Dr. Anxin Ma

This group focuses on building physical mechanism guided continuum level modeling approaches. Inside crystal plasticity FEM framework we try to achieve modeling results for special materials systems and boundary conditions which can be tested by smaller length scale modeling methods such as Discrete Dislocation Dynamics and Molecular Dynamics. For one integration point locally we may introduce one Repetitive Volume Element with proper homogenization approach, non-locally we also try to consider strain gradient and possible dislocation density flux between different integration points to study possible deformation heterogeneity inside multiphase materials.

The nonlocal effect across GBs (with misfit angle>15˚) cannot be modelled by GND approach. The penetration energy for each slip systems are formulated to consider additional hardening along GB normal direction. Plots show lattice rotations of one bi-crystal with 33˚ GB.

Group Members

We are interested in the following topics:

  • Physical principle guided constitutive model development in continuum level
  • Multiphase materials modeling include slip and phase transformation
  • Superalloy creep, thermal and mechanical fatigue
  • Texture and anisotropy evolution  during forming process

Current projects:

TRIP steel deformation modelling Superalloy Creep Modelling (see image below)

Deformation of turbine blades of superalloy single crystal is modeled by unit cell approach at integration points. At different temperature and stress regions precipitation strengthening work (creep only in matrix channels), failure (precipitates can be sheared), and variation (precipitate rafting) are considered.

Dislocations and Grain Boundaries
Study of the plastic deformation of polycrystalline under micrometer length scale requires the consideration of nonlocal effect caused by grain boundary (GB). The dislocation-GB interaction may generate dislocation pile-ups and stress concentrations. The internal stress adding up with the external load can produce plastic deformation and activate additional dislocation sources within the GB. The nonlocal effect from GB can also be strain gradient relaxation. Physically this relaxation manifests itself as dislocation penetration, absorption at the GB and GB slide. We pursue theoretical and numerical (FEM) modelling approaches to understand plastic deformation under micrometer length scale. Input data for current study is the interaction criteria between dislocation and GB and coherent zone model form atomic simulations. Dislocation dynamics (DD) simulations will be employed as a parallel method to fit parameters for continuum level equations concerning dislocation-GB interaction. The plots show misorientation across GB of Al bicrystal with 33° misorienation.

Recent Publications

S. Gupta, R. Twardowski, P. Kucharczyk, S. Münstermann Experimental and Numerical Investigations of the TRIP Effect in 1.4301 Austenitic Stainless Steel Under Static Loading, Steel research international, 85, 793-802, (2014)

E. Borukhovich, P. S. Engels, T. Böhlke, O. Shchyglo et al. Large strain elasto-plasticity for diffuse interface models, Modelling and Simulation in Materials Science and Engineering, 22, 034008, (2014)

A. Ma, A. Hartmaier On the influence of isotropic and kinematic hardening caused by strain gradients on the deformation behaviour of polycrystals, Philosophical Magazine, 94, 125-140, (2014)

P. S. Engels, C. Begau, S. Gupta, B. Schmaling et al. Multiscale modelling of nanoindentation: from atomistic to continuum models, A. Tiwari Nanomechanical Analysis of High Performance Materials, 203, 285-322, (2014)

M. Sharaf, P. Kucharczyk, A. Ma, N. Vajragupta et al. Assessment of fatigue microcrack initiation and growth capabilities in structural steels: an interdisciplinary experimental and numerical method, Proceedings of the 3rd International Conference of Engineering Against Failure, (2013)

  • Prof. Wolfgang Bleck
    RWTH Aachen
  • Prof. Dierk Raabe
    MPIE Düsseldorf
  • Prof. Steve Roberts
    University of Oxford
  • Prof. Bob Svendson
    TU Dortmund

Contact: Dr. Anxin Ma
Ruhr-Universität Bochum
44780 Bochum
Tel: +49 234 32 29376
Fax: +49 234 32 14984