demands familiarity with the most common properties of  various  metals. PROPERTIES OF METALS This section is devoted primarily to the terms used in  describing  various  properties  and  characteristics  of metals  in  general.  Of  primary  concern  in  aircraft maintenance are such general properties of metals and their  alloys  as  hardness,  brittleness,  malleability, ductility,   elasticity,   toughness,   density,   fusibility, conductivity, and contraction and expansion. You must know the definition of the terms included here because they form the basis for further discussion of aircraft metals. Hardness Hardness refers to the ability of a metal to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by working the metal and, in the case of steel and certain titanium and aluminum alloys, by heat treatment and cold-working (discussed later). Structural parts are often formed from metals in their soft state and then heat treated to harden them  so  that  the  finished  shape  will  be  retained. Hardness  and  strength  are  closely  associated  properties of all metals. Brittleness Brittleness is the property of a metal that allows little bending or deformation without shattering. In other words, a brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected  to  shock  loads,  brittleness  is  not  a  very desirable property. Cast iron, cast aluminum, and very hard steel are brittle metals. Malleability A  metal  that  can  be  hammered,  rolled,  or  pressed into various shapes without cracking or breaking or other  detrimental  effects  is  said  to  be  malleable.  This property is necessary in sheet metal that is to be worked into curved shapes such as cowlings, fairings, and wing tips. Copper is one example of a malleable metal. Ductility Ductility is the property of a metal that permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are  greatly  preferred  for  aircraft  use  because  of  their ease  of  forming  and  resistance  to  failure  under  shock loads. For this reason, aluminum alloys are used for cowl  rings,  fuselage  and  wing  skin,  and  formed  or extruded parts, such as ribs, spars, and bulkheads. Chrome-molybdenum steel is also easily formed into desired  shapes.  Ductility  is  similar  to  malleability. Elasticity Elasticity is that property that enables a metal to return to its original shape when the force that causes the  change  of  shape  is  removed.  This  property  is extremely   valuable,   because   it   would   be   highly undesirable  to  have  a  part  permanently  distorted  after  an applied load was removed. Each metal has a point known as the elastic limit, beyond which it cannot be loaded without causing permanent distortion. When metal is loaded beyond its elastic limit and permanent distortion does result, it is referred to as strained. In aircraft construction, members and parts are so designed that the maximum loads to which they are subjected will never stress them beyond their elastic limit. NOTE: Stress is the internal resistance of any metal to distortion. Toughness A  material  that  possesses  toughness  will  withstand tearing or shearing and may be stretched or otherwise deformed  without  breaking.  Toughness  is  a  desirable property in aircraft metals. Density Density is the weight of a unit volume of a material. In aircraft work, the actual weight of a material per cubic inch  is  preferred,  since  this  figure  can  be  used  in determining   the   weight   of   a   part   before   actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part and still maintain the proper weight and balance of the  aircraft. Fusibility Fusibility is defined as the ability of a metal to become liquid by the application of heat. Metals are fused  in  welding.  Steels  fuse  at  approximately  2,500°F, and aluminum alloys at approximately 1, 110°F. 1-23