and  repair  procedures  for  reinforced  plastic  are  covered in chapter 14 of this TRAMAN. COMPOSITE MATERIAL Composites  are  materials  consisting  of  a  com- bination of high-strength stiff fibers embedded in a common  matrix  (binder)  material;  for  example,  graphite fibers and epoxy resin. Composite structures are made of a number of fiber and epoxy resin laminates. These laminates can number from 2 to greater than 50, and are generally bonded to a substructure such as aluminum or nonmetallic  honeycomb.  The  much  stiffer  fibers  of graphite, boron, and Kevlar® epoxies have given com- posite  materials  structural  properties  superior  to  the metal  alloys  they  have  replaced. The use of composites is not new. Fiber glass, for example, has been used for some time in various aircraft components.  However,  the  term  advanced   composites applies to graphite, boron, and Kevlar®, which have fibers of superior strength and stiffness. The use of these advanced composite materials does represent a new application  for  naval  aircraft. Composite  materials  are  replacing  and  supple- menting metallic materials in various aircraft structural components.   The   first   materials   were   used   with laminated  fiber  glass  radomes  and  helicopter  rotor blades. In recent years, the replacement of metallic materials  with  more  advanced  composite  materials  has rapidly  accelerated.  This  has  become  particularly evident with the advent of the F/A-18, AV-8B, SH-60B, and CH-53E aircraft; and it is anticipated that composite materials  will  continue  to  comprise  much  of  the structure in future aircraft. As a result, there is a growing requirement  to  train  you  in  the  use  of  advanced composite   materials. There  are  numerous  combinations  of  composite materials being studied in laboratories and a number of types  currently  used  in  the  production  of  aircraft components. Examples of composite materials are as follows: graphite/epoxy, Kevlar®/epoxy, boron poly- amide,   graphite   polyamide,   boron-coated   boron aluminum,  coated  boron  titanium,  boron  graphite  epoxy hybrid,  and  boron/epoxy.  The  trend  is  toward  minimum use of boron/epoxy because of the cost when compared to current generation of graphite/epoxy composites. Composites  are  attractive  structural  materials because they provide a high strength/weight ratio and offer  design  flexibility.  In  contrast  to  traditional materials  of  construction,  the  properties  of  these materials  can  be  adjusted  to  more  efficiently  match  the Figure 1-32.—Sandwich construction. requirements  of  specific  applications.  However,  these materials  are  highly  susceptible  to  impact  damage,  and the  extent  of  the  damage  is  difficult  to  determine visually.  Nondestructive  inspection  (NDI)  is  required  to analyze  the  extent  of  damage  and  the  effectiveness  of repairs.  In  addition,  repair  differs  from  traditional metallic  repair  techniques.  A  more  detailed  explanation of  advanced  composites  and  their  inspection  and  repair procedures are covered in chapter 14 of this TRAMAN. SANDWICH  CONSTRUCTION From  the  standpoint  of  function,  sandwich  parts  in naval aircraft can be divided into two broad classes: (1) radomes and (2) structural. The first class, radomes, is a reinforced plastic sandwich construction designed primarily to permit accurate and dependable functioning of the radar equipment. This type of construction was discussed  in  the  preceding  section  under  “Reinforced Plastics.” The second class, referred to as structural sandwich, normally has either metal or reinforced plastic facings on cores of aluminum or balsa wood. This material is found in a variety of places such as wing surfaces, decks, bulkheads, stabilizer surfaces, ailerons, trim tabs, access doors, and bomb bay doors. Figure 1-32 shows one type of  sandwich  construction  using  a  honeycomb-like aluminum  alloy  core,  sandwiched  between  aluminum alloy  sheets,  called  “facings.”  The  facings  are  bonded  to the lightweight aluminum core with a suitable adhesive so as to develop a strength far greater than that of the components  themselves  when  used  alone. 1-44