EGR 281 Winter 2007Tensile Testing Polymers

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EGR 281 Winter 2007 Tensile Testing Polymers

• For next week
• Read the handout on vacuum forming procedure
• Read the handout on ceramics to prep for group
  activity
• Quiz next week will cover the material on the
  ceramics handout - last quiz for the year
• Note Tonight there will be a two-question quiz
  on the material from these slides. This quiz is
  available for extra credit there is no need to
  take this quiz unless you want the extra credit

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EGR 281 Winter 2007 Tensile Testing Polymers

• As with metals, we would like to know the
  mechanical properties of polymers so that we can
  design components effectively
• How heavy must a polymer components
  cross-section be to carry a certain load?
• Will stress relaxation cause failure late in the
  life of the component?
• For this reason, we tensile test polymers in the
  same basic manner as metals
• Tensile tester with moving cross-head
• Increase tensile load until failure occurs and
  record load and extension
• The thermoplastic materials we will be testing
  tend to be fairly ductile
• Stress and strain follow the same equations as
  those used for metals
• s F/A (Stress force divided by area)
• e ?L/Lo (Strain change in length divided
  by original length)

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EGR 281 Winter 2007 Tensile Testing Polymers
brittle polymer
?FS of polymer ca. 10 that of metals
plastic
elastomer
elastic modulus less than metal
Adapted from Fig. 15.1, Callister 7e.
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EGR 281 Winter 2007 Tensile Testing Polymers
Tensile behavior for a crystalline polymer
s
necking starts
e
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EGR 281 Winter 2007 Tensile Testing Polymers
Tensile behavior for an amorphous polymer
s
e
No crystalline rotation or sliding is involved,
simply stretching and straightening the
long-chain molecules molecules may also slide
over each other
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EGR 281 Winter 2007 Tensile Testing Polymers

• Stress relaxation testing tells us whether a
  polymer can maintain a load over time
• Strain the part to a constant value (that is,
  halt the tensile test)
• Record how the load changes with time
• A variety of equations have been developed to
  predict how stress will behave in materials under
  stress relaxation conditions
• 4th order polynomial with time
• Exponential with time
• Logarithmic with time

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• The Zwick-Roell Tensile Tester
• Safety
• Wear safety glasses at all times (although
  thermoplastic polymers are unlikely to generate
  shrapnel)
• The Zwick has wedge grips like the Tinius Olsen,
  so watch out for pinch points
• No long hair, dangling jewelry, loose clothing,
  etc
• The upper crosshead moves to pull the samples be
  careful that your hands dont get caught by the
  moving crosshead
• The Zwick is top-heavy dont lean on the
  uprights as it might topple. Leaning on the
  Zwick is a bad idea anyway because this might
  cause misalignment, which could skew test results
• Note the position of the emergency stop button
  there is also a stop icon on the computer screen

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• The Zwick-Roell Tensile Tester Notes on
  Operation
• Refer to handout for detail on procedure
• Load cell is designed for high precision at low
  loads cannot handle more than about 1100 pounds
• Metal object at the upper grip shaped like a
  block 2
• Instrumented to measure and record force by the
  use of strain gages (essentially printed circuit
  resistors whose resistances change when they are
  deformed)
• THE LOAD CELL IS FRAGILE AND EXPENSIVE never
  drop it, never crush it with the grips!
• The grips are operated using the knob use it to
  lift the handle over the step
• There are machine stops along the uprights to
  protect the load cell
• There are also stops programmed into the
  software, again aimed at protecting the load cell

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• The Zwick-Roell Tensile Tester Notes on
  Operation (continued)
• Note that the LE button sends the crosshead
  back to its starting position
• The software calculates stress and strain and
  saves the info to a file, but U of M computers
  would need the Zwick software to open files, so
  we will have to print out graphs and data tables
  with a copy for each student
• Keep track of the order in which samples are run.
  The Zwick software is written for a quality
  control lab it assumes that each sample is part
  of a large group of the same thing
• For tensile testing, we will pull samples until
  they break
• Use the crosswire function to find the load and
  elongation at the yield point if these are not
  automatically recorded by the software
• For stress relaxation, we will pull samples until
  they have begun to yield, then halt the test and
  record the load every 10 seconds for 90 seconds.
  The software will not record this data (will
  treat this as an interrupted test and delete the
  data)

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• Directions for Generating Data
• To generate a stress-strain curve,
• Measure and record the width and thickness of the
  sample at the center of the gage section
• Load sample into the grips and hit the start icon
• Input the thickness and width values (the
  software will prompt you for this information
  before it starts the test)
• Hit Okay and the test will start. Let it run
  until the sample breaks.
• When the sample breaks, an icon of a broken
  tensile bar will appear at the upper left side of
  the screen along with the sample number. Click
  on this to highlight it.
• Right-click on the graph and choose Layout
  Next.
• Right-click again and choose Activate
  Crosswire.
• Now by using the arrow keys on the keyboard, you
  can find out the load and extension values for
  any point on the curve. Use this function to
  find the yield point if needed. You can return
  to the standard view by right-clicking and
  choosing Layout Back.
• Print out the graph and table when the test is
  complete. Note the material, width, and
  thickness on the graph.
• Remove the sample and hit the LE button to
  bring the grips back to the starting point

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• Directions
• For stress relaxation,
• Measure and load the sample as above
• Input the width and thickness and start the test
• Halt the test when the sample begins to yield
  (when it forms a neck or when its stress-strain
  curve shows a knee)
• Load in pounds will be displayed at the bottom of
  the screen.
• One student watches and records the load while
  the other student calls out the time at 10-second
  intervals
• Record load at 0, 10, 20, 90 seconds
• Stop the test, hit the LE button, and remove
  the sample

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• Scenario (note this scenario covers two weeks)
• We are a materials laboratory and a customer has
  approached us requesting a recommendation. He
  needs a plastic material for his new line of
  outdoor advertising signs. The requirements for
  the material are shown below.
• The material must be strong (high yield strength)
  to resist wind and hail damage
• Stress relaxation must be minimal because the
  design depends on a snap fit within a metal rim.
• The material must also have excellent vacuum
  forming characteristics in order to pick up the
  intricate shape that the customers art
  department has designed.
• April 2 Evaluate yield strength and stress
  relaxation behavior. For yield strength, highest
  is best. For stress relaxation, the material
  whose load stays highest is best. Write an
  interim report on yield stress and stress
  relaxation. This report will be due on April 9.
  If time permits, begin work on vacuum forming.
  Also two questions on tonights material for
  extra credit if desired.
• April 9 Work as a group to run vacuum forming
  trials evaluate the vacuum formability of the
  polymers. Rate the polymers based on all three
  parameters strength, stress relaxation, and
  vacuum formability and make a group
  recommendation on the best material to use.
• Individually, write a report covering the
  overall material recommendation this report will
  be due on April 16. Like your metallography
  reports, this report should refer to the results
  of your earlier report, but there is no need to
  re-type the entire report on YS and stress
  relaxation. If time permits, students may also
  make a sign to take home.
• NOTE -- Additionally on April 9, there will be a
  short quiz on the material in the ceramics
  handout this will prepare you for the group
  activity focused on ceramics.

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• Snap ring construction relies on
• the rim of the stressed part
• shape restitution in the stressed part
• to hold the part in place

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• Data Analysis
• Interim report on April 2
• Tabulate (make a table out of) the engineering
  yield strength and the engineering strain at
  break for the three materials.
• Include a copy of the graph showing the tensile
  tests to failure on the three materials.
• Tabulate the load-time data for the stress
  relaxation performance of all three materials.
• Normalize this data by converting it to
  percentages. That is, assume that each
  materials starting load is 100. Then convert
  the remaining loads to a percentage of this
  initial load.
• Graph the normalized data for all three materials
  on the same graph.
• Recommend the best material for yield strength
  and stress relaxation. For yield strength,
  highest is best. In the case of stress
  relaxation, the best material loses the least
  strength (that is, retains the highest stress).

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• Data Analysis
• Overall report on April 9
• After completing your vacuum forming trials, rate
  each material on its vacuum formability. A
  material with good vacuum formability
• Will pick up fine detail from the mold
• Will not form webs
• Will not tear
• Will not have any other unspecified defects.
• As a group, make up and fill out an overall
  ranking table similar to the one that you did for
  the casting lab. Consider the performance of the
  three polymers regarding yield strength, stress
  relaxation, and vacuum formability. Of these,
  the most important is vacuum formability,
  followed by stress relaxation . Yield strength
  is lowest priority because (within reason) we can
  always use a thicker cross section to reduce
  stress. Develop a group recommendation based on
  these factors, along with an explanation for why
  you rated the materials the way you did.
• Make a final, group vacuum formed sign using the
  large M based on your recommendation. As with
  the casting lab, the group grade will be based on
  the quality of this part as well as the rating
  chart and explanations.
• For your individual lab report, you do not
  necessarily need to follow the groups
  recommendation