INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS Int. J. Numer. Anal. Meth. Geomech. (2015) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/nag.2370 ISA model: A constitutive model for soils with yield surface in the intergranular strain space W. Fuentes * ,† and Th. Triantafyllidis Institute of Soil Mechanics and Rock Mechanics, Karlsruhe Institute of Technology KIT, Engler-Bunte-Ring 14, 76131 Karlsruhe, Germany SUMMARY In this article, a new constitutive model for soils is proposed. It is formulated by means of plasticity, but in contrast to the precedent works, it presents a yield function describing a surface within the intergranular strain space. This latter is a state variable providing information of the recent strain history. An expression for the plastic strain rate has been proposed to guarantee the stress rate continuity. Under the application of medium or large strain amplitudes, the constitutive equation becomes independent of the intergranular strain and delivers a mathematical structure similar to some Karlsruhe hypoplastic models. Some simulations of monotonic and cyclic triaxial test are provided to evaluate and analyze the model performance. Copyright © 2015 John Wiley & Sons, Ltd. Received 29 May 2014; Revised 1 February 2015; Accepted 3 February 2015 KEY WORDS: ISA model; hypoplasticity; saturated sands; cyclic loading; mechanical behavior 1. INTRODUCTION In the last decades, several theories have been developed to capture the mechanical behavior of the soils. The most popular approaches are usually ‘stress-based’ models; that is, the model presents a single or multiple yield surfaces within the stress space. Among them, one can find conventional critical state models [1, 2], bounding surface models [3, 4], multisurface plasticity models [5–7], generalized plasticity [8, 9], or subloading plasticity models [10–12]. Although they have shown to be very competent, some limitations are often observed especially under cyclic loading: models with very narrow yield surface (e.g., [3, 4, 13]) are not able to ‘remember’ the stress history of the material as shown by the experiments [14, 15] or might present overshooting when incorporating ‘loading-initialization’ tensors. In contrast, models with greater yield surface (as in [16]) do not catch the observed plastic accumulation within this surface or cannot capture accurately the stiffness degradation under cyclic loading with small strain amplitudes (k " k<1  10 3 ) [17, 18]. These and other shortcomings allowed some alternative frameworks to emerge. Examples of them are the hyperplastic models [19–21], the Karlsruhe hypoplastic models [22–24] extended with inter- granular strain [25, 26], endochronic models [27, 28], and other models defining the yield surface within the strain space [29–32]. All these frameworks are said to be strain based because they do not require a yield condition describing a surface within the stress space. By doing this, they have shown that some of the mentioned limitations are addressed and a more realistic description of the elastic locus is in many cases provided. Moreover, these models seem to avoid complicated inte- gration algorithms considering that their formulation is compatible with most of the finite element codes whereby a strain increment is given as input. *Correspondence to: W. Fuentes, Institute of Soil Mechanics and Rock Mechanics, Karlsruhe Institute of Technology KIT, Engler-Bunte-Ring 14, 76131 Karlsruhe, Germany. † E-mail: william.lacouture@kit.edu Copyright © 2015 John Wiley & Sons, Ltd. Journal: International Journal for Numerical and Analytical Methods in Geomechanics Year: 2015