A. Niemunis

Hypoplastic model for cohesionless soils with elastic strain range

SUMMARY In order to eliminate ratcheting a so-called intergranular strain has been added to a hypoplastic constitutive model. This additional state variable represents the deformation of the interface layer between the grains. The new concept is outlined and comparisons with and without intergranular strain are presented. Some comments on numerical implementation and determination of material constants are made. A discussion on the uniqueness of the solution and objectivity of the rate of intergranular strain is added. # 1997 John Wiley & Sons, Ltd. Mech. of Cohes.-Frict. Mater., 2, 279‐299 (1997)

Prediction of Permanent Deformations in Pavements Using a High-Cycle Accumulation Model

The present paper discusses the application of a high-cycle accumulation (HCA) model originally developed for sand for the prediction of permanent deformations in an unbound granular material (UGM) used for base and subbase layers in pavements. Cyclic triaxial tests on precompacted samples of an UGM have been performed in order to validate and calibrate the model. The stress amplitude, the initial density, and the average stress were varied. The test results are compared to those of air-pluviated samples of sand (subgrade material). Some significant differences in the behavior of both materials under cyclic loading are outlined. It is demonstrated that the functions describing the intensity of accumulation can be maintained for an UGM with different material constants, but that the flow rule must be generalized in order to describe the anisotropy. Recalculations of the laboratory tests show a good prediction of the modified HCA model.

BEYOND FAILURE IN GRANULAR MATERIALS

Recent investigations on the hypoplastic constitutive model for granular materials show that the failure surface can be surpassed by some stress paths. This is contradictory to the conventional definition of failure surface in plasticity, according to which the stress is allowed to move on the failure surface but never across it. In the present paper, the interrelations among the different constitutive models are discussed with special reference to failure and stability. For the hypoplastic constitutive equation, the accessible stress states and the stable stress states are found to be enclosed by a bound surface and a stability surface in the stress space, respectively. Theoretical findings about the bound surface and the stability surface are verified qualitatively by presenting results of triaxial tests on dry sand.

On hyperelasticity for clays

Abstract Under purely elastic conditions a constitutive model for clays should be conservative. This can be achieved by using a hyperelastic stress–strain relation derived from an elastic potential. To circumvent problems connected with formulation of a suitable potential a simple concept of back-stress elasticity is proposed. Several elastic models are critically reviewed, two of which had been originally incorrectly classified as hyperelastic. A novel elastic potential which can be used to generate various hyperelastic models is proposed. Its salient feature is that the resulting stiffness is a homogeneous function of order 1 with respect to stress. An example of Mathematica calculation with tensorial differentiation is enclosed (Wolfram S. Mathematica. A system for doing mathematics by computer. Addison-Wesley Publishing, 1988). ©

Towards the FE prediction of permanent deformations of offshore wind power plant foundations using a high-cycle accumulation model

The paper discusses a possible application of the authors’ high-cycle accumulation (HCA) model for the prediction of long-term deformations of offshore wind power plant (OWPP) foundations. The calibration of the HCA model parameters for a typical North Sea fine sand is presented. These parameters have been used for exemplary finite element calculations of a monopile foundation, with variation of soil density, average load and cyclic load amplitude.

An averaging procedure for layered materials

In order to model thin-layered soils a special averaging procedure is proposed. It works with very few restrictions imposed on the constitutive equations used for individual materials. General considerations are followed by an example from open pit mining industry. Geometrical non-linearity is considered. The proposed method is not necessarily restricted to the problems of soil mechanics. It could also be applied on composite materials. Copyright

On the estimation of the amplitude of shear strain from measurements in situ

Abstract It is well known that the shear strain amplitude caused by an elastic wave can be estimated on the basis of measurements of the particle velocity. In this aspect, the difference between a vertically polarized plane shear wave and a plane Rayleigh wave is presented. A particular case is studied in which the vertical component of the velocity of a particle lying on the ground surface is known. The relation between the particle velocity and the strain amplitude is derived and a diagram for evaluation of this amplitude is given.

Peak Stress Obliquity in Drained and Undrained Sands. Simulations with Neohypoplasticity

The difference between undrained and drained peak friction angle is considerable (up tp \(10^\circ \)), despite identical densities and pressures (at peak). This cannot be explained using the elastoplastic formalism. An attempt is made to describe this effect with neohypoplasticity. For this purpose two types of nonlinearity are used, the well-known term \(Y m_{ij} \Vert \dot{\epsilon }\Vert \) and the novel skew-symmetric correction tensor which is added to the elastic stiffness.

Neohypoplasticity—Estimation of Small Strain Stiffness

Behaviour of soils under small cycles is examined in the triaxial apparatus and the results are used for the calibration of several constitutive relations. The small strain relation is not exactly linear and stiffness \( E_{ijkl}\) in \(\dot{\sigma _{ij}}= E_{ijkl}\dot{\varepsilon _{kl}}\) is not constant. The popular hypoplastic (HP) model describes the small strain behaviour using the intergranular strain (Niemunis, Herle, Mech Cohesive-Frictional Mater 2(4):279–299 1997). However, this idea with an additional strain has several shortcomings. A better approach is the paraelastic (PE) model (Niemunis et al, Acta Geotech 6(2):67–80 2011; Prada Sarmiento, Paraelastic description of small-strain soil behaivour 2012). In this study the paraelasticity has been used already while evaluating of the raw data from triaxial test results. Similarly a simplified high cycle accumulation (HCA) formula (Niemunis et al, Comput Geotech 32(4):245–263 2005) and a simple assumption of stress dependence of \( E_{ijkl}\) have been used to purify the measured test data. A general curve-fitting strategy for testing of different constitutive models is developed. Some shortcomings of PE and HCA could be observed.

Computer Aided Calibration, Benchmarking and Check-Up of Constitutive Models for Soils. Some Conclusions for Neohypoplasticity

A computer aided calibration, benchmarking and testing of constitutive models is presented. A large collection of test data for Karlsruhe sand is used together with the neohypoplastic model as an example. The essential feature of the procedure is an automatic evaluation of the discrepancies between a test and its simulation. They can be minimized by modifications of material constants or used as a benchmark for different models. Apart from the curve-fitting part one may check whether a constitutive model violates the Second Law. For this purpose several specially designed stress loops are tried out. A shake down of all state variables is established. The neohypoplasticity and the barotropic hypoelasticity are used as examples in the thermodynamic tests. In conclusion several modification to neohypoplastic models are presented.