Material characterization and residual stresses simulation during
the manufacturing process of epoxy matrix composites
Y. Abou Msallem
a
, F. Jacquemin
a,
*
, N. Boyard
b
, A. Poitou
a
, D. Delaunay
b
, S. Chatel
c
a
GeM, Centrale Nantes, Université de Nantes, UMR CNRS 6183, France
b
LTN, Ecole Polytechnique de l’Université de Nantes, UMR CNRS 6607, France
c
Mechanical Modeling Research Team, EADS France CTO/IW/SP/ME, France
article info
Article history:
Received 1 December 2008
Received in revised form 2 June 2009
Accepted 18 September 2009
Keywords:
A. Thermosetting resin
B. Residual/internal stress
D. Thermal analysis
B. Thermomechanical
abstract
A thermal, rheological and mechanical material characterization of an aeronautic epoxy resin from com-
mercial prepreg is reported in this article. The kinetic of the crosslinking reaction of the resin is charac-
terized and modeled. The specific heat, the glass transition temperature, the thermal expansion
coefficients, the chemical shrinkage coefficients and the thermo-mechanical properties have been inves-
tigated as a function of temperature and degree of cure. Dynamic mechanical measurements are used to
determine the gel point. Finally, the residual stresses developed during the curing process are calculated
using a finite element simulation, taking into account the material properties evolutions according to
proposed models. The results highlight the importance of the characterization accuracy and the associ-
ated models.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
The use of composite materials is growing rapidly especially for
aeronautic structures. Nevertheless, the curing of thick composite
parts, for example, remains a challenging task because it can lead
to temperature and degree of polymerization gradients through
the thickness. These phenomena are due to the combination be-
tween the low thermal conductivity of the composite and the large
heat of reaction released during the crosslinking reaction. The tem-
perature and degree of cure gradients, the chemical shrinkage, the
evolution of properties of the matrix during the curing process
and the material anisotropy lead to the development of residual
stresses in the manufactured part. For example, Bogetti and Gilles-
pie [1] have proposed a methodology for predicting the residual
stress development during the curing of thick pieces and that by
adding the effect of volumetric shrinkage. The residual stresses
can have a significant effect on the mechanical performance of com-
posites structures by inducing delaminations, distortions and initi-
ating cracks [2,3] especially for the complex shape parts.
Generally speaking, studies concerning residual stress forma-
tion in thermosetting composite materials take into account only
the thermal effect [4,5]. Moreover, uniform temperatures and
properties through the thickness of the piece are often supposed.
The residual stresses are determined by calculating the tempera-
ture difference between the cure and ambient temperatures
assuming that no stress occurs before the cooling.
Another mechanism contributing to the development of resid-
ual stress is the chemical shrinkage of the resin during the cross-
linking polymerization reaction. Some authors have emphasized
the importance of the volumetric shrinkage [6,7]. Others have
studied the influence of this phenomenon on the curing stress
development for thick composite pieces [1,8].
During the curing process, the thermal, physical, rheological
and mechanical properties of the resin vary making the analysis
difficult. The modeling has to be improved for a better representa-
tion of the variations of the properties and advances in experimen-
tal characterizations, especially for the new resin generation, have
to be made.
In this paper, an appropriate characterization and modeling of
the reaction kinetic of an aeronautic epoxy resin from prepreg (the
commercial name cannot be revealed for confidentiality reason) is
presented. The variations of the specific heat, the volumetric varia-
tion, the glass transition temperature (T
g
) and the mechanical prop-
erties of the resin with cure are also determined. T
g
is identified as a
key factor: when the temperature is below T
g
the reaction rate is
strongly reduced and the material behavior change from rubbery
state to glassy state. In addition, a rheological study is performed
to correctly identify the gel point from which residual stresses are
developed, and a new model is proposed to represent the cure-
dependent variations of the mechanical properties of the resin. Fi-
nally, a finite element approach is used to simulate the residual
stresses generated by the curing process of composite materials.
1359-835X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compositesa.2009.09.025
* Corresponding author.
E-mail address: Frederic.jacquemin@univ-nantes.fr (F. Jacquemin).
Composites: Part A 41 (2010) 108–115
Contents lists available at ScienceDirect
Composites: Part A
journal homepage: www.elsevier.com/locate/compositesa