Experiment #1: TENSILE TEST LABORATORY MAE 650:431 Mechanical Engineering Laboratory Department of Mechanical and Aerospace Engineering Rutgers: The State University of New Jersey SAFETY/PRECAUTIONS 1. Eye protection must be worn during the tensile test. 2. Do not begin the laboratory exercise until the Teaching Assistant demonstrates the use of the test equipment. 3. Before beginning a test or moving/resetting the Instron Testing Machine, all items including hands, hair, etc. should be clear of machine. 4. The extensometer should be handled with care. It is a fragile, and expensive (~$3,000) piece of equipment. 1. OBJECTIVES The purpose of this laboratory is to determine characteristic mechanical properties of three different metals, i.e. aluminum, brass and copper, by performing uniaxial tensile tests using given specimens. 2. BACKGROUND Material selection is a central task of the overall design process. Engineers must decide which materials are the most appropriate for a particular design. Physical properties such as thermal, magnetic, electrical, optical, acoustical, and mechanical properties such as tensile behavior, toughness, ductility, hardness, along with cost, availability and durability are the most common selection criteria used in design of mechanical systems. Mechanical properties are those characteristics revealed by the application of a force system. Often these properties depend strongly on temperature and loading rate. For metals, the mechanical properties can be further subdivided to elastic and inelastic or plastic properties. Elastic properties are in general less sensitive to temperature variations and loading rates. 2.1. Elastic Properties Most metallic systems (polycrystalline structures) used in mechanical design are linear isotropic solids, characterized by the Young’s modulus, E and the Poisson’s ratio n. The Young’s modulus is the ratio of the stress along the loading axis s axial to the corresponding axial strain, eaxial, recorded during a uniaxial test, i.e.

E=

s axial e axial

The Poisson’s ratio relates the lateral strain elateral to the axial strain eaxial during a uniaxial test, i.e.

n =-

e lateral e axial

Most metals exhibit a Poisson’s ratio around 0.3. The magnitude of these properties is of fundamental importance in the design of mechanical components since they relate the deformation of a material with the magnitue of the applied load. 2.2. Inelastic Properties The inelastic behavior of metals, which is dominated by the motion of linear defects such as dislocations and vacancies, plays a central role on assessing the safety conditions of a mechanical system. Yield

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stress, ultimate stress; fracture stress, elongation, toughness, and ductility are among the properties used to identify the inelastic behavior of metals. These properties can be greatly affected by thermal and mechanical treatments, which are usually utilized to confer the metal with optimal properties for specific applications. For example cold working during lamination of a metal sheet can significantly increase the yield stress and decrease the elongation to fracture. Annealing, on the other hand, can restore the material to the initial conditions. Alloying is also an effective and commonly used methodology to alter the postyielding behavior. 2.3. Uniaxial Tensile Test Many of the elastic and plastic properties of metals (and other material such as polymers) are obtained by performing a simple test: the uniaxial tensile test. In this test, a specimen (see Figure 1a for typical shape) is stretched at a predetermined rate until breakage. During the test, the applied tensile load versus 60000

50000

s

40000

30000

20000

10000

0

0

0.05

0.1

0.15

0.2

0.25

0.3

e (a)

(b)

Figure 1. (a) Typical shape of testing specimen for axial load. (b) Examples of engineering stress-strain curves for the materials to be tested elongation data are recorded allowing the determination of the material load stress and strain characteristics. Stress vs. strain curves for the metals to be tested metals are shown in Figure 1a. Additional information on the details of the test can be found in your textbooks on Strength of Materials (650:291) on Design of Mechanical Components (650:342). Material data sheets may be found on the web at http://www.matweb.com/.

3. LABORATORY EQUIPMENT The most relevant parts of the equipment available in the Materials Testing Laboratory is listed below: Instron Material Testing System (Instron 4411)

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Instron extensometer Personal Computer with GPIB Interface Calipers Digital Mass Scale or Mechanical Balance Series XI Instron Software The Instron Model 4411 MTS (Materials Testing System) machine is designed for testing a wide range of materials in tension and compression. A test specimen is secured between the machine rigid base and the moving crosshead, and a system motor drive moves the crosshead in an upward or downward direction to apply a tensile or compressive load to the specimen. The applied load is measured by a load transducer, which is mounted between the specimen and the crosshead. The 4411 system is also equipped with a oneinch length gauge extensometer, which can be attached to the specimen to accurately measure the initial small specimen elongation along the one-inch gauge length. The machine also automatically records large specimen elongations by monitoring the position of the moving crosshead. A General-Purpose Interface Bus (GPIB) interface allows automatic test control and data logging using a personal computer. 4. INSTRUCTIONS FOR USING THE INSTRON / PROCEDURE The steps described below should be followed closely. Words in square brackets (i.e. [Enter]) indicate physical buttons on the Instron computer keyboard, or radio buttons on the computer screen (activated by pressing the left mouse button with the cursor placed over the button). It is important to note that when entering information following a computer prompt the Enter button on the keyboard should not be pressed until all values have been changed on the screen. You can generally move from one prompt to the other using the mouse and left mouse button, or by pressing the Tab key on the keyboard. 4.1 Sample Preparation Two samples of three metal specimens will be distributed to each group for testing under uniaxial tensile conditions. Specimens will be labeled A1, A2, B1, B2, C1, and C2. 1. Using the calipers provided, measure and record the dimensions of each specimen. 2. Clean the body (long narrow width) of the specimen so that it is free of oil so that the extensometer does not slip when it is attached. 3. Using the precision scale, weigh and record the weight of each specimen. 4.2 Initializing the Equipment 1. Turn on the Instron 4411 using the switch located on the right side. Turn on the PC and printer if they are not already on. While the system is booting-up, note where the emergency cut-off switch (large round red button) is located on the Instron. 2. After the Instron 4411 has initialized hold one of the specimens next to the Instron grips to set the appropriate distance between them. Use the [Up] and [Down] JOG buttons to move the top grip until approximately 1 to 2 mm of the specimen head is visible from the top and bottom grips when the specimen is clamped. Do not clamp the specimen in the grips at this time. 3. Once the distance is set, press the [GL RESET] button. This sets the gage length, which will be used by the Instron machine. After pressing the button, the AT G.L. LED should light up indicating that the gage length has been set.

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4. Press the [IEEE 488] button (an LED will be illuminated beside the button). This will pass control from the Instron 4411 control panel to the computer through the GPIB interface. 4.3 Using the Instron Series IX program 1. On the PC double click the left mouse button on the [Series IX] icon on the Windows Desktop. 2. In the logon screen type in the following: User Name: mae431 Password: test 3. Select [OK] after the quantities are entered. 4. From the Home Screen select the [Test] button. 5. In the file dialog screen that appears automatically (Enter sample name), enter the name and folder to store the raw data used by the Instron machine. Under the Folders: menu, select the following folder which has been set-up for you c:\mae431\ your section # \ your group letter. Under the Filename: menu, enter a file name that will identify your data (an extension of .mrd will automatically be added to the name you set). Press [OK] after the information is entered. 6. On the next screen you will set the test method. Verify the following in the test method panel. Sample ID: your file name.mrd Operator Name: mae431 The indicator beside Tensile should be selected at the top of the screen Test Method Description: 31 Tensile Test for Materials Testing Lab (MAE 431) Choose Test Method: 31 Tensile Test for Materials Testing Lab (MAE 431) highlighted Live Displays: checked Auto Sample ID: unchecked Change any settings that are incorrect and press [OK] 7. In the file dialog screen that appears automatically (ASCII test data file name), enter the name and folder to store the ASCII data. Under the Folders: menu select the same folder as before c:\mae431\ your section # \ your group letter. Under the Filename: menu, enter a file name that will identify your data (an extension of .TXT will automatically be added to the name you set so you can use the same name as before which is the default value). Press [OK] after the information is entered. 8. Select the test specimen and place it in the center of the jaw grips. The top grip should be tightened first. Make sure the specimen is tightly secured before proceeding. For the first run, ask your TA to check for correct placement before proceeding. 9. Attach the extensometer to the specimen (see figure 2). To do this first set the gauge length by pressing the two round buttons on the extensometer making sure the cone is pressed against the cone seat. Holding the extensometer in this manner, attach it to the middle of the specimen with the wire clamps. Make sure the extensometer clamp is secure and does not slip from the specimen. Be very careful when using extensometer, dropping it can make it loose its calibration.

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Figure 2. Positioning of the extensometer on a sample. 10. Press the [Start Test] menu item at the top of the screen. 11. In the specimen information dialog box, which appears set the following values. Width: your width measurement of the specimen in inches Thickness: your thickness measurement of the specimen in inches Ext. gauge length: 1.000000 in Specimen: Name of the specimen Press the [OK] button after the specimen information is entered. 12. A message will appear on the screen “Test will now start. The crosshead will move.” Press [OK]. 13. The Instron machine will take out the residual force on the specimen by tightening the gripper and automatically zeroing the extensometer. A message will appear briefly on the screen “Unloading and balancing extensometer. Click to stop” Don’t press any buttons. 14. A plot will be shown on the screen as the tensile test progresses. 15. At a strain of 0.05 a message will appear on the screen “Remove the extensometer.” Immediately remove the extensometer from the specimen and press [OK]. Note that if you do not do this and the specimen breaks with the extensometer attached, the extensometer may be damaged. 16. After a short time the specimen will break. 17. A message appears after specimen fractures “Remove specimen. Click to return crosshead”. Be sure to first remove the specimen and then press [OK] (the gripper will immediately return to the Instron gage length). Do not press [OK] until after the specimen has been removed or it may damage the equipment when the gripper returns. 18. If you have additional specimens to measure, repeat steps 8 to 17 for the remaining specimens. 19. If you are done testing press [End Sample] on the top menu of options. 20. Press [Continue] on the top menu of options. 21. A report dialog box will appear. Verify the following settings. Report To: select Both which sends the information to the file and printer Copies To Print: enter the number of group of members Press the [Report File] button. In the file dialog screen that appears (Enter Report file Name) enter the name and folder to store the report information. Under the Folders: menu select the

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same folder as before c:\mae431\ your section # \ your group letter. Under the Filename: menu, enter a file name that will identify your data (an extension of .rep will automatically be added to the name you set so you can use the same name as before which is the default value). Press [OK] after the information is entered. Once these settings have been made Press [OK]. A file dialog box will automatically appear (Results output file name) to set the results output file name. Under the Folders: menu select the same folder as before c:\mae431\ your section # \ your group letter. Under the Filename: menu, enter a file name that will identify your data (an extension of .RLT will automatically be added to the name you set so you can use the same name as before which is the default value). Press [OK] after the information is entered. 22. The program will then prompt, “Do you want to Test another sample”. Select [No] to indicate you are finished testing. 23. Press the [Exit] button in the Home Screen and press [OK] to exit the Series IX program. 24. Turn off the Instron 4411. 25. From Windows Desktop double click on the [Windows Explorer] icon. Go to the directory where you have placed all of the data, and copy all the files to a floppy disk, which you place in the A: drive. Do not use your own disks; copy the files for the other group members in a PC lab (D116). This helps prevent viruses from spreading to the lab computers and other students. 4.4. Before leaving the laboratory Check the following: 1. Floppy disk contains all the data from the tests 2. Take the broken specimens 3. Take printouts

5. DATA PROCESSING There are two files needed for the data analysis portion of the laboratory. The file with the raw ASCII data in it is the one that you named with a .TXT extension. This file has two sections: -A header section, where all test conditions are recorded and a data section where the information from the test itself is organized in 3 columns. The first column corresponds to the point number, the second to the strain [in/in] (or displacement based on a one inch gauge length), and the third to the applied load [lbs]. Since this is an ASCII file, any text editor can easily edit it. Also, it can be read directly into a spreadsheet program or imported to a graphic program. The data from each specimen is recorded sequentially and separated by horizontal lines. -A second file with a .rep extension containing calculated results generated by the Instron program (ultimate stress, maximum strain, yield stress, etc.). This information is also in the .RLT file. It should be noted that the displacement and strain are based on a four-inch gage length so the values may be slightly different from those in the raw data file, which are based on a one-inch gage length from the extensometer. All data is in ASCII format and may be viewed in Excel or text editor (Notepad or WordPad). The file with the .mrd extension is binary and can only be used by the Instron program to replay the data.

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5. REPORT The report following items should at least include the following: 1. Definitions of the following terms, including a description of how they are calculated from the stress vs. strain data, and what information they provide about the material: Proportional limit 0.2% conventional yield stress Ultimate stress Fracture stress Elongation to fracture Young modulus Modulus of resilience Modulus of toughness 2. Engineering stress vs. strain curves plotted from the raw data for each material. There should be three graphs one for each material, with two curves in each graph, representing each specimen for that material. Each plot should show the: a. The proportional limit b. The ultimate stress c. The fracture stress 2. A linear fit of the linear stress-strain portion of the data for each material. This will enable: d. Finding Young’s modulus for each material. e. Identification of the 0.2% conventional yield stress. f. Using the data within the linear stress-strain portion estimation of the 90% confidence limits for the values of Young modulus found in 2d. 3. For each specimen the plots in (3) superimposed on those generated by the Instron testing system. 4. For each material a Table with the following quantities showing calculated using the raw data and also the corresponding values computed by the Instron testing software (printout): The proportional limit The yield point The ultimate stress The fracture stress The 0.2% yield stress The Young’s modulus 5. For each specimen a Table with the dimensions, the weight the volume and the density. 6. Calculation of the Modulus of Resilience and Modulus of Toughness for each material computed by numerically integrating the appropriate area under the stress-strain curve. The equations and methodology utilized to obtain the results should be shown, and the values obtained values should be tabulated for each specimen. A discussion of what information these quantities provide about the material should be included. Finding the Modulus of Toughness will require evaluating the area under stress-strain diagram using numerical integration. Numerical integration can be easily carried out using the data in an Excel spreadsheet. Example calculations and description of how the integral was evaluated should be included. 7. A discussion of: - The agreement between results computed from the raw data and those generated by the Instron - Any peculiar behavior in the stress-strain curves plotted in (2) or (3) above

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8. Based on the property values of each specimen describe some possible different uses of each of the materials in the design of mechanical components. 9. Sample calculations for all computed results.

For the write up use the Template posted on the course web site. Length of report should be between 3 (min) to 5 (max) pages. Sample calculations attached in appendix may be handwritten or typed and should not be included in the report number of pages.

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