Purpose:
The purpose of this lab is to perform a stress-strain test on two different plastics. Several parameters such as the yield stress (Sy), the ultimate strength (Su), the fracture stress and strain, toughness, and Youngís modulus (E) of these materials can be obtained from the stress-strain tests.
Procedure:
The main instrument used in performing the stress-strain test is the Test Resources Q series system. The Q series system is used to apply specific displacements or loads on the different samples and to measure the corresponding load or displacement. Both load and displacement values as well as the sampling points are acquired by XY software on the adjacent computer. These values are then processed and plotted in MATLAB/EXCEL. The stress versus strain curve is obtained to measure the mentioned parameters.
The stress versus strain curve will be obtained at different strain rates and used to determine the above mentioned parameters. The following is a step by step procedure to carrying out this experiment. Please note the following procedure is subject to change according to TA instructions.
Experimental Steps:
Measure and record the width, length, and thickness of the polymer samples.
Turn on machine and zero it.
Select ìData Acqî. Select ìLog Rateî (4). Set ìTest Lengthî(1) to 60 seconds. Press ESC twice.
Select ìControlî. Select ìTest Profileî(1).
-Set ìControl Channel SegAî to Position and press ENT
-Set ìSegAî to the deformation rate determined by TA and press ENT
-Set ìEnd CH SegAî to Position and press ENT
-Set ìEnd AmtAî to 0 and press ENT
-Select NO(2) for ìCreate SegBî
Set ìDirection(2) to Tension and press ESC.
Insert your polymer sample into the clamps.
Measure and record the length of the sample between the clamps.
Zero the load and position and run the test. The test will be finished when the sample completely breaks apart. Press ESC immediately once the wire breaks.
Measure and record the diameter of the wire at the break for final cross-sectional area.
Open XY Plot in the Desk Top.
Select ìPRINTî from the control pad, and select ìXY Data->Hostî (1) to send results to USB port.
Right click in the plot on the desktop, and select ìExportî. Save data as ìCommon Separated Valuesî to computer. Open file, and delete Columns C, D, and F. Zero the Position column by subtracting all positions values by the initial position. Position should be converted to meters by dividing by 1000.
Repeat the test for the second sample.
Data Processing:
Once the data for the various tests have been collected, you can retrieve it into MATLAB for processing as follows.
1. The data set in .CSV format (EXCEL file) can be imported into MATLAB. This data set is in the format: I column ñ sampling time (sec), II column ñ Load (N) and III column ñ position (mm).
2. You can import the comma separated variable file with your data directly into MATLAB.
To import the EXCEL file into MATLAB.
Save your files (as EXCEL). Open the file; delete everything above the columns, including the column titles.
Re-save and rename the data (name it what the specimen is such as : Wire)
Now open up MATLAB and under the File Menu, select Import data. Select the file and open it. (If the computer needs you to make a selection, make sure itís a comma separated variable on the top left and you should see the data in the window. Select the next button and make sure the file is selected in the next screen with a little check mark in the box next to the file).
Select finish. You should see the file in your ëworkspaceí with a little yellow matrix symbol next to it. If you canít see it, try typing in the file name exactly how you saved it (case sensitive, here) and you should see a multitude of numbers on your screen in your command window; should be three columns and the same data as you had saved.
3. For the engineering stress and strain, divide the displacement array by the original length to obtain the strain and divide the load by the cross sectional area of the specimen to obtain the stress.
s=Load/(Area_initial )
?=Position/(Length_initial )
4. Plot the engineering and true curves of stress vs. Strain for your specimen. A title and axes labels are expected for each plot. Include a hard copy of these curves in your report.
5. From the stress vs. strain curves, find the following: yield stress, ultimate strength, stress and strain of breaking, youngís modulus, and toughness. The yield strength can be calculated by 0.2% offset method. This method might not apply to all materials, especially highly elastic materials. The ultimate strength is the maximum stress reached by the specimen. The toughness of the material which is area under the stress-strain curve can be calculated by curve fitting the stress-strain curve and then integrating the equation using the trapz-command. These values and strain rate should be placed in a table and labeled appropriately with all units indicated.
Lab Report:
Your lab report should include the introduction and objective, procedure, results, discussion, and conclusion. Include MATLAB code in appendix. The introduction should tell an overview of the lab and give some background on the materials being tested. The procedure should be an abridged version of the lab manual procedure with notes on any deviations from the lab manual procedure. The results section should not only have all the plots and tables but explain what is in each. This implies all text and formulas necessary to make it clear what you are presenting. The discussion section is where you compare your results to known results and each material with the other materials. Specifically for the discussion please cover the following:
What is the behavior of the different materials as compared to each other?
Determine which sample is HDPE and which is PET.
How do your results compare to known results?
The conclusion should have a summary of what you have learned in this lab, and how it applies to your work as an engineer.
Note: Include at least 3 ñ 4 references (Articles/ Journals/ books) that support your introduction and discussion part.
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