ENME317_Lab_2_Tension_Test_Updated

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ME439

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Mechanical Engineering

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Nov 24, 2024

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S CHULICH S CHOOL OF E NGINEERING Department of Mechanical and Manufacturing Engineering ENME317 M ECHANICS OF D EFORMABLE S OLIDS I L ABORATORY #2: T ENSION T EST I NTRODUCTION AND O BJECTIVES : The term tension test is usually taken to refer to a test in which a prepared specimen is subjected to gradually increasing uniaxial (along one axis) load until failure occurs. In the test, a sample of material to be tested is placed between the grips of a load frame that pulls the sample apart. During the load application, the load, and the corresponding elongation of the sample in a direction parallel to the applied load are recorded. A tension test is the most fundamental test conducted in stress analysis since its results are critical in mechanical component design. That is, the load-elongation results can be analyzed to determine the mechanical properties, such as the strength and stiffness, of the material being tested. In fact, tension test procedures have been standardized so that the results obtained for different materials, from different laboratories can be compared. The American Society of Testing and Materials (ASTM) provides details of tension testing procedures in their standard ASTM E8. The objectives of this lab are to gain experience with conducting tension tests and analyzing the results to determine the mechanical behavior of three different materials. B ACKGROUND : T HE S TRESS -S TRAIN C URVE AND THE M ECHANICAL P ROPERTIES OF M ATERIALS : The tension test will give us a force versus displacement (elongation) curve for the specimen tested. The average normal true stress in an axially loaded member (ie. The lab sample) as the load P increases can determined from: , (1) where A is the cross-sectional area of the member at load level P. Although the area A changes with the test, if the deformations within the sample are small, engineering stress is often used. Engineering stress references the initial cross-sectional area, A o and it is defined by: . (2) P A s = 0 P A s =

Engineering stress therefore changes only with the applied load. At any given load P, the elongation of the sample from its initial length, D l can be calculated using: , (3) where l o is the initial gauge length and l is the gauge length at load level P. At any given load level P, the average normal engineering strain, e in the loading direction can be found by dividing original length of the sample, or: . (4) The plot of the stress vs. strain, as defined by Eqns. (2) an (4) is called the engineering stress- strain curve of the material being tested. Different materials exhibit different stress-strain curves. Ductile materials are those materials that undergo significant permanent deformations before failure. A typical stress strain curve for a ductile material is shown below. Figure 1: Engineering Stress-Strain Curves for a ductile material (adapted from Hibbeler) There are four main regions of the engineering stress strain curve. These regions are labelled in Figure 1, and described below. o D = - l l l o D e = l l

Region 1: Elastic Region: • Sample returns to its original shape after load is removed. • Linearly Elastic Behavior (material is linear in the elastic region) • Hooke’s Law: relation between stress and strain in the elastic region: • E: “Young’s Modulus” / “Elastic Modulus”/ “Modulus of Elasticity” Ø Slope of s - e curve in the elastic region Ø It is a material constant Ø Same units as stress (Pa or psi) Region 2: Yielding: • Elastic region continued until the yield stress, s y , is reached. • Plateau in plot represents a region where the material slips, ie with no increase in load, the strain increases. • If load is removed after the yield stress is reached, the sample will NOT return to its original configuration. The deformation is called plastic deformation. Region 3: Strain Hardening: • Slippage stops and further load can be applied (curve rises but becomes flatter) • Unloading along linear line parallel to the elastic curve. • Reloading – yields at a value greater than original yield stress • Cross-sectional area decreases uniformly. • Strain hardening continues until the ultimate tensile strength, s U is reached. Region 4: Necking: • Cross-sectional area decreases in a localized region • Smaller area can only carry a decreasing load (downward turn) • s B = stress level when fracture (Breaking) occurs in the necked region. Some materials, such as aluminum, do not exhibit a well-defined yield limit. In such cases, the 0.2% offset method, as depicted in Figure 2 below is used to define the yield strength. E s e =

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Figure 2: 0.2% Offset Method for Determining Yield Strength. Brittle materials such as cast iron, glass, or stone do not exhibit significant plastic deformation prior to fracturing. A typical stress-strain curve for a brittle material is shown in Figure 3 below. Figure 3: Typical Stress-Strain Curve of a Brittle Material The percentage that a sample elongates prior to fracture is a measurement of the ductility of the sample. It can be calculated as: As a reference, a ductile structural steel has a percentage elongation of around 20%. B o o % Elongation x100% é ù - = ê ú ë û l l l

E QUIPMENT : Load Frame: In this lab, tensile tests will be preformed on a Tinius Olsen Model 25 ST universal material testing machine that is shown below in Figure 4. This electromechanical device has a maximum load capacity of 25kN and can be operated manually on the attached panel, or remotely using the Horizon software suite. The associated software, Horizon, will be used to set the test parameters and collect data. To apply the load, the crosshead pulls up on the sample that is gripped to the stationary frame base. The load is recorded along with the displacement of the crosshead. It should be noted that although the crosshead displacement is used as a measure of sample elongation, it is not an exact representation since the stiffness of the machine will affect the results. An attempt has been made to isolate the samples stiffness from the machines in the Horizon software, but associated errors in the elastic modulus may still be present. Figure 4: Apparatus Diagram

Samples: Three different materials, and therefore three different samples will be tested. The three materials to be tested are designated by the American Iron and Steel Institute (AISI) as: AISI 1018 hot rolled steel (low carbon steel), AISI 1074 Hot Rolled (medium carbon steel), and AISI 6061-T6 (Aluminum). Figure 5 shows a drawing of the samples to be tested. These samples have been designed roughly in accordance with ASTM E8. It should be noted that the area is reduced in the middle of the sample. This is to ensure that the samples will fail in this location, where the stress field is uniform, and not influenced by the grips. WARNING: the samples may have sharp edges, be careful not to cut yourself or others. Figure 5: Sample Drawing P ROCEDURE : SAFETY FIRST! Keep clear of the machine while tests are running , or any time the crosshead is moving. Using the Horizon software interface, there is a no indicator sound while the machine is moving - you must communicate with your group so that everyone is aware when the crosshead is moving. You will know when the test is complete, and the machine will automatically stop. In the event of a problem, press the red safety stop button on the base of the machine. This is a twist-on switch (clockwise) that cuts all power to the frame. Before running a test, ensure that this switch is in the on position (extended). There is risk of material debris being ejected from the samples during testing, so safety glasses are required always during the lab. These will be provided however students can bring their own protective eyewear if they wish, at the discretion of the lab supervisor. 1.5R

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P ROCEDURE CONT ’ D : The following steps should be executed during the lab. 1. Mark the initial gauge length ℓ 0 (as indicated on Figure 5) on the sample using the marker provided. 2. Measure the relevant dimensions of the tensile specimen using calipers and record them in Table 1 below. Table 1: Sample Dimensions Dimensions [mm] AISI 1018 (hot rolled) low carbon steel AISI 1074 (hot rolled) medium carbon steel 6061-T6 Aluminium Fillet radius (R) 14 12 14 Initial gauge length ℓ 0 Width (W) Thickness (T) 3. Ensure no one is touching the controls before placing the sample between the grips and securing it with 3/8” pins. It may be necessary to jog the crosshead up or down manually so that the holes line up – this should be done by the person installing the sample. Keep hands clear at all times while the crosshead is moving. 4. Ensure that the sample is unloaded and zero the force and position values in the Horizon software. 5. Check the parameters for the test in the Horizon software, input geometry as necessary. 6. Confirm that everyone in the group has protective eyewear on and is clear of the machine, and perform the test by clicking start (play button next to test) in Horizon. The load frame will pull the sample to failure and automatically shut off. 7. Observe the Force - Position (load – elongation) diagram produced during the test by the software. Note how the curve changes prior to failure and record your observations below. Did the curve give you an indication that failure was about to occur? Record these observations in Table 2 below. Table 2: Observations on the load-elongation plot during the test AISI 1018 (hot rolled) low carbon steel AISI 1074 (hot rolled) medium carbon steel 6061-T6 Aluminium

8. When the test is over, output your data to a .csv file. You may either save the file to a USB stick or email it to yourself from the lab computer. 9. Remove both pieces of the sample from the grips. 10. Observe the failed faces of the test sample and record your observations below. Specifically, is the fracture face flat? Is it partially flat but some material is at an angle? Record your observations in Table 3 below. Table 2: Observations on the fracture surfaces AISI 1018 (hot rolled) low carbon steel AISI 1074 (hot rolled) medium carbon steel 6061-T6 Aluminium 11. Put the two surfaces back together and measure the gauge length at fracture, ℓ B . ℓ B = _________________________ mm 12. Repeat Steps 1-11 for each sample that your group is testing.

Guidelines for the Preparation of the Laboratory Writeup Objectives and Procedure 1. Provide a brief summary of the objectives of the lab and state briefly the procedure. (max 6 sentences) Results and Analysis of Results 2. Include Tables 1-3 from above. 3. Plot the engineering stress vs. engineering strain separately for each sample. 4. From your stress-strain curve calculate the yield stress, the elastic modulus, the ultimate tensile strength, and the percent elongation for each sample. Look in the available literature for the above reported values. Calculate the percentage difference between your values and those found in the literature. Present all of these results in the form of a table. 5. For each sample, indicate whether you believe that the material is ductile or brittle. Base your answer on the stress-strain curve produced, your calculated percent elongation and your observations of the appearance of the fracture faces and during the test of each sample. Conclusions 6. Conclusions: Briefly (in maximum 6 sentences) state your major takeaways from this lab.

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