The mostwidely used composite laminates remain based on thermosetting organic resins,which in addition to contributing to the enhancement of mechanical properties, showvery good thermal properties. However, despite their very improved mechanicalproperties, thermoset based laminate composites have the major disadvantage ofpoor out of plane properties, especially with regards to their impact response28. During an impact test, the energy absorbed by the composite plate is usedmostly to generate damage in the composite plate 29. This damage is mainlycomposed of matrix cracking, delamination at the interface between layers andfibre breakage 30. Thus, properties that contribute to preventing one ofthese phenomena lead to the improvement of the impact resistance of the composite.Various methodshave been used to improve the damage resistance of composite laminates.
Theseapproaches include fibres/filler hybrid systems 31, reinforcing of thethermoset matrix, the introduction of a fine thermoplastic film at the interfacebetween plies, or the use of z-fibre pinning for the prevention of delamination32. More recently some authors reported a new method that consisted of addingnanoparticles or tri block-copolymers into the thermoset matrix. To avoiddelamination, it is desirable to use a polymer matrix with a good toughness. In fact, since delamination is initiated by the extensionand the bridging of matrix cracks, the use of a tough matrix thus leads toprevent this type of damage. Anotherstudy from Reis, et al. on damage tolerance of Kevlar/epoxy-based nanoclay foundthat adding nanoclays contributed to the increase of the maximum load anddamage area by about 29% 30.Compositematerials 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 velocityimpacts, the damage initiates with matrix cracks. These cracks causedelamination at the interfaces among plies that have different fiberorientations than each other. For thin specimens, the bending stresses, due tothe impact, cause matrix cracking in the lowest ply, and the damage spreadsfrom the bottom through the other plies up to the impacted face. The damage is characterizedby the matrix cracks and the delamination in the ply boundaries, like areversed pine tree 32. For stiffer specimens, the matrix cracks initiate onthe impacted surface of the specimen due to the high contact stresses.
Thedamage propagates from the top surface to the bottom one through the otherplies like a pine tree 33. Such damages are very difficult to be detectedwith the naked eye and can lead to severe reductions in the stiffness and thestrength of the structures. Consequently, the study of the behaviour of thecomposite structures subjected to low velocity impact is essential to avoidloss of performance.Theutilization of nanoclay as filler in polymers has attracted extensive attentionof researchers due to the advanced static, dynamic, thermal, flame retardantand gas barrier properties of the resulting composites 34-35. Anbusagar etal. 36 have investigated the effect of nanoclay content on sandwichcomposites under flexural and impact loading. The effect of a nanoclay improvedepoxy matrix on Kevlar composites laminates under the low velocity impact hadbeen studied by Reis et al.
37. The laminates which have been manufacturedwith epoxy resin and were filled with 6% of nanoclay resulting the bestperformance in terms of elastic convalescence and penetration threshold. Anbusagar etal. 38 have investigated the effect of nanoclay modified by polyester resinon flexural, impact, hardness and water absorption properties of untreated wovenjute and glass fabric hybrid sandwich laminates experimentally. The test resultshave indicated that the flexural properties were substantially increased at 4%of nanoclay loading while impact, hardness and water absorption properties haveincreased at 6% of nanoclay loading. Hosur et al. 39 and Njuguna et al. 40showed that by adding a small amount of nanoclay as filler, major improvementsin foam failure strength and energy absorption could be achieved.
The lowvelocity impact tests carried out on fiber reinforced epoxy clay nanocompositeswere found to have significant influence on damage resistance and load bearingproperties. According toIqbal et al. 41 CFRP laminates with 3 wt% clay showed highest incipientenergy and highest energy absorbed as compared to 0 and 5 wt%. This wasattributed to exfoliated dispersion of 3 wt% clay in the epoxy matrix.
It wasalso noted that 3 wt% clay showed the least damage area while the damageincreased with increasing impact energy. Similarly, Alomari et al. 42 andAvila et al. 43 reported that higher filler content increased the impactdamage resistance but higher clay loadings were not as efficient in resistingdelamination as lower clay loadings. They attributed this to clay agglomerationwhich acted as stress concentrators.
Higher clay loadings deteriorated theproperties which was again attributed to nanoparticle agglomeration. On theother hand, researchers like Aymerich et al. 44 and Esfahani et al. 45found that addition of nanoclay promotes the damage initiation and propagationinside the fiber based nanocomposites with higher damage areas thereby makingnanoclay addition essentially a negative influence.