Interfacial loading

III will be discussed. In section IV modeling of interfacial load transfer between CNT and polymer in nanocomposite will be introduced and finally we conclude the review by emphasizing the cmrent challenges and future research directions. 

The interfacial load transfer during the stress-strain deformation of the 5 wt% CNT/UHMWPE composite fiber is shown in terms of Raman shift at different deformations at a draw ratio of 30. Raman shift versus strain for the asymmetric C-C stretching band of PE (1060cm ) and the G-band (1595 cm ) of CNT inside the 5wt% MWCNT/UHMWPE composite fiber at a draw ratio of 30 are depicted in Figs 10.8(a) and 10.8(b), respectively. The Raman shift of the pure UHMWPE fiber at a draw ratio of 30 is also shown in Fig. 10.8(a) for comparison. The data are divided into three regions. In region I, the G-band shows a rapid... 

A numerical tool capable of predicting the mechanical behaviour of SWCNTs reinforced rubber. The formulation is based in a micromechanical, non-linear, multi-scale finite element approach and utilizes a Mooney-Rivlin material model for the rubber and takes into account the atomistic nanostructure of the nanotubes. The interfacial load transfer characteristics were parametrically approximated via the use of joint elements of variable stiffness. The SWCNTs improve significantly the composite strength and toughness especially for higher volume fractions. 

Introductory paragraphs similar to the above can be found in hundreds of nanocomposite papers. With the exception of reinforced elastomers, nanocomposites have not lived up to expectations. Although claims of modulus enhancement by factors of 10 exist, these claims are offset by measurements that show little or no improvement... The lackluster performance of nanocomposites has been attributed to a number of factors including poor dispersion, poor interfacial load transfer, process-related deficiencies, poor alignment, poor load transfer to the interior of filler bundles, and the fractal nature of filler clusters [5]. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Fig. 3. Load—deflection curve for a SiC—C—SiC composite in four-point bending. Note the extreme change in behavior fora composite fabricated with a 0.17-p.m carbon layer between the SiC fiber and the SiC matrix as compared with a composite with no interfacial layer (28).

In a mechanical test, interfacial strength may be quantified in terms of either the minimum load required for interface disruption or the total integral energy or work expended. In many situations, due to non-uniformity of chemical or morphological conditions over the area of the interface or to non-uniformity of the applied stress in a given test [7], the two criteria are different. The investigator must thus strive to minimize or deal with both of the above complications, i.e. the interfaces studied should be chemically and morphologically uniform, and the stresses applied in the test should be uniform or distributed in way which is quantitatively describable. 

Contact mechanics, in the classical sense, describes the behavior of solids in contact under the action of an external load. The first studies in the area of contact mechanics date back to the seminal publication "On the contact of elastic solids of Heinrich Hertz in 1882 [ 1 ]. The original Hertz theory was applied to frictionless non-adhering surfaces of perfectly elastic solids. Lee and Radok [2], Graham [3], and Yang [4] developed the theories of contact mechanics of viscoelastic solids. None of these treatments, however, accounted for the role of interfacial adhesive interactions. 

When the surfaces are in contact due to the action of the attractive interfacial forces, a finite tensile load is required to separate the bodies from adhesive contact. This tensile load is called the pull-off force (P ). According to the JKR theory, the pull-off force is related to the thermodynamic work of adhesion (W) and the radius of curvature (/ ). 

Fig. 4. Schematic of the JKR treatment of contact mechanics calculations. The point (, a, P) corresponds to the actual state under the action of interfacial forces and applied load P. P is the equivalent Hertzian load corresponding to contact radius a between the two surfaces, ( o, P) and (S], a, P ) are the Hertzian contact points. The net stored elastic energy and displacement S are calculated as the difference of steps 1 and 2.

The JKR theory is essentially an equilibrium balance of energy released due to interfacial bond formation and the stored elastic energy. For simple elastic solids the deformation as a function of load, according to the JKR theory is given by... 

In the JKR experiments, a macroscopic spherical cap of a soft, elastic material is in contact with a planar surface. In these experiments, the contact radius is measured as a function of the applied load (a versus P) using an optical microscope, and the interfacial adhesion (W) is determined using Eqs. 11 and 16. In their original work, Johnson et al. [6] measured a versus P between a rubber-rubber interface, and the interface between crosslinked silicone rubber sphere and poly(methyl methacrylate) flat. The apparatus used for these measurements was fairly simple. The contact radius was measured using a simple optical microscope. This type of measurement is particularly suitable for soft elastic materials. 

The premise of the above analysis is the fact that it has treated the interfacial and bulk viscoelasticity equally (linearly viscoelastic experiencing similar time scales of relaxation). Falsafi et al. make an assumption that the adhesion energy G is constant in the course of loading experiments and its value corresponds to the thermodynamic work of adhesion W. By incorporating the time-dependent part of K t) into the left-hand side (LHS) of Eq. 61 and convoluting it with the evolution of the cube of the contact radius in the entire course of the contact, one can generate a set of [LHS(t), P(0J data. By applying the same procedure described for the elastic case, now the set of [LHS(t), / (Ol points can be fitted to the Eq. 61 for the best values of A"(I) and W. 

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