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CMT
program
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| Overview
One specimen each of Aluminum and Steel are pulled in tension to failure.
Load and crosshead extension data are recorded automatically through load cells and
strainometers into the computer, to be converted to stress and strain and
plotted during analysis.
prelab assignment |
 Typical 8-in. long
by 0.35-in. diameter specimens Objectives:
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Demonstration of the stress - strain relationship of steel and aluminum
as industry standard civil engineering materials. Analysis includes
yield point, yield strength computed by the 0.2% offset method, proportional
limit, modulus of elasticity, ultimate strength, ductility as measured by %
reduction in area, and ductility as measured by permanent % elongation.
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Practice with highly sensitive digital equipment (strainometers)
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Practice manipulating large data sets recorded by computer.
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Practice calculating and plotting stress - strain data with Microsoft
Excel.
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Fig. 3-1 - Stress-strain diagram for
A36 steel
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Safety Info |
Safety!
- Safety glasses are required at all times while equipment is turned on. Not
only are safety glasses a good precaution, they prepare students for industry where they
are also required.
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Equipment
Used
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Procedure [excel data sheets]
right-click, save target as
At your table:
- Obtain a test specimen of 6061-T6 aluminum or A-36 structural
steel.
- Measure the diameter of the specimen at four points on the reduced diameter portion using a micrometer and compute the average
diameter.
- Lightly mark a 4"± gage length with a punch. Failure can occur at a punch mark
if you make it too heavy.
- Accurately measure and record the length between the punch marks using dial
calipers.
At the testing machine: Instron
4400R or Instron 4485
- Place one end of the specimen in the top grip of the Instron testing machine.
- The lab instructors will help you
place the Strainometer on the specimen. It should be facing toward the front of
the machine.
- Zero the strain (Strain Bal and <enter>)
- Apply the bottom grip to the specimen. This will result in some load and strain.
- Press the IEEE button to transfer control of the machine from the console to the
computer.
- Use a load application rate of 0.1 in/min.
- At approximately 10% strain, remove the extensometer.
- After failure, stop the test.
The excel data sheet has four columns of data:
Time
(milliseconds) |
Load
(kips) |
Crosshead
Displacements
(inches) |
Strain (%)
**NOT decimally correct |
- Measure and record the permanent elongation between punch marks, and measure the necked down diameter with
dial calipers.
** the strain numbers are in percent, but not decimally correct.
For example, 50% = 0.50. To be correct, you should divide the listed
strain numbers by 100 and then display them in percent format.
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Data
Development of Results Examples follow each part.
For both the Steel and Aluminum tests:
- Plot load versus crosshead extension for the entire range of data.
This plot will be used to determine the ultimate strength.
- Plot the stress-strain graph up to a point a just past yield (when the load reading
started to fluctuate). This plot will be used to compute the modulus of elasticity
and yield strength.
Determine the following for both the steel and aluminum specimens.
- The yield strength (0.2% offset method for aluminum).
- The modulus of elasticity.
- The ultimate strength.
- The ductility as measured by % reduction in area.
- The ductility as measured by permanent % elongation.
An example of the steel stress-strain curve
marked up with best fit E line and yield.
The videos posted in Excel 2007 for the
plastics lab show essentially all you
need to know to plot the steel and aluminum results.
Howtodoit in Excel 2003 videos in Macromedia Flash format. Download the
free player from
www.macromedia.com.
Spreadsheet calculations and load-deformation plot [play
video part 1 of 2] or 3.4Mb / 4.37 minutes
Stress-strain plot and magnified stress-strain plot for E and yield [play video part 2 of 2] or
2.7Mb / 6.38 minutes
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