The the extension and the bridging of matrix cracks,

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 31. 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 32. 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 33. 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.

The
utilization of nanoclay as filler in polymers has attracted extensive attention
of researchers due to the advanced static, dynamic, thermal, flame retardant
and gas barrier properties of the resulting composites 34-35. Anbusagar et
al. 36 have investigated the effect of nanoclay content on sandwich
composites under flexural and impact loading. The effect of a nanoclay improved
epoxy matrix on Kevlar composites laminates under the low velocity impact had
been studied by Reis et al. 37. The laminates which have been manufactured
with epoxy resin and were filled with 6% of nanoclay resulting the best
performance in terms of elastic convalescence and penetration threshold.

Anbusagar et
al. 38 have investigated the effect of nanoclay modified by polyester resin
on flexural, impact, hardness and water absorption properties of untreated woven
jute and glass fabric hybrid sandwich laminates experimentally. The test results
have shown that the flexural properties were substantially increased at 4% of
nanoclay loading while impact, hardness and water absorption properties have
increased at 6% of nanoclay loading. Hosur et al. 39 and Njuguna et al. 40
showed that by adding a small amount of nanoclay as filler, major improvements
in foam failure strength and energy absorption could be attained. The low
velocity impact tests carried out on fiber reinforced epoxy clay nanocomposites
were found to have substantial effect on damage resistance and load bearing
properties.

According to
Iqbal et al. 41 CFRP laminates with 3 wt% clay showed highest emerging energy
and highest energy absorbed as compared to 0 and 5 wt%. This was attributed to
exfoliated dispersion of 3 wt% clay in the epoxy matrix. It was also distinguished
that 3 wt% clay showed the least damage area while the damage increased with
increasing impact energy. Similarly, Alomari et al. 42 and Avila et al. 43 stated
that higher filler content increased the impact damage resistance but higher
clay loadings were not as effective in resisting delamination as lower clay
loadings. They attributed this to clay agglomeration which acted as stress
concentrators. Higher clay loadings weakened the properties which was again
attributed to nanoparticle agglomeration. On the other hand, researchers like
Aymerich et al. 44 and Esfahani et al. 45 found that addition of nanoclay excites
the damage initiation and propagation inside the fiber based nanocomposites
with higher damage areas thus making nanoclay addition essentially a negative
influence.