Jaan-Willem Simon

Finite Element Analysis of Layered Fiber Composite Structures Accounting for the Material’s Microstructure and Delamination

The present paper focuses on composite structures which consist of several layers of carbon fiber reinforced plastics (CFRP). For such layered composite structures, delamination constitutes one of the major failure modes. Predicting its initiation is essential for the design of these composites. Evaluating stress-strength relation based onset criteria requires an accurate representation of the through-the-thickness stress distribution, which can be particularly delicate in the case of shell-like structures. Thus, in this paper, a solid-shell finite element formulation is utilized which allows to incorporate a fully three-dimensional material model while still being suitable for applications involving thin structures. Moreover, locking phenomena are cured by using both the EAS and the ANS concept, and numerical efficiency is ensured through reduced integration. The proposed anisotropic material model accounts for the material’s micro-structure by using the concept of structural tensors. It is validated by comparison to experimental data as well as by application to numerical examples.

Shakedown Analysis Combined With the Problem of Heat Conduction

This paper deals with the computation of the shakedown load of engineering systems subjected to varying loads. In p ticular, we focus on thermal loading and the resulting heat c onduction problem in combination with shakedown analysis. Th e analysis is based on the lower bound shakedown theorem by Melan. The calculation is carried out by use of an interior-p oint algorithm. Emphasis is placed on the presentation of theoretical deriv ations whereas numerical aspects are out of scope and will be p resented elsewhere. The methodology is illustrated by the app lication to a simplified model of a tube sheet in heat exchangers.

A starting-point strategy for interior-point algorithms for shakedown analysis of engineering structures

Lower-bound shakedown analysis leads to nonlinear convex optimization problems with large numbers of unknowns and constraints, the solution of which can be obtained efficiently by interior-point algorithms. The performance of these algorithms strongly depends on the choice of the starting point. In general, starting points should be located inside the feasible region. In addition, they should also be well centred. Although there exist several heuristics for the construction of suitable starting points, these are restricted, as long as only the mathematical procedure is considered without taking into account the nature of the underlying mechanical problem. Thus, in this article, a strategy is proposed for choosing appropriate starting points for interior-point algorithms applied to shakedown analysis. This strategy is based on both the mathematical characteristics and the physical meaning of the variables involved. The efficiency of the new method is illustrated by numerical examples from the field of power plant engineering.

Interior-Point Method for the Computation of Shakedown Loads for Engineering Systems

A new interior-point algorithm for the computation of shakedown loads has recently been developed by the authors. The analytical formulation is based on the statical shakedown theorem by Melan which leads to a nonlinear convex optimization problem. The algorithm’s efficiency results from the close adaption of the solution procedure to the specific problem of shakedown analysis. This paper focuses on algorithmic aspects of the proposed method. A numerical example of practical interest is used for validation purposes.Copyright

Numerical and experimental investigation of the structural behavior of a carbon fiber reinforced ankle-foot orthosis

Ankle-foot orthoses (AFOs) are designed to enhance the gait function of individuals with motor impairments. Recent AFOs are often made of laminated composites due to their high stiffness and low density. Since the performance of AFO is primarily influenced by their structural stiffness, the investigation of the mechanical response is very important for the design. The aim of this paper is to present a three dimensional multi-scale structural analysis methodology to speed up the design process of AFO. The multi-scale modeling procedure was applied such that the intrinsic micro-structure of the fiber reinforced laminates could be taken into account. In particular, representative volume elements were used on the micro-scale, where fiber and matrix were treated separately, and on the textile scale of the woven structure. For the validation of this methodology, experimental data were generated using digital image correlation (DIC) measurements. Finally, the structural behavior of the whole AFO was predicted numerically for a specific loading scenario and compared with experimental results. It was shown that the proposed numerical multi-scale scheme is well suited for the prediction of the structural behavior of AFOs, validated by the comparison of local strain fields as well as the global force-displacement curves.

Numerical analysis of layered fiber composites accounting for the onset of delamination

Since delamination is a major failure mode of layered composites, predicting its initiation is essential for the design of composite structures. Evaluating delamination onset criteria based on stress-strength relations requires an accurate representation of the through-the-thickness stress distribution, which is delicate for thin shell-like structures. Therefore, in this paper, a solid-shell finite element is utilized, which allows for incorporating a fully three-dimensional, anisotropic, micro-mechanically motivated material model, still being suited for application to thin structures. Moreover, locking phenomena are cured by using both the enhanced assumed strain (EAS) and the assumed natural strain (ANS) concept, and numerical efficiency is ensured through reduced integration.

Displacement-based multiscale modeling of fiber-reinforced composites by means of proper orthogonal decomposition

Many applications are based on the use of materials with heterogeneous microstructure. Prominent examples are fiber-reinforced composites, multi-phase steels or soft tissue to name only a few. The modeling of structures composed of such materials is suitably carried out at different scales. At the micro scale, the detailed microstructure is taken into account, whereas the modeling at the macro scale serves to include sophisticated structural geometries with complex boundary conditions. The procedure is crucially based on an intelligent bridging between the scales. One of the methods derived for this purpose is the meanwhile well established FE

Micro-Macro Modelling of Metallic Composites

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