Heiko Andrae - Academia.edu (original) (raw)

Papers by Heiko Andrae

PAMM

The viscoelastic behavior of short fiber reinforced polymers (SFRPs) partly depends on different ... more The viscoelastic behavior of short fiber reinforced polymers (SFRPs) partly depends on different microstructural parameters such as the local fiber orientation distribution. To account for this by simulation on component level, two‐scale methods couple simulations on the micro‐ and macroscale, which involve considerable computational costs. To circumvent this problem, the generation of a viscoelastic surrogate model is presented here. For that purpose, an adaptive sampling technique is investigated and data are obtained by creep simulations of representative volume elements (RVEs) using a fast Fourier transform (FFT) based homogenization method. Numerical tests confirm the high accuracy of the surrogate model. The possibility of using that model for efficient material optimization is shown.

Macromolecular Materials and Engineering, 2021

4D printing of shape memory polymers enables the production of thermoresponsive objects. In this ... more 4D printing of shape memory polymers enables the production of thermoresponsive objects. In this contribution, a facile printing strategy is followed for an in‐house synthesized thermoplastic poly(ether urethane). Processing by means of fused filament fabrication, in which the difference between nozzle temperature and material‐specific glass transition temperature of the polymer is kept as low as possible, allows to obtain highly shrinkable objects whose shape and thermoresponsiveness can be precisely controlled. The effectiveness of the method also applies to the printing material polylactic acid. One possible application lies in highly shrinkable objects for assembly purposes. As proof‐of‐concept, lightweight hands‐free door openers for healthcare applications are functionally simulated and developed. Once printed, such devices shrink when heated to fit on door handles, allowing an easy assembly. At the end‐of‐use, a heating‐initiated disassembling and mechanical recycling are pro...

Advanced Materials, 2021

Shape morphing implicates that a specific condition leads to a morphing reaction. The material th... more Shape morphing implicates that a specific condition leads to a morphing reaction. The material thus transforms from one shape to another in a predefined manner. In this paper, not only the target shape but rather the evolution of the material's shape as a function of the applied strain is programmed. To rationalize the design process, concepts from informatics (processing functions, for example, Poisson's ratio (PR) as function of strain: ν = f(ε) and if‐then‐else conditions) will be introduced. Three types of shape morphing behavior will be presented: (1) achieving a target shape by linearly increasing the amplitude of the shape, (2) filling up a target shape in linear steps, and (3) shifting a bulge through the material to a target position. In the first case, the shape is controlled by a geometric gradient within the material. The filling kind of behavior was implemented by logical operations. Moreover, programming moving hillocks (3) requires to implement a sinusoidal fu...

This report discusses two approaches for a posteriori error indication in the linear elasticity s... more This report discusses two approaches for a posteriori error indication in the linear elasticity solver DDFEM: An indicator based on the Richardson extrapolation and Zienkiewicz-Zhu-type indicator. The solver handles 3D linear elasticity steady-state problems. It uses own input language to describe the mesh and the boundary conditions. Finite element discretization over tetrahedral meshes with first or second order shape functions (hierarchical basis) has been used to resolve the model. The parallelization of the numerical method is based on the domain decomposition approach. DDFEM is highly portable over a set of parallel computer architectures supporting the MPI-standard.

ABSTRACT A 3D multiscale method is proposed to model the residual stresses in multiphase material... more ABSTRACT A 3D multiscale method is proposed to model the residual stresses in multiphase materials under the hybrid-semiconcurrent multiscale framework. As an illustration, the quenching residual stresses in SiCp/2124Al composite are modeled. The total residual stresses are separated into the macro, elastic misfit and thermal misfit residual stresses by means of the present multiscale model. In this multiscale model, one macroscale model is connected to two microscale models via scale transition boundary conditions. The predicted total residual strains in the metal matrix and the reinforcing particles coincide with reported experimental data very well. The predicted total, macro, elastic misfit and thermal misfit residual stresses agree reasonably well with the reported experimental ones. The present model provides a new tool to gain a deep insight into the residual stresses in multiphase materials.

publication.editionName, 2015

This paper presents numerical methods for the characterization of fiber orientation and fiber bun... more This paper presents numerical methods for the characterization of fiber orientation and fiber bundles of medium density wood fiberboards (MDF). The strength and stiffness of MDF is significantly affected by the fiber orientation and fiber bundles. Proposed methods and results are necessary to virtually generate realistic fiber networks and optimize MDF by using computer simulations. Based on 3D \mu$$μCT images for laboratory manufactured MDF with oriented fibers, the fiber orientation is calculated in two ways. Firstly, we use an image processing method based on Hessian matrix directly on \mu$$μCT image. Secondly, we computed the effective heat conductivity by solving PDEs on a segmentation of the \mu$$μCT image to estimate the fiber orientation. A fiber bundle segmentation method based on local fiber orientations is introduced. Fiber bundles, which are segmented by this method show good agreement with manually segmented ones. It was observed that fiber bundles are oriented in MDF plane with log-normal distribution of bundle length. The proposed methods are general and can be used also to calculate fiber orientation and segment fiber bundles in fiber concrete, paper, glass and carbon fiber composites.

The desire to simulate more and more geometrical and physical features of technical structures an... more The desire to simulate more and more geometrical and physical features of technical structures and the availability of parallel computers and parallel numerical solvers which can exploit the power of these machines have lead to a steady increase in the number of grid elements used. Memory requirements and computational time are too large for usual serial PCs. An a priori partitioning algorithm for the parallel generation of 3D nonoverlapping compatible unstructured meshes based on a CAD surface description is presented in this paper. Emphasis is given to practical issues and implementation rather than to theoretical complexity. To achieve robustness of the algorithm with respect to the geometrical shape of the structure authors propose to have several or many but relatively simple algorithmic steps. The geometrical domain decomposition approach has been applied. It allows us to use classic 2D and 3D high-quality Delaunay mesh generators for independent and simultaneous volume meshing. Different aspects of load balancing methods are also explored in the paper. The MPI library and SPMD model are used for parallel grid generator implementation. Several 3D examples are shown.

The paper presents a mathematical description of shape memory alloys within the framework of cont... more The paper presents a mathematical description of shape memory alloys within the framework of continuum mechanics, where the spatially multidimensional case is considered. A new simple model is introduced, which correctly describes not only all well-studied shape memory effects but also the more complex behavior. The key idea is a new set of internal state variables, which are averaged values

Solid State Phenomena, 1991

Proceedings of the Eighth International Conference on Computational Structures Technology, 2006

... This work was supported by the Semiconductor Research Corporation Center of Excel-lence for M... more ... This work was supported by the Semiconductor Research Corporation Center of Excel-lence for Microscience and Technology. &#x27;John С. С. ... 40, 295 (1979). I0Y. S. Touloukian and EH Buyco, Thermophysical Properties of Matter (IFI/PLENUM, New York, 1970), Vol. 2. &quot;KM. ...

PAMM, 2002

Unidirectional, long fiber carbon / carbon composites fabricated by chemical vapor infiltration (... more Unidirectional, long fiber carbon / carbon composites fabricated by chemical vapor infiltration (CVI) consisting of carbon fibers in a pyrolytic carbon matrix are anisotropic materials. It is practically impossible to identify experimentally the elastic properties (modules) of this anisotropic material. The aim of this investigation is to predict the elastic properties of this composite theoretically. The study of this material with the help of microscopy gives information about the very complicated anisotropic structure of this composite at each length scale. That is the reason that a hierarchical model for this material is developed, which consists of four length levels. A methodology for identification of the elastic properties for such composites is proposed. The problem is solved with the help of a homogenization procedure for each level. 

Numerische Mathematik, 1997

A Galerkin approximation of both strongly and hypersingular boundary integral equation (BIE) is c... more A Galerkin approximation of both strongly and hypersingular boundary integral equation (BIE) is considered for the solution of a mixed boundary value problem in 3D elasticity leading to a symmetric system of linear equations. The evaluation of Cauchy principal values (v. p.) and finite parts (p. f.) of double integrals is one of the most difficult parts within the implementation of such boundary element methods (BEMs). A new integration method, which is strictly derived for the cases of coincident elements as well as edge-adjacent and vertex-adjacent elements, leads to explicitly given regular integrand functions which can be integrated by the standard Gauss-Legendre and Gauss-Jacobi quadrature rules. Problems of a wide range of integral kernels on curved surfaces can be treated by this integration method. We give estimates of the quadrature errors of the singular four-dimensional integrals.