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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.8, #3)
Compactness of the Cement Microstructure Versus Crack Bridging in Mortars Reinforced with Amorphous Cast Iron Fibers and Silica Fumes by Carl Redon; Jean-Louis Chermant (pp. 149-161).
Both mechanical compaction and addition of pozzolanic silica fumes can provide low permeability interfacial transition zones around the fibers which reinforce a mortar matrix. This paper deals with the controversial effect of achieving a higher matrix compactness and its influence on the fracture behaviour of a mortar reinforced with amorphous cast iron fibers. Test were conducted in uniaxial tension on notched composite mortar prisms in order to plot load versus crack opening curves and evaluate the bridging energy provided by the fibers across a single opening crack. These measures were correlated to SEM observations of the microstructure of fiber/mortar interface, depending on the compaction energy and/or the mortar composition. It is relatively difficult to establish a compromise between ductility and high performance in terms of durability for the material system tested. Indeed, fibers were pulled out of low compactness mortars exhibiting large porous interfacial transition zones (ITZs) along the fiber surface. These zones mainly comprised fibrillous CSH, ettringite and large portlandite crystals. Conversely, when the ITZs around the fibers where filled with compact CSH, resulting from the pozzolanic reaction between silica fume and portlandite, no fiber slippage was observed, but the reinforced mortar broke in a quasi-brittle manner.
Keywords: fiber pull-out; uniaxial notched tensile test; crack-opening curves; crack bridging energy; interface transition zone; silica fume; pozzolanic reaction; cement microstructure
Compaction and Transverse Permeability of Glass Rovings by P. J. Bates; D. Taylor; M. F. Cunningham (pp. 163-178).
Continuous reinforcements such as glass rovings are used in a variety of polymer processes such as filament winding and compression molding. In all these processes, impregnation of the roving fiber bundle by the liquid polymer is essential. Modeling polymer impregnation requires an estimate of the void fraction-compressive stress relationship and the transverse Darcy permeability. The void fraction of 2400 tex glass rovings of different glass fiber diameters and roving thicknesses was assessed as a function of the applied compressive stress. The results indicate that existing void fraction-compressive stress models can be used to adequately fit the experimental data. The transverse permeability of these rovings was also measured as a function of applied compressive stress, fiber diameter, roving thickness, fluid velocity and viscosity using two different permeameters developed for this research. The permeability was observed to depend strongly on the compressive stress, and hence void fraction. As expected, the permeability was independent of the roving thickness, fluid velocity and viscosity. A Kozeny constant value of 7 was found to reasonably fit the data from both permeameters. Scatter of the experimental data was observed. It is hypothesized that this may be due to variations in the roving void fraction resulting from twists, cross-overs and waviness among the fibers.
Keywords: composites; permeability; compaction; roving; glass; Darcy; Carman–Kozeny
The Wear Behaviour of Composite Materials with Epoxy Matrix Filled with Hard Powder by I. Crivelli Visconti; A. Langella; M. Durante (pp. 179-189).
The wear behaviour of composite materials, sliding under dry conditions against smooth steel counterface, has been investigated. The composite materials consisted of glass woven fabric reinforcing three different systems of matrix: epoxy resin, epoxy resin filled with powders of silica and epoxy resin filled with powders of tungsten carbide. The powders were mixed in a volumetric fraction of 6% with the epoxy resin. Three laminates were manufactured by hand lay up technology. The sliding tests have been conducted on the specimens, cut from the three laminates, with a pin on disk apparatus. The results put in evidence different wear behaviours of the composite materials observed at different values of sliding speed and pressure. The presence of different wear mechanisms has been appreciated by SEM-micrographic examinations.
Keywords: composite materials; wear; filled matrix; hard powders
Damage Modeling of Notched Graphite/Epoxy Sandwich Panels in Compression by José M. Mirazo; S. Mark Spearing (pp. 191-216).
Open-hole honeycomb sandwich panels with woven graphite/epoxy facesheets and Nomex™ cores were tested uniaxially in compression to characterize their damage tolerance. A plain weave T-300 graphite fiber fabric was used for the facesheets in two stacking sequences: [45/02] and [03]. Observations of macroscopic sub-critical damage behavior were different in the two material systems. Linear damage zones (LDZ), consisting of fiber micro-buckles and extensive delamination, were typically observed in the [03] material. The [45/02] material exhibited a delamination/bulge zone (DBZ), which consisted of an out-of-plane curved deformation of the outer 45° ply accompanied by a delamination from the interior 0° plies. Modeling of these apparently distinct failure modes, and comparison to experimental data, revealed that the only mode representative of damage tolerant behavior is linear damage zone formation and propagation for both material systems, and that the delamination/bulge behavior is a secondary phenomenon.
Keywords: linear damage zone; kink-band; micro-buckling; delamination-buckling; damage tolerance; graphite; honeycomb sandwich panel; compression