Research project

Isogeometric finite elements for shells

Development of locking-free finite elements based on NURBS shape functions for the analysis of shell structures.

Overview

  • Integration of design (CAD) and analysis (FEM)
  • Finite Elemente Method with NURBS shape functions of high continuity
  • Exploit the higher continuity for the development of innovative shell elements
  • Simple representation of large rotations for shear deformable shell formulations
  • Intrinsical avoidance of geometrical locking effects
"Pinched Cylinder" benchmark problem, computation with isogeometric 5-parameter shell elements (c)
"Pinched Cylinder" benchmark problem, computation with isogeometric 5-parameter shell elements

Projekt Description

Isogeometric Analysis (IGA) received a great deal of attention in the past decade and presents a suitable numerical method to solve a number of problems of academic and industrial interest. There are many benefits coming along with smooth function spaces (as NURBS) and higher inter-element continuity. In particular for shell finite elements significant benfits come along with a smoother basis like NURBS. For instance, a NURBS discretization of higher continuity enables a pointwise unique description of the director field within a whole patch.

director definition (c)
left: IGA – unique director definition, right: FE – multiple director definition

Hierarchic, shear deformable shell formulations

The higher continuity of isogeometric discretizations provoked a renaissance of classical shell models neglecting transverse shear deformation, for instance Kirchhoff-Love, because corresponding requirements for the smoothness of ansatz spaces (min. C1 within a patch) are easily met. However, shear deformable shell formulations with Reissner-Mindlin kinematics and threedimensional shell models enable the use of C0-continuous ansatz spaces. Nevertheless, the higher continuity can be exploited to develop innovative shear deformable shell elements.

shear deformable shell formulation with hierarchic rotations (RM-hr) (c)
Shell’s midsurface for small and large rotations, shear deformable shell formulation with hierarchic rotations (RM-hr)

Hierarchic shell formulations of Reissner-Mindlin type are developed through hierarchic enrichment of the Kirchhoff-Love model. Via hierarchic reparametrization of the kinematic equations transverse shear locking can be a priori avoided within a pure primal method. The concept is equally applied for shear deformable beam, plate and shell formulations, whereas two hierarchic parameterizations can be distinguished.

The parametrization of hierarchic rotations (RM-hr) as well as the parametrization of hierarchic displacements (RM-hd) results in formulations being intrinsically free from transverse shear locking.

Simple representation of large rotations for shear deformable shell formulations

A geometrically nonlinear analysis with shear deformable shell formulations requires a special treatment and a multiplicative decomposition of large rotations. Via elegant reparametrization in hierarchic shell formulations, also in this context an innovative concept can be developed. The underlying assumption is that transverse shear rotations are rather small for typical shell problems, whereas total rotations may be arbitrarily large. This assumption results in a additive decomposition of the Green-Lagrange strains into fully nonlinear parts of the roation-free Kirchhoff-Love type and hierarchically added, linearized transverse shear parts. Thus the total rotations are additively decomposable and can be represented in a simple fashion.

Snap-trough of ring with 5p shell elements (c)
Snap-trough of ring under rotational loading, computation with isogeometric 5-parameter shell elements

Membrane locking

Higher inter-element continuities of isogeometric ansatz spaces may also have disadvantages with respect to quality of results. In particular membrane locking proved to be extremely dominant. For maximum continuity the occurring oscillations in membrane forces do not vanish with higher orders and lead for primal isogeometric shell elements under certain circumstances to very bad results.

Classical concepts known from finite element technology are usually not directly transferable to isogeometric finite elements because the higher inter-element continuity prohibits elementwise treatment. With the Mixed-Dipslacement (MD) concept a variational method has been developed in order to efficiently avoid geometric locking effects. The combination of hierarchic shell formulations based on hierarchic displacements and the MD concept to avoid membrane locking leads to fully locking free shear deformable shell formulations (RM-hd-MD). The snap-through benchmark problem of a cylindrical segment under large rotations displays the superior behavior of the presented RM-hd-MD formulation compared to a primal Kirchhoff-Love ( KL) formulation. Independent of the polynomial order, with the RM-hd-MD formulation smooth results of high quality can be obtained for the membrane force. The primal KL elements allow only a bad approximation of the expected solution and show strong oscillations in the results for the membrane force.

Snap-through of cylindrical segement, RM-hd-MD shell elements (c)
Snap-through of cylindrical segement, contour plot of the membrane force in ring direction, 10 isogeometric RM-hd-MD shell elements of polynomioal order p=3
Snap-through of cylindrical segement, RM-hd-MD shell elements, various polynomial oder (c)
Intermediate step: contour plot of the membrane force in ring direction, 10 isogeometric RM-hd-MD shell elements of polynomial order p=2/3/4
Snap-through of cylindrical segement, RM-hd-MD shell elements, various polynomial oder (c)
Intermediate step: contour plot of the membrane force in ring direction, 10 isogeometric KL shell elements of polynomial order p=2/3/4/8/10/15

Publications

  1. Simon Bieber, Bastian Oesterle, Ekkehard Ramm, Manfred Bischoff. A variational method to avoid locking – independent of the discretization scheme. International Journal for Numerical Methods in Engineering, 114. 2018. DOI: 10.1002/nme.5766
  2. Bastian Oesterle. Intrinsisch lockingfreie Schalenformulierungen. Doctoral Thesis. Bericht Nr. 67, Institut für Baustatik und Baudynamik, Universität Stuttgart. 2018. DOI: 10.18419/opus-10046
  3. Bastian Oesterle, Simon Bieber, Renate Sachse, Ekkehard Ramm and Manfred Bischoff. Intrinsically locking-free formulations for isogeometric beam, plate and shell analysis. Proc. Appl. Math. Mech., 18. 2018. DOI: 10.1002/pamm.201800399
  4. Bastian Oesterle, Renate Sachse, Ekkehard Ramm, Manfred Bischoff. Hierarchic isogeometric large rotation shell elements including linearized transverse shear parametrization. Computer Methods in Applied Mechanics and Engineering, 321. 2017. DOI: 10.1016/j.cma.2017.03.031
  5. Bastian Oesterle, Renate Sachse, Simon Bieber, Ekkehard Ramm, Manfred Bischoff. Isogeometric analysis with hierarchic shell elements – intrinsically free from locking by alternative parametrizations. Proceedings of the IASS Annual Symposium 2017. Annette Bögle, Manfred Grohmann (eds.) "Interfaces: architecture.engineering.science". 25-28th September, 2017, Hamburg, Germany. 2017.
  6. Bastian Oesterle, Ekkehard Ramm, Manfred Bischoff. A shear deformable, rotation-free isogeometric shell formulation. Computer Methods in Applied Mechanics and Engineering, 307. 2016. DOI: 10.1016/j.cma.2016.04.015
  7. Bastian Oesterle, Manfred Bischoff, Ekkehard Ramm. Hierarchic isogeometric analyses of beams and shells. In: M. Kleiber, T. Burczynski, K. Wilde, J. Gorski, K. Winkelmann, L. Smakosz (Eds.) "Advances in Mechanics: Theoretical, Computational and Interdisciplinary Issues". Proceedings of the 3rd Polish Congress of Mechanics (PCM) & 21st International Conference on Computer Methods in Mechanics (CMM), Gdansk, Poland, 8-11 September 2015. 2016.
  8. R. Echter, B. Oesterle, M. Bischoff. A hierarchic family of isogeometric shell finite elements. Computer Methods in Applied Mechanics and Engineering, 254. 2013. DOI: 10.1016/j.cma.2012.10.018
  9. Ralph Echter. Isogeometric analysis of shells. Doctoral Thesis. Bericht Nr. 59, Institut für Baustatik und Baudynamik, Universität Stuttgart,. 2013. DOI: 10.18419/opus-510
  10. Ralph Echter, Manfred Bischoff. Numerical efficiency, locking and unlocking of NURBS finite elements. Comp. Meth. Appl. Mech. Eng., 199. 2010. DOI: 10.1016/j.cma.2009.02.035

Researcher:

Bastian Oesterle
Dr.-Ing.

Bastian Oesterle

Postdoc

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