Guide to the Mechanical Properties of Metals
Table of Mechanical Properties of Metals
Mechanical properties of metals are shown in table 7-2.
Mechanical Properties of Metals - Table 7-2
Mechanical Properties of Metals. Values depend on Heat Treatment of Mechanical Condition or Mass of the Metal.
Stress and Strain
Metal stress and strain are one of the primary mechanical properties of metals. Another way to think about the concept is load/area. The deformation of metal can be measured directly, although stress cannot.
- Strain: deformation of the component/original length
- Stress: torsional, shear and direct.
Examples of Direct Stress on Metal
Metal Tensile Strength
Typical Universal Testing Machine
Tensile strength is defined as the maximum load in tension a material
will withstand before fracturing, or the ability of a material to resist
being pulled apart by opposing forces. Also known as ultimate strength,
it is the maximum strength developed in a metal in a tension test. (The
tension test is a method for determining the behavior of a metal under
an actual stretch loading. This test provides the elastic limit,
elongation, yield point, yield strength, tensile strength, and the
reduction in area.) The tensile strength is the value most commonly
given for the strength of a material and is given in pounds per square
inch (psi) (kiloPascals (kPa)). The tensile strength is the number of
pounds of force required to pull apart a bar of material 1.0 in. (25.4
mm) wide and 1.00 in. (25.4 mm) thick (see figure 7-1 below).
Metal Tensile Strength - Figure 7-1
Shear Stress and Strain
Shear strength is the ability of a material to resist being fractured by
opposing forces acting of a straight line but not in the same plane, or
the ability of a metal to resist being fractured by opposing forces not
acting in a straight line (see figure 7-2 below).
Metal Shear Strength Diagram - Figure 7-2
Shear Stress and Strain
shear stress, t = Shear Load / Area
shear strain, g = angle of deformation (radians)
shear modulus, G = t /g (elastic region)
Fatigue strength is the maximum load a material can withstand without
failure during a large number of reversals of load. For example, a
rotating shaft which supports a weight has tensile forces on the top
portion of the shaft and compressive forces on the bottom. As the shaft
is rotated, there is a repeated cyclic change in tensile and compressive
strength. Fatigue strength values are used in the design of aircraft
wings and other structures subject to rapidly fluctuating loads. Fatigue
strength is influenced by micro-structure, surface condition, corrosive
environment, and cold work.
Compressive strength is the maximum load in compression a material will
withstand before a predetermined amount of deformation, or the ability
of a material to withstand pressures acting in a given plane (figure 7-3).
The compressive strength of both cast iron and concrete are greater
than their tensile strength. For most materials, the reverse is true.
Compressive Strength of Metal - Figure 7-3)
Poisson's Ratio, v
Elasticity is the ability of metal to return to
its original size, shape, and dimensions after being deformed,
stretched, or pulled out of shape. The elastic limit is the point at
which permanent damage starts. The yield point is the point at which
definite damage occurs with little or no increase in load. The yield
strength is the number of pounds per square inch (kiloPascals) it takes
to produce damage or deformation to the yield point.
Measured using Poisson's ratio (the ratio of lateral to axial strain), which states that when a metal is strained in one direction, there are corresponding strains in all other directions.
Modulus of Elasticity
Modulus of Elasticity Stiffness, Stress and Strain Chart
The modulus of elasticity is the ratio of the internal stress to the strain produced.
The concept of elastic deformation refers to deformation that is not permanent. The load on the metal creates the deformation which returns to its original form and dimensions when the load is removed.
When it comes to the majority of metals, the region which is elastic is linear. Non-linear metals include cast iron. Hooke's law is applied to linear elastic behavior where E is the modulus of elasticity.
MEtal Ductility - AR% and ER%/h4>
The ductility of a metal is that property which
allows it to be stretched or otherwise changed in shape without
breaking, and to retain the changed shape after the load has been
removed. It is the ability of a material, such as copper, to be drawn or
stretched permanently without fracture. The ductility of a metal can be
determined by the tensile test by determining the percentage of
elongation. The lack of ductility is brittleness or the lack of showing
any permanent damage before the metal cracks or breaks (such as with
Brittleness is the property opposite of
plasticity or ductility. A brittle metal is one than cannot be visibly
deformed permanently, or one that lacks plasticity.
- Metals are brittle is the EL% < 5% (approximate)
- Metals are ductile if the EL% > 8% (approximate)
Brittle vs. Ductile Materials
Microstructural Origins of Metal Plasticity
Plasticity is the ability of a metal to be deformed extensively without rupture. Plasticity is similar to ductility. It measure the slide, climb and slip of atoms in the crystal structure. The climb and slip occur at dislocations while slide occurs at the grain boundaries.
Malleability is another form of plasticity,
and is the ability of a material to deform permanently under compression
without rupture. It is this property which allows the hammering and
rolling of metals into thin sheets. Gold, silver, tin, and lead are
examples of metals exhibiting high malleability. Gold has exceptional
malleability and can be rolled into sheets thin enough to transmit
Reduction of Area
This is a measure of ductility and is
obtained from the tensile test by measuring the original cross-sectional
area of a specimen to a cross-sectional area after failure.
Resilience and Toughness
Toughness is a combination of high strength
and medium ductility. It is the ability of a material or metal to resist
fracture, plus the ability to resist failure after the damage has
begun. A tough metal, such as cold chisel, is one that can withstand
considerable stress, slowly or suddenly applied, and which will deform
before failure. Toughness is the ability of a material to resist the
start of permanent distortion plus the ability to resist shock or absorb
Note that both the toughness and resilience equations are determined as:
(J/m3 or N.mm/mm3= MPa)
Toughness Chart, Ut
Resilience is the measure of the ability of a material to absorb energy without plastic or permanent deformation.
(J/m3 or N.mm/mm3= MPa)
Resilience Chart, Ur
Weldability and Machinability
The property of weldabiity and machinability is the difficulty or ease with which a material can be welded or machined.
Abrasion resistance is the resistance to wearing by friction.
Resistance of a metal to impacts is
evaluated in terms of impact strength. A metal may possess satisfactory
ductility under static loads, but may fail under dynamic loads or
impact. The impact strength of a metal is determined by measuring the
energy absorbed in the fracture.
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.
Table 7-3 below illustrates the hardness of various metals.
Hardness Conversion Table - Table 7-3
The concept of elastic deformation is determined by Hooke's Law. The notion is that elastic deformation is not permanent. If a load is removed, the part returns to its original dimensions and shape.
Metal Harness Tests
Brinell Harness Test
In this test, a
hardened steel ball is pressed slowly by a known force against the
surface of the metal to be tested. The diameter of the dent in the
surface is then measured, and the Brinell hardness number (bhn) is
determined by from standard tables (see table 7-3)
Rockwell Harness Test
This test is based upon the
difference between the depth to which a test point is driven into a
metal by a light load and the depth to which it is driven in by a heavy
load. The light load is first applied and then, without moving the
piece, the heavy load is applied. The hardness number is automatically
indicated on a dial. The letter designations on the Rockwell scale, such
as B and C, indicate the type of penetrator used and the amount of
heavy load (table 7-3). The same light load is always used.
Scleroscope Hardness Test
This test measures hardness by
letting a diamond-tipped hammer fall by its own weight from a fixed
height and rebound from the surface; the rebound is measured on a scale.
It is used on smooth surfaces where dents are not desired.