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Testing and Failure Analysis Chapter 10

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Title: Testing and Failure Analysis Chapter 10


1
Testing and Failure Analysis Chapter 10
Professor Joe Greene CSU, CHICO
MFGT 144
2
Chapter 10 Topics
  • Testing
  • Electrical
  • Physical
  • Mechanical
  • Thermal
  • Failure Analysis
  • Stress
  • Differential Scanning Calorimeter
  • Glass Content

3
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 gt 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.

4
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 the surface of a specimen. (Fig 10-5)

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

6
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.

Ref C-MOLD Design Guide
7
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
  • Waters 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

8
Physical Testing
  • Density
  • Method,
  • Based upon Archemedes principle that the buoyancy
    force is equal to the weight of fluid displaced

9
Physical Testing

10
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 a particular dimension on part, usually a
    scribed in mark in the mold.
  • Measure cold part.
  • Place sample in water
  • Measure dimension every hour
  • for the first 12 hours.
  • Measure after 1 day,
  • Measure after 1 week, 1 month

11
Physical Testing
  • Melt Flow Index

12
Melt Index
  • Melt index test measure the ease of flow for
    material
  • Procedure (Figure 3.6)
  • 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

13
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

14
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

compression
tension
torsion
flexure
15
Modulus
  • Modulus of Elasticity (E) or Youngs Modulus is
    the ratio of stress to corresponding strain
    (within specified limits).
  • A measure of stiffness
  • Stainless Steel E 28.5 million psi (196.5 GPa)
  • Aluminum E 10 million psi
  • Copper E 16 million psi
  • Molybdenum E 50 million psi
  • Nickel E 30 million psi
  • Titanium E 15.5 million psi
  • Tungsten E 59 million psi
  • Carbon fiber E 40 million psi
  • Glass E 10.4 million psi
  • Composites E 1 to 3 million psi
  • Plastics E 0.2 to 0.7 million psi

16
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.

17
Testing Procedure
  • Tensile tests yield a tensile strain, yield
    strength, and a yield stress
  • Tensile modulus or Youngs modulus or modulus of
    elasticity
  • Slope of stress/strain
  • Yield stress
  • point where plastic
  • deformation occurs
  • Some materials do
  • not have a distinct yield point
  • so an offset method is used

Yield stress
1000 psi
Stress
Yield strength
Slopemodulus
0.002 in/in
Strain
18
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 true stress, except where large plastic
    deformation occurs.
  • Ductile materials can endure a large strain
    before rupture
  • Brittle materials endure a small strain before
    rupture
  • Toughness is the area under a stress strain curve

19
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 of the material in service are
    preferred.
  • Mark the sample in two locations for a length
    dimension.
  • Apply a load
  • Measure the marks over a time period and record
    deformation.

20
Creep Results
  • Creep versus time

21
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.
  • Hysteresis energy that is lost after repeated
    loadings. The loading exceeds the elastic limit.

Stress
Strain
Elastic strain
Inelastic strain
22
Impact Testing
  • Two Basic Methods- notched or unnotched samples
  • Izod (vertical beam)
  • Charpy (horizontal beam)

http//www.ccsi-inc.com/new/html-instruments.htm
23
Thermal Testing
  • Five Thermal Properties
  • Melting Point
  • heat deflection temperature
  • Vicant Softening Temperature
  • Flamability
  • Oxygen Index

24
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.
  • Temp is raised until needle penetrates sample a
    given amount.

25
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

26
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
    and the weight is reduced and meaured.
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