|Composites Design and Manufacture (Plymouth University teaching support materials)
Interfaces. Interphases. Meso-mechanics (fibre clustering and resin-rich volumes). Voids.
Interfaces (also see fibre coatings)
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].
An interphase is formed when there is a reaction between the reinforcement and the matrix [8, 9]. For example, titanium carbide may be formed when carbon fibres are used in a titanium matrix. Jancar  has reviewed the role of the interphase in the control of composite performance.
Meso-mechanics (including clustering of reinforcement and consequent resin-rich volumes)
Mesomechanics  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:
The nature of fibre-reinforced composites is such that there is generally dual-scale structure with clustering of fibres in bundles (tows) and larger features dictated by the reinforcement architecture (e.g. chopped strand mat, woven fabrics or stitched non-crimp fabrics (NCF)). This data required for rules-of-mixture may be insufficient for a full description of the meso-structures where clustering of fibres occurs. The use of image processing and analysis for the characterisation of composite micro-/meso-structures has been the subject of a number of publications (e.g. Guild and Summerscales . Pyrz [13, 14]. Summerscales . Summerscales et al . The structures within clustered populations have been described using a variety of parameters, e.g.
Mulligan et al  reviewed the experimental and theoretical work relevant to the effects of fibre-bundling on the mechanical properties of a short-fibre composite. They identified a variety of techniques that have been used to control fibre-bundling in composite materials, but that none of those techniques were entirely satisfactory to study the effect of fibre-bundling on the mechanical properties. Davy and Guild  have studied the distribution of inter-particle distances. Summerscales et al  have used Voronoi tessellation and fractal dimension to correlate mechanical properties and processability to the microstructure of fabric-reinforced polymer matrix composites.
In the context of natural fibre composites, Baley et al  stated that "Fiber clusters and bundles promote damage initiation and fracture propagation; to improve composite quality and performance, it is necessary to improve the separation and dispersion of fibers by optimising both the extraction procedures and manufacturing conditions"
Varghese and Whitcomb  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.
Virtual testing is an emerging framework for integrated modelling of multiple lengthscales (Table 1), typically from atom to macroscale.
The process by which composites are manufactured can entrap air (or volatiles) in the laminate as porosity or voids. The causes may include (i) air in the resin mix, (ii) volatiles in the resin "boiling", (iii) the degree of impregnation of pre-impregnated materials, (iv) trapping air between prepreg layers, or (v) race-tracking in liquid composite moulding processes. Thorfinnson and Biermann  proposed that the degree of impregnation (DI) could be calculated using the interstitial volume (IV: the space between the dry fibres) and the pore volume (PV) of the prepreg using DI = (IV-PV)/IV.
Judd and Wright , Ghiorse , Baley et al , Liu and Chen  and Mehdikhani et al  have reviewed porosity/voids in composites. Judd and Wright 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  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 Vv (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 Vv < 0.5%, the voids are spherical with a diameter of 10 μm and are due to trapped volatiles. As Vv 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 Vv and void thickness, where the thickness is related to fibre diameter.
Stone and Clarke  reported that below Vv = 1.5% voids tend to be volatile-induced and hence spherical with diameters in the range 5-20 μm, while above Vv = 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 Vv. Mayr et al  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  have presented a good summary of the options for the characterisation of voids in composites. The techniques available and their respective issues are shown in Table 2:
|Archimedes density||Matrix burn-off||Chemical digestion||Water Absorption at 100°C||Ultrasonic scanning||Ultrasonic & DWT ||Microscopy||X-ray CT|
|Data requirements||fibre and matrix densities||fibre and matrix densities||fibre and matrix densities||Limited use ||calibration samples|
|Can report negative void content||✗|
|Can lose some fibre||✗||✗||✗|
|Preparation time and costs||✗|
|2D shape and size information||✓||✓|
|3D shape and size information||✓|
|Applicable standards/guidance||ASTM D3171||ISO 62:2008||CRAG test method 1001||CMH-17 §126.96.36.199.4|
Madsen et al  considered that the porosity in plant (and other hollow) fibre composites can be divided into three components:
Madsen et al  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.
Click here for TalisList
| Hot-linked references may only return an
abstract, which will allow you to judge the relevance of the paper to your
Should you need to login to access the full publication, then it is recommended that you use TalisList with Athens authentication.