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Interfaces

An interface is the distinct region where the reinforcement (fibre) and the matrix (polymer) meet. The topic of interfaces has been reviewed by several authors [1-7].

1. AC Metcalfe, Interfaces in Metal Matrix Composites (LJ Broutman and RH Crock - editors, Composite Materials series volume 1), Academic Press, New York and London, 1974.  ISBN 0-12-136501-8.
2. EP Plueddemann, Interfaces in Polymer Matrix Composites (LJ Broutman and RH Crock - editors, Composite Materials series volume 6), Academic Press, New York and London, 1974.  ISBN 0-12-136506-9.
3. DL Caldwell, Interfacial analysis, In SM Lee - editor, Encyclopedia of Composites, VCH Publishers, New York, 1990, volume 2, 361-377.  ISBN 0-89573-732-9.
4. JDH Hughes, The carbon fibre/epoxy interface: a review, Composites Science and Technology, 1991, 41(1), 13-45.
5. Jang-Kyo Kim and Yiu-wing Mai, High strength, high fracture toughness fibre composites with interface control: a review, Composites Science and Technology, 1991, 41(4), 333-378.
6. MC Andrews, DJ Bannister and RJ Young, The interfacial properties of aramid epoxy model composites, Journal of Materials Science, 1 August 1996, 31(15), 3893-3913.
7. LT Drzal, PJ Herrera-Franco and Henjen Ho. Fibre-Matrix Interface Tests, Chapter 5 in A Kelly and C Zweben - editors, Comprehensive Composite Materials volume 5, Pergamon, Oxford, 2000, 71-111.  ISBN 0-08-043723-0.

Interphases

An interphase is formed when there is a reaction between the reinforcement and the matrix [1, 2].  For example, titanium carbide may be formed when carbon fibres are used in a titanium matrix. Jancar [3] has reviewed the role of the interphase in the control of composite performance.

1. WD Bascom, Interphase in fiber-reinforced composites, In SM Lee - editor, Encyclopedia of Composites, VCH Publishers, New York, 1990, volume 2, 411-422. ISBN 0-89573-732-9.
2. ST Mileiko, Interphases, Chapter 4.09.2.3 in A Kelly and C Zweben - editors, Comprehensive Composite Materials volume 4, Pergamon, Oxford, 2000, 268-270.  ISBN 0-08-042993-9.
3. J Jancar, Review of the role of the interphase in the control of composite performance on micro- and nano-length scales, Journal of Materials Science, October 2008, 43(20), 6747-6757.

Voids

Judd and Wright [1] reviewed 47 papers and concluded that "although there is a considerable scatter in results (reflecting in part the difficulties of accurate void content determination) the available data show that the interlaminar shear strength of composites decreases by about 7 per cent for each 1 per cent voids up to at least the 4 per cent void content level, beyond which the rate of decrease diminishes.  Other mechanical properties may be affected to a similar extent.  This is true for all composites regardless of the resin, fibre or fibre surface treatment used in their fabrication".  See Table 1 of the reference for a comprehensive analysis of the data.

Purslow [2] proposed a novel classification system for voids. He suggested that the current system is only significant for fairly uniformly distributed voids. For example, to quote a V_v (void volume fraction) of 0.5% for a composite of generally high quality (voids < 0.2%) but with an occasional very large void could be very misleading and potentially dangerous. It is difficult to measure void contents to such low values. He suggested that the void content should be quoted as "0<voids<0.2%; infrequent local voids > 0.5%". His studies have suggested that when V_v < 0.5%, the voids are spherical with a diameter of 10 μm and are due to trapped volatiles.  As V_v increases, the voids due to trapped volatiles decrease in number and are replaced by large intra-tow/intra-lamina voids. The results suggested a linear relationship between V_v and void thickness, where the thickness is related to fibre diameter.

Stone and Clarke [3] reported that below V_v = 1.5% voids tend to be volatile-induced and hence spherical with diameters in the range 5-20 μm, while above V_v = 1.5% the voids are flattened and elongated in the in-plane direction due to the limitation of space between the fibre bundles and are also significantly larger than those voids at a lower V_v.  Mayr et al [4] have recently reported that small pores in CFRP with porosity levels <1.8% often have roughly circular cross-sections and found an abrupt increase in the out-of-plane shape factors at this percentage porosity.

Little et al [5] have presented a good summary of the options for the characterisation of voids in composites.  The techniques available and their respective issues are shown below:

Madsen et al [6] considered that the porosity in plant (and other hollow) fibre composites can be divided into three components:

• fibre-correlated porosity, including (a) fibre porosity (the lumen), (b) fibre/matrix interface porosity and (c) impregnation porosity (where the reinforcement architecture denies the matrix resin access to inter-fibre spaces),
• matrix correlated porosity (bubbles in the resin), and
• structural porosity (where insufficient matrix is available to fill the free space in a fully compacted fibre assembly.

They [6] suggest there is a transition value of fibre weight fraction which gives an optimal combination of high fibre volume fraction, high composite density and low porosity.  They studied natural fibres (flax, hemp and jute) in polymer matrix (polypropylene or polyethyleneterephthalate) composites and observed that the thermoplastic matrix is not able to impregnate the fibre lumen.

The webpage on Resin Transfer Moulding includes a section on void formation and transport.

Meso-mechanics (including clustering of reinforcement)

Mesomechanics [7] is the area that bridges the microstructure-property relationship of materials with non-continuum mechanics.  It is aimed at developing the fundamental principles and the associated methodologies which can guide the creation of multiphase materials with desired microstructures balanced by prediction of their in-service microscopic and macroscopic behaviours.  The US Air Force Office of Scientific Research (AFOSR) research initiative encompassed fundamental studies in the following general areas:

• constitutive modelling of multiphase materials
• damage mechanics of multiphase materials
• stress waves and dynamic responses
• very high temperature behaviour
• non-linear structural behaviour
• computational and experimental mechanics.

Davy and Guild [8] have studied the distribution of inter-particle distances. Summerscales et al [9] have used Voronoi tessellation and fractal dimension to correlate mechanical properties and processability to the microstructure of fabric-reinforced polymer matrix composites.

Varghese and Whitcomb [10] have used a local averaging procedure to determine effective properties for a reinforcement that has microstructure, specifically using finite element analysis to study the effect of homogenisation for the modelling of hollow fibres.  The homogenised properties can potentially be used to eliminate one level of microstructure when modelling such systems.

References

1. NCW Judd and WW Wright, Voids and their effects on the mechanical properties of composites - an appraisal, SAMPE Journal, January/February 1978, 14(1), 10-14.
2. D Purslow, On the optical assessment of the void content in composite materials, Composites, 1984, 15(3), 207-210.
3. DEW Stone and B Clarke, Ultrasonic attenuation as a measure of void content in carbon-fibre reinforced plastics, Non-Destructive Testing, 1975, 8(..), 137-145.
4. G Mayr, B Plank, J Sekelja and G Hendorfer, Active thermography as a quantitative method for non-destructive evaluation of porous carbon fibre reinforced polymers, NDT&E International, 2011, 44(7), 537-543.
5. JE Little, X Yuan and MI Jones, Characterisation of voids in fibre reinforced composite materials, NDT&E International, March 2012, 46(1), 122-127.
6. B Madsen, A Thygesen and H Lilholt, Plant fibre composites - porosity and volumetric interaction, Composites Science and Technology, 2007, 67(7-8), 1584-1600.
7. GK Haritos, JW Hager, AK Amos, MJ Salkind and ASD Wang, Mesomechanics: the microstructure-mechanics connection, International Journal of Solids and Structures, 1988, 24(11), 1081-1096.
8. PJ Davy and FJ Guild, The distribution of interparticle distance and its application in finite element modelling of composite materials, Proceedings of the Royal Society of London, 8 July 1988, 418(1854), 95-112.
9. J Summerscales, FJ Guild, NRL Pearce and PM Russell, Voronoi cells, fractal dimensions and fibre composites, Journal of Microscopy, February 2001, 201(2), 153-162.
10. J Varghese and J Whitcomb, Effective properties of composites whose reinforcement has microstructure, Mechanics of Advanced Materials and Structures, July 2006, 13(3), 227-235.

Further reading

1. DR Mulligan, SL Ogin, PA Smith, GM Wells and CM Worrall - Fibre-bundling in a short-fibre composite, Composites Science and Technology, 2003, 63(5), 715-725.
2. Jang-Kyo Kim and Yiu-wing Mai - High strength, high fracture toughness fibre composites with interface control: a review, Composites Science and Technology, 1991, 41(4), 333-378.
3. JDH Hughes - The carbon fibre/epoxy interface: a review, Composites Science and Technology, 1991, 41(1), 13-45.