JEC COMPOSITES MAGAZINE - Issue #111 - March 2017 - Special JEC World - o - 116

TECHNOLOGY behaviour

Life cycle prediction of composite structures

NOLWENN LERAY,
Project manager, Mechanical
Component Fatigue Department,
SOPHIE TOILLON,
Technical expert, Polymer and
Composite Engineering Department,
Cetim

Forecasting the long-term life cycle of organic-matrix composite materials (OMC)
is a key priority for designers (optimisation) and users (maintenance strategies)
of composite materials for structural applications.
Cetim holds extensive experience and knowledge of the mechanical behaviour
and ageing of composite materials with thermoplastic and thermosetting
matrices, and is currently studying methods for evaluating the life cycle of parts,
particularly the parts subject to fatigue stresses in various environments.

W

hen forecasting the durability of an organic-matrix composite material, one should take into account not only the
mechanical stresses applied in service (static, fatigue or
creep stress), but also the environmental ageing phenomena (particularly temperature and humidity).
Composites have long been considered as materials particularly
resistant to fatigue, however the current trend is to use them with
higher levels of stresses, and the issue of composite fatigue has to be
reconsidered. With composites and polymers, fatigue rupture can
occur in several ways, due to mechanical damage (crack initiation
and spreading, delamination) or to thermal damage after a material
self-heats or is subjected to heat cycles. Fatigue phenomena can also
worsen due to physico-chemical ageing factors such as UV radiation,
but also due to ageing in a humid environment or when completely
immersed. All these phenomena lead to damage and must be taken
into consideration to determine the life cycle of a part (Figure 1).

Fig. 1: General
introduction
to composite
materials and
forecasting
methods for
the ONERA life
cycle
(J.F. Maire,
Onera)

116

jec composites magazine / No111 March 2017

Physical and physico-chemical ageing
When the materials are in use, the environment can cause physical
or chemical damage in addition to the mechanical stresses applied
to the composite parts (fatigue, creep or static stress). The influence of ageing on mechanical characteristics is widely recognised.
Forecasting the life cycle of reinforced composite systems subjected to combined mechanical and environmental loads is also
complex due to possible interactions between the mechanical and
physico-chemical causes of damage.
With composite structures, most cases of physico-chemical ageing
occur in two types of environments: wet (or aqueous) ageing and
thermal ageing. In fact, photochemical ageing (UV) often only affects the surface layer of the composite (downgraded matrix) and
has little effect on the mechanical performance of the part.
Wet ageing
In the case of ageing in a wet or aqueous environment, the absorption and diffusion of water or humidity within the composite are
decisive factors, governing the long-term behaviour of the material. These factors not only depend on the water absorbed in the
matrix, but also on the fibre density, the fibre/matrix interfaces,
and the porosity levels.
Water absorption within the composite can have various consequences, in particular:
- differential expansion of the material, creating internal stresses
and, in some cases, leading to damage;
- plasticising of the material, particularly with a reduction in the
glass transition temperature (Tg);
- changes to certain mechanical characteristics.
Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #111 - March 2017 - Special JEC World - o
