Stress Transfer Efficiency

More recently, Liu et al. (47), compared OH- covalently functionalized SWNTs to SDS stabilized SWNTs. The authors used Raman spectroscopy to understand why the improvements of the Young s modulus and strength are greater for covalently functionalized nanotubes. The Raman shift was plotted versus the tensile strain in the two composites PVA/OH-SWNTs and PVA/ SDS dispersed SWNTs, in the elastic regime (for strain values below 1.2%). The shift is linear in this regime, and a larger slope is measured 

More recently, a similar percolation threshold of 0.67 wt% was also measured in a composite containing MWNTs. The composites were prepared by gelation/crystallization from solutions in the mixture solvent of dimethyl-sulfoxide (DMSO) and water (51). 

Thermo-electrical properties of PVA composite fibers with a large fraction of carbon nanotubes were studied by Miaudet et al. (52,53). Low temperature conductivity measurements showed that the conductivity depends on several factors the electronic properties of the nanotubes (54), the number and properties of intertube contacts, like in other polymer composites (55). The authors investigated also the behavior at high temperature. A strong increase of conductivity is observed in the vicinity of the glass transition of 

close to 80°C. The polymer chains become mobile at the glass transition, allowing a partial re-organization of the nanotubes in the matrix and a modification of the conducting network. The polymer mobility allows some relaxation in the structure with possible better intertube contacts and loss of nanotube alignment which favors the contact probability. Both mechanisms can explain improvements of conductivity at the glass transition of the polymer. 

---

The second reason for the better mechanical properties of cross-linked composites is the higher quality of the fiber—matrix interface which improves the stress transfer efficiency between these two components. 

George et al. [27] studied stress relaxation behaviour of pineapple fibre-reinforced polyethylene composites. They found stress relaxation to be decreased with an increase of fibre content due to better reinforcing effect It is also reported by George et al. [28] that properties of fibre-reinforced composites depend on many factors like fibre-matrix adhesion, volume fraction of fibre, fibre aspect ratio, fibre orientation as well as stress transfer efficiency of the interface. Luo and Netravah [29] found an increase in the mechanical properties of green composites prepared from PALFs and poly(hydroxybutyrate-co-valerate) resin (a biodegradable polymer) with the fibres in the longitudinal direction. However, the researchers reported a negative effect of the fibres on the properties in the transverse direction. 

Optimization of interfacial adhesion in the development of natural fiber-reinforced polymer composites has been the subject of extensive research of the past two decades. Many techniques have been developed and tested and the principal aim of various modification strategies has been to reduce the fiber-fiber interaction through aiding improved wetting and dispersion, as well as improving interfacial adhesion and the resulting stress transfer efficiency from the matrix to the fiber. 

Equation 1 assumes that the shear stress at the interface is constant as a result of complete interfacial debonding. With good adhesion, only partial debonding or other micro-mechanical events such as transverse matrix cracking are observed, which invalidate the assumption of a constant interfacial shear stress. As a result, alternative data reduction techniques have been developed. For example, Tripathi and Jones developed the cumulative stress-transfer function, which deals with the limitations given above. This has been further refined by Lopattananon et al into the stress-transfer efficiency from which an ineffective length of that fibre in that resin can be determined. In this model, the matrix properties and frictional adhesion at debonds can be included in the analysis. It is also possible to use the three-phase stress-transfer model of Wu et al to include the properties of an interphase. 

The properties of composites depend on many factors, such as, filler content, size, shape and aspect ratio, uniform dispersion of the filler in the matrix, filler/matrix interfacial bonding, and stress/transfer efficiency through the interface [59]. Through in-situ polymerization technique, a uniform filler dispersion and effective interfacial bonding can be achieved. 

All polymer composites absorb substantial amounts of moisture or water in humid environment as well as in water. The most important concern in indoor and outdoor applications of natural fiber-based biocomposites with polymer matrices is their sensitivity to water absorption, which can reduce considerably their mechanical, physical, and thermal properties and performances. The water absorption of biocomposites results in the debonding or gap in the natural fiber-polymer matrix interfacial region, leading to poor stress transfer efficiency from the matrix to the fiber and reduced mechanical and dimensional stabilities as well [158]. It has been known that the hemiceUulose component in cellulose-based natural fibers may be mainly responsible for water absorption because it is more susceptible to water molecules than the crystalline cellulose component. Also, poor interfacial adhesion... 

As already mentioned, CNT dispersion and stress transfer must all be optimized to reach maximum mechanical properties. One of the most challenging issues in polymer nanocomposites is the determination of the stress transfer efficiency through the interface between CNTs and polymer matrix. This is a prerequisite to take advantage of the extremely high modulus and strength of CNTs. In addition, the high aspect ratio of CNTs necessitates huge interfacial areas available for stress transfer compared to traditional micrometer-size fiber composites. 

---