2.2 PROPERTIES OF METALS

The internal reactions of a metal to external forces are known as mechanical properties. The mechanical properties are directly related to each other. A change in one property usually causes a change in one or more additional properties. For example, if the hardness of a metal is increased, the brittleness usually increases and the toughness usually decreases. Following is a brief explanation of the mechanical properties and how they relate to each other.

Tensile Strength. Tensile strength is the ability of a metal to resist being pulled apart by opposing forces acting in a straight line (Figure 2-1). It is expressed as the number of pounds of force required to pull apart a bar of the material 1 inch wide and 1 inch thick.


Figure 2-1. Tensile strength.

Shear Strength. Shear strength is the ability of a metal to resist being fractured by opposing forces not acting in a straight line (Figure 2-2). Shear strength can be controlled by varying the hardness of the metal.


Figure 2-2. Shear strength.

Compressive Strength. Compressive strength is the ability of a metal to withstand pressures acting on a given plane (Figure 2-3).


Figure 2-3. Compressive strength.

Elasticity. Elasticity is the ability of metal to return to its original size and shape after being stretched or pulled out of shape (Figure 2-4).


Figure 2-4. Elasticity.

Ductility. Ductility is the ability of a metal to be drawn or stretched permanently without rupture or fracture (Figure 2-5). Metals that lack ductility will crack or break before bending.


Figure 2-5. Ductility.

Malleability. Malleability is the ability of a metal to be hammered, rolled, or pressed into various shapes without rupture or fracture (Figure 2-6).


Figure 2-6. Malleability.

Toughness. Toughness is the ability of a metal to resist fracture plus the ability to resist failure after the damage has begun. A tough metal can withstand considerable stress, slowly or suddenly applied, and will deform before failure.

Hardness. Hardness is the ability of a metal to resist penetration and wear by another metal or material. It takes a combination of hardness and toughness to withstand heavy pounding. The hardness of a metal limits the ease with which it can be machined, since toughness decreases as hardness increases. The hardness of a metal can usually be controlled by heat treatment.

Machinability and Weldability. Machinability and weldability are the ease or difficulty with which a material can be machined or welded.

Corrosion Resistance. Corrosion resistance is the resistance to eating or wearing away by air, moisture, or other agents.

Heat and Electrical Conductivity. Heat and electrical conductivity is the ease with which a metal conducts or transfers heat or electricity.

Brittleness. Brittleness is the tendency of a material to fracture or break with little or no deformation, bending, or twisting. Brittleness is usually not a desirable mechanical property. Normally, the harder the metal, the more brittle it is.