Konstantinos Karapiperis

Bio: Konstantinos Karapiperis is an academic researcher from California Institute of Technology. The author has contributed to research in topics: Discrete element method & Granular material. The author has an hindex of 5, co-authored 14 publications receiving 102 citations. Previous affiliations of Konstantinos Karapiperis include University of California, Davis & ETH Zurich.

Data-Driven multiscale modeling in mechanics

Abstract: We present a Data-Driven framework for multiscale mechanical analysis of materials. The proposed framework relies on the Data-Driven formulation in mechanics (Kirchdoerfer and Ortiz 2016), with the material data being directly extracted from lower-scale computations. Particular emphasis is placed on two key elements: the parametrization of material history, and the optimal sampling of the mechanical state space. We demonstrate an application of the framework in the prediction of the behavior of sand, a prototypical complex history-dependent material. In particular, the model is able to predict the material response under complex nonmonotonic loading paths, and compares well against plane strain and triaxial compression shear banding experiments.

Combined loading of caisson foundations in cohesive soil: Finite element versus Winkler modeling

Abstract: The undrained response of massive caisson foundations to combined horizontal, vertical and moment loading is parametrically investigated through a series of 3D finite element analyses. The parameters are: (a) the embedment ratio (D/B), (b) the factor of safety against initial vertical loading (FSV) and (c) the ratio of the overturning moment to the horizontal force applied at the top of the caisson (M/Q). Emphasis is given on: (i) the identification of all possible failure mechanisms in M–Q–N space, (ii) the developed stress distributions along the caisson walls for various load levels up to complete failure conditions. The results are then used as a feedback for calibrating the parameters of a generalized four-type spring model, originally proposed by Gerolymos and Gazetas (2006), through a genetic algorithm-based optimization procedure. The predictions of the Winkler model compare very well with the FE results, not only at the local response level (in terms of stress distributions along the caisson shafts), but at a global response level (in terms of force–displacement curves and M–Q–N failure envelopes at the top of the caisson) as well. Contrary to established lateral soil resistance theories, it is shown that both the ultimate horizontal soil reaction and resisting moment per unit depth do not solely depend on the strength properties of soil and geometry of the caisson but are also functions of the applied load ratio M/Q and initial soil yielding due to vertical loading. Interesting conclusions are also drawn regarding the transition from the elastic to the ultimate limit state (hardening). Quantifying through analytical expressions the contribution of each of the two basic lateral resisting mechanisms to the response of the caisson, a classification method for embedded foundations is then proposed. The capabilities of the Winkler model are further demonstrated through comparison with FE analysis of the caisson cyclic lateral response.

Generalized failure envelope for caisson foundations in cohesive soil: Static and dynamic loading

Abstract: The response of massive caisson foundations to combined vertical (N), horizontal (Q) and moment (M) loading is investigated parametrically by a series of three-dimensional finite element analyses. The study considers foundations in cohesive soil, with due consideration to the caisson-soil contact interface conditions. The ultimate limit states are presented by failure envelopes in dimensionless and normalized forms and the effects of the embedment ratio, vertical load and interface friction on the bearing capacity are studied in detail. Particular emphasis is given on the physical and geometrical interpretation of the kinematic mechanisms that accompany failure, with respect to the loading ratio M/Q. Exploiting the numerical results, analytical expressions are derived for the capacities under pure horizontal, moment and vertical loading, for certain conditions. For the case of fully bonded interface conditions, comparison is given with upper bound limit equilibrium solutions based on Brinch Hansen theory for the ultimate lateral soil reaction. A generalized closed-form expression for the failure envelope in M–Q–N space is then proposed and validated for all cases examined. It is shown that the incremental displacement vector of the caisson at failure follows an associated flow rule, with respect to the envelope, irrespective of: (a) the caisson geometry, and (b) the interface conditions. A simplified geometrical explanation and physical interpretation of the associativity in M-Q load space is also provided. Finally, the derived failure envelope is validated against low (0.67 Hz) and high frequency (5 Hz) dynamic loading tests and the role of radiation damping on the response of the caisson at near failure conditions is unraveled.

Investigating the incremental behavior of granular materials with the level-set discrete element method

Abstract: A computational framework is presented for high-fidelity virtual (in silico) experiments on granular materials. By building on i) accurate mathematical representation of particle morphology and contact interaction, ii) full control of the initial state of the assembly, and iii) discrete element simulation of arbitrary stress paths, the proposed framework overcomes important limitations associated with conventional experiments and simulations. The framework is utilized to investigate the incremental response of sand through stress probing experiments, focusing on key aspects such as elasticity and reversibility, yielding and plastic flow, as well as hardening and fabric evolution. It is shown that reversible strain envelopes are contained within elastic envelopes during axisymmetric loading, the yield locus follows approximately the Lade-Duncan criterion, and the plastic flow rule exhibits complex nonassociativity and minor irregularity. Hardening processes are delineated by examining the stored plastic work and the fabric evolution in the strong and weak networks. Special attention is given to isolating in turn the effect of particle shape and interparticle friction on the macroscopic response. Interestingly, idealization of particle shape preserves qualitatively most aspects of material behavior, but proves quantitatively inadequate especially in anisotropic stress states. The results point to the importance of accurately resolving particle-scale interactions, that allows macroscopic behavior to emerge free from spurious micromechanical artifacts present in an idealized setting.

Data-Driven nonlocal mechanics: Discovering the internal length scales of materials

Abstract: Nonlocal effects permeate most microstructured materials, including granular media, metals and foams. The quest for predictive nonlocal mechanical theories with well-defined internal length scales has been ongoing for more than a century since the seminal work of the Cosserats. We present here a novel framework for the nonlocal analysis of material behavior , which bypasses the need to define any internal length scale. This is achieved by extending the Data-Driven paradigm in mechanics, originally introduced for simple continua, into generalized continua. The problem is formulated directly on a material data set, comprised of higher-order kinematics and their conjugate kinetics, which are identified from experiments or inferred from lower scale computations. The case of a micropolar continuum is used as a vehicle to introduce the framework, which may also be adapted to strain-gradient and micromorphic media. Two applications are presented: a micropolar elastic plate with a hole, which is used to demonstrate the convergence properties of the method, and the shear banding problem of a triaxially compressed sample of quartz sand , which is used to demonstrate the applicability of the method in the case of complex history-dependent material behavior.

A review on recent advancements of substructures for offshore wind turbines

Abstract: The sustainable development of offshore wind energy requires thorough investigations on technological issues. The substructure, which acts as the natural link between technologies and environments, is a critical topic for the offshore wind industry. This paper presents a comprehensive review of variable types of offshore wind substructures associate with their corresponding example projects. The study is complemented with a special attention to a novel foundation, namely suction bucket foundation. Main technological issues related to this concept are integrated. In the paper, bearing behaviors of offshore wind turbines (OWTs) with the suction bucket foundation under lateral loads, vertical loads, combined loads, and extreme loading conditions are discussed. Two installation methods are introduced. The geometric and improved design is illustrated by considering capabilities in transportation and installation. Research methods, including field tests, laboratory tests, centrifuge tests, theoretical analysis and numerical simulations, are listed; these methods are employed in previous studies to investigate behaviors of the OWT. This review integrates most relevant aspects and recent advancements together, which aims to provide a reference frame for future studies and projects.

Closure of "Strain Localization and Undrained Steady State of Sand"

Abstract: Experimental results are presented that characterize the behavior of a loose, fine-grained, water-saturated sand tested under globally undrained conditions is a plane strain apparatus. Together with local measurements of boundary forces and deformations, stereophotogrammetry is used to track the progressive localization of strain. The constitutive behaviors in the tests prior to localization can be characterized as either undrained local softening or undrained load softening and subsequent rehardening. In both cases, a consistent pattern of onset of the formation of the persistent shear band, mobilization of the maximum effective friction, and the complete formation of the band was observed. Conditions approximating steady state only were observed in the shear bands. The effect of strain localization on the conventionally derived steady state line was found to be minimal when undrained load softening behavior was observed, but significantly different steady-state lines were found from results of plane strain and axisymmetric tests. The differences between tests conducted under these two stress conditions assessed to evaluate the practical implications of the observed differences.

Data-Driven multiscale modeling in mechanics

Abstract: We present a Data-Driven framework for multiscale mechanical analysis of materials. The proposed framework relies on the Data-Driven formulation in mechanics (Kirchdoerfer and Ortiz 2016), with the material data being directly extracted from lower-scale computations. Particular emphasis is placed on two key elements: the parametrization of material history, and the optimal sampling of the mechanical state space. We demonstrate an application of the framework in the prediction of the behavior of sand, a prototypical complex history-dependent material. In particular, the model is able to predict the material response under complex nonmonotonic loading paths, and compares well against plane strain and triaxial compression shear banding experiments.

4th Ishihara lecture: Soil–foundation–structure systems beyond conventional seismic failure thresholds

Abstract: A new paradigm has now emerged in performance-based seismic design of soil–foundation–structure systems. Instead of imposing strict safety limits on forces and moments transmitted from the foundation onto the soil (aiming at avoiding pseudo-static failure), the new dynamic approach “invites” the creation of two simultaneous “failure” mechanisms: substantial foundation uplifting and ultimate-bearing-capacity slippage, while ensuring that peak and residual deformations are acceptable. The paper shows that allowing the foundation to work at such extreme conditions may not only lead to system collapse, but it would help protect (save) the structure from seismic damage. A potential price to pay: residual settlement and rotation, which could be abated with a number of foundation and soil improvements. Numerical studies and experiments demonstrate that the consequences of such daring foundation design would likely be quite beneficial to bridge piers, building frames, and simple frames retrofitted with a shear wall. It is shown that system collapse could be avoided even under seismic shaking far beyond the design ground motion. Three key phenomena are identified as the prime sources of the success; they are illustrated for a bridge–pier: (i) the constraining of the transmitted accelerations by the reduced ultimate moment capacity of the foundation, to levels of about one-half of those developing in a conventional design; (ii) the beneficial action of the static vertical load of the structure which pushes down to “re-center” the leaning (due to uplifting and soil yielding) footing, instead of further distressing the plastic hinge of the column of the conventional design; and (iii) the substantial increase of the fundamental natural period of the system as uplifting takes place, which brings the structure beyond the significant period range of a ground motion, and hence leads to the abatement of its severe shaking.

Coarse-Graining of a Physical Granular System

Abstract: Results, including displacement, strain and stress fields, obtained by applying a resolution-controlled coarse-graining method to an experiment, comprised of a bidisperse system of photoelastic disks under pure shear, are presented. The paper reviews the experimental methods as well as the philosophical and technical bases of the coarse-graining methods employed in this study. Some fields reveal the emergence of a shear band while others do not. Correlations of the displacement fluctuations are shown to decay on a very small scale, of the order of a few particle diameters, even close to jamming. An unexpectedly simple relation between the particle rotation angles and the rotation field is reported. Implications of these and other findings are discussed.