Guide to the Mechanical Properties of Metals

https://weldguru.com/wp-content/uploads/2017/07/Do-You-Have-What-It-Takes-To-Become-A-Welder_2.png
Do You Have What It Takes To Become A Welder_(2)
https://weldguru.com/wp-content/uploads/2017/01/WeldGuru-slider02.png
WeldGuru-slider02
https://weldguru.com/wp-content/uploads/2017/01/WeldGuru-slider01.png
WeldGuru-slider01

Guide to the Mechanical Properties of Metals

Summary:

The mechanical properties of metals determine the range of usefulness of
the metal and establish the service that can be expected. Said another way it refers to how metals will respond to external loads.  This includes how they deform (twist, compress, elongate) or break as a function of applied temperature, time, load and other conditions.

Mechanical
properties are also used to help specify and identify the metals. They
are important in welding because the weld must provide the same
mechanical properties as the base metals being joined. The adequacy of a
weld depends on whether or not it provides properties equal to or
exceeding those of the metals being joined.

Mechanical
properties  are characterized by stress and strain (tension, compression, shear, torsion), elastic deformation and plastic deformation (yield strength, tensile strength, ductility, toughness, hardness).

  • Hardness: Resistance to abrasion and indentation.
  • Toughness and Resilience: Measure of how metal absorbs energy
  • Ductility: measure of the ability to deform plastically without fracture (fracture strain, area reduction, elongation)
  • Strength: Offset yield, proof, fracture, yield, ultimate – measured as stress
  • Stiffness: Young’s Modulus or Elastic Modulus
  • Loading: Tensile (pulling on each end of a metal bar is tensile strength), compressive, shear, torsion)
  • Stress and Strain: tension and compression, shear and torsion

The mechanical properties of metals are almost always given in MPa or Ksi. (1000 psi = 1 ksi = 6.89 MPa).

For detail on each mechanical properties of metals concept continue reading below or read the free eBook at the bottom of this page.

Modern Materials Testing System

Extensometer attached to metal specimen for testing mechanical properties of metal.

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 Strength

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

Diagram:
shear stress, t = Shear Load / Area
shear strain, g = angle of deformation (radians)
shear modulus, G = t /g (elastic region)

Fatigue Strength

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

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)

Elasticity

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.

Elastic Deformation

Hooke's Law

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.

Ductility

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
cast iron).

Elongation Equation

Area Reduction

Brittleness

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

Plasticity

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

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
light.

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
energy.

Note that both the toughness and resilience equations are determined as:

energy/unit volume

Toughness Equation:

 

(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.

Resilience Equation:

(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

Abrasion resistance is the resistance to wearing by friction.

Impact Resistance

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

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

Elastic Deformation

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.

NEXT: Types Of Welding Metals >>