Implementation of a two-surface plasticity model for cyclic loading on sand into the finite element method
Authors
Nielsen, Jonas Vestergaard ; Melin, Guillaume Maxime
Term
4. term
Education
Publication year
2016
Submitted on
2016-06-08
Pages
92
Abstract
This thesis addresses the limitations of conventional soil models such as Mohr–Coulomb under cyclic loading, which do not capture stress history or phenomena like pore pressure build-up, cyclic mobility, and potential liquefaction. The objective is to implement the Critical State two-surface plasticity model for sands by Manzari and Prachathananukit (2000) within the Finite Element Method (FEM) in Matlab. Foundational plasticity concepts and three stress-update schemes (Forward Euler, Modified Forward Euler, and Radial Return) are presented. The implementation begins with von Mises plasticity—both perfectly plastic and with linear hardening—to establish and compare update schemes using a simple patch test and a strip footing example, assessing bearing capacity and computational time. The advanced two-surface model is then implemented with an explicit Forward Euler integration and is intended to be verified under drained and undrained conditions, for monotonic and cyclic loading at different confining pressures. The work emphasizes numerical implementation and a testing/verification framework to support engineering use of advanced sand models for cyclic loading, with relevance to offshore foundations.
Denne afhandling adresserer, at traditionelle jordmodeller som Mohr–Coulomb ofte er utilstrækkelige under cyklisk last, fordi de ikke indregner spændingshistorik og fænomener som poretrykakkumulation, cyklisk mobilitet og mulig liquefaktion. Formålet er at implementere Manzari & Prachathananukits (2000) kritisk tilstand to-overflade plasticitetsmodel for sand i den endelige elementmetode (FEM) i Matlab. Som grundlag gennemgås plasticitetsteori og tre integrationsstrategier til spændingsopdatering (Forward Euler, Modified Forward Euler og Radial Return). Implementationen indledes med von Mises-plasticitet, både perfekt plastisk og med lineær hærdning, for at etablere og sammenligne opdateringsskemaer via en simpel patch test og et stribefundament, hvor bæreevne og beregningstid vurderes. Den avancerede to-overflade model implementeres herefter med en eksplicit Forward Euler-integration og tilsigter at blive verificeret i drænede og udrænede forsøg under monotone og cykliske belastninger ved varierende indeslutningstryk. Afhandlingen fokuserer primært på den numeriske implementering og et test- og verifikationssetup, der skal understøtte ingeniørmæssig anvendelse af avancerede sandmodeller ved cyklisk belastning, eksempelvis for offshore fundamenter.
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