Название: Aircraft engineering (Морозова М. А.)
Жанр: Авиационные технологии и управление
Polyethylene (PE) fiber resists impact better than glass or carbon. PE/epoxy shows a low dielectric constant: 30\% less than that of aramid/epoxy, and half that of glass/epoxy. Like aramid (previously mentioned as Kevlar), PE is resistant to projectile penetration leading to use in ballistic protection for ar- mor, radomes, etc. PE fibers melt at relatively low temperatures up to 230°F (110°C) and they absorb very little moisture. Typical types of PE fibers are Spectra 900 and Spectra 1000.
• Spectra 900 — specific strength is 35 \% greater than glass or aramid, twice that of carbon — specific modulus is less than carbon, but 30\% greater than that of aramid and three times that of glass
• Spectra 1000 — It is 15\% stronger and 40\% stiffer than Spectra 900
• Polyethylene is difficult to «wet» and processing is in the development
CARBON (or Graphite)
Although the names «carbon» and «graphite» are used interchangeably when describing fibers, carbon fibers are 93-95\% carbon, and graphite fibers are more than 95\% carbon. Use of either graphite (preferred by U.S. users) or carbon (preferred by European users) is acceptable from an engineering stand- point. Graphite fibers are among the strongest and stiffest composite materials being combined with matrix systems for high-performance structures. The out- standing design properties of carbon/matrix composites are their high strength- to-weight and stiffness-to-weight ratios. With proper selection and placement of fibers, composites can be stronger and stiffer than equivalent steel parts at less than half the weight. Carbon fibers are classified into three categories; PAN- polyacrylonitrile, pitch, and rayon-based fibers.
(a) PAN-derived fibers have been available for many years. For several of the lower modulus varieties, large data bases have been developed through their use in aircraft and aerospace programs. PAN-based fibers offer the highest strength and best balance of mechanical properties in composites. Moduli up to
130 Msi (897 Gpa) are available commercially. They are available in standard, intermediate, and high modulus grades. These fibers are generally selected for their high strength and efficient retention of properties.
(b)The pitch-based fibers are newer and, while they are not as strong as the PAN fibers, the ease with which they can be processed to a higher modulus makes them attractive for stiffness-critical applications. Favorable cost projec- tions for volume production of pitch fibers because of lower raw material cost has not been realized.
(c) Carbon fibers based on a rayon precursor do not have the high mechan- ical properties available in PAN and pitch-based fibers, and recently have been used almost exclusively as reinforcements in C/C composites for rocket nozzle throats, aircraft brakes, nose cones and ablative applications.
Carbon composite laminates offer fatigue limits far in excess of aluminum, or steel, along with superior vibration damping. Further, the thermal expansion coefficients of carbon composite fibers become increasingly negative with in- creasing modulus. This allows the design of structures with virtually no thermal expansion or contraction across ranging thermal cycles. As with fiberglass, car-
bon fiber products are available as prepreg, molding-compound, and other stan
dardized product forms.
Carbon or graphite is generally available in three forms:
(a) HTS-High tensile strength fiber; (b) HM-High modulus fiber; (c) UHM-Ultra high modulus fiber.