materials notes

1. PROPERTIES OF THE ENGINEERING MATERIAL
MECHANICAL PROPERTIES
 Mechanical Engineers – calculate the forces
subjected to materials.
 Material Scientists – show how materials
deform (elongate, compress, and twist) or break
as a function of applied load, time, temperature,
and other conditions.
 Standards – common procedures which are
published by the American Society for Testing
and Materials (ASTM).
Concepts of Stress and Strain
 Normalization to the area – the load is
calculated per unit area to compare specimens
of different sizes.
 Stress – force divided by area.
 Tension & Compression Tests – the relevant
area is perpendicular to the force.
 Shear or Torsion Tests – the area is
perpendicular to the axis of rotation.
σ = F/A0 (tensile or compressive stress)
τ = F/A0 (shear stress)
MPa = 106
Newtons/m2
A0 = initial area
 Deformation Elongation – change in dimensions
as a result of a tensile or compressive stress.
ε = ΔL/L (strain)
 True Stress – force divided by the actual area.
Stress-Strain Behavior
 Elastic Deformation – when the stress is
removed, the material returns to the dimension
it had before the load was applied.
 Elastic Deformation – Deformation is reversible,
non-permanent.
 Plastic Deformation – when the stress is
removed, the material does not return to its
previous dimension but there is a permanent,
irreversible deformation.
 Elastic – in tensile tests, the stress-strain
relationship is called Hooke’s Law:
σ = E ε
*where E (slope of the stress-strain curve) is
Young’s modulus or Modulus of elasticity.
τ = G γ
*where G is the shear modulus.
 Elastic moduli – measure the stiffness of the
material. Related to the second derivative of the
interatomic potential or first derivative of the
force vs. inter-nuclear distance.
 Elastic modulus – decreases with temperature. E
is large for ceramics (stronger ionic bond), small
for polymers (weak covalent bond). E depends
on direction for single crystals. For randomly
oriented policrystals, E is isotropic.
 Proportional Limit – the region in the strain
curve which obeys Hooke’s Law; the ratio of
stress with strain gives proportionality constant
known as Young’s Modulus.
 Elastic Limit – the point in the graph up to which
the material returns to its original position when
the load acting on it is completely removed.
 Yield Point or Yield Stress Point – the point at
which the material starts to deform plastically;
there is permanent deformation.
Two Yield Points – upper yield point and lower yield
point.
 Yield Point Stress – the stress corresponding to
the yield point.
 Ultimate Stress Point – the point corresponding
to the maximum stress that a material can
handle before failure.
 Fracture or Breaking Point – the point in the
stress strain curve at which the failure of the
material takes place.
 Yield Point – the strain deviates from being
proportional to stress; deform permanently
(plastically).
Hooke’s Law is not valid beyond the yield point.
 Yield Stress – measures the resistance to plastic
deformation.
Plastic deformation is caused by the motion of
dislocations.

2.  Tensile Strength – the maximum where the
stress-strain passes through when stress
continues in the plastic regime.
 Ductility – the property of a solid material which
indicates how easily a material gets deformed
under tensile stress; increases with the rise of
temperature.
 Strength – the property of a material which
opposes the deformation or breakdown of
material in presence of external forces or load.
 Toughness – the ability of a material to absorb
energy and gets plastically deformed without
fracturing.
 Hardness – the ability of a material to resist to
permanent shape change due to external stress.
Various measure of Hardness:
 Scratch Hardness – oppose the scratch to outer
surface layer due to external force
 Indentation Hardness – oppose the dent due to
punch of external hard and sharp object.
 Rebound Hardness – aka dynamic hardness;
height of the “bounce” of a diamond-tipped
hammer dropped from a fixed height on the
material.
 Hardenability – the ability of a material to attain
the hardness by heat treatment processing;
inversely proportional to the weld-ability of a
material.
 Brittleness – indicates how easily a material gets
fractured when subjected to a force or a load.
 Malleability – property of solid material which
indicates how easily a material gets deformed
under compressive stress.
 Creep and Slip – Creep – property of material
which indicates the tendency to move slowly and
deform permanently under the influence of
external mechanical stress; Slip – a plane with
high density of atoms.
 Resilience – the ability of a material to absorb
the energy when it is deformed elastically by
applying stress and release the energy when
stress is removed.
 Proof Resilience – the maximum energy that can
be absorbed without permanent deformation.
 Modulus of Resilience – the maximum energy
that can be absorbed per unit volume without
permanent deformation.
 Fatigue – the weakening of material caused by
the repeated loading of material.
DUCTILE
 Low and medium carbon steel
 High capacity to impact test
 Has high resistance to deformation
 Basically soft
 Fails by yielding or necking
 Has defined yield point
BRITTLE
 High carbon
 Low capacity to impact loads
 Low resistance to deformation
 Basically hard
 Fails by fracture
 Has no define yield point
AISI AND SAE DESIGNATIONS OF STEEL
X X X X  points of carbon
X X X X  approximate percentage of alloying element
X X X X  class of steel
Class of Steel
1. Carbon
2. Nickel
3. Chrome Nickel
4. Molybdenum
5. Chromium
6. Chrome Vanadium
7. Tungsten
8. Triple Alloy Steel
9. Silicomanganese
 Prefixes – indicate the method of producing
steel
A – basic open hearth alloy steel
B – acid Bessemer carbon steel
C – basic open hearth carbon steel
D – acid open hearth carbon steel
E – electric furnace
NE – national emergency steel
 Suffixes:
F – free machining steel
H – hardened

3. Chemical Properties
 pH – measure of the acidity and basicity of a
solution; pH less than 7 is acidic while pH
greater than 7 is basic or alkaline.
 Hygroscopy – the ability of a substance to
attract and hold water molecules from the
surrounding environment through either
absorption or adsorption.
 Hygroscopic substances: sugar, honey, glycerol,
ethanol, methanol, diesel fuel, sulfuric acid,
methamphetamine, many salts (including table
salt).
Engineering polymers (hygroscopic): nylon,
ABS, polycarbonate, cellulose, poly(methyl
methacrylate), polyethylene, polystyrene.
 Surface Tension – property of the surface a
liquid that allows it to resist an external force;
caused by the cohesion of like molecules.
 Specific Internal Surface Area – a material
property of solids which measures the total
surface area per unit of mass, solid or bulk
volume, or cross-sectional area.
 Reactivity – the rate at which a chemical
substance tends to undergo a chemical reaction
in time.
Metals which have naturally slow reaction kinetics,
even though their corrosion is thermodynamically
favorable:
 Zinc
 Magnesium
 Cadmium
Thermal Properties
 Thermal Conductivity (k) – the property of a
material reflecting its ability to conduct heat.
 Thermal Resistivity – reciprocal of thermal
conductivity.
 Thermal Diffusity – the thermal conductivity
divided by the volumetric heat capacity or
“thermal bulk”.
 Thermal Expansion – the tendency of matter to
change in volume in response to a change in
temperature.
 Seebeck Coefficient (thermopower) – measure
of the magnitude of an induced thermoelectric
voltage in response to a temperature difference
across that material.
 Seebeck Effect – the conversion of temperature
differences directly into electricity.
 Emissivity – a dimensionless quantity; the
relative ability of a material’s surface to emit
energy by radiation; depends on factors such as
temperature, emission angle and wavelength.
 True Black Body – has emissivity of 1
 Real Object – has emissivity less than 1
 Heat Capacity – the measurable physical
quantity that characterizes the amount of heat
required to change a body’s temperature by a
given amount.
 Molar Heat Capacity – heat capacity per mole of
a pure substance
 Specific Heat Capacity – “specific heat”; heat
capacity per unit mass of a material
 Heat of Vaporization – aka enthalpy of
vaporization or heat of vaporization; energy
required to transform a given quantity of a
substance into a gas at a given pressure (often
atmospheric pressure).
 Heat of Fusion – aka enthalpy of fusion, specific
melting heat or latent heat of fusion; change in
enthalpy resulting from the addition or removal
of heat from 1 mole of a substance to change its
state from a solid to a liquid (melting) or the
reverse processes of freezing.
 Pyrophoricity – ignite spontaneously in air;
water reactive and will ignite when in contact
with water or humid air. e.g. creation of sparks.
 Flammability – how easily something will burn
or ignite, causing fire or combustion.
 Autoignition Temperature – or kindling point of
a substance; the lowest temperature at which it
will spontaneously ignite in a normal
atmosphere without an external source of
ignition.
 Inversion Temperature – the critical
temperature below which a non-ideal gas (all
gases in reality) that is expanded at constant
enthalpy will experience a temperature
decrease, and above will experience a
temperature increase.
 Joule-Thomson Effect – temperature change
when a non-ideal gas is expanded at constant
enthalpy; exploited in the liquefaction of gases.

4.  Critical Point (critical state) – specifies the
conditions (temperature, pressure and
sometimes composition) at which a phase
boundary ceases to exist.
Critical Point of Water
 647 K (374 degrees Celsius or 705 degrees
Fahrenheit) and 22.064 MPa (3200 psia or 218
atm)
 Glass Transition Temperature – the reversible
transition in amorphous materials from a hard
and relatively brittle state into a molten or
rubber-like state.
 Eutectic Point – the minimum freezing point
attainable corresponding to the eutectic mixture
(which means lowest melting point); this is the
point where all the three phases of the solid-
liquid system namely, liquid melt of the two
metals and the solid phases of each of the
components respectively co-exist at equilibrium.
 Melting Point – the temperature at which the
vapor pressure of the solid and liquid are equal.
 Freezing Point or Crystallization Point – the
temperature of the reverse change from liquid to
solid.
 Boiling Point – the temperature at which the
vapor pressure of the liquid equals the
environmental pressure surrounding the liquid.
 Normal Boiling Point (atmospheric boiling point
or atmospheric pressure boiling point) – of a
liquid is the special case in which the vapor
pressure of the liquid equals the defined
atmospheric pressure at sea level, 1
atmosphere.
 Saturation Temperature – “boiling point”; the
temperature for a corresponding saturation
pressure at which a liquid boils into its vapor
phase.
 Triple Point – the temperature and pressure at
which three phases of a substance coexist in
thermodynamic equilibrium.
Triple Point of Water
 273.16 K (0.01 degrees Celsius), 611.73 Pa (ca.
6.1173 millibars, 0.0060373057 atm).
 Flash Point – the lowest temperature at which it
can vaporize to form an ignitable mixture in air;
requires an ignition source.
 Curie Point – the temperature at which a
ferromagnetic or a ferromagnetic material
becomes paramagnetic on heating; the effect is
reversible.
ELECTRICAL PROPERTIES
 Electrical Conductivity – a measure of a
material’s ability to conduct an electric current;
reciprocal of electrical resistivity.
 Permittivity – the measure of how much
resistance is encountered when forming an
electric field in a medium; relates to a material’s
ability to transmit or permit an electric field.
 Dielectrics – materials possessing high electrical
resistivities.
 Dielectric Constant – relative permittivity of a
material for a frequency of zero; ratio of the
amount of electrical energy stored in a material
by an applied voltage.
 Dielectric Strength – the maximum electric field
strength that a material can withstand
intrinsically without breaking down.
Factors Affecting Dielectric Strength
 Thickness of specimen (directly proportional)
 Operating temperature (inversely proportional)
 Frequency (inversely proportional)
 Humidity (inversely proportional)
 Piezoelectric Constant – the measure of charge
which accumulates in certain solid materials in
response to applied mechanical strain.
 Piezoelectricity – electricity resulting from
pressure
Applications of Piezoelectricity
 Electric Cigarette lighter
 (sensor application) Piezoelectric Microphones
& Piezoelectric pickups for Acoustic-electric
guitars
 Loudspeakers
 Inkjet Printers

5. MAGNETIC PROPERTIES
 Diamagnetism – a property of all materials and
opposes applied magnetic fields, but is very
weak.
 Paramagnetism – stronger than diamagnetism
and produces magnetization in the direction of
the applied field, and proportional to the applied
field.
 Ferromagnetic – its effects are very large,
producing magnetizations sometimes orders of
magnitude greater than the applied field and
larger than either diamagnetic or paramagnetic
effects.
 Optical Properties – a material’s response to
exposure to electromagnetic radiation and, in
particular, to visible light.
Optical Properties of Nonmetals
 Reflection – bouncing of light as it hits the
surface of a material
 Refraction – a change in direction and speed in a
ray of light as it passes through a material
 Luminescence – a phenomenon in which
materials are capable of absorbing energy and
then reemitting visible light.
MATERIAL TESTING
 Tensile Testing – aka tension testing; a
fundamental materials science test in which a
sample is subjected to a controlled tension until
failure; measures strength and ductility.
 Universal Testing Machine (UTM) – most
common testing machine used in tensile testing;
has two crossheads; one is adjusted for length of
specimen and the other is driven to apply
tension.
 Elongation measurement – used to calculate the
engineering strain.
 Compression Testing – a method for
determining the behavior of materials under a
compressive load.
 Impact Testing – measure an object’s ability to
resist high-rate loading; most commonly consists
of Charpy and IZOD Specimen Configurations.
Impact Test Specimen Types (Notch Configurations)
 V-Notch
 U-Notch
 Key-Hole Notch
 Un-notched
 ISO (DIN) V-Notch
(with capabilities of impact testing subsize specimens
down to ¼ size)
 Izod Impact Test – consists of a pendulum with a
determined weight at the end of its arm swinging
down and striking the specimen while it is held
securely in a vertical position; the notch is
positioned facing the striker.
 Georges Charpy – modified the Izod Impact Test
to hold the specimen in a horizontal rather than
a vertical position.
The IZOD Impact Test, like the Charpy Impact Test, is also
used to test materials at low temperature to try to
simulate conditions that may occur in the actual use of
the material.
 Charpy Impact Test – measures the energy
absorbed by a standard notched specimen while
breaking under an impact load; used as an
economical quality control method to determine
notch sensitivity and impact toughness.
 Hardness Test – measures a material’s strength
by determining resistance to penetration.
Hardness tests are performed more frequently bcos:
 Simple and inexpensive
 Nondestructive
 Other mechanical properties may be estimated
from hardness data, such as tensile strength
 Rockwell Hardness Test – measured according
to depth of indentation under a constant load;
most widely used hardness test in US & generally
accepted due: (1) its speed (2) freedom from
personal error (3) ability to distinguish small
hardness difference (4) small size of indentation
 The dial contains 100 divisions, each division
representing a penetration of 0.002mm.
 Rockwell Number – represents the difference in
depth from the zero reference position as a
result of the applied major load.
 The Brinell Hardness Test – consists of indenting
the test material with a 10mm diameter
hardened steel or carbide ball subjected to a

6. load of 3000kg; the best for achieving the bulk or
macro-hardness of a material, particularly those
with heterogeneous structures.
 Brinell Ball – makes the deepest and widest
indentation.
 Vickers Hardness Test – consists of indenting the
test material with a diamond indenter, in the
form of a right pyramid with a square base and
an angle of 136 degrees bet. opposite faces
subjected to a load of 1 to 100kgf.
 Microhardness Test – refers to static
indentations made with loads not exceeding
1kgf; the indenter is ether the Vickers diamond
pyramid or the Knoop elongated diamond
pyramid.
 Knop Indenter – a diamond ground to pyramidal
form that produces a diamond shaped
indentation having approximate ratio between
long and short diagonals of 7:1.; depth of
indentation = 1/30 of its length.
 Non Destructive Testing (NDT) – a wide group of
analysis techniques used in science and industry
to evaluate the properties of a material,
component or system without causing damage.
 Ultrasonic Testing (UT) – very short ultrasonic
pulse-waves that detect internal flaws or to
characterize materials (center frequencies
ranging from 0.1 to 15MHz)
Two Methods of receiving Ultrasound waveform:
 Reflection (pulse-echo)
 Attenuation (through-transmission)
 Magnetic Particle Inspection (MPI) – inducing a
magnetic field in a ferromagnetic material and
then dusting the surface with iron particles
which concentrate near imperfections,
previewing a visual indication of the flaw.
 Radiographic Testing (RT) – or industrial
radiography, is a nondestructive testing (NDT)
method of inspecting materials for hidden flaws
by using the ability of short wavelength
electromagnetic radiation (high energy photons)
to penetrate materials.
 Neutron radiographic testing (NRT) – a variant
of radiographic testing which uses neurons
instead of photons to penetrate materials.
 Dye Penetrant Inspection (DPI) or Liquid
Penetrant Inspection (LPI) or Penetrant Testing
(PT) – widely applied and low-cost inspection
method used to locate surface-breaking defects
in all non-porous materials (metals, plastics, or
ceramics).
Characteristics of a Penetrant
 Spread easily over the surface of the material
being inspected
 Be drawn into surface breaking defects by
capillary action
 Remain in the defect but remove easily from the
surface of the part
 Remain fluid so it can be drawn back to the
surface through drying and developing steps
 Be highly visible or fluoresce brightly
 Not be harmful to the material or the inspector
Types of Penetrant Materials
 Fluorescent Penetrants – contain a dye or
several dyes that fluoresce when exposed to UV
radiation.
- more sensitive than visible penetrants
because the eye is drawn to the glow of the
fluorescing indication.
 Visible penetrants – contain red dye that
provides high contrast to white developer
background.
- do not require a darkened area and an UV
light in order to make an inspection.
- less vulnerable to contamination from
things.
Methods of Penetrant Materials
 Water Washable – can be removed from the
part by rinsing with water alone.
 Post-Emulsifiable, Lipophilic – the penetrant is
oil soluble and interacts with the oil-based
emulsifier to make removal possible.
 Solvent Removable – require the use of a
solvent to remove the penetrant from the part.
 Post-Emulsifiable, Hydrophilic – uses an
emulsifier that is a water soluble detergent
which lifts the excess penetrant from the surface
of the part with a water wash.