METALLIC MATERIALS The     most     common     metals     used     in     aircraft construction    are    aluminum,    magnesium,    titanium, steel, and their alloys. Aluminum Aluminum    alloys    are    widely    used    in    modern aircraft   construction.   Aluminum   alloys   are   valuable because   they   have   a   high   strength-to-weight   ratio. Aluminum alloys are corrosion resistant and comparatively    easy    to    fabricate.    The    outstanding characteristic of aluminum is its lightweight. Magnesium Magnesium is the world's lightest structural metal. It is a silvery-white material that weighs two-thirds as much   as   aluminum.   Magnesium   is   used   to   make helicopters.  Magnesium's  low  resistance  to  corrosion has limited its use in conventional aircraft. Titanium Titanium    is    a    lightweight,    strong,    corrosion- resistant  metal.  Recent  developments  make  titanium ideal  for  applications  where  aluminum  alloys  are  too weak  and  stainless  steel  is  too  heavy.  Additionally, titanium is unaffected by long exposure to seawater and marine atmosphere. Alloys An alloy is composed of two or more metals. The metal present in the alloy in the largest amount is called the base metal. All other metals added to the base metal are   called alloying    elements.   Adding   the   alloying elements may result in a change in the properties of the base metal. For example, pure aluminum is relatively soft   and   weak.   However,   adding   small   amounts   or copper,    manganese,    and    magnesium    will    increase aluminum's  strength  many  times.  Heat  treatment  can increase or decrease an alloy's strength and hardness. Alloys   are   important   to   the   aircraft   industry.   They provide  materials  with  properties  that  pure  metals  do not possess. Steel Alloys Alloy steels used in aircraft construction have great strength, more so than other fields of engineering would require. These materials must withstand the forces that occur on today's modern aircraft. These steels contain small     percentages     of     carbon,     nickel,     chromium, vanadium,  and  molybdenum.  High-tensile  steels  will stand stress of 50 to 150 tons per square inch without failing. Such steels are made into tubes, rods, and wires. Another type of steel used extensively is stainless steel. Stainless steel resists corrosion and is particularly valuable for use in or near water. NONMETALLIC MATERIALS In   addition   to   metals,   various   types   of   plastic materials  are  found  in  aircraft  construction.  Some  of these   plastics   include   transparent   plastic,   reinforced plastic, composite, and carbon-fiber materials. Transparent Plastic Transparent plastic is used in canopies, windshields,   and   other   transparent   enclosures.   You need  to  handle  transparent  plastic  surfaces  carefully because  they  are  relatively  soft  and  scratch  easily.  At approximately 225°F, transparent plastic becomes soft and pliable. Reinforced Plastic Reinforced  plastic  is  used  in  the  construction  of radomes, wingtips, stabilizer tips, antenna covers, and flight controls. Reinforced plastic has a high strength-to-weight ratio and is resistant to mildew and rot. Because it is easy to fabricate, it is equally suitable for other parts of the aircraft. Reinforced plastic is a sandwich-type material (fig. 4-4).  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 4-4 HONEYCOMB CORE FACINGS (MULTIPLE LAYERS OF GLASS CLOTH) Anf0404 Figure 4-4.—Reinforced plastic.