JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition - 68

simulation
chain of composites materials made of thermoset or thermoplastic matrix and continuous fibers, to conduct for the last 4
years R&D projects focusing on shape distortion of thermoset
composites materials. A dozen of industrial projects completed in the last 2 years validated the developed approach and
resulted in the industrialization of the software solution named
PAM-DISTORTION as a complement of the existing simulation chain (PAM-FORM and PAM-RTM).

Shape distortion of thermoset composite parts
We will now be focusing on the manufacturing chain described
in figure 2 in which several layers of dry textiles are placed in a
forming tool to obtain the so-called preform that is then placed
in a RTM mold for the injection of the thermoset resin and
curing (that might start during the injection) before demolding.
Warping and spring-in of the final product are a consequence
of the stresses induced by manufacturing. Stresses for its major
part are created in the curing stage during which the thermoset
matrix will first go from liquid state to rubbery state and then
from rubbery state to glassy state. The uncured part is heated to
reach cured temperature and then cooled down to room temperature. During this cycle two nature of strains that directly
impact geometrical distortions are observed:
- Thermal strains due to the change of temperature of the resin
and the reinforcement. These strains depend on the phase of
the resin: thermal expansion of the resin is highly different in
the rubbery phase than in the glassy phase.
- Chemical shrinkage that occurs due to phase transformation
of the thermoset matrix that goes from liquid to glassy (solid)
state.
The release, during demolding, of the stresses generated during
curing results in the geometrical distortion of the final product
A simple cure cycle and the physical phenomena observed

(phase transformation, degree of cure and temperature evolution) are summarized in figure 3.
The basics for accurate prediction on how the part will deform
is an accurate prediction of stresses. Accurate prediction of
stresses can only come from a complete simulation of the
involved physics. Most of the available software on the market
neglect the phase transformation involved in the process, which
means that evolution of physical properties of the resin during
curing is not considered and that chemical shrinkage is not
taken into account. This first simulation approach is not useless,
it can give first indications on distortions. However, it can also
lead to the conclusion that simulation tools in general cannot
properly determine how your part will deform.
The second approach as integrated in PAM-DISTORTION
aims at considering all physics involved in curing process but
still allowing simplified first approach when data are missing.
Indeed, the counterpart of this second approach is the need for
material properties of the thermoset resin in rubbery as well as
in glassy (solid) phase. Whereas glassy data are usually known,
obtaining rubbery data might be more tricky: they can be
estimated from glassy data based on experience or they can be
obtained through additional material testing.
Material data being the core input of simulation and standards
being missing for some of the characteristics (as for instance the
coefficient of chemical shrinkage), it is very important to follow
a step by step approach for a proper use of the simulation tool
on industrial parts. First step is to get confident in the material data used. This first step goes through simple geometrical
samples that will be used to validate and tune material data to
be entered in the application. Once material data used validated,
all physics being considered in the software tool, industrial cases
can be analyzed with confidence.

Simulation of geometrical distortion
This chapter describes the three main stages of the simulation of
geometrical distortion of thermoset composite parts. Whatever
the software used (considering that this software account for all
the physics involved in the process - see the second approach
described above), the three following stages will be respected:
- Creation of solid mesh of the part;
- Simulation of the curing process to get the temperature and
degree of cure history in each location of the part;
- Simulation of the distortion using temperature and degree of
cure history computed in the previous stage as an input.

Fig. 3: Evolution of properties during a simple curing cycle

68 jec composites magazine / No88 April 2014

The mesh creation is a critical stage that is also the most time
consuming. Two modeling strategies are possible at this stage:
whereas each ply of the stacking is represented by one layer of
solid mesh; whereas one layer of solid mesh represents several
plies. The second option implies an homogenization of the
material data that can either be done "manually" or through

JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition