innovative fe-analysis of the roller burnishing process for different ...

X International Conference on Computational Plasticity
COMPLAS X
E. Oñate and D. R. J. Owen (Eds)
© CIMNE, Barcelona, 2009
INNOVATIVE FE-ANALYSIS OF THE ROLLER BURNISHING
PROCESS FOR DIFFERENT GEOMETRIES
F. Klocke * , V. Bäcker * , H. Wegner * , A. Timmer *
* Chair of Manufacturing Technologies, Laboratory for Machine Tools and Production Engineering
RWTH Aachen University
Steinbachstr. 19, 52074 Aachen, Germany
e-mail: V.Baecker@wzl.rwth-aachen.de, web page: http://www.wzl.rwth-aachen.de
Key words: Incremental Forming Process, Finite Element Analysis, Fan Blades.
Summary This article shows the model development using the FEA for roller burnishing
applied on different geometries. The aim of this work is the development of valid and
effective FE-models, which are able to provide a quantitative prediction of the rime zone state
of roller burnished workpieces. These FE-models are verifiable through the comparison of the
calculated residual stress state with the experimental results of roller burnishing tests.
1 INTRODUCTION
The small damages by unavoidable impacts of foreign objects on heavily loaded turbojet
engine components, like fan and compressor blades, can result in oscillating fatigue and
breakage during continued operation. This type of damage represents more than 25% of the
most frequent causes for cost-extensive engine overhauling [7].
Previous research results [2], [5], [6] show that applying a mechanical strain to harden the
rime zone leads to a significant increase of the damage tolerance. Such a treatment can be
achieved with the manufacturing process of roller burnishing. This process distinguishes itself
by substantial advantages from other mechanical strain hardening methods: high and deeply
reaching compressive residual stresses, high strain hardening and excellent surface quality [6].
However, the determination of optimal process parameters for a given load still requires an
elaborate experimental set-up and subsequent time-consuming and cost-extensive
measurements of the rime zone’s state. Previous publications [1], [3], [4] proposed the
application of the Finite Element Analysis (FEA) as an effective and cost reducing alternative
to an experimental set-up. However, these works were only able to provide a qualitative
prediction of the rime zone state due to the simplifications they imposed to the model
geometry, boundary conditions and material.
This article shows the model development using the FEA for different geometries of an
analogue test specimen (Figure 1). These geometries were chosen specifically to represent
typical shapes of turbine blade and turbine disk components. Two common materials of the
aerospace industry were used (Ti-6Al-4V and IN718). The developed FE-models are
verifiable through the comparison of the calculated residual stress state with the experimental
results of the roller burnishing tests.

F. Klocke, V. Bäcker, H. Wegner and A. Timmer.
2 MODEL DEVELOPMENT
In the following sections the developed FE-models will be described (Figure 1).
Figure 1: Modelling of the roller burnishing process for different geometries
The determination of an accurate stress distribution in the rime zone requires a high
number of elements, which makes a simulation of the complete geometry impossible.
Therefore cut-out dimensions for each geometry were defined. This allows a robust and timeefficient
roller burnishing simulation. In order to ensure a minimal influence of the boundaries
on the stress development, symmetric boundary conditions were applied on these surfaces.
Each section was modelled as a deformable 3D-body. The roller burnishing tools were
simplified and modelled as rigid balls. This assumption is valid, because of the high hardness
of the tungsten carbide roller ball (~2400 HV) compared to the processed material (~450 HV).
Contact between tool and workpiece was modelled using a finite-sliding formulation where
separation and sliding of finite amplitude, and arbitrary rotation of the surfaces may arise. The
normal contact behaviour between the contact surfaces was modelled using the hard contact
formulation implemented in ABAQUS. The contact constraint is enforced with a Lagrange
multiplier representing the contact pressure in a mixed formulation. The friction was modelled
using an extended version of the classical isotropic Coulomb friction model. The extensions
include an additional limit on the allowable shear stress, anisotropy, and the definition of a
“secant” friction coefficient. Thereby, the friction model is able to consider stick-slip-effects,
which can occur during roller burnishing at high pressures.
The behaviour of the used materials, IN718 and Ti-6Al-4V, was modelled using the
implemented ABAQUS material models for metals subjected to cyclic loading. The elastic
material behaviour was described using a linear elasticity model, whereas for the plastic
behaviour a nonlinear, isotropic/kinematic hardening model was used. The model parameters
for the description of the elastic material behaviour and for the plastic hardening were
determined from material examinations and cyclic tests.
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F. Klocke, V. Bäcker, H. Wegner and A. Timmer.
3 MODEL VALIDATION
To achieve quantitative comparison with experimental results, highly accurate simulation
results are required. In order to obtain statistically firm simulation results, an evaluation of a
single validation line is insufficient. The heterogeneity of the numerical results makes an
evaluation of a previously defined validation volume necessary.
Figure 2: Concept of the evaluation methodology for plane and curved geometries
The concept of the evaluation methodology is shown in Figure 2. The predefined
validation volume consists of a number of surfaces, which are determined by the mesh
density. Each validation surface consists of a number of validation lines, which lie within this
surface. By averaging these results for each surface a representative distribution over the
depth can be obtained.
Figure 3: Contour plots and residual stress distributions for roller burnishing
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F. Klocke, V. Bäcker, H. Wegner and A. Timmer.
In order to provide a quantitative comparison of the simulated and experimental results,
this evaluation methodology was applied. The results of this comparison are shown
exemplarily for the roller burnishing of Ti-6Al-4V (Figure 3). They achieve generally a very
good correlation between calculated and measured results for all geometries.
The distribution of the residual stresses for thin walled geometries differs significantly
from the stress distribution observed and calculated for plane geometry, which is affected
only in the rime zone of the processed surface. Thin walled geometries are influenced over
their complete thickness. Due to the roller burnishing process, negative residual stresses are
induced in the rime zone. These stresses are compensated by positive residual stresses in the
inner part of the workpiece.
The contour plot of the residual stress in the simulation using curved geometry shows that
the roller burnishing process does not induce stresses only in the processed curved zone. The
stress induction is extended to the adjacent plane areas.
4 CONCLUSIONS
The high stress gradients in the rime zone after roller burnishing require fine meshing of
this area, which limits the model size and renders a macro-modelling of the roller burnishing
process as time-inefficient or even impossible. The model simplifications and the assumptions
made in the modelling phase cause additional scatter to the numerical errors and have to be
chosen carefully. A quantitative prediction of the residual stress state of the rime zone
requires a sophisticated and representative validation strategy based on statistically firm
evaluation of the model results, which is what these FE-models achieve. All developed FEmodels
were validated and are able to predict the residual stress state after roller burnishing
with sufficient accuracy.
5 AKNOWLEDGEMENT
The authors gratefully acknowledge the financial support from the European 6th
Framework Programme through the research project VERDI (Virtual Engineering for Robust
Manufacturing with Design Integration; http://www.verdi-fp6.org).
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