Interfacial Micromechanics

It has been demonstrated that Raman spectroscopy is not only a very powerful technique for following the deformation of high-performance fibres and composites but is also of use in the study of the deformation of isotropic polymers. It has been shown that the shifts in the Raman bands is related dire y to deformation of the bonds in the polymers, and so the technique offers a unique method of following molecular deformation in polymers. The relationship between the shift 

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Shjmg Y T, Bennett J A, Young R J, Davies R J and Eichhorn S J (2006) Analysis of interfacial micromechanics of model composites using sjmehrotron microfocus X-ray diffraction, J Mater Sci 41 6813-6821. 

N Melanitis, C Galiotis. Interfacial micromechanics in model composites using laser Raman spectroscopy. Proc Roy Soc London A 440 379-389, 1993. 

Other aspects of interfacial science and chemistry are examined by Owen and Wool. The former chapter deals with a widely used chemistry to join disparate surfaces, that of silane coupling agents. The latter chapter describes the phenomenon of diffusion at interfaces, which, when it occurs, can yield strong and durable adhesive bonds. Brown s chapter describes the micromechanics at the interface when certain types of diffusive adhesive bonds are broken. The section on surfaces ends with Dillingham s discussion of what can be done to prime surfaces for adhesive bonding. 

The commercial composite materials being marketed today are optimized in order to make the interfacial properties acceptable in the sense that they will not fail at such low levels as to detract from the overall composite behavior. Considering a unidirectional specimen, where the fibers are all aligned parallel to each other, commercial systems can be described by a rule of mixtures661 relationship (Fig. 10). Properties of the matrix and fiber can be linearly combined based on the volume fraction of each constituent. For example, the longitudinal tensile modulus is the sum of the proportion of each component. The interface in these systems is considered ideal in that it efficiently transmits forces between fiber and matrix without failure. Using this model as a basis for micromechanical analysis and discussion, the magnitude of the forces present at the interface can be predicted. 

Brenner H (1979) A Micromechanical Derivation of the Differential Equation of Interfacial Statics. Journal of Colloid and Interface Science 68 (3) 422-439... 

Mavrovouniotis GM, Brenner H (1993) A Micromechanical Investigation of Interfacial Transport Processes. I. Interfacial Conservation Equations. Phil Trans R Soc bond A 345 165-207... 

Jones, D.B. and Middelberg, A.P.J. Micromechanical testing of interfacial protein networks demonstrates ensemble behavior characteristic of a nanostructured biomaterial, Langmuir, 18, 5585, 2002. 

The same effect is possible when the interfacial strength is low (reduced coupling). Therefore, from the micromechanical point of view there is no difference between void formation by cavitation and by debonding. 

Furthermore, the absorption of water by the interphasal polymer can reduce its yield strength below the interfacial bond strength. Thus, the apparent interfacial shear strength will be reduced, and a yield front, rather than a debond, will propagate along the fibre interface modifying the stress transfer micromechanics at a fibre break. A consequence is that the stress concentrations in adj acent fibres to the fibre break will be reduced, and the probability of the formation of a flaw of critical dimensions is also reduced. The number of interacting fibre breaks associated with a flaw of critical dimensions will increase. 

Three test methods, the fragmentation, pull-out and microbond methods, were used to analyse the micromechanics for carbon fibre/epoxy composites and Raman spectroscopy was used to determine the variation of fibre strain with position along a carbon fibre in a resin. It was demonstrated that the latter technique was capable of revolutionising interpretation of composite micromechanics and the different micromechanical test methods. In particular, it was shown that the technique could be used to distinguish between elastic deformation, interfacial debonding and shear yielding of the matrix at the interface. 19 refs. (FRC 94, Institute of Materials, Newcastle, March 1994)... 

Vignes-Alder, M. and Brenner, H., A micromechanical derivation of the differential equations of interfacial statics. III. line tension, 7. Colloid Interface Sci., 103,11,1985. Vogler, E.A., Practical use of concentration-dependent contact angles as a measure of solid-liquid adsorption. 1. Theoretical aspects, Langmuir, 8, 2005, 1992. Vrbanac, M.D. and Berg, J.C., The use of wetting measurements in the assessment of acid-base interactions at solid-liquid interfaces, J. Adhesion Sci. TechnoL, 4, 255, 1990. 

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. 

For a good understanding of the fibre-matrix interface mechanics in relation to its physico-chemical properties, it is therefore necessary to determine the interface capacity to transfer the stresses from the matrix to the fibre. This is usually carried out by means of specific mechanical tests on real or model composites. The aim of the first part of this paper is to present and discuss briefly the most common micromechanical tests and theoretical approaches used nowadays for studying the mechanical behaviour of interfaces in composites. A second part will be concerned with the influence on the magnitude of the fibre-to-matrix stress transfer capacity, defined in terms of interfacial shear strength, of the interfacial adhesion between both constituents, on the one hand, and of the matrix alterations (interfacial layers) near the interface, on the other hand. 

A tentative model has been proposed to relate the interfacial shear strength at the fibre-matrix interface, measured by a fragmentation test on single fibre composites, to the level of adhesion between both materials. This last quantity has been estimated from the surface properties of both the fibre and the matrix and was defined as the sum of dispersive and acid-base interactions. This new model clearly indicates that the micromechanical properties of a composites are mainly determined by the level of physical interactions established at the fibre-matrix interface and, in particular, by electron acceptor-donor interactions. Moreover, to a first approximation, our model is able to explain the stress transfer phenomenon through interfacial layers, such as crystalline interphases in semi-crystalline matrices and interphases of reduced mobility in elastomeric matrices. An estimation of the elastic moduli of these interphases can also be proposed. Furthermore, recent work [21] has shown that the level of interfacial adhesion plays a major role on the final performances (tensile, transverse and compressive strengths and strains) of unidirectional carbon fibre-PEEK composites. 

Finally, it can be concluded that the reversible work of adhesion at the fibre-matrix interface is an important parameter in determining the micromechanical behaviour of model composites as well as the ultimate properties of unidirectional laminates. Such a general approach could allow us to further improve the performances of advanced composites by controlling both the processing conditions and the level of interfacial interactions. 

Still another micromechanical method for single fiber simulation of a polymer composite interfacial adhesion is the microbond test. This method was developed by Miller et al. [52] and initially applied for S3mthetic fibers. As mentioned by Craven et al. [53], the microbond test is suitable for any fiber that can carry only low loads. This is the particular case of silk, a strong natural fiber but with limited load bearing capacity due to diameters that can be finer than 50 pm. This could be the case of some lignocellulosic fibers such as the ramie with diameters of the order of 10 pm. 

Joung MP, Son TQ, Byung SH, Lawrence KD (2006) Interfacial evaluation of modified jute and hemp fibers olypropylene (PP)-maleic anhydride polypropylene copolymers (PP-MAPP) composites using micromechanical technique and nondestructive acoustic emissitm. Cranpos Sci Technol 66 2686-2699... 

As the performance of the composite is profoxmdly dominated by the micromechanical deformation process, its knowledge and control are critical for the improvement of composite properties. The effect of particle characteristics and interfacial adhesion on the micromechanical deformation processes in PP-wood composites was investigated by Renner et al. [7]. They proposed a failure map as well as the practical results and considered the influence of matrix characteristics on deformation and failure in PP-natural fiber composites in other research [24]. Hietala et al. [78] studied the effect of chemical pre-treatment and moisture content of wood chips on the wood particle aspect ratio during the processing and mechanical properties of WPCs. The use of pretreated wood chips enhanced the flexural properties of the wood chip-PP composites. Moreover, the use of undried wood chips compared to dried one can improve and reduce the flexural strength and flexural modulus, respectively. On the other hand, they concluded that the use of pretreated and undried wood chips lead to the highest aspect ratio after compounding. The effect of composition and the incorporation... 

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