JEC COMPOSITES MAGAZINE - Issue #101 - November/December 2015 - 44

simulation

Different material models for intra-ply shear
and double-dome FE forming analysis
In the study presented in this paper, different
methods for the forming simulation of fibre-reinforced thermoplastics using Abaqus/Explicit are
investigated and compared. Current FE methods
either assume a continuum of fibres and thermoplastic with orientation-dependent properties or
define the fibres and thermoplastic separately.
Furthermore, several mechanisms can be decoupled, e.g. the shear behaviour and the stiffness of
the material, to avoid the over-prediction of bending stiffness. In this case, shear behaviour can be
represented by 2D elements and stiffness using 1D
elements sharing the same nodes.

By
PRof. DR.-ing BeRnD engel
M.eng. JasMin gRaef
chaiR in foRMing technologY
univeRsitY of siegen

ith the focus on fast and cost-efficient production
methods, e.g. for lightweight automotive parts, fibrereinforced thermoplastics are becoming more important compared to reinforced thermosets. After heating over
their melting temperature, these materials are formable several
times and can be processed using known stamping methods
with suitable modifications. This makes it possible to use semifinished products in the form of pre-consolidated sheets.
Forming simulations of fibre-reinforced thermoplastics (FRTs)
are helpful to understand and modify the forming process. To
reach accurate results, the determination of specific material
parameters is required to describe the tensile, shear and bending stiffness as well as the friction behaviour (tool-ply and plyply) at the forming temperature of FRTs. The testing methods

described in the literature by several authors are often deduced
from methods used for dry textiles. Thus, the methods need to
be adapted for the characterization of impregnated textiles.
Shear stiffness is often identified by picture-frame tests or
bias-extension tests. But there are no standardized methods for
the temperature-dependent characterization of FRTs and the
implementation of the characteristic values into a finite element
(FE) model.
In the literature, different approaches for FE models are investigated, such as homogenized modelling strategies [1, 2] and
discrete approaches. The latter consist in a parallel connection
between several elements, e.g. to separate the shear and tensile
or bending stiffness of FRTs by a combination of 1D and 2D
elements. [3] and [4] use beam and shell elements to decouple
the shear and tensile behaviour of the fabric while [5] uses truss
elements instead of beam elements. [6] introduces a FE model
for dry fabrics with membranes for in-plane properties and an
offset of shells to represent the bending behaviour. The authors
extended this model by another shell element using the Reuss
model for the simulation of impregnated fabrics [7].
Abaqus/Explicit has its own fabric material model (*Fabric) to
define the in-plane behaviour of a fabric material under plane
stress conditions. It also assumes that the shear behaviour is
independent from the tensile response in the fibre direction.
The shear and tensile behaviour can be defined based on test
data depending on the temperature for membrane elements [8].
Thus, the influence of the melted thermoplastic on the shear
response of the fabric will be considered without the need for
an individual definition of the matrix component.
The bending stiffness of FRTs at the forming temperature is
much lower than the tensile stiffness in the fibre direction.
Therefore, the fabric model of Abaqus/Explicit and several
authors that describe FE models for dry fabrics or FRTs assume
that the bending stiffness is negligible [8-10].
Forming simulations are performed to predict the fibre orientation of reinforcements and wrinkling. [7] and [11-13] explain the
influence of the bending stiffness on the formation of wrinkles.
Wrinkles are due to a low bending and shear stiffness of FRTs,
while the bending stiffness defines the amount and size of
wrinkles [12].
In this work, different FE models for the forming simulation of
fibre-reinforced thermoplastics (2/2 twill with an areal weight
of 600 g/m², PA 6) using Abaqus/Explicit are investigated and
compared. They consist of different elements such as trusses,

44 jec composites magazine / No101 November - December 2015

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #101 - November/December 2015