Testing and Failure Analysis
Thermal Analysis (TA)
Professor Joe Greene
CSU, CHICO
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Topics
• Testing
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Electrical
Physical
Mechanical
Thermal
• Failure Analysis
– Stress
– Differential Scanning Calorimeter
– Glass Content
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Testing
• Electrical Testing
– Plastics are good insulators, handles for screw divers etc.
– Ability to withstand exposure to electrical current.
• Conditioning samples
– ASTM D-618: 73F (23C) and RH of 50% for > 40 hours
– Dry samples to get consistent results
• Dielectric Strength
– Amount of voltage required to arc through a specimen of
plastic (figure 10-1)
– Voltage starting at 0 Volts is applied to one side of
specimen and increased until it arcs through.
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Testing
• Dielectric Constant
– The electrical capacitance of a specific plastic cross
section as a ratio to that of a similar cross section of air.
• Volume Resistivity
– Ability of a plastic to resist an electric current through its
bulk. (Fig 10-3) Used for electrical insulators.
• Surface Resistivity
– Ability of a plastic to resist current across its surface.
(Fig 10-5)
• Arc Resistance
– Amount of time required for an electrical arc to carbonize
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the surface of a specimen. (Fig 10-5)
Physical Testing
• Shrinkage Rate (ASTM D-955)
– Test is used to measure the amount of shrinkage that
occurs in a plastic after it has been heated and injected
into a mold, then allowed to cool. (Figure 10-6)
– Dimensions are measured in two dimensions from marks
in tool or dimensions of mold hot and of part.
• Plastic parts can shrink much as 20 percent by
volume, when measured at the processing
temperature and the ambient temperature.
– Crystalline materials shrink more
– Amorphous materials shrink less
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Physical Testing
• Crystalline and semi-crystalline materials are particularly
prone to thermal shrinkage
– Molecules arrange themselves in a more orderly way, forming
crystallites when cooled below Tg.
– Greater difference in specific volume from phase change
• Amorphous materials tend to shrink less
– The microstructure of amorphous materials does not change with
the phase change.
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Ref: C-MOLD Design Guide
Physical Testing
• Density
– Test to determine the weight of a specific volume of a
particular plastic.
– Measures grams per cubic cm (g/cc)
– Plastics range from approximately 0.90 to 1.60 g/cc
– Water’s density is 1.0 g/cc
– Specific gravity is density of material divided by the
density of water at that temperature and pressure.
– Method,
• Based upon Archemedes principle that the buoyancy force is
equal to the weight of water displaced
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Physical Testing
• Density
– Method,
Mass _ air
Density 
Mass _ water
• Based upon Archemedes principle that the buoyancy force is
equal to the weight of fluid displaced
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Physical Testing
•
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Physical Testing
• Water Absorption (ASTM D-570)
– Test determines amount of moisture that is absorbed by a
plstic material over a 24-hour period. Can be extended to
1 week or even 1 month.
– Method
– Measure dimension every hour
for the first 12 hours.
– Measure after 1 day,
– Measure after 1 week, 1 month
% gain
• Measure a particular dimension on part, usually a scribed in
mark in the mold.
Moisture Content of Polymer
• Measure cold part.
0.5
0.4
• Place sample in water
0.3
Series1
0.2
0.1
0
0
500
1000
1500
Time, hours
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Physical Testing
• Melt Flow Index
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Melt Index
• Melt index test measure the ease
of flow for material
• Procedure (Figure 3.6)
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Heat cylinder to desired temperature (melt temp)
Add plastic pellets to cylinder and pack with rod
Add test weight or mass to end of rod (5kg)
Wait for plastic extrudate to flow at constant rate
Start stop watch (10 minute duration)
Record amount of resin flowing on pan during time
limit
– Repeat as necessary at different temperatures and
weights
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Melt Index and Viscosity
• Melt index is similar to viscosity
• Viscosity is a measure of the materials resistance to flow.
– Viscosity is measured at several temperatures and shear rates
– Melt index is measured at one temperature and one weight.
• High melt index = high flow = low viscosity
• Low melt index = slow flow = high viscosity
• Example, (flow in 10 minutes)
Polymer Temp Mass
– HDPE 190C
10kg
– Nylon 235C
1.0kg
– PS
200C
5.0Kg
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Mechanical Test Considerations
• Principle factors are in three main areas
– manner in which the load is applied
– condition of material specimen at time of test
– surrounding conditions (environment) during testing
• Tests classification- load application
– kind of stress induced. Single load or Multiple loads
– rate at which stress is developed: static versus dynamic
– number of cycles of load application: single versus fatigue
• Primary types of loading
shear
compression
tension
torsion
flexure
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Modulus
• Modulus of Elasticity (E) or Young’s Modulus is the ratio of
stress to corresponding strain (within specified limits).
– A measure of stiffness
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Stainless Steel
Aluminum
Copper
Molybdenum
Nickel
Titanium
Tungsten
Carbon fiber
Glass
Composites
Plastics
E= 28.5 million psi (196.5 GPa)
E= 10 million psi
E= 16 million psi
E= 50 million psi
E= 30 million psi
E= 15.5 million psi
E= 59 million psi
E= 40 million psi
E= 10.4 million psi
E= 1 to 3 million psi
E= 0.2 to 0.7 million psi
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Modulus Types
• Modulus: Slope of the stress-strain curve
– Initial Modulus: slope of the curve drawn at the origin.
– Tangent Modulus: slope of the curve drawn at the tangent
of the curve at some point.
– Secant Modulus: Ratio of stress to strain at any point on
curve in a stress-strain diagram. It is the slope of a line
from the origin to any point on a stress-strain curve.
Initial Modulus
Tangent Modulus
Stress

Secant Modulus
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Strain
Testing Procedure
• Tensile tests yield a tensile strain, yield strength, and
a yield stress
• Tensile modulus or Young’s modulus or modulus of
elasticity
– Slope of stress/strain
– Yield stress
1000 psi
– point where plasticStress
deformation occurs

– Some materials do
not have a distinct yield point
so an offset method is used
Yield stress
Yield strength
0.002 in/in
Strain

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Expected Results
• Stress is measured load / original cross-sectional
area.
• True stress is load / actual area.
• True stress is impractical to use since area is
changing.
• Engineering stress or stress is most common.
• Strain is elongation / original length.
• Modulus of elasticity is stress / strain in the linear
region
• Note: the nominal stress (engineering) stress equals
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true stress, except where large plastic deformation
Creep Testing
• Creep
– Measures the effects of long-term application of loads
that are below the elastic limit if the material being
tested.
– Creep is the plastic deformation resulting from the
application of a long-term load.
– Creep is affected by temperature
• Creep procedure
– Hold a specimen at a constant elevated temperature under
a fixed applied stress and observe the strain produced.
– Test that extend beyond 10% of the life expectancy 19of the
material in service are preferred.
Creep Results
• Creep versus time
Fixed
l0
lF
Tertiary Creep
Constant
Load
Creep
(in/in)
Secondary Creep
Primary Creep
Time (hours)
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Energy Capacity
• Energy Capacity: ability of a
material to absorb and store
energy. Energy is work.
• Energy = (force) x (distance)
• Energy capacity is the area
under the stress-strain curve.
Stress

Strain

• Hysteresis: energy that is lostStress

after repeated loadings. The
loading exceeds the elastic
Strain 
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limit.
Elastic strain
Inelastic strain
Impact Testing
• Two Basic Methods- notched or unnotched samples
– Izod (vertical beam)
– Charpy (horizontal beam)
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http://www.ccsi-inc.com/new/html-instruments.htm
Thermal Testing
• Five Thermal Properties
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Melting Point
heat deflection temperature
Vicant Softening Temperature
Flamability
Oxygen Index
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Thermal Testing
• Five Thermal Properties
– Melting Point, Tm, and Glass Transition, Tg (DSC)
• Measures the temperature difference, and energy necessary to
establish a “zero” temperature difference, between as sample
and a reference sample. Figures 10-16 a and b
– Heat Deflection Temperature (HDT)
• 3-Point bending test on a sample in a temperature environment.
• Temperature at which the sample deflects at specified amount.
– Vicant Softening Temperature
• Similar to the HDT test except the sample is not supported, but
placed flat at the base of the apparatus, which is placed in a
temperature environment.
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• Temp is raised until needle penetrates sample a given amount.
Thermal Testing
• Five Thermal Properties
– Flammability
• Measures the condition of the sample as it is exposed to an
ignition source. Dripping, smoking, or other condition is
recorded, as well as the speed and distance the flame travels.
– Limited Oxygen Index (LOI)
• Measure the minimum amount of ozygen that will support
flaming combustion of a plastic product.
• Specimen is ignited with a flame source, then the source is
removed. The oxygen level is adjusted upward or downward to
determine the minimum level that will sustain burning.
• The level is stated as the percentage of oxygen contained in the
airstream
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DSC and TGA
• DSC Measures
– Tg
– Tm
– Crystallinity
• Thermogravimetric Analysis (TGA) Measures
– Filler content, resin content, additives content
– Place small specimen in a chamber that is part of the
TGA apparatus (Figure 10-26a)
– The chamber weighs sample as it is slowly heated to
1000F and sample decomposes
– As the sample is heated the sample slowly burns and26 the