Transcript Why Do We Look at Tests

Materials II Properties
and Mechanics
Module 1
Properties and Tests
Why Do We Look at Tests
 The importance of understanding the test is
at the heart of understanding the materials
and what they are capable of performing
 Data sheets describe the material properties
but do not describe the test methods.
 Tests must be regulated to ensure accuracy
in the testing of the materials.
ASTM Standard
ASTM Standard
Mechanical Properties
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Mechanical properties are the most important
properties looked at when considering a given
material because virtually all service conditions
and the majority of the end-use applications
involve some form of mechanical loading
These values are almost always listed on material
data sheets, it is important to remember that the
recoded values are typically at room temperature
and do not represent the different effect of
temperature and other environmental changes
Also important to remember that the product is
typically subjected to more than one type of
deformation at once
Mechanical Properties
 Describe how the material acts with
applications of force or load
 There are three different types of force
Tension
Compression
Shear
Mechanical Properties
 Comprised of three parts
 Stress, the force over the original cross
sectional area
 Strain, the deformation in a percentage of the
change in length compared to the original
length
 Modulus, is the toughness of the material and
is the ratio stress/strain
 When reviewing stress/strain curves the more
area under the curve the tougher the material
Mechanical Properties
 Stress-Strain Curve for a ductile plastic
Mechanical Properties
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Stress – force applied to a given area
to produce deformation
Strain – change in length per unit of the
original length
Elongation – the increase in length
produced by a tensile load
Yield point – the first point on the
stress-strain curve where an increase
in strain occurs with out an increase in
stress
Yield strength – the stress at the yield
point
Mechanical Properties
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Proportional limit – the greatest stress a
material is capable of without any deviation
from proportionality of stress to strain
Modulus of elasticity – the ratio of stress to
strain under the proportional limit of the
curve
Ultimate strength – the maximum unit of
stress a material will withstand when
subjected to an applied load in compression,
tension, or shear
Secant modulus – ratio of the total stress to
corresponding strain a specific point on the
curve
Mechanical Properties
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Stress-strain curves are used to classify
the general properties of a material
 Soft and weak
 Hard and brittle
 Soft and tough
 Hard and strong
 Hard and tough
Mechanical Properties
 Tensile Strength
 Single most important indication of strength in
a material
 The force necessary to pull apart a specimen
along with how much the material stretches
before breaking
Mechanical Properties
Tensometer and Specimen
Mechanical Properties
 Flexural Strength
 Stress-strain in flexure is also important to
designers of plastic parts
 It is the ability of a material to withstand bending
forces applied perpendicular to the part
 Stresses are a combination of compression and
tension
 The result of a flexural test is the maximum stress
and strain that occur on the outer surface that is in
tension
Mechanical Properties
 Flexural Strength
 Most polymers do not break therefore many times
the test is maximum stress when the strain for the
outer surface is 5%
 This test is good because it represents beams or
similar structures
Mechanical Properties
 Shear Strength
 Force needed to produce a fracture by a
shearing action, an example is scissors
 Force over the cross sectional area being
sheared. Expressed in force per area (psi) based
on the area of the sheared edge
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Test specimen and apparatus
 The specimen is a round or square washer
approximately two inches in diameter or square
with a 7/16 diameter thru hole in the center
Mechanical Properties
 Impact strength
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Indicates the amount of energy required to
break a given material
Impact strength is directly related to the ability
of a material to absorb and distribute energy.
Impact strength is directly related to the
chemical structure of the polymeric material.
Mechanical Properties
 Impact strength
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Two main types of impact testing
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The falling mass test consists of dropping a ball
shaped mass onto the test sample
Pendulum test consists of dropping a pendulum
into a test specimen
 Charpy
 Izod
 Notched Izod
Mechanical Properties
 Falling Mass Impact strength
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Impact must be on a flat surface
Indicates a good for direct indicator
Does not take into consideration the design or
built in stresses.
Does not take into account velocity increases.
Mechanical Properties
 Notched Izod Impact strength
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Utilizes a vertically placed specimen in a direct
path to the pendulum
A 90 degree is notched into the speciman
where the point of impact is going to occur
The 90 degree notch will induce failure,
important for design consideration because of
material notch sensitivity.
Mechanical Properties
 Fatigue and Flexing
 Fatigue life is defined as the number of cycles of
deformation required to bring a part to failure
under a given set of conditions
 Materials strength is greatly reduce by cyclic
loading
 Since many materials see this kind of force this
type of test is a popular one
 Failures occur from repeated applications of
stress in different directions
 Values are normally given in numbers of cycles
to failure at a given stress level
Mechanical Properties
 Hardness
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Resistance to deformation particularly
permanent, indentation or scratching
Is a relative term, no units but a scale “relative
hardness”
Two main tests
Rockwell for relative harder materials
Durometer for relatively softer materials
Mechanical Properties
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Rockwell hardness
 For relative harder materials such as nylons,
acetals, polycarbonates, and acrylics
 M scale very hard
 R scale hard
 C scale is used for metals
Mechanical Properties
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Durometer hardness
 For relatively softer materials
 Shore scale D for harder
 Shore scale A for soft rubbery types
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Based on the penetration of a specific indenter
under certain conditions
 Indenter is spring loaded and protrudes from a base
 Sharper indenter used for harder materials
 Larger flat on point used for softer materials
Mechanical Properties
 Abrasion resistance
 Abrasion is related to force, load, and area of
contact
 The hardness of material also has a big affect
 Abrasion resistance is the ability to withstand
mechanical action such as rubbing, scraping, or
erosion
 The test is complicated by the fact that as the
material is abraded friction will cause the
material to heat up which gives it different
characteristics
Mechanical Properties
 Abrasion resistance
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Abrasion resistance is typically measured
by a weight loss when a material is
abraded with a given abrader
Physical Properties
 Specific gravity or density
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Mass per unit volume lbs/in3 or kg/m3
Relative density, ratio of mass of a given
volume of material over the mass of an equal
volume of water with a density of 1
Physical Properties
 Tensile Creep
 The mechanical tests that we have noted to this
point measure the strength of plastic in a short
period of time
 Short time tests are irrelevant due to the fact
that most plastics are in continuous use over a
long period of time
 Creep measures the deformation of a material
over a period of time
Physical Properties
 Glass transition temperature
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The temperature when the material looses its
rigidity and becomes pliable, all materials have
a glass transition temperature, it is the window
in which they exist is vastly different.
(crystalline vs. amorphous)
 Melt temperature
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The temperature at which the material
becomes liquid loosing 90% of its’ viscosity.
Thermal Properties
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Thermal properties are how the temperature
affects different mechanical, electrical, optical, and
other properties
Different things effect how the temperature effects
the properties
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Molecular orientation
 Orientation decreases thermal stability
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Molecular weight
 Low temperature flexibility and brittleness
Thermal Properties
 Thermal conductivity
 The rate at which the material will transmit
heat
 Given by a k factor, aluminum has a k factor of
122, it transfers heat very well, some plastic
foams have a k factor of 0.01
 This is important in insulation materials
 Specific heat or heat capacity
 The amount of heat required to raise the
temperature of one unit of mass by one
degree Celsius
Thermal Properties
 Heat Deflection Temperature
 The highest continuous temperature that a
material can withstand without deforming
 Softening Point
 This test is done by placing a needle against a
sample of material, the temperature is
increased 50 C per hour and when the needle
penetrates the temperature is recorded
 Thermal Expansion
 A coefficient used to determine expansion in
length, area, or volume
Thermal Properties
 Mold Shrinkage
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The amount by which a molded part is
smaller than the cavity space where it was
produced
Typically given in in/in, mm/mm, or %
Thermal Properties
 Brittleness
Temperature
At low temperatures the material
approaches it’s glass transition
temperature it becomes hard and
brittle
 The temperature at which a material
exhibits a brittle failure in an impact
test.
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Environmental Properties
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Environmental refers to the area that the plastics
products are used in
The environment can have drastic effects on the
properties and appearance of different materials
The major environmental properties are
 Solar radiations
 Caused from different type of solar energy
 Ultraviolet radiation can cause fracture of the
molecular chains which promotes thermal oxidative
degradation
 This degradation results in embrittlement,
discoloration and loss of mechanical properties
Environmental Properties
UV stabilizers are used to combat these
effects
 Absorbers are both organic and
inorganic pigments that absorb the
harmful radiation and dissipate it, a
common one is carbon black
 Stabilizers inhibit the rupture of the
chains by chemical means, basically
dissipating the energy to lower less
damaging levels
Environmental Properties
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Microorganisms, bacteria, fungus, and mold
 Polymers by themselves are typically not
effected by microorganisms but the lower
molecular weight additives such as
plasticizers, lubricants and stabilizers are
 As these additives migrate to the surface of
the part they can come under attack
 Degradation can also show up as loss of
aesthetics, mechanical properties, and
increase of embrittlement
Environmental Properties
 Weathering
 Test sample are exposed to heat, sunlight,
and humidity
 Samples are rated on color change, gloss
level, and loss of physical properties
 Two main types environmental weathering and
accelerated weathering
Environmental Properties
 Ultraviolet resistance (ASTM D-2565, G-23)
 Going with weatherability it’s the resistance or
the effects of sunlight
 Stress cracking
 Stress cracking may be caused by solvents,
radiation, or strain
Environmental Properties
 Moisture content
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All plastic materials either collect moisture or
absorb it (hygroscopic) from the atmosphere
A moisture analyzer is a piece of equipment
used for this test
Materials require drying prior to processing
otherwise you will have poor properties and
difficulty processing
Optical Properties
 Specular gloss
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Directs light at different angles (20, 45, 60)
and compares the results to the reflection of a
mirror
 Luminous transmittance
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Measure the clarity of the plastic
Optical Properties
 Color
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Color perception requires three things
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Light source
Object
Observer
Color is created by the selective reflection
and absorption of specific light waves
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When light strikes an object the light waves that
are reflected is the color that we see
Example an object absorbs all colors accept
blue so we see the object as blue
Optical Properties
 Color
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Color defined by three terms
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Value
 Referred to as neutral colors, ranging from white to
black
 Also called lightness
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Hue
 The attribute of color perception
 Red – blue – green – yellow
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Chroma
 Also referred to as saturation
 How far the color is from the neutral axis
Optical Properties
 Value
Optical Properties
 Hue
Optical Properties
 Chroma
Optical Properties
 Color
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Note that color is affected by the light
source or the illuminant
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CIE has standard illuminates
 Daylight
 Noon light
 Florescent light
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Ways to measure color
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Tristimulus system or L, a, b,
 L is where it lies on the neutral axis, 100 = perfect
white and 1 = black
 a is green verses red
 b is blue verses yellow
Optical Properties
 Color
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Tristimulus system or L, a, b,
L*=100 white
+ b* = yellow
- a* = green
+ a* = red
- b* = blue
L*=0 black
Optical Properties
 Color
 Note that all different colors produce different
series of light waves
 Instrumented color measurement done with a
spectrophotometer
=
red
object
reflectance %
 Uses a specific light source
 Gives a spectral read out
wavelength
(nm.)
Optical Properties
 Color
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Also must do visual color evaluation due to
different light sources
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Color appears different depending on light
source
Referred to as Metamerism
Use a light booth for this evaluation
daylight
incandescent
Flammability
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Need to understand how polymers burn
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When exposed to a flame or when it starts to
burn the material decomposes or the molecular
chains start to come apart
This produces volatile polymer fragment or
short polymer chains on the surface of the part
These short chains are fuel which goes to the
flame front
At the flame front it mixes with the oxygen in
the atmosphere and produces more heat and
more fire
Flammability
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Flammability of a material is reduced by
breaking the cycle
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Additives to disrupt the flame generation
Additives to promote the retention of the fuel
Additive that act as a heat sink like hydrated
alumina
Flammability
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Materials flammability is based on the
following criteria
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Ease of ignition
Flame spread – spreads across a surface
Fire endurance - penetrates
Rate of heat release – how much and quickly
Ease of extinction
Smoke evolution
Toxic gas generation
Analytical
 Rheology
As stated earlier melt index measure at a
given temperature and a specific flow rate
 Rheology is the study of flow and viscosity
is the resistance to flow due to friction
between layers
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The more friction between layer the greater the
resistance to flow
The more force is needed to “move” the
material, this force is referred to as shear
Analytical
 Rheology
 Shearing occurs when the fluid is poured or
mixed
 The rate of speed that the layers move is called
the velocity gradient and is called the shear
rate
 Shear stress is the stress caused from the
layers moving
 When the shear rate increases at the same rate
as the shear stress the fluid is considered a
Newtonian fluid
 Water is Newtonian
 Plastic materials are non-Newtonian
Analytical
 Rheology
Rheology takes into account this shear
stress and measures flow of a material at
different shear rates (variations in injection
pressure)
 Different types of rheometers
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Torque rheometers
Rotational rheometers
Capillary rheometers
Electrical Properties
 Arc resistance
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Measure of time for the plastic material to arc
or to short
 Resistivity
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The resistance of two conductors with an
insulator of the given material between them
 Dissipation factor
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Measures the power lost in the in the plastic
insulator
Electrical Properties
 Dielectric strength
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The electrical voltage required to break down
or arc through a test sample of plastic
 Dielectric constant
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Measure the ability of the material to store
electricity
Data Sheets
 Contain the property values for the specific
materials along with other pertinent
information concerning the material
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Melt flow rate
Specific gravity
Melt temperature
Drying conditions
 Typically received from the material
supplier but can also be found on the
internet and other sources