mfgt104 materials and quality chap 14 tensile testing viscosity and melt index l.
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MFGT104 Materials and Quality Chap 14: Tensile Testing Viscosity and Melt Index. Professor Joe Greene CSU, CHICO. MFGT 104. Chap 14: Tensile Testing. Objectives Recognize and define common terms related to tensile testing.

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mfgt104 materials and quality chap 14 tensile testing viscosity and melt index
MFGT104Materials and Quality Chap 14: Tensile TestingViscosity and Melt Index

Professor Joe Greene


MFGT 104

chap 14 tensile testing
Chap 14: Tensile Testing
  • Objectives
    • Recognize and define common terms related to tensile testing.
    • Describe the terms stress and strain and the effects of both in tensile loading.
    • List the equipment necessary to conduct a tensile test.
    • Describe the operation of various equipment related to tensile testing.
    • Relate the general procedures used in conducting a tensile test.
    • Perform the necessary calculations related to tensile testing.
    • Recognize expected tensile test results.
    • Describe common variations in standard tensile test procedures including creep testing.
  • Tensile tests are conducted on standard tensile specimen “dog-bone” shape, but any rectangular cross section will work.
  • Tensile tests are used to determine
    • elastic limit and percent elongation
    • tensile strength and modulus of elasticity
    • proportional limit
    • yield point and yield strength
  • Tensile test are conducted with ASTM standards
    • metals (Section E8)
    • plastics (Section D 638
    • fibers (Section D 2343)
    • adhesives (Section D 897)
    • paper (Section D 987)
    • rubber (Section 412)
tensile testing principles
Tensile Testing Principles
  • Tensile loads are those that tend to pull a sample apart
  • Tensile loads produce deformations.
  • Deformation is a change in the form of a specimen that is produced by the applied load.
  • Tensile stress is load per unit area.

For rectangular bars

For circular specimens

tensile test examples
Tensile Test Examples
  • Example
  • What is the stress developed in a rectangular specimen that is 0.5in x 0.5in at 1000lb tensile load?
  • What is the stress developed in a round specimen with a 0.505 in. diameter at 1000lb tensile load?
elongation and strain
Elongation and Strain
  • When the tensile test begins and the load is increased the specimens grows longer as it is pulled.
  • Initial cross sectional area is used.
  • A two point gage punch is typically used.
  • A extensometer or strain gage can be used.
  • Elongation is the final length minus the original length.
  • Strain is the elongation divided by the initial length.
  • Example, Given the final length of a specimen as 1.005 in and the original length of 1.000 in, what is the strain?
poisson s ratio
Poisson’s Ratio
  • Poisson’s ratio is the ratio of lateral strain to axial strain
  • Deformation occurs in two directions during tensile testing
    • Lateral direction- at right angles to the test
    • Axial direction- at the ends of the tensile bar
    • Example

If the lateral strain is 0.005 and the axial strain is 0.010, what is the Poisson’s ratio?

  • Most engineering materials, the values for Poisson’s ratio (P.R.) range from 0.25 to 0.7. Approx. 0.3 for plastics
testing procedure
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
    • point where plastic

deformation occurs

    • Some materials do

not have a distinct yield point

so an offset method is used

Yield stress

1000 psi


Yield strength


0.002 in/in


tensile calculation examples
Tensile Calculation Examples
  • Suppose we want to use the length of a cable car to pull a cable car weighing 1200 lb plus 6 passengers at 175 lbs each. What would be the minimum diameter of the cable have to be if the cable’s yield strength was 32,000 psi?
test specimens
Test Specimens
  • Tensile test specimens
    • Standard shape is a tensile bar with narrow midsection.
      • Smaller midsection ensures proper necking at the center and not necking at the grips or off centered.
    • Ends can be flat or threaded
    • Alternative bar shapes
      • Straight bar
      • round
      • square
testing machines
Testing Machines
  • Testing machines are employed to apply measurable loads
    • portable hand-held devices with capacities 2,000 to 5,000 lb
    • stationary machines 300,000 lb or more
  • Tensile test is used to determine the tensile properties of a material, tensile strength, elongation, modulus
  • Procedure
    • measure specimen’s length, width, cross sectional area
    • place specimen is grippers
    • apply a steady load on the sample, ie. Pull sample at 0.05in per min
    • Note: very fast pulling rates can lead to different results.
    • measure the deformation or length of the specimen
    • continue test until fracture
expected results
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
creep testing
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.
creep results
Creep Results




Tertiary Creep



Secondary Creep



Primary Creep

Time (hours)

  • Creep versus time
physical testing
Physical Testing
  • Melt Flow Index
melt index
Melt Index
  • Melt index test measure the ease of flow for material
  • Procedure (Figure 3.6 from MFGT041 book)
    • 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
melt index and viscosity
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)


    • HDPE 190C 10kg
    • Nylon 235C 1.0kg
    • PS 200C 5.0Kg










Ln shear rate,

  • Kinematic viscosity, , is the ratio of viscosity and density
  • Viscosities of many liquids vary exponentially with temperature and are independent of pressure
      • where, T is absolute T, a and b
      • units are in centipoise, cP
  • Tensile test provide important data, which is often used to identify the physical and mechanical properties of materials.
  • Tensile test can be used to construct stress-strain curves.
  • Important information gathered from tensile testing are
    • tensile strength, yield, ultimate
    • tensile elongation, ultimate
    • tensile modulus or Young’s modulus
    • Poisson’s ratio
  • During tensile testing materials exhibit elastic and plastic deformations
  • Creep testing is a long term test which can be conducted at elevated temperatures.