M.G. Holmquist et al./ Composites: Part A 34(2003)163-170 Table 1 Summary of composite tubes Tube Thickness Fibre volume fraction, V Composite porosity, P Matrix porosity, Pm Pressure at failure Tangential stress at failure 4 32.8 1234 34.2 354 53.4 45.7 789 390 >81.3 ued with epoxy along the line formed by the edge of the outer ply. Q is given by contained three layers of cloth. All thickness measurements were made in the two-layer region of the tube Based on weight measurements the mullite-to-alumina (2) volume ratio decreased from 70/30 to 64.6/35.4 after three cycles of precursor infiltration and pyrolysis. Average fibre where @o is the volumetric flow rate of the gas measured on volume fraction in the final composite tubes was determined the atmospheric pressure side of the test set-up and P is the to be 37% and the average matrix porosity was 44%.As mean of internal and surrounding gas pressure. A viscosity shown in Fig. 2, the fibre bundles appeared to be well of 17.5 x 10-N s/m was assumed for nitrogen at room infiltrated by the matrix slurry. The matrix within the fibre temperature [37] bundles has a relatively uniform distribution of pores, Pressure was increased manually at approximately 25 I um in diameter. This observation is consistent with 50 kPa/s. The flow meter was restricted to a maximum flow reports from others made on similar materials [18, 21]. rate of 1.67 Is(100 I/min), but by using a by-pass valve Occasional large pores(-50X -100 um")were observed twice as high flow rates could be evaluated. The pressure at between the cloth plies. These pores are certainly related to the specimen inlet and the flow rate at the outlet were air bubbles trapped between the plies during the wrapping monitored as the pressure at the tank regulator procedure. Some cracklike faws, perpendicular to the fibres increased. Several pressure cycles were performed during and with regular spacing, were observed in matrix rich the test, in particular when a pressure drop occurred at the regions. These cracks were more than likely due to the pecimen inlet Such a pressure drop indicated an increase in constraint the fibres impose on the matrix shrinkage during flow rate in the line, the pressure at the tank outlet being drying and heating [6, 23]. Because the openings of these fixed. This pressure drop was used as an indication of a cracks are relatively large it was not possible to eliminate damage event in the composite tube and it was usually also them in the subsequent impregnation/pyrolysis steps. The accompanied by an audible sound. When this occurred, the transition from the section containing three layers of cloth to pressure was decreased and the tube was visually inspected two layers was generally smooth, although some specimens to determine type and location of failure. The pressure was had a more abrupt step-like transition then increased again to study the progression of damage Permeability of the as-processed tubes was measured to Nine tubes were tested in this manner be about 5.1 x 10-6m at a differential pressure of 138 kPa(20 psi), which is in reasonable agreement with 3. Result The tubes were composed of two layers of cloth and were produced by winding an initial single strip of prepreg cloth around the mandrel two times. A summary of the tubes evaluated is given in Table 1. The length of the tubes fabricated by the described method varied between 57 and 68 mm and their inner diameter averaged 12. 8 mm thickness of all the tubes was 0.49+ 0.02 mm. The sm 0.5mm section of the tube(approximately one-fifth of its circ ference or 8 mm) where ends of the cloth overlapped Fig. 2. Microstructure of as processed tube (cross-section)Q
is given by Q
¼ QoPo P
ð2Þ where Qo is the volumetric flow rate of the gas measured on the atmospheric pressure side of the test set-up and P
is the mean of internal and surrounding gas pressure. A viscosity of 17.5 £ 1026 N s/m2 was assumed for nitrogen at room temperature [37]. Pressure was increased manually at approximately 25– 50 kPa/s. The flow meter was restricted to a maximum flow rate of 1.67 l/s (100 l/min), but by using a by-pass valve twice as high flow rates could be evaluated. The pressure at the specimen inlet and the flow rate at the outlet were monitored as the pressure at the tank regulator was increased. Several pressure cycles were performed during the test, in particular when a pressure drop occurred at the specimen inlet. Such a pressure drop indicated an increase in flow rate in the line, the pressure at the tank outlet being fixed. This pressure drop was used as an indication of a damage event in the composite tube and it was usually also accompanied by an audible sound. When this occurred, the pressure was decreased and the tube was visually inspected to determine type and location of failure. The pressure was then increased again to study the progression of damage. Nine tubes were tested in this manner. 3. Results The tubes were composed of two layers of cloth and were produced by winding an initial single strip of prepreg cloth around the mandrel two times. A summary of the tubes evaluated is given in Table 1. The length of the tubes fabricated by the described method varied between 57 and 68 mm and their inner diameter averaged 12.8 mm. The thickness of all the tubes was 0.49 ^ 0.02 mm. The small section of the tube (approximately one-fifth of its circum￾ference or 8 mm) where ends of the cloth overlapped contained three layers of cloth. All thickness measurements were made in the two-layer region of the tube. Based on weight measurements, the mullite-to-alumina volume ratio decreased from 70/30 to 64.6/35.4 after three cycles of precursor infiltration and pyrolysis. Average fibre volume fraction in the final composite tubes was determined to be 37% and the average matrix porosity was 44%. As shown in Fig. 2, the fibre bundles appeared to be well infiltrated by the matrix slurry. The matrix within the fibre bundles has a relatively uniform distribution of pores, ,1 mm in diameter. This observation is consistent with reports from others made on similar materials [18,21]. Occasional large pores (,50 £ , 100 mm2 ) were observed between the cloth plies. These pores are certainly related to air bubbles trapped between the plies during the wrapping procedure. Some cracklike flaws, perpendicular to the fibres and with regular spacing, were observed in matrix rich regions. These cracks were more than likely due to the constraint the fibres impose on the matrix shrinkage during drying and heating [6,23]. Because the openings of these cracks are relatively large it was not possible to eliminate them in the subsequent impregnation/pyrolysis steps. The transition from the section containing three layers of cloth to two layers was generally smooth, although some specimens had a more abrupt step-like transition. Permeability of the as-processed tubes was measured to be about 5.1 £ 10216 m2 at a differential pressure of 138 kPa (20 psi), which is in reasonable agreement with Table 1 Summary of composite tubes Tube Thickness (mm) Fibre volume fraction, Vf (%) Composite porosity, pc (%) Matrix porosity, pm (%) Pressure at failure (MPa) Tangential stress at failure (MPa) 1 0.48 38.9 28.0 45.8 2.48 32.8 2 0.48 34.2 28.5 43.3 3.38 44.7 3 0.50 35.4 27.5 42.5 4.21 53.4 4 0.52 35.0 29.2 44.9 3.81 45.7 5 0.48 36.7 27.0 42.6 4.21 55.6 6 0.48 36.8 27.4 43.3 3.93 52.0 7 0.51 37.8 27.6 44.4 3.96 49.4 8 0.52 36.0 28.8 45.0 3.45 42.1 9 0.49 39.0 28.0 45.6 .6.27 .81.3 Tube 9 was glued with epoxy along the line formed by the edge of the outer ply. Fig. 2. Microstructure of as processed tube (cross-section). 166 M.G. Holmquist et al. / Composites: Part A 34 (2003) 163–170