Delamination of moisture saturated graphite/polyimide composites due to
rapid heating
Michael W. Czabaj
*
, Alan T. Zehnder, Chung-Yuen Hui
Sibley School of Mechanical and Aerospace Engineering, Cornell University Ithaca, NY 14853-7501, United States
article info
Article history:
Received 12 January 2010
Received in revised form 3 May 2010
Accepted 19 May 2010
Available online 16 July 2010
Keywords:
A. Polyimide-matrix composites (PiMC’s)
B. Delamination
B. Thermomechanical
Hygrothermal performance
abstract
In this study, experimental methods and a theoretical model are developed with aim of understanding
and predicting the onset of steam pressure-induced delamination. Experiments are performed by rapid
heat-up of moisture saturated T650-35/HFPE-II-52 graphite/polyimide laminates, pre-implanted in the
midplane with circular starter cracks. The deformation of these flaws and subsequent delamination
growth is measured using custom designed, transverse extensometers. A theoretical model for calcula-
tion of internal steam pressure within a deforming circular cavity is derived and combined with a linear
elastic fracture mechanics approach to predict the onset of delamination growth. The experimental
results and theoretical calculations are used to highlight the effect of heating rate, initial flaw size, initial
moisture content, and material toughness on steam-induced delamination. A parametric study is con-
ducted and used to identify conditions required for transition between steam-induced delamination
and steam-induced blistering damage.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Due to their high thermal resistance, specific strength and spe-
cific stiffness, polyimide-matrix composites (PiMC’s) extend the
application of composite materials into extreme temperature envi-
ronments. Due to their excellent properties, PiMC’s can be utilized
in the next-generation space propulsion systems, rocket engine
and missile components, and advanced turbine blades [1,2]. Em-
ployed in these structures, PiMC laminates are expected to with-
stand very high rates of heating and prolonged service at
temperatures in vicinity of their glass transition temperature, T
g
.
Increased understanding of the high temperature performance of
PiMC’s is therefore crucial to adopting them in real-world
applications.
Among the concerns surrounding the use of PiMC’s is that they
are prone to absorption of moisture, typically up to 1–1.5% by
weight in extended exposure to high humidity environments.
When moisture-saturated laminates are heated too quickly for
the absorbed moisture to escape, large internal water vapor pres-
sures can develop, leading to plasticization and hydrolysis of the
matrix [3,4], void nucleation and instability, interlaminar delami-
nation, propagation of pre-existing flaws, and in worst case global
failure of the structure [1,3,5,6].
To-date, the majority of research has focused on initiation and
progression of the steam-induced damage in initially undamaged
composites. More specifically, in most investigations, the mois-
ture-saturated laminates contain no significant internal flaws prior
to rapid heat-up and any accumulated damage is the result of the
internal steam pressure. Based on experiments performed on mois-
ture-saturated samples of polyimide neat resin, the authors of
[1,3,6] concluded that at high enough heating rates, internal steam
pressure will lead to nucleation of internal damage in form of
voids. Additionally, it was determined that initiation and evolution
of steam-induced damage in moisture-saturated laminates will oc-
cur primarily in the regions of high resin concentration, i.e. be-
tween the plies of the laminate [6]. Continued heating of
laminates containing internal blisters may result in further damage
and weakening of the bond between plies, eventually leading to
interlaminar delamination.
An additional mode in which PiMC’s can fail during a severe
hygrothermal cycle involves laminates containing pre-existing
flaws. Such flaws, mainly in the form of interlaminar delamina-
tions, may result from fabrication defects, impact damage or an
earlier hygrothermal cycle. When rapidly heated, the moisture
trapped within a laminate containing a delamination flaw will
pressurize the delamination, potentially leading to delamination
fracture.
Under different conditions one of these failure modes will be fa-
vored over the other. For example, in a relatively thick laminate
containing a delamination in the midplane, one may expect that
blistering will occur prior to delamination growth. Here, the
1359-8368/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compositesb.2010.05.008
* Corresponding author. Address: 212 Kimball Hall, Cornell University, Ithaca, NY
14853-1502, United States. Tel.: +1 607 255 0824; fax: +1 607 255 2011.
E-mail address: mwc35@cornell.edu (M.W. Czabaj).
Composites: Part B 41 (2010) 568–577
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