KAYENTA : theory and user's guide.

Abstract

The physical foundations and domain of applicability of the Kayenta constitutive model are presented along with descriptions of the source code and user instructions. Kayenta, which is an outgrowth of the Sandia GeoModel, includes features and fitting functions appropriate to a broad class of materials including rocks, rock-like engineered materials (such as concretes and ceramics), and metals. Fundamentally, Kayenta is a computational framework for generalized plasticity models. As such, it includes a yield surface, but the term "yield" is generalized to include any form of inelastic material response including microcrack growth and pore collapse. Kayenta supports optional anisotropic elasticity associated with ubiquitous joint sets. Kayenta supports optional deformation-induced anisotropy through kinematic hardening (in which the initially isotropic yield surface is permitted to translate in deviatoric stress space to model Bauschinger effects). The governing equations are otherwise isotropic. Because Kayenta is a unification and generalization of simpler models, it can be run using as few as 2 parameters (for linear elasticity) to as many as 40 material and control parameters in the exceptionally rare case when all features are used. For high-strain-rate applications, Kayenta supports rate dependence through an overstress model. Isotropic damage is modeled through loss of stiffness and strength. We are grateful to our family members for their patience and support during the ongoing development of this model.

Relaxing the multi-stage nested return algorithm for curved yield surfaces and nonlinear hardening laws

“…2-6 are generated in a Mathematica implementation of the multi-stage nested return algorithm. A piecewise yield function is used to construct a simple two-surface model comprising a linear Drucker-Prager region and a curved cap, which together represent the effect of pressure-dependent shear strength and porosity, respectively (Brannon et al 2009). The fast return was computed differently for each of the three shaded regions shown in Fig.…”

“…In our two-surface model, an example of a state variable is the hydrostatic compressive strength X , which defines the intercept of the cap along the hydrostat. The evolution of strength with porosity is defined by a crush curve (Brannon et al 2009), which defines the dependence of X on ε p v . The evolution law is theṅ…”

Mesoscale modeling of nonlinear elasticity and fracture in ceramic polycrystals under dynamic shear and compression

“…Parameters listed in Tables 8 and 10 can be used directly in macroscopic models of inelasticity and failure of ceramic materials incorporating Weibull statistics and pressure-dependent shear strength (Brannon et al, 2007(Brannon et al, , 2009Leavy et al, 2010). The present results may be particularly valuable for AlON, for which experimental data (statistical and pressure-strength) are not as readily available.…”

“…In each material, the increase in shear strength with compressive pressure or confinement is evident. Previous models (Brannon et al, 2007(Brannon et al, , 2009Leavy et al, 2010) have assumed that variability in strength decreases with an increase in pressure. This phenomenon is not apparent from the present results.…”

“…Of particular interest in the present study are the following physical phenomena: dependence of dynamic shear strength on pressure, sensitivity of peak strength to grain morphology and elastic anisotropy, and dependence of statistical variations in peak strength on specimen size (i.e., number of grains) and local material properties. Pressure-dependent strength statistics from mesoscale simulations can be used to provide parameters entering macroscopic constitutive models for ceramic material behavior incorporating statistical failure criteria (Brannon et al, 2007(Brannon et al, , 2009Leavy et al, 2010;Graham-Brady, 2010). This paper is structured as follows.…”