Transcript MFGT104 Materials and Quality Chap 14: Tensile Testing Viscosity and Melt Index

MFGT104
Materials and Quality
Chap 14: Tensile Testing
Viscosity and Melt Index
Professor Joe Greene
CSU, CHICO
MFGT 104
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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.
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Introduction
• Tensile tests are conducted on standard tensile specimen “dogbone” shape, but any rectangular cross section will work.
• Tensile tests are used to determine
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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
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metals (Section E8)
plastics (Section D 638
fibers (Section D 2343)
adhesives (Section D 897)
paper (Section D 987)
rubber (Section 412)
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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.
F

A
load
stress 
cross  sect _ Area
stress ( psi) 
load (lb)
A(in 2 )
P
 
WD

For rectangular bars
P
d 4
2
For circular specimens
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Tensile Test Examples
• Example
• What is the stress developed in a rectangular specimen that
is 0.5in x 0.5in at 1000lb tensile load?
P
1000 lb
2


 4000 lb in  4000 psi
WD (0.5in)( 0.5in)
• What is the stress developed in a round specimen with a
0.505 in. diameter at 1000lb tensile load?

P
 d2 4

1000
 (0.505 ) 2 4
 4993 lb / in 2  4993 psi
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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?
  (l f  l0 ) l0  (1.005in 1.000in) / 1.000in  0.005(in / in)
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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?
P.R.  lateral _ strain / axial _ strain  0.005 / 0.001  0.5
• Most engineering materials, the values for Poisson’s ratio
(P.R.) range from 0.25 to 0.7. Approx. 0.3 for plastics
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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
Yield stress
1000 psi
Stress
– point where plastic
deformation occurs
– Some materials do
not have a distinct yield point
so an offset method is used
Yield strength

0.002 in/in
Strain

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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?
1200  6(175 )
area  force / stress 
 0.070 in 2
32,000 psi
diameter  4 (0.070 )  0.300 in
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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
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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
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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
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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 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
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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. 13
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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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 from MFGT041 book)
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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
– HDPE
– Nylon
– PS
Temp
190C
235C
200C
Mass
10kg
1.0kg
5.0Kg
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Viscosity
• 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
e
T=200
a b lnT
Ln

T=300
T=400
0.01
0.1
1
Ln shear rate,
10

100
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Summary
• 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
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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.
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