Imperial College London

Dr Anton Köllner

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Marie Skłodowska-Curie Individual Fellow



+44 (0)20 7594 6028a.koellner




425Skempton BuildingSouth Kensington Campus





Publication Type

15 results found

Köllner A, 2021, Predicting buckling-driven delamination propagation in composite laminates: An analytical modelling approach, Composite Structures, Vol: 266, Pages: 1-17, ISSN: 0263-8223

Robust and efficient predictions of buckling-driven delamination propagation, enabled by a novel analytical modelling approach, are presented. The model considers full mechanical coupling (extension-shear, extension-bend, extension-twist/shear-bend and bend-twist), contact and mode-mixity and thus significantly enhances the capabilities of current analytical approaches. A problem description in cylindrical coordinates enables the evaluation of the energy release rate along the delamination boundary. The model uses an energy formalism to determine the post-buckling deformation and a crack-tip element analysis employing force and moment resultants acting on the delamination boundary to determine the energy release rate. Composite panels with circular thin-film delaminations and various multi-directional stacking sequences are investigated for in-plane compressive loading. Predictions of applied strains causing delamination growth, i.e. threshold strain, show good agreement with published experimental data and 3D finite element analysis. A parametric study varying the ratio of delamination size to depth is performed. Based on the findings obtained, governing deformation characteristics of buckling-driven delamination growth are identified and insight into damage tolerant design of composite laminates is obtained, which is of particular interest for compression after impact (CAI) strength of composite structures.

Journal article

Köllner A, Nielsen MWD, Srisuriyachot J, Rhead AT, Butler Ret al., 2021, Buckle-driven delamination models for laminate strength prediction and damage tolerant design, Thin Walled Structures, Vol: 161, ISSN: 0263-8231

Two state-of-the-art analytical compression after impact (CAI) modelling approaches are presented and evaluated for the problem of thin-film buckle-driven propagation of a delamination in composite laminates. Characteristic phenomena are investigated by evaluating the behaviour of the energy release rate of an anisotropic sublaminate above a 2D embedded delamination. These characteristics include extension-bend, shear-bend and bend-twist coupling as well as contact of sublaminate and base laminate. A holistic approach with the aid of a detailed analysis of deformation characteristics from artificial delamination experiments and finite element analysis provide strong validation of the modelling approaches. Suggestions are made regarding analytical methods suitable for use in the initial aerospace structural design stage. It is found that models which capture the mode-mixity and post-buckled energy terms accurately will allow for better design decisions to be made that are not overly conservative. Whereas methods, which do not account for such mixity and post-buckling, can nevertheless be used to design for damage tolerance.

Journal article

Köllner A, Kashtalyan M, Guz I, Völlmecke Cet al., 2020, On the interaction of delamination buckling and damage growth in cross-ply laminates, International Journal of Solids and Structures, Vol: 202, Pages: 912-928, ISSN: 0020-7683

The compressive behaviour of cross-ply laminates with delaminations and matrix cracked layers is investigated by means of an analytical modelling approach. Insight into the post-buckling and damage growth behaviour is obtained owing to comprehensive parametric studies varying delamination length and depth as well as matrix crack density for cross-ply laminates with different layups. The efficient modelling approach comprises the well-known total potential energy principle and the Equivalent Constrained Model for determining reduced stiffness properties depending on the matrix crack density. Thus, unlike previous studies on delamination buckling, the effect of matrix cracked layers is taken into consideration. The analysis of the energy release rates for delamination and matrix crack growth enables the identification of configurations (e.g. delamination depth and length, total thickness of the laminates) which are prone to delamination growth and matrix crack growth, respectively. Beyond that, relationships between post-buckling and damage growth behaviour are identified and discussed.

Journal article

Köllner A, Todt M, Ganzosch G, Völlmecke Cet al., 2019, Experimental and numerical investigation on pre-stressed lattice structures, Thin-Walled Structures, Vol: 145, ISSN: 0263-8231

The effect of pre-stress on the buckling behaviour of geometric unit cells of collinear square lattices is investigated experimentally and numerically. The geometric unit cells are manufactured using fused deposition modelling. Manufacturing strategies are presented which incorporate fibres subjected to pre-stress within the unit cell. The effect of pre-stressed fibres is analysed by comparing the compressive behaviour of unit cells with and without fibre reinforcement. The buckling behaviour of the unit cells is also investigated numerically by employing a parametric study within Abaqus varying the pre-stress in the fibres. The experimental test series shows that the addition of pre-stressed fibres to the system results in an increase in buckling and maximum load of 260%–480% and 220%–350% respectively. The increase strongly relates to the manufacturing quality, i.e. the bonding between the lattice material and the fibres, where a sufficient bonding yields significantly larger loads. The experimental findings on the qualitative and quantitative buckling behaviour correspond well with results obtain from the numerical study.

Journal article

Köllner A, Forsbach F, Völlmecke C, 2019, Delamination buckling in composite plates: an analytical approach to predict delamination growth, New achievements in continuum mechanics and thermodynamics a tribute to Wolfgang H. Müller, Editors: Abali, Altenbach, dell’Isola, Eremeyev, Öchsner, Publisher: Springer, Pages: 241-255, ISBN: 9783030133061

An analytical modelling approach is presented which is capable of determining the post-buckling responses as well as the onset of delamination growth of multi-layered composite plates with an embedded circular delamination. In order to overcome current drawbacks of analytical models regarding embedded delaminations, the model employs a problem description in cylindrical coordinates and a novel geometric representation of delamination growth in conjunction with a Rayleigh-Ritz formulation and the so-called crack-tip element analysis. The modelling approach is applied to study the compressive response of composite plates with thin-film delaminations loaded under radial compressive strain. Post-buckling responses and the onset of delamination growth are determined for several layups. The results are in very good agreement with finite element simulations while requiring low computational cost.

Book chapter

Köllner A, Völlmecke C, 2018, Post-buckling behaviour and delamination growth characteristics of delaminated composite plates, Composite Structures, Vol: 203, Pages: 777-788, ISSN: 0263-8223

The problem of a delaminated composite plate subjected to in-plane compressive loading is investigated by employing a novel analytical framework previously developed by the authors. The framework is capable of modelling the post-buckling behaviour considering damage growth by using a set of generalized coordinates only. Therefore, in order to model the post-buckling responses of delaminated composite plates a Rayleigh–Ritz formulation is employed. Thus, the post-buckling behaviour as well as the delamination growth characteristics are determined by solving a set of non-linear algebraic equations only. For the cases investigated, the study reveals that delamination growth is associated with the global buckling response. So long as stable delamination growth is present, the post-buckling response remains also stable. However, unstable delamination growth may be caused which would occur unexpectedly yielding sudden failure of the structure. This underlines the importance of considering delamination growth when studying the structural stability behaviour of these structures.

Journal article

Köllner A, Kashtalyan M, Guz I, Völlmecke Cet al., 2018, Modelling cracked cross-ply laminates with delamination buckling, Key Engineering Materials, Vol: 774, Pages: 60-65, ISSN: 1013-9826

The mechanical behavior of cross-ply laminates loaded under in-plane compression containing matrix cracks and delaminations is investigated in order to study their influence on the structural stability behavior. This is done by employing a semi-analytical modelling approach which comprises an analytical framework for a structural stability analysis of damageable structures and the Equivalent Constrained Model for deriving reduced stiffness properties of the cracked layers. Cross-ply laminates with varying delamination depths as well as varying matrix crack densities are studied.</jats:p>

Journal article

Köllner A, Völlmecke C, 2018, Structural stability behaviour of delaminated composite plates, Eighth International Conference on Thin-Walled Structures

Conference paper

Ganzosch G, Todt M, Köllner A, Völlmecke Cet al., 2018, Experimental investigations on pre-stressed stayed columns on smaller length scales, Eighth International Conference on Thin-Walled Structures

Conference paper

Köllner A, Völlmecke C, 2017, An analytical framework to extend the general structural stability analysis by considering certain inelastic effects-theory and application to delaminated composites, Composite Structures, Vol: 170, Pages: 261-270, ISSN: 0263-8223

An analytical framework which incorporates damage propagation/growth into the general structural stability analysis is presented. Therefore, the conventional total potential energy approach is extended by introducing an extended total potential energy-like functional capable of describing inelastic processes in which equilibrium holds between available and the required force for producing a change in structure. The work deals with systems which are described by I generalized coordinates and K damage parameters. The damage parameters are found to be functions of I generalized coordinates and M load parameters. The underlying variational principle for inelastic solids may be solved using discrete formulations or approximate methods such as a Rayleigh–Ritz formulation. This leads to a set of non-linear algebraic equations, comprising post-critical equilibrium paths and damage propagation. In order to verify the framework, it is applied to the well-known problem in which a delaminated composite strut/plate is subjected to an in-plane compressive load.

Journal article

Köllner A, Jungnickel R, Völlmecke C, 2017, Delamination growth in buckled composite struts, Defect and Material Mechanics, Editors: Kienzler, Publisher: Springer, ISBN: 9783319516318

This book contains selected papers from the International Symposium on Defect and Material Mechanics (ISDMM 15) held in Bremen, Germany, September 14-17, 2015.

Book chapter

Köllner A, Völlmecke C, 2017, Buckling and postbuckling behavior of delaminated composite struts, International Journal for Computational Methods in Engineering Science and Mechanics, Vol: 18, Pages: 25-33, ISSN: 1465-8763

The buckling and postbuckling behavior of composite struts under uniaxial compression is investigated. A geometrically nonlinear model comprising only four generalized coordinates is applied to multi-layered struts built up of transversally isotropic unidirectional layers. Laminates with a cross-ply layup are investigated. By minimizing the total potential energy of the system, equilibrium paths and critical buckling loads for varying lengths and depths of delamination are determined. Thus, the response of the system in the postbuckling range is analyzed and areas of stable and unstable behavior are determined. The outcome of the work provides detailed information about the influence of delaminations on the buckling behavior of composite struts.

Journal article

Köllner A, Jungnickel R, Völlmecke C, 2016, Delamination growth in buckled composite struts, International Journal of Fracture, Vol: 202, Pages: 261-269, ISSN: 0376-9429

Existing analytical models dealing with buckling and postbuckling phenomena of delaminated composites comprise one limitation: the restriction to stationary delaminations. In the current work, an analytical framework is presented which allows to model the postbuckling response of composites without such limitation. Therefore, the well-known problem of a composite strut with a through-the-width delamination is studied. The system is fully described by a set of I generalized coordinates. The postbuckling response for a stationary delamination is modelled using the conventional total potential energy approach. The postbuckling response for a non-stationary delamination, i.e. once delamination growth occurs, is modelled using an extended total potential energy functional in which the delamination length is expressed by the generalized coordinates and the load parameters. By solving the underlying variational principle the postbuckling response is obtained. Implementing the Rayleigh–Ritz method yields a set of non-linear algebraic equations which is solved numerically. Postbuckling responses for a cross-ply laminate are provided until the strut fails. Depending on delamination depth and length additional load bearing capacities of such composite struts are documented before failure due to unstable delamination growth occurs.

Journal article

Koellner A, Cameron CJ, Battley MA, 2014, Structural responses on a BMX racing bicycle, 10th Conference of the International-Sports-Engineering-Association, Publisher: ELSEVIER SCIENCE BV, Pages: 618-623, ISSN: 1877-7058

Conference paper

Koellner A, Cameron CJ, Battley MA, 2014, Measurement and analysis system for bicycle field test studies, 10th Conference of the International-Sports-Engineering-Association, Publisher: ELSEVIER SCIENCE BV, Pages: 350-355, ISSN: 1877-7058

Conference paper

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