The damage in the composite plate [29]. This damage

The most
widely used composite laminates remain based on thermosetting organic resins, which
in addition to contributing to the enhancement of mechanical properties, show very
good thermal properties. However, despite their very improved mechanical
properties, thermoset based laminate composites have the major disadvantage of
poor out of plane properties, especially with regards to their impact response
28. During an impact test, the energy absorbed by the composite plate is used
mostly to generate damage in the composite plate 29. This damage is mainly
composed of matrix cracking, delamination at the interface between layers and
fibre breakage 30. Thus, properties that contribute to preventing one of
these phenomena lead to the improvement of the impact resistance of the
composite.

Various methods
have been used to improve the damage resistance of composite laminates. These
approaches include fibres/filler hybrid systems 31, reinforcing of the
thermoset matrix, the introduction of a fine thermoplastic film at the interface
between plies, or the use of z-fibre pinning for the prevention of delamination
32. More recently some authors reported a new method that consisted of adding
nanoparticles or tri block-copolymers into the thermoset matrix. To avoid
delamination, it is desirable to use a polymer matrix with a good toughness. In fact, since delamination is initiated by the extension
and the bridging of matrix cracks, the use of a tough matrix thus leads to
prevent this type of damage. Another
study from Reis, et al. on damage tolerance of Kevlar/epoxy-based nanoclay found
that adding nanoclays contributed to the increase of the maximum load and
damage area by about 29% 30.

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

Composite
materials are often used for impact applications 4. Low velocity impact
(defined as events in the range 1–10 m/s) can cause matrix cracking,
delamination and fiber breach. New studies put in evidence as, during the low velocity
impacts, the damage initiates with matrix cracks. These cracks cause
delamination at the interfaces among plies that have different fiber orientations
than each other. For thin specimens, the bending stresses, due to the impact,
cause matrix cracking in the lowest ply, and the damage spreads from the bottom
through the other plies up to the impacted face. The damage is characterized by
the matrix cracks and the delamination in the ply boundaries, like a reversed
pine tree 14. For stiffer specimens, the matrix cracks initiate on the
impacted surface of the specimen due to the high contact stresses. The damage
propagates from the top surface to the bottom one through the other plies like
a pine tree 15. Such damages are very difficult to be detected with the naked
eye and can lead to severe reductions in the stiffness and the strength of the
structures. Consequently, the study of the behaviour of the composite
structures subjected to low velocity impact is essential to avoid loss of
performance.