
Theoretical model with anisotropy
Luding and Perdahcioglu CIT (2011), Magnanimo and Luding, Granular Matter (2011) Simple constitutive model
ANISOTROPIC SYSTEM undrained biaxial test
O.I. Imole et al. KONA (2013), Kumar et al. Acta Mechanica (under review) Shear Stress Evolution of stress and fabric Deviatoric fabric dev =√3 J 2 F dev =√3 J 2 Fc definition of fabric
We apply purely deviatoric perturbations along the path Elastic moduli
Incremental elastic behavior
Small enough before any local rearrangement happens How small is small?
Bulk modulus
Anisotropy modulus
Shear modulus
Theoretical prediction undrained cyclic biaxial test
Incremental elastic behavior
F dev =√3 J 2 Fc
Luding and Perdahcioglu CIT (2011), Magnanimo and Luding, Granular Matter (2011) Simple constitutive model
Initial state: isotropic volume fraction Shear Stress Deviatoric Fabric Undrained biaxial test: calibration
Undrained CYCLIC biaxial test: prediction Initial state: isotropic volume fraction Shear Stress Deviatoric Fabric
Isobaric compression
Conclusions We analyzed the incremental behavior of isotropic granular samples and we linked is with the coordination number We analyzed the incremental behavior of anisotropic granular samples and we linked is with volumetric and deviatoric fabric When a simple constitutive model with evolution equation for fabric is introduced and calibrated with the moduli cyclic shear can be predicted
Thank you