LAB 3: Stress/Strain Analysis of unknown Metallic wire

| June 19, 2015

LAB 3: Stress/Strain Analysis of unknown Metallic wire

Purpose:
The purpose of this lab is to perform a stress-strain test on unknown metallic wire. Students are to plot both the engineering stress-strain curve and the true stress-strain curve for the material being tested. Several parameters such as the yield stress (Sy), the ultimate strength (Su), the fracture stress and strain, toughness, and Young’s 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.
Sample Preparation:
Measure the original length and the diameter of the sample to be tested using Vernier caliper scale. It is important to measure the section of the specimen that is receiving the load and not the length of the specimen used by the grips. Also, measure the average cross sectional area by taking measurements at several locations along the length of specimen subjected to the load.
Method Setup:
Step 1: Turn on the controller, and Zero position and load by pressing <1) Both>.
Step 2: Press <Control> from the Indicating screen.
Step 3: Press <1) Test Profile> and set the following: Press <1> to toggle between
Position and Load control. Press <ENT> to continue. Enter test rate (in/min or lbf/min). Press <ENT> to continue. Press <1> to toggle between Position, Load and time to set as END CHANNEL (Select position to end test at a certain displacement, Select load to end test at a certain force, or Select time to end test after a certain amount of time). Press <ENT> to continue. Enter End amount (this will define the maximum amount before it stops). Enter 0 to continue until break. Press <ENT> to continue.
Step 4: Press <2> to select the “Direction” for Compression or Tension.
Step 5: Set the jog rate, home rate and preload levels if desired by pressing <4>, <6> and <7> respectively. Press <ESC> when done.
Step 6: Press <UTILS> from the Indicating screen. Clear the results from memory.
Note: If the stored results are important, print before clearing. Once the number of stored results reaches 80, then the results need to be cleared.

Testing the specimen:
Step 1: Select appropriate settings for test to be completed.
Step 2: Insert the specimen in the machine and move the crosshead to the desired starting position using the JOG toggle switch.
Step 3: Place the Indicator Status in READY Mode and tare any residual load by pressing the <ZERO> key.
Step 4: Push the Start/Home toggle switch to START to begin loading the specimen. Once the load exceeds the THRESHOLD value, the Test Status will change to TESTING and the Sample break detector is armed. This can be observed by watching the Indicator Status change from “RT or RL” to “TT or TL”.
Step 5: Once the end of test condition is detected, the test is complete and Test Status will change to DONE (“D” will appear in the display). If the Auto Store is turned ON, the results will be saved to memory and/or printed automatically. IF Auto Store is turned OFF, press <STORE> to save the current test results. Note: If AUTOSTORE is not enabled and you do not press <STORE> before pressing <ZERO> the test results will be lost.
Step 6: Press <PRINT> from the Indicating screen.
Step 7: Ensure cables are connected as described in section 3 and you have also initiated upload from XY software.
Step 8: Press the <1> numeric key (XY Data ->Host) to send results to USB port.
Step 9: Measure the diameter of the sample after it has been tested.
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 – sampling time (sec), II column – Load (N) and                     III column – extension (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 it’s 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 ‘workspace’ with a little yellow matrix symbol next to it. If you can’t 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.
4)    For the true stress and strain, find the natural log of the instantaneous length over the original length for the strain, and divide the load by the final cross sectional area of the specimen to obtain the stress.
5)    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.
6)    From the stress vs. strain curves, find the following: yield stress, ultimate strength, stress and strain of breaking, young’s 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 quad 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 MATLAB code in appendix. The introduction should tell an overview of the lab and give some background on metals in biomedical engineering. 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 contain plots/tables and an explanation of what is in each. Include equations and explain how you calculated your results. The discussion section is where you compare your results to known results and predict what your material is. Specifically for the discussion please cover the following:

a)    What is the behavior of this material? And how does it compare to the behavior of polymers?
b)    What is the MYSTERY material? Justify your answer.
c)    Write about Elastic and Plastic deformation and identify these regions from the obtained graphs?

Note: Include at least 3 – 4 references (Articles/ Journals/ books) that support your introduction and discussion part.

Lab 4 Pre-lab Assignment
We will be using the Kelvin Model to model viscoelasticity in this lab. Find some other viscoelastic models other than the Kelvin Model and mention the components included in these models. Also, show the diagram corresponding to these models. Hypothesize why we use the Kelvin Model as opposed to another elastic/spring models. What kind of materials do we use the Kelvin Model for?

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