JEC COMPOSITES MAGAZINE - Issue #103 - March 2016 - 76

testing

Study of the bending stiffness of fibrereinforced thermoplastics at forming temperature
This paper presents different cantilever bending
tests used to characterize the bending stiffness
of fibre-reinforced thermoplastics at forming
temperature. The ASTM D1388 standard describes
a standard test method for determining the stiffness of fabrics (dry or impregnated) in a standard
atmosphere (21°C and 65% relative humidity).
This test was modified in this work for experimental studies above the melting temperature of
the polymer matrix.

Pr. Dr. bernD engeL, cHair of forMing tecHnoLogy
M. eng. JasMin graef, cHair of forMing tecHnoLogy
university of siegen

everal authors have described the influence of bending
stiffness on the formation of wrinkles. Thus, it is important to consider bending stiffness in finite element (FE)
forming analyses for better prediction of wrinkling.
Fibre-reinforced thermoplastics (FRTS) are gaining importance in the production of lightweight parts. After heating over
their melting temperature, they are formable several times and
can be processed using known stamping methods with suitable modifications. This makes it possible to use semi-finished
products in the form of pre-consolidated sheets. They do not
need a long curing time compared to reinforced thermosets.
Forming simulations of fibre-reinforced thermoplastics are
performed to understand and modify the forming process.
Simulation tools require the determination of specific material
parameters such as tensile, shear and bending stiffness at the
forming temperature of FRTS.
Shear stiffness is often identified by means of picture-frame
tests or bias-extension tests. However, there are no standardized

76 jec composites magazine / No103 March 2016

methods for the temperature-dependent characterization of
bending stiffness [1], but only a standard test method for the stiffness of fabrics (dry or impregnated) in a standard atmosphere
(21°C and 65% relative humidity) - ASTM D1388-96 [2].
The bending stiffness of FRTS at forming temperature is much
lower than their tensile stiffness in the fibre direction. Therefore, several authors who describe FE models for dry fabrics
assume that bending stiffness is negligible [3, 4].
[5-9] describe the influence of bending stiffness on the formation of wrinkles. Wrinkles occur due to the low bending and
shear stiffness of FRTS, whereas bending stiffness defines the
amount and size of the wrinkles [7].
Forming simulations are performed to predict the fibre orientation of reinforcements and wrinkling. Thus, it is important
to determine the shear, tensile and bending behaviour with
experimental tests for the validation of FE models.
In prior studies [1], several discrete approaches for the FE modelling of dry and impregnated fabrics as well as the fabric material model of Abaqus/Explicit were investigated and compared
for simulations of picture-frame shear test data.
There was a good agreement between discrete models and
experimental test results up to shear angles of about 40°. For
higher shear angles, there was a significant increase in the shear
stiffness of simulation results. The fabric material model has the
advantage that the shear and tensile response can be defined
based on test data [10], resulting in a better agreement with
experimental test data in the study. However, it just works for
membrane elements having no bending stiffness.
In further studies, beam elements or an offset of shell elements
according to Nishi et al. [11] will be added to the fabric model

Fig. 1: Cantilever bending test (fixed) (see [12, 13])

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #103 - March 2016