Johanna Waimann

• Professor
• University Professor at Ruhr University Bochum

In various engineering applications, components subjected to high mechanical or multi‐physical loads such as thermal, thermo‐mechanical, or electro‐thermal loads are predominantly made of metals or composites. These materials are characterized by polycrystalline or multi‐phase microstructures which determine the overall material response. However,...

In general, the overall macroscopic material behavior of any structural component is directly dependent on its underlying microstructure. For metal components, the associated microstructure is given in terms of a polycrystal. To enable the simulation of the related microstructural and overall elasto‐viscoplastic material behavior, a two‐scale simul...

The process of electrochemical machining uses chemical reactions to dissolve material of the surface layer. This special kind of processing avoids undesired microstructural changes in the surface, such as the formation of dislocations. ECM is thus a very promising processing technique for high-strength materials. To model the complex chemical react...

Electro‐chemical machining (ECM) allows the removal of material based on the effect of anodic dissolution and without mechanical contact. Thus, it avoids tool abrasion as well as influencing the surface quality, for instance due to formed dislocations and/or damage. Due to that, ECM is a very attractive machining process for high strength materials...

The mechanical behavior of a periodic heterogeneous microstructure may be predicted by using a fast Fourier transform (FFT) based simulation approach. To reduce the computational effort of this method, we introduced a model order reduction (MOR) technique utilizing a reduced set of Fourier modes for the computations in Fourier space. To increase th...

Two evolving surfaces define the moving boundary value problem in electrochemical machining. On the side of the work piece, the anodic dissolution yields the evolution of the machined surface. A novel approach for modeling the dissolution process of the anode has recently been proposed by [6], which is based on internal variables and effective mate...

Due to the general pursuit of technological advancement, structural components need to meet increasingly higher standards. In order to optimize the performance behavior of the used materials, detailed knowledge of the overall as well as microscopic material behavior under certain mechanical and thermal loading conditions is required. Hence, we pres...

This work presents a new numerical approach to efficiently model the cathode's moving surface in the moving boundary value problem of electrochemical machining. Until recently, the process simulation with finite elements had the drawback of remeshing required by the changing surface geometries. This disadvantage was overcome by an innovative model...

Processes, such as deep rolling or induction hardening, have a remarkable influence on the material properties within the surface layer of a work piece. Our overall goal is to develop efficient two-scale methods, which are able to show the microstructural evolution of the machined material. The calculation of a spatially resolved microstructure com...

The overall, macroscopic constitutive behavior of most materials of technological importance such as fiber-reinforced composites or polycrystals is very much influenced by the underlying microstructure. The latter is usually complex and heterogeneous in nature, where each phase constituent is governed by non-linear constitutive relations. In order...

Phase transformations in steel have a high impact on the material properties. The material is modified by a changed stiffness as well as residual stresses which are a result of transformation strains and plastic deformations during the phase transition. In the current work, we propose a variational material model which is able to display the phase...

This work presents a new approach to efficiently model the cathode in the moving boundary value problem of electrochemical machining. Until recently, the process simulation with finite elements had the drawback of remeshing required by the changing surface geometries. This disadvantage was overcome by a novel model formulation for the anodic dissol...

We present a modified model order reduction (MOR) technique for the FFT-based simulation of composite microstructures. It utilizes the earlier introduced MOR technique (Kochmann et al., 2019), which is based on solving the Lippmann–Schwinger equation in Fourier space by a reduced set of frequencies. Crucial for the accuracy of this MOR technique is...

To capture the material behavior of composite microstructures, Moulinec and Suquet [5] proposed a homogenization scheme making use of fast Fourier transforms (FFT) and fixed-point iterations. To reduce the computational effort of this spectral method, Kochmann et al. [3] introduced a model order reduction technique, which is based on using a fixed...

The FFT-based method introduced by Moulinec and Suquet [9] serves as an alternative for the classical finite element based simulation of periodic microstructures. This simulation approach makes use of fast Fourier transforms (FFT) as well as fixed-point iterations to solve the microscopic boundary value problem which is captured by the Lippmann-Sch...

The manufacturing process of electrochemical machining (ECM) allows for the precise machining of high-strength materials. In order to save time and reduce experimental costs, one depends on efficient simulation tools. Numerous modeling approaches consider the anodic material dissolution in ECM based on a high resolution of the material (see e.g. [2...

This work presents a novel approach to efficiently model anodic dissolution in electrochemical machining. Earlier modeling approaches employ a strict space discretization of the anodic surface that is associated with a remeshing procedure at every time step. Besides that, the presented model is formulated by means of effective material parameters....

We present a modified model order reduction (MOR) technique for the FFT-based simulation of composite microstructures. It utilizes the earlier introduced MOR technique (Kochmann et al. [2019]), which is based on solving the Lippmann-Schwinger equation in Fourier space by a reduced set of frequencies. Crucial for the accuracy of this MOR technique i...

This work presents a novel approach to efficiently model anodic dissolution in electrochemical machining. Earlier modeling approaches employ a strict space discretization of the anodic surface that is associated with a remeshing procedure at every time step. Besides that, the presented model is formulated by means of effective material parameters....

The purpose of this work is the development of an efficient two-scale numerical scheme for the prediction of the local and overall mechanical behavior of polycrystalline materials with elasto-viscoplastic constitutive behavior at finite strains. Assuming scale separation, the microstructural deformations are prescribed by the kinematics of the macr...

To capture all the individual microstructural effects of complex and heterogeneous materials in structural finite element simulations, a two‐scale simulation approach is necessary. Since the computational effort of such two‐scale simulations is extremely high, different methods exist to overcome this problem. In terms of a FFT‐based microscale simu...

In forming processes of metals, different physical phenomena are observable which may alter the material properties and behavior severely. These observations can be attributed to changes in the surface layer of the material. This research is a part of the transregional collaborative research center SFB/TRR 136 which deals with surface layer modific...

Based on our previous works, we present the finite-element implementation of an energy-based material model that displays the effect of functional fatigue of shape memory alloys during cyclic loading. The functional degradation is included in our model by taking account of irreversible martensitic volume fractions. Three internal variables are used...

In forming industry, the generation of desired material properties in the surface layer is a challenging task. The results are highly dependent on the process parameters such as the tool geometry or the applied loading. As these parameters change from process to process, it is merely impossible to directly compare different processes as e.g. deep r...

We present a model order reduction (MOR) method for finite strain FFT solvers to reduce the computational costs of the FFT simulation scheme of a two‐scale FE‐FFT simulation. The underlying method is based on a reduced set of frequencies which leads to a reduced fixed‐point scheme. The reduced set of frequencies is determined offline, based on the...

Due to their special material behavior– namely the superelasticity as well as the one‐way and two‐way effect– shape memory alloys are very attractive materials for industrial applications. The solid/solid phase transformation between austenite and martensite is however accompanied by a formation of dislocations which influence the cyclic behavior o...

Heterogeneous materials are important for a vast amount of applications e.g. in automotive industry or in aerospace. For instance when producing components, it can be desired to use materials with a heterogeneous microstructure in order to achieve specific material properties. Thus, it is beneficial to take the materials' microscopic structure into...

We present an efficient and accurate solution scheme for a FE-FFT-based two-scale simulation which is based on a microstructural convergence analysis leading to a minimal number of grid points, which needs to be considered in terms of the two-scale simulation. While performing the entire simulation with this coarse discretized microstructure, the m...

Shape memory alloys show the effect of functional fatigue under cyclically loading. This fatigue comes along with a decrease of the stress plateaus in the characteristic hysteresis curve which is also accompanied by an accumulated permanent strain. Cyclic experiments detected that a formation of dislocations trigger a stabilization of martensite an...

Due to the effects of pseudoelasticity and pseudoplasticity, shape memory alloys (SMAs) are very promising materials for the industrial usage. However, applications of SMA are still challenging due to the functional degradation during cyclic loading. The related effect of functional fatigue which occurs during pseudoelastic loading is modeled by su...

The phenomenon of functional fatigue occurs during cyclic loading of pseudoelastic shape memory alloys. We model this effect by considering an irreversible martensitic volume fraction in addition to the reversible amounts of austenite and martensite based on variational principles. The inclusion of irreversible martensitic volume fractions coincide...

Shape memory alloys show a very complex material behavior associated with a diffusionless solid/solid phase transformation between austenite and martensite. Due to the resulting (thermo‐)mechanical properties – namely the effect of pseudoelasticity and pseudoplasticity – they are very promising materials for the current and future technical develop...

This work presents a variational material model for transformation-induced plasticity in steels. We will use the principle of the minimum of the dissipation potential to develop a coupled material model for plastic deformations and phase transformations that simultaneously accounts for the hardening effects that play an important role. We will use...

Due to the effect of transformation induced plasticity (TRIP) , TRIP-steels are very promising materials, e.g. for the automobile industry. The material behavior is characterized by very complex inner processes, namely phase transformation coupled with plastic deformation and kinematic hardening. We establish a micromechanical model which uses the...

The pseudoelastic material behavior is one outstanding feature of shape memory alloys. This effect comes along with the forming of two plateaus in the stress/strain diagram of a tension test. Cyclic loading leads to a decrease particularly of the upper stress-plateau due to the evolution of plastic deformations which also implies fatigue of the mat...

Shape memory alloys show the well known effect of pseudo-elasticity associated with the formation of two stress plateaus in the stress/strain diagram for tension tests. Due to cyclic loading, the stress plateaus decrease with every load cycle, particularly the upper one. This important effect of functional fatigue results from plastic deformations...