M.G. Holmquist et al./Composites: Part A 34(2003)163-170 short-range repulsive potential, and thus a weakly attractive tubes were subsequently impregnated with the alumina particle network [25]. Slurries were returned to the precursor solution under vacuum. The impregnation step mechanical roller for another 12 h. Pressure filtration was performed in a dry nitrogen atmosphere to prevent (4 MPa)was used to consolidate the slurry; the consolidated premature gelation of the precursor due to atmospheri body was subsequently fluidised again via vibration after it water vapour. The composites were left in the precursor was placed in a plastic bag to prevent drying. The solution for 2 h at atmospheric pressure and transferred into consolidation pressure of 4 MPa was below the critical ammoniated water(pH 10) to gel the precursor throughout pressure where a large number of particles are pushed into the body and prevent it from re-distributing to the surface ontact. which would obviate fluidisation after consolida- during the evaporation of the solvent [32, 33]. After 4 h the tion [25, 30). Using the weight difference method, the composites were removed, dried and heated to 900"C in volume fraction of solids within the water saturated order to pyrolyze the precursor. This was repeated three consolidated bodies was determined to 56.4+ 0.4 times and following the last cycle, the composites were Cloth, cut into-102x 76 mm? rectangles with the two given a final sintering treatment at 1200"C for 2h, which outer most tows along all four edges removed, were put in served to crystallise the precursor to the corundum (a) separate plastic bags; an excess of the pre-consolidated structure. In this manner, the strength of the connection slurry was dispensed on both sides of each cloth. The plastic between mullite particles could be increased without any bag served to prevent the slurry from drying and from shrinkage of the mullite network adhering to tools used in subsequent manufacture. Assisted by vibration, the slurry was manually rolled across the 23. Characterisation surface of the fibre cloth with a piece of aluminium rod until the cloth was fully infiltrated. Since the fluidised body The porosity was measured using the Archimedes exhibits shear-rate thinning, the vibration reduces the method in water. Fibre volume fraction was then calculated viscosity and allows rapid intrusion of the particles into from knowledge of the fibre cloth volume and total tube the fibre cloth [22, 24). Prepregs were frozen to aid removal volume. Microstructures were studied by optical from the plastic bag, i.e. they popped apart due to the microscopy and scanning electron microscopy of cross differential thermal expansion sections of as processed and tested tubes. Damage evolution To produce the composite tube, the prepreg was clamped during testing was monitored by visual observations of the into a three-part collapsible mandrel(diameter 12.7 mm) tube surfaces. In one case, a thin red lacquer coating was made of stainless steel using a plastic extension tab and applied on the outer surface of a tube to ascertain the failure placed atop a vibrating aluminium table at room tempera ture. as the matrix material thawed and became fluid once ain,the prepreg section was slowly wound around the 2. 4. Testing procedure mandrel as shown in Fig. 1. Gentle pressure was applied to Brass testing fixtures were prepared to enable pressure the prepreg in order to squeeze out a small portion of the testing of the CFCC tubes. Tubes were joined to the test fluid powder matrix and create a wake of matrix just in fror of the composite tube that was forming around the mandrel. fixtures using a rapid curing epoxy resin. The fixture This wake of matrix slurry served to fill in air gaps as the included a central rod that prevented axial loading of the tubes and premature failure at the epoxy joint. Internal prepreg layer wrapped on itself, thus preventing large-scale porosity in the final composite. Once the cloth was fully pressure was applied to the composite tube from a nitrogen source(2500 bar). The gas pressure was controlled by wound around the mandrel, the strip of protective plastic was tightly wrapped around the prepreg tube and placed in a The absolute pressure near the inlet of the tube and drying oven at 70C. The mandrel was collapsed and volumetric flow rate at the exhaust port were monitored emoved and the tubes were heated to 900"C for 2 h to The permeability, K, of the composite material was partially sinter the alumina particles to the mullite particles calculated at a low pressure difference(138 kPa)using the and give the porous matrix some strength. The composite following equation, which is applicable to laminar flow of compressible fluids through porous media [ 39] =/s (P1-P) (1) where Pi is the internal pressure, Po, the surrounding Fig 1 Schematic illustration showing how the prepreg is wrapped around a pressure(atmospheric), t, the tube wall thickness, A, the collapsible mandrel. viscosity of the gas, S, the surface area of the tube wall andshort-range repulsive potential, and thus a weakly attractive particle network [25]. Slurries were returned to the mechanical roller for another 12 h. Pressure filtration (4 MPa) was used to consolidate the slurry; the consolidated body was subsequently fluidised again via vibration after it was placed in a plastic bag to prevent drying. The consolidation pressure of 4 MPa was below the critical pressure where a large number of particles are pushed into contact, which would obviate fluidisation after consolida￾tion [25,30]. Using the weight difference method, the volume fraction of solids within the water saturated consolidated bodies was determined to 56.4 ^ 0.4%. Cloth, cut into ,102 £ 76 mm2 rectangles with the two outer most tows along all four edges removed, were put in separate plastic bags; an excess of the pre-consolidated slurry was dispensed on both sides of each cloth. The plastic bag served to prevent the slurry from drying and from adhering to tools used in subsequent manufacture. Assisted by vibration, the slurry was manually rolled across the surface of the fibre cloth with a piece of aluminium rod until the cloth was fully infiltrated. Since the fluidised body exhibits shear-rate thinning, the vibration reduces the viscosity and allows rapid intrusion of the particles into the fibre cloth [22,24]. Prepregs were frozen to aid removal from the plastic bag, i.e. they popped apart due to the differential thermal expansion. To produce the composite tube, the prepreg was clamped into a three-part collapsible mandrel (diameter 12.7 mm) made of stainless steel using a plastic extension tab and placed atop a vibrating aluminium table at room tempera￾ture. As the matrix material thawed and became fluid once again, the prepreg section was slowly wound around the mandrel as shown in Fig. 1. Gentle pressure was applied to the prepreg in order to squeeze out a small portion of the fluid powder matrix and create a wake of matrix just in front of the composite tube that was forming around the mandrel. This wake of matrix slurry served to fill in air gaps as the prepreg layer wrapped on itself, thus preventing large-scale porosity in the final composite. Once the cloth was fully wound around the mandrel, the strip of protective plastic was tightly wrapped around the prepreg tube and placed in a drying oven at 70 8C. The mandrel was collapsed and removed and the tubes were heated to 900 8C for 2 h to partially sinter the alumina particles to the mullite particles and give the porous matrix some strength. The composite tubes were subsequently impregnated with the alumina precursor solution under vacuum. The impregnation step was performed in a dry nitrogen atmosphere to prevent premature gelation of the precursor due to atmospheric water vapour. The composites were left in the precursor solution for 2 h at atmospheric pressure and transferred into ammoniated water (pH 10) to gel the precursor throughout the body and prevent it from re-distributing to the surface during the evaporation of the solvent [32,33]. After 4 h the composites were removed, dried and heated to 900 8C in order to pyrolyze the precursor. This was repeated three times and following the last cycle, the composites were given a final sintering treatment at 1200 8C for 2 h, which served to crystallise the precursor to the corundum (a) structure. In this manner, the strength of the connection between mullite particles could be increased without any shrinkage of the mullite network. 2.3. Characterisation The porosity was measured using the Archimedes method in water. Fibre volume fraction was then calculated from knowledge of the fibre cloth volume and total tube volume. Microstructures were studied by optical microscopy and scanning electron microscopy of cross sections of as processed and tested tubes. Damage evolution during testing was monitored by visual observations of the tube surfaces. In one case, a thin red lacquer coating was applied on the outer surface of a tube to ascertain the failure location. 2.4. Testing procedure Brass testing fixtures were prepared to enable pressure testing of the CFCC tubes. Tubes were joined to the test fixtures using a rapid curing epoxy resin. The fixture included a central rod that prevented axial loading of the tubes and premature failure at the epoxy joint. Internal pressure was applied to the composite tube from a nitrogen source (2500 bar). The gas pressure was controlled by manually opening or closing the regulator on the gas supply. The absolute pressure near the inlet of the tube and volumetric flow rate at the exhaust port were monitored. The permeability, K; of the composite material was calculated at a low pressure difference (138 kPa) using the following equation, which is applicable to laminar flow of compressible fluids through porous media [39] Q
¼ KS mt
ðPi 2 PoÞ ð1Þ where Pi is the internal pressure, Po; the surrounding pressure (atmospheric), t; the tube wall thickness, m; the viscosity of the gas, S; the surface area of the tube wall and Fig. 1. Schematic illustration showing how the prepreg is wrapped around a collapsible mandrel. M.G. Holmquist et al. / Composites: Part A 34 (2003) 163–170 165