Tension Test Introduction: Mechanical testing plays an important role in evaluating fundamental properties of engineering materials, as well as in developing new materials and in controlling the quality of materials for use in design and construction. If a material is to be used as part of an engineering structure that will be subjected to a load, it is important to know that the material is strong enough and rigid enough to withstand the loads that it will experience in service. As a result, engineers have developed several experimental techniques for mechanical testing of engineering materials subjected to tension, compression, bending or to rsion loading. The tension test is the most commonly performed and is the simplest among of all the mechanical tests. The purpose of this test is to determine fundamental mechanical properties for use in design, including: the elastic modulus (E), the proportional limit (σ), the yield strength (σ,), the ultimate strength (σ,), the fracture strength (a), and the general stress-strain behavior. A specimen is subjected to a gradually increasing or quasi-static uniaxial load until failure occurs. This is accomplished by gripping opposite ends of the specimen and pulling it apart. The specimens used may have either a circular or a rectangular cross section, which is approximately uniform over a gage length (the length within which elongation measurements are made). The ends of tensile specimen should be suitable to fit properly the gripping device and they are usually enlarged to provide an extra area for gripping and to avoid the specimen break at the gripping location. A typical specimen (such as shown in Figure 1) is held rigidly between a fixed beam (the crosshead) and a moving beam (the actuator platform). A load cell (sensor) is used to measure the resultant force that builds up in the material as its length is increased by moving the actuator. The change in the gage length of the sample as pulling proceeds is measured from either the change in actuator position (stroke or overall change in length) or a sensor attached to the sample (called an extensometer). O Distance between shoulders Gage length Fillet Figure 1: A typical tension specimen with pin ends. In this experiment, we will test the quasi-static tensile mechanical properties of four metallic materials by utilizing a universal material tester Model IK-16 (shown in Fig.2). The IK-16 tester is capable of tension and compression testing within its capacity of 1,000 lbs. over the 2.5-inch stroke range. The machine is screw driven and it its movement is controlled with the 'Control Box'. Feedback can be monitored through a Lab VIEW vi on the desktop computer. The tensile tests will be run utilizing a ramp function under constant displacement (stroke) rate until failure. e Test.pdf and... Test Specimen Control Box Crosshead Figure 2: Tensile Testing Machine (Model 1K-16) Apparatus: IK-16 Tension testing machine Calipers Safety glasses 4 standard test specimens of the following materials: Aluminum Cold-Rolled Steel 0 D D Stainless Steel Ο Brass Lock Handles Grips Actuator Platform 13 Procedure: 2. 3. 4. 5. 6. Switch on the machine by pressing the switch located at the rear end of the machine. Press the <ENT> button twice to initialize the testing machine. Determine the initial cross-section area (A,) of the gauge length section of the specimen using the calipers. Take 5 measurements of the width and the thickness and average their values. Measure the gage length (Lo) of the specimen. See figure 1 for gauge length definition. The specimen is first fixed on to the lower grip, the crosshead is then lowered to appropriate height and then the specimen is fixed to the upper grip. Lock the crosshead. On the 1K-16 controller, select the following options: 6.1. <MNU/CAN> 6.2. the arrows to navigate to <7) Setup> 6.3. Press <ENT> 6.4. Use the arrows to navigate to <3) Actuator Rate> 6.5. Press <ENT> 6.6. Enter '0.050' in/min 6.7. Press <ENT> 6.8. Press <MNU/CAN> Switch on the machine by pressing the switch located at the rear end of the machine. 1. 2. Press the <ENT> button twice to initialize the testing machine. 3. Determine the initial cross-section area (A) of the gauge length section of the specimen using the calipers. Take 5 measurements of the width and the thickness and average their values. 4. Measure the gage length (L.) of the specimen. See figure I for gauge length definition. 5. 6. The specimen is first fixed on to the lower grip, the crosshead is then lowered to appropriate height and then the specimen is fixed to the upper grip. Lock the crosshead. On the 1K-16 controller, select the following options: 6.1. MNU/CAN> 6.2. the arrows to navigate to <7) Setup> 6.3. Press ENT 6.4. Use the arrows to navigate to <3) Actuator Rate> 6.5. Press <ENT> 6.6. Enter 0.050' in/min 6.7. Press <ENT> 6.8. Press MNU/CAN> 13 7. 8. 9. On the computer: 7.1. Create a folder to save your group's data into 7.2. Open the 1K-16.00.vi from the desktop 7.3. Click the 'RUN' arrow 7.4. Click the 'chart' tab 7.5. Click the Record to file switch 7.6. Select your group's data folder, and name the file as a CSV (i.e.: aluminum.csv) 7.7. Click OK On the IK-16 controller, select the following options: 8.1. Use the arrows to navigate to <2) Setpoint> 8.2. Press <ENT> 8.3. Enter: 0.5' in 8.4. Press <ENT> to begin the test Observe closely how the specimen necks under the tensile force exerted on it. As soon as the specimen breaks hit the "STOP" button on the LabVIEW.vi to stop the test data recording. 10. Unload the specimen from the 1K-16 tensile testing machine. 11. Measure the cross section of the fractured specimen at the neck (A) by joining the two pieces together (take an average of 5 measurements) and calculate the percent reduction in area: %RA = 100 (A - Ar) Ao 12. Change the <3) Actuator Rate> to 3 in/min. 13. Use the control box to drive the Actuator Platform back to Home Position ('0.00' in). 14. Open the .csv file in which the data is stored with excel. Delete all the negative load data along with the corresponding stroke data. 7. On the computer 8. 7.1. Create a folder to save your group's data into 7.2. Open the 1K-16.00.vi from the desktop 7.3. Click the 'RUN arrow 7.4. Click the 'chart' tab 7.5. Click the Record to file switch 7.6. Select your group's data folder, and name the file as a CSV (i.e.: aluminum.csv) 7.7. Click OK On the IK-16 controller, select the following options: 8.1. Use the arrows to navigate to <2) Setpoint> 8.2. Press <ENT> 8.3. Enter: 0.5' in 8.4. Press ENT to begin the test Observe closely how the specimen necks under the tensile force exerted on it. As soon as the specimen breaks hit the "STOP" button on the LabVIEW vi to stop the test data recording. 10. Unload the specimen from the 1K-16 tensile testing machine. 11. Measure the cross section of the fractured specimen at the neck (A) by joining the two pieces together (take an average of 5 measurements) and calculate the percent reduction in area: %RA 100 Ao 12. Change the <3) Actuator Rate> to 3 in/min. 13. Use the control box to drive the Actuator Platform back to Home Position ('0.00' in). 14. Open the .csv file in which the data is stored with excel. Delete all the negative load data along with the corresponding stroke data. 15. Plot the graph for load vs. stroke from the remaining data available. 16. Copy the data to your portable storage device. 17. Repeat steps 3-16 for the other three specimen. Analysis: For each material tested perform the following analysis. 1) Explain why the initial portion of load versus displacement curve is not linear. 2) (3) From the data stored in the file calculate engineering stress σ, and engineering strain E, (see the hint below). Then, construct two σ-ɛ graphs, one (2a) which covers the initial part of the stress-strain curve (up to 1% strain) and the second (26) showing the entire stress-strain curve (until fracture). Determine: a) the elastic modulus (E) b) the proportional limit (op) c) the yield strength (oo) d) the ultimate strength (ou) and associated strain (eu) e) the fracture strength (o) and associated strain (e) 4) Calculate energy capacity in terms of resilience (u) and tensile toughness (us). 5) Also calculate and plot true stress versus true strain curve. 6) Plot both engineering and true stress-strain curves on one graph. Discuss the differences between engineering and true stress-strain curves. 7) Discuss and explain the differences between two specimens in terms of ductility (%RA), strain hardening ratio (σu/Go), and tensile toughness (us). ***Hint **s It is reasonable to assume that all the grip parts and the specimen ends are nearly rigid. In this case, virtually all the displacement in actuator motion AL is due to elongation within the straight section ("the gage length", Lo) of the test specimen. (Note that actual elongation AL over the gage length is preferable.). Strain may therefore be calculated as a = AL/Lo. Also, as the test proceeds, the applied load P must increase to enforce specimen's elongation. This load P divided by the cross-section area Ao is used to calculate stress σ= P/Ao. Stress and strain as above, based on the initial (undeformed) dimensions, Ao and Lo, are called engineering stress and strain Engineering Strain: E= True Stress: Before necking: Engineering Stress: Resilience: AL Lo P A 2E True Strain: Tensile Toughness: = σ(1 + €) € = In(1+ €) After necking: Ao A A₁ At fracture: Ao P σ = σf Ar Af €₁ = In 10 A A u 2