JEC COMPOSITES MAGAZINE - Issue #111 - March 2017 - Special JEC World - o - 117

Life cycle prediction of composites structures

The kinematics of water diffusing within the material can, in
some cases, obey Fick's law or a Langmuir-type model. In both
cases, the absorption phenomenon is considered as reversible. In
other cases, the diffusion anomalies observed can be attributed
to irreversible chemical damage to the material (hydrolysis) or
damage (cracks) generated within the composite under the effect
of mechanical stresses, for example.
At high temperatures or in specific pH conditions, hydrolytic
damage can occur. Such hydrolysis phenomena depend on the
type of material and, in particular, the presence or absence of
hydrolysable functional groups such as esters or amides. These
phenomena will cause irreversible damage and make the material more brittle. Chemical ageing by hydrolysis can occur a long
time after the material is saturated with water.
Thermal ageing
Thermal and/or thermo-oxidative ageing of composites (with
thermoplastic or thermosetting matrices) mainly affects the matrix. The degradation processes generally involve free radicals
(Figure 2). These phenomena are due to the decomposition of
the polymer above a certain temperature (chain cleavage) or the
oxidation of the material in the presence of oxygen (this phenomenon is controlled by the diffusion of oxygen in the material) and make the material more brittle.
In some cases, the life cycle is forecast based on the principle of
time/temperature equivalence and using Arrhenian-type methods. This involves monitoring changes in an essential property
for the application in question, up to a relevant end-of-life criterion, in the context of isothermal ageing at high temperatures
to determine the life cycle based on extrapolation from lower
temperatures. However, this method is only valid if the degradation mechanisms likely to occur over the long-term use of the
material are not modified within the range of temperatures considered, and if the method is representative of all the oxidation
mechanisms involved. Therefore, this approach is limited.
Other methods based on a kinetic model are used to model the
material's condition at the molecular, macromolecular and mor-

phological scales, and subsequently to use structure-property relations to predict properties at the macroscopic scale. This kinetic model is derived from the mechanistic diagrams identified in
the context of the material's oxidation over time. This model incorporates oxygen diffusion phenomena within the material and
the progress of the reactions will depend not only on time, but
also on the depth of the elementary layer under consideration.
These methods can be used to establish the degradation kinetic mechanisms and oxidation profiles over the entire thickness
of the composite. These approaches are still being developed in
order to better represent the interactions between the oxidation
process and the anaerobic heat damage deeper towards the core
of the material.
Mechanical fatigue
Unlike metal materials, the fatigue mechanisms affecting composites are quite similar to static damage. However, the damage
processes are quite complex and change as life progresses, as
shown in Figure 3. The fatigue initiation phase is quite short,
and cracks appear in the material from the earliest cycles. The
material accepts these cracks well, and they continue to spread
throughout the matrix until a fibre-matrix delamination appears. The fibres then break one by one, and the forces are transferred to the remaining fibres until the internal stress is too high
and the specimen is destroyed. The propagation phase is much
shorter with composites than with metallic materials, and the
whole thickness of the material is damaged, not just the surface.
Fatigue curves show stresses over a number of cycles. The shape
and gradient of these curves can vary substantially depending
on the type of matrix and reinforcement used. As shown in Figure 4, fatigue properties depend on the type of forming method
and on the fibre orientation. Long unidirectional fibres, subjected to stresses in the fibre direction, have longer life cycles
than short fibres, which are randomly distributed throughout
the structure. All composite materials have their own fatigue
characteristics, determined depending on external forces, and
internal characteristics.

Fig. 2: Mechanistic
scheme of
polymer oxidation
developed by
Bolland & Gee

Fig. 3:
Fatigue damage
variation in
composite materials

No111 March 2017

/ jec composites magazine

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Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #111 - March 2017 - Special JEC World - o