1.Maintain a vacuum of 650 to 750 mmHg. 2.Apply a contact pressure of 0.5 MPa. 3.Simultaneously ramp the temperature as rapidly as possible to 390C. 4.Apply a consolidation pressure of 1.4 MPa. 5.Hold the pressure at a temperature of 390C for 5 min per 8 plies, but not for more than 30 min. 6.Cool the laminate rapidly to room temperature.The degree of crystallinity for crystalline polymers is influenced by cooling rate. Pressure can be released as the laminate temperature falls below the glass transition temperature of the matrix(=140C). 3.3 Determination of Volume Fractions of Fibers,Resin, and Voids As discussed in Chapter 2,the stiffness and strength properties of composites are strongly dependent on the fiber volume fraction,and this parameter thus constitutes an important quality measure of such materials.This section details measurement of fiber volume fraction for polymer matrix composites reinforced with glass,carbon,or aramid fibers. The fiber volume fraction of a composite may be determined by chemical matrix digestion,the burn-off technique,or by photomicrographic techni- ques.The matrix digestion method is standardized(ASTM D3171 [13])and consists of dissolving the (polymer)matrix in a hot digestion medium- concentrated nitric acid for epoxy matrix composites or sulfuric acid followed by hydrogen peroxide for polyimides and PEEK or other digestion media(see ASTM D 3171 [13]).Care must be taken to select a medium that attacks the matrix,but does not attack the fibers.After the matrix is dissolved, the fibers are weighed.The volume fractions are calculated from the weights and densities of the constituents.The resin burn-off method (ASTM D 2584 [14])is sometimes used for glass fiber composites because glass fibers (as opposed to carbon and Kevlar [E.I.du Pont de Nemours and Company] fibers)are resistant to oxidation at the temperatures required to burn off the matrix(500 to 600C).Similar to the chemical matrix digestion method,the fibers are weighed after the matrix has been removed to enable calculations of fiber volume fractions. The photomicrographic method is not an ASTM standard,but it provides an independent estimate of the fiber volume fraction.The method requires a photograph of a polished cross section of a composite and many samples to produce reliable results,because the area viewed is only about a hundredth of a square millimeter.On the other hand,it is possible to obtain an image of the distribution of fibers and to detect voids. ©2003 by CRC Press LLC1. Maintain a vacuum of 650 to 750 mmHg. 2. Apply a contact pressure of 0.5 MPa. 3. Simultaneously ramp the temperature as rapidly as possible to 390°C. 4. Apply a consolidation pressure of 1.4 MPa. 5. Hold the pressure at a temperature of 390°C for 5 min per 8 plies, but not for more than 30 min. 6. Cool the laminate rapidly to room temperature. The degree of crystallinity for crystalline polymers is influenced by cooling rate. Pressure can be released as the laminate temperature falls below the glass transition temperature of the matrix (≈140°C). 3.3 Determination of Volume Fractions of Fibers, Resin, and Voids As discussed in Chapter 2, the stiffness and strength properties of composites are strongly dependent on the fiber volume fraction, and this parameter thus constitutes an important quality measure of such materials. This section details measurement of fiber volume fraction for polymer matrix composites reinforced with glass, carbon, or aramid fibers. The fiber volume fraction of a composite may be determined by chemical matrix digestion, the burn-off technique, or by photomicrographic techni￾ques. The matrix digestion method is standardized (ASTM D 3171 [13]) and consists of dissolving the (polymer) matrix in a hot digestion medium — concentrated nitric acid for epoxy matrix composites or sulfuric acid followed by hydrogen peroxide for polyimides and PEEK or other digestion media (see ASTM D 3171 [13]). Care must be taken to select a medium that attacks the matrix, but does not attack the fibers. After the matrix is dissolved, the fibers are weighed. The volume fractions are calculated from the weights and densities of the constituents. The resin burn-off method (ASTM D 2584 [14]) is sometimes used for glass fiber composites because glass fibers (as opposed to carbon and Kevlar [E.I. du Pont de Nemours and Company] fibers) are resistant to oxidation at the temperatures required to burn off the matrix (500 to 600°C). Similar to the chemical matrix digestion method, the fibers are weighed after the matrix has been removed to enable calculations of fiber volume fractions. The photomicrographic method is not an ASTM standard, but it provides an independent estimate of the fiber volume fraction. The method requires a photograph of a polished cross section of a composite and many samples to produce reliable results, because the area viewed is only about a hundredth of a square millimeter. On the other hand, it is possible to obtain an image of the distribution of fibers and to detect voids. TX001_ch03_Frame Page 49 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC