Materials Science & Engineering for Mechanical Engineers
Lecture 3:Mechanical Properties of Materials
Types of Loading
1. Axial Loading
➢ Forces are directed along the axis of the member. Could be
Mechanical Property
➢ a measure of a material’s ability to carry or resist mechanical
forces or stresses
➢ is usually determined by subjecting prepared specimens to
standard laboratory tests
tensile (“pulling”) or compressive (“squeezing”).
2. Transverse Loading
➢ Forces are applied perpendicular to the axis of a member.
➢ relates to on how a material respond to applied loads (or forces)
Mechanical Property of Material
3. Torsional Loading
➢ Twisting action caused by a pair of externally applied equal and
oppositely directed force couples acting on parallel planes
1. Machinability
6. Creep
10. Hardness
2. Brittleness
7. Resilience
11. Fatigue
3. Ductility
8. Toughness
12. Strength
4. Elasticity
9. Malleability
13. Stiffness
5. Plasticity
Stress
➢ is defined as the resistance offered by a material to external
forces or loads.
2. Shearing Stress
➢ Member is subjected to a transverse load applied perpendicular
to the centroid of the cross section.
where:
F - Applied load
A - surface area where the
load is applied
Types of Stress
3. Bearing Stress
➢ When one surface is compressed into another, a bearing stress
results.
4. Bending Stress
➢ Is the normal stress that an object withstands when it is subjected
to external load at any cross-section.
➢ The bending stress is also defined as the ratio between the
Bending moment and the section modulus of the section.
1. Normal Stress
➢ Member is subjected to an axial load applied through the
centroid of the cross-section.
Types of Stress
Modulus of Elasticity in Tension, E
➢ Part of stress-strain diagram that is
straight, stress is proportional to strain and
E is constant of proportionality.
➢ Also called Young’s Modulus
Strength Properties
Strain
➢ is the deformation of a material
from stress.
➢ is simply a ratio of the change in
length to the original length.
➢ deformations that are applied
perpendicular
to
the
cross-section
are
normal
strains,
while
deformations
applied
parallel
to
the
cross-section are shear strains.
➢ is dimensionless.
Stress-Strain Diagram
Modulus of Elasticity in Shear, G
➢ This property indicates a material stiffness under loading.
➢ Resistance to shear deformation
𝑤ℎ𝑒𝑟𝑒:
𝐸 = 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 𝑜𝑓 𝑒𝑙𝑎𝑠𝑡𝑖𝑐𝑖𝑡𝑦 𝑖𝑛 𝑡𝑒𝑛𝑠𝑖𝑜𝑛
𝑣 = 𝑃𝑜𝑖𝑠𝑠𝑜𝑛′𝑠 𝑅𝑎𝑡𝑖𝑜
Poisson’s Ratio
➢ the ratio of the lateral strain (contraction) to the longitudinal strain
(extension) when the element is loaded with a longitudinal tensile
force.
Elastic Limit
➢ After being loaded, materials no longer return to its original
shape when load is removed.
Permanent Deformation – Stress beyond material is no longer elastic.
Yield Point (Yield Stress/Strength)
Yield Point
➢ is the stress value reached where additional strain occurs without
further in stress.
Yield Strength
➢ Portion of the stress-strain diagram where there is a large
increase in strain with little or no increase in stress. (elongated
plastically, permanently)
Ultimate Stress or Tensile Strength
➢ Highest stress on stress-strain curve.
➢ The peak of the stress-strain curve is considered the ultimate
tensile strength sometimes called ultimate strength.
Working Stress
➢ The stress that is capable of preventing failure is called working
stress or allowable stress.
➢ Maximum safe axial stress used in design.
Ultimate Strength
Ultimate Tensile Strength
➢ (often called Tensile Strength) is the maximum stress, in
tension, that may be sustained without fracture.
Breaking or Fracture Strength
➢ is the stress at which fracture occurs.
Rupture Stress
➢ Failure occurs.
➢ Also known as breaking strength.
1. Machinability
➢ refers to the relative ease with which a material can be cut.
2. Brittleness
➢ is one that lacks significant ductility. It is the tendency of a
material to fracture without appreciable deformation.
3. Ductility
➢ is a measure of the degree of plastic deformation that has
been sustained before ultimate fracture.
➢ For repeated loads, ductile materials is better than brittle
materials.
Order of Ductility
1. Cooper
2. Iron
3. Aluminum
4. Zinc
5. Tin
6. Lead
4. Elasticity
➢ is the ability of a material to strain under load and then return to
its original size when unloaded.
5. Plasticity
➢ is the ability of a material to strain under load and will not return
to its original dimension when the load is removed.
6. Creep
➢ is a slow process of plastic deformation that takes place when
a material is subjected to a constant condition of loading
below its normal yield point.
7. Resilience
➢ is the capacity of a material to absorbed energy when it is
elastically deformed and then upon unloading, to have this
energy recovered.
Modulus of Toughness
➢ is equal to the total area under the stress-strain curve to the point
of rupture.
9. Malleability
➢ is the ability of a material to be severely deformed plastically in
compression without fracture.
Order of Malleability
1. Copper
2. Aluminum
3. Tin
4. Lead
5. Iron
6. Zinc
10. Hardness
➢ is a measure of material’s resistance to permanent
deformation (indentation) under static or dynamic loading.
Modulus of Resilience
➢ is the amount of energy that a unit volume of material can
absorb while in the elastic range.
8. Toughness
➢ is the capacity of material to withstand a
shock load without breaking. It is the ability
of a material to absorb energy during plastic
deformation up to rupture.