the base metal. All other metals added to the alloy are called  alloying  elements.  Alloying  elements,  in  either small or large amounts, may result in a marked change in the properties of the base metal. For example, pure aluminum  is  relatively  soft  and  weak.  When  small amounts of other elements such as copper, manganese, and  magnesium  are  added,  aluminum’s  strength  is increased many times. An increase or a decrease in an alloy’s strength and hardness may be achieved through heat  treatment  of  the  alloy.  Alloys  are  of  great importance  to  the  aircraft  industry.  Alloys  provide materials with properties not possessed by a pure metal alone. Alloy steels that are of much greater strength than those found in other fields of engineering have been developed. These steels contain small percentages of carbon,  nickel,  chromium,  vanadium,  and  molybdenum. High-tensile steels will stand stresses of 50 to 150 tons per  square  inch  without  failing.  Such  steels  are  made into tubes, rods, and wires. Another type of steel that is used extensively is stainless  steel.  This  alloy  resists  corrosion  and  is particularly valuable for use in or near salt water. COMMON NONMETALLIC MATERIALS In  addition  to  metals,  various  types  of  plastic materials are found in aircraft construction. Transparent plastic is found in canopies, windshields, and other transparent  enclosures.  Handle  transparent  plastic surfaces  with  care,  because  this  material  is  relatively soft  and  scratches  easily.  At  approximately  225°F, transparent  plastic  becomes  soft  and  very  pliable. Reinforced  plastic  is  made  for  use  in  the  con- struction  of  radomes,  wing  tips,  stabilizer  tips,  antenna covers, and flight controls. Reinforced plastic has a high strength-to-weight ratio and is resistant to mildew and rot. Its ease of fabrication make it equally suitable for other parts of the aircraft. Reinforced  plastic  is  a  sandwich-type  material.  See figure 1-22. It is made up of two outer facings and a center layer. The facings are made up of several layers of glass cloth, bonded together with a liquid resin. The core material (center layer) consists of a honeycomb structure  made  of  glass  cloth.  Reinforced  plastic  is fabricated into a variety of cell sizes. High-performance aircraft require an extra high strength-to-weight   ratio   material.   Fabrication   of composite  materials  satisfies  the  special  requirement. This construction method uses several layers of bonding Figure 1-22.—Reinforced plastic. materials  (graphite  epoxy  or  boron  epoxy).  These materials are mechanically fastened to conventional substructures.  Another  type  of  composite  construction consists  of  thin  graphite  epoxy  skins  bonded  to  an aluminum  honeycomb  core. METALLIC  MATERIALS Learning   Objective:   Identify   properties   of metallic  materials  used  in  aircraft  con- struction. Metallurgists  have  been  working  for  more  than  a half  century  improving  metals  for  aircraft  construction. Each metal has certain properties and characteristics that make it desirable for a particular application, but it may have other qualities that are undesirable. For example, some metals are hard, others comparatively soft; some are brittle, some lough; some can be formed and shaped without fracture; and some are so heavy that weight alone  makes  them  unsuitable  for  aircraft  use.  The metallurgist’s  objectives  are  to  improve  the  desirable qualities  and  tone  down  or  eliminate  the  undesirable ones. This is done by alloying (combining) metals and by  various  heat-treating  processes. You do not have to be a metallurgist to be a good AM,  but  you  should  possess  a  knowledge  and  under- standing of the uses, strengths, limitations, and other characteristics  of  aircraft  structural  metals.  Such knowledge  and  understanding  is  vital  to  properly construct  and  maintain  any  equipment,  especially airframes. In aircraft maintenance and repair, even a slight  deviation  from  design  specifications  or  the substitution of inferior materials may result in the loss of both lives and equipment. The use of unsuitable materials  can  readily  erase  the  finest  craftsmanship.  The selection of the specific material for a specific repair job 1-22