Interfacial stress transfer

Gong L, Kinloch IA, Young RJ, Riaz I, Jalil R, Novoselov KS. Interfacial stress transfer in a graphene monolayer nanocomposite. Adv Mater. 2010 Jun 25 22(24) 2694-7. 

Several parameters affect the mechanical properties of the composites, including proper dispersion and a large aspect ratio of the filler, interfacial stress transfer, a good alignment of reinforcement, and solvent selection. 

More modestly, to a first approach, the aim of the present part is to analyse the influence of the interfacial adhesion W on the magnitude of the fibre-to-matrix stress transfer capacity. For all the study, these interfacial stress transfer capacity is defined in term of interfacial shear strength x, measured by means of a fiagmentation test on single fibre composites. According to the previous part of this study, x is given by the following expression ... 

However, the slope of the straight lines obtained are different for each system and, as already stated, could depend on the intrinsic properties of the fibre and the matrix, in particular their mechanical properties. In agreement with recent developments of the fully elastic model of interfacial stress transfer proposed by Cox [16], it has been shown that k is directly related to the elastic moduli Ef and Em of the fibre and the matrix respectively, in order that T can be expressed as ... 

This is the well-known rule-of-mixture which describes a rather idealised situation and can predict the modulus only for continuous fibre-reinforced composites where there is sufficient stress transfer from the matrix to the fibre. However, short fibres are usually much shorter than the specimen length. For short fibres we must consider the matrix-fibre stress transfer. When the matrix is under stress, the maximum stress transferred to the fibre is described by the interfacial stress transfer (t). The stress transfer depends on the fibre length (/), so that at some critical length, /, the stress transferred is large enough to break the fibre. The stress transferred to the fibre builds up to its maximum value (o that which causes breakage) over a distance 1 from the end of the fibre. This means that the long fibres carry load more efficiently than short fibres. 

When the nanocomposite matrix is semi-ciystalline, incorporation of [nano)particles such as CNTs frequently aims at modifying the crystallization behavior of the polymer in order to improve its properties like, for example, its mechanical performance, and/or to shorten processing cycle times. This way, high levels of mechanical reinforcement can be achieved at low CNT loadings due to the formation of a highly crystalline layer in the immediate vicinity of the CNT walls, ensuring effective interfacial stress transfer. In addition, dispersion of electrically conductive particles into a semi-crystalline [as well as amorphous) polymer matrix also leads to the production of conductive materials. 

Irrespective of the method of preparation there are two fundamental and critical issues associated with translating or transferring the unique properties of carbon nanotubes to a polymer matrix. Firstly, the nanotubes must be uniformly distributed and dispersed throughout the polymer matrix, and secondly, there must be enhanced interfacial interaction/wetting between the polymer and the nanotubes. For example, any load applied to the polymer matrix should be transferred to the nanotube. This load relies on the effective interfacial stress transfer at the polymer-nanotube interface, which tends to be polymer dependent (7). Three general approaches have been adopted in attempts to modify the surface of CNTs to promote such interfacial interactions chemical, electrochemical and plasma treatment. For example, Castafio et al. (8) placed different organofunctional groups... 

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

In the macrocomposite model it is assumed that the load transfer between the rod and the matrix is brought about by shear stresses in the matrix-fibre interface [35]. When the interfacial shear stress exceeds a critical value r0, the rod debonds from the matrix and the composite fails under tension. The important parameters in this model are the aspect ratio of the rod, the ratio between the shear modulus of the matrix and the tensile modulus of the rod, the volume fraction of rods, and the critical shear stress. As the chains are assumed to have an infinite tensile strength, the tensile fracture of the fibres is not caused by the breaking of the chains, but only by exceeding a critical shear stress. Furthermore, it should be realised that the theory is approximate, because the stress transfer across the chain ends and the stress concentrations are neglected. These effects will be unimportant for an aspect ratio of the rod Lld> 10 [35]. 

CNT can markedly reinforce polystyrene rod and epoxy thin film by forming CNT/polystyrene (PS) and CNT/epoxy composites (Wong et al., 2003). Molecular mechanics simulations and elasticity calculations clearly showed that, in the absence of chemical bonding between CNT and the matrix, the non-covalent bond interactions including electrostatic and van der Waals forces result in CNT-polymer interfacial shear stress (at OK) of about 138 and 186MPa, respectively, for CNT/ epoxy and CNT/PS, which are about an order of magnitude higher than microfiber-reinforced composites, the reason should attribute to intimate contact between the two solid phases at the molecular scale. Local non-uniformity of CNTs and mismatch of the coefficients of thermal expansions between CNT and polymer matrix may also promote the stress transfer between CNTs and polymer matrix. 

Apart from the elastic stress transfer at the perfectly bonded interface, another important phenomenon that must be taken into account is the stress transfer by friction, which is governed by the Coulomb friction law after the interface bond fails. Furthermore, matrix yielding often takes place at the interface region in preference to interfacial debonding if the matrix shear yield strength, Xm is significantly smaller than the apparent interface bond strength, tb. It follows thus... 

In particulate-filled thermoplastics, the matrix is the load-bearing component and all deformation processes take place in the matrix. Particulate fillers are, in most cases, not capable of carrying any substantial portion of the load due to the absence of interfacial friction as the means of stress transfer. This is evidenced by the lack of broken particles on the surfaces of fractured filled thermoplastics. Hence, it seems appropriate to start this volume with a brief overview of the basic structural levels and manifestation of these levels in governing the mechanical properties of semicrystaUine thermoplastics used in compounding. 

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