Tensile Strength Testing of Aluminum: Lab Experiment Overview

University of Central Oklahoma Tension Test - Experiment 2 Dylan Robinson, Joshua Jackson, and Kaleb Paddock Dr. Adnan Al-Ibadi Thursday 1:00 PM 9/29/22

Abstract Tension is used in many engineering applications. For example, a bridge deck might be suspended in tension using bound steel cables. In order for engineers to compare different materials’ tensile strength it is economically beneficial to test smaller pieces of material. For laboratory experiment 2, we took a specimen of aluminum in a dogbone shape, and applied a tensile load until the material failed. Using the Universal Testing Machine or ZPM, we collected data from the specimen during tensile loading. Values like displacement and force were collected. Given these values, we were able to determine the normal stress, strain, modulus of elasticity, and a few other values. Machines and Instruments ‘ Image 1’ shows the main machine used in this experiment. This machine will be referred to as the Universal Testing Machine or ZPM. The ZPM is able to take very precise measurements (down to the nearest 1/100 millimeter). This is obviously a very important tool to have access to when looking to determine stress, strain, and the modulus of elasticity - which are all the key components we are looking to resolve in this laboratory experiment. Without a test stand like this or similar, comparing different materials to one another would be significantly more expensive. By understanding how the ZPM works and how different materials compared to one another, engineers can perform complex trade studies at minimal cost.

Image 1: Test Specimen in ZPM Machine.

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Experimental Method In this experiment, we rely heavily on the concepts of stress, strain, modulus of elasticity, and a few other key concepts specific to strength of materials. Here we conduct a tensile test on an aluminum specimen, and using the ZPM, collect the values of the precise force being applied as well as displacement seen in the Aluminum. Our part in the actual testing of the specimen was fairly limited, since we did not interact with the machine as far as calibration, and overall equipment setup is concerned. After the specimen is tested, we take the data given and compute our values. In this particular experiment, we ran into a few issues with the supports for the universal testing machine. The specimen was not able to lock into the machine properly, and we were forced to switch to a different machine, as shown below in Figure 2. This second, older, ZPM was limited in the tensile force it could apply. We were unable to fracture the material.

Figure 2: Specimen Loaded into the secondary testing machine.

Test Procedure This procedure assumes the ZPM has been set up and calibrated. A dog bone shape was cut from an Aluminum block. The dog bone was cut in such a way that it could easily be mounted to the ZPM. Mounting of the specimen included opening TestBuilder and setting the parameters for the tension test. Setting the parameters of the ZPM consisted of inputting the necessary information into the universal testing machine, such as velocity of the ZPM actuator, rate of tensile force and what data needed to be collected. Once the parameters in TestBuilder were set the experimental procedure was carried out by the Lab Technician. From there, the Aluminum dog bone was put onto the supports in the machine. From there the ZPM was started, and a tensile load was applied to the specimen. Data from this tensile loading was recorded. Ansys procedure To begin the work in Ansys, first open the software and then click on the preprocessor menu. From here click on element type, then add/edit/delete. This will allow you to choose the material being tested. The material for this experiment is the Brick 20node 186. Once this material has been selected you can close out of the element types. Next, select material props, then material models, and choose structural, linear, elastic, and finally isotropic. After these values are defined you can close the window. After that, you will create a volume by going to modeling, create, volumes, blocks, and then by 2 corners and z. After putting in the dimensions, click on the isometric view to see the 3D object. Now it is time to add the mesh. Select meshing, then mesh tool and set the mesh element size length to be 0.25 inches. After setting the size you will pick all three volumes to add the mesh. Now that the mesh is applied, you can add constraints. In the solution menu, select define loads, apply, structural, displacement, and the on areas. Select the top area of the object. This will fix the location and constrain all translational degrees of freedom. Next, in the structural menu, select pressure, then on areas. Select the bottom surface to apply a pressure of -10psi. Now it is time to solve the system, go to solution, then solve, and then current LS. After solving you will plot the deformed shape. Go to general postproc, then plot results, and then deform shape. From this plot you can find the maximum deflection or DMX. Specimen Specifications The specimen used for this experiment was a piece of aluminum cut into shape with the CNC machine. The length of the specimen was 21.3mm, with a width of 1.9mm, and a thickness of 2.3mm. This gives the specimen a cross- sectional area of 4.37mm 2 .

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Results Table 1 shows the data collected from the tensile loading applied to the Aluminum dog bone. Here you can see the ZPM was unable to cause the material to fail entirely. Table 1

Ansys Results Shown below in Figure 3, is the deformed shape of the material after the load was applied to the specimen. You can see the deformation by looking at the meshing on the block. Next to it is the DMX value of 0.265e-5 which is the maximum deflection calculated by the Ansys software. Figure 3

Validation and Discussion Shown in Graph 1 is the stress vs. strain diagram from the data obtained during the experiment. We are not given direct values for these from the test machine, and were hence calculated using the following equations for stress (EQ. 1) and strain (EQ. 2). EQ. 1: 𝛔 =F/A Here, 𝞂 is the normal stress value (measured in MPa) F is the force being applied applied to the specimen A is the cross-sectional area of the specimen at its thinnest point. EQ. 2: ϵ = 𝛿 /L Here, ϵ is the measured value for strain - which is a unitless value, 𝛿 is the gauge length of the specimen (in meters), and L is the original length of the specimen. With these two equations, and the provided values for the force, and gauge length/displacement, we can calculate the values for 𝞂 and ϵ needed to create our graph shown below. The graph was created using Excel and the table of data points that was provided to us from the testing device. Figure 1: Stress vs. Strain graph

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From the stress-strain graph shown above in figure 1, several assumptions can be made. Per the experiment, we were asked to find the approximate modulus of elasticity (E) for the particular material being used. We can find this value by looking at the approximate point on the graph where the line stops being linear (again, this is an approximation, so there is room for error). In this particular instance, negating the interval on the graph, [0, 0.02) ∪ ( 0.02, 25] we can estimate that the proportional limit for this material is somewhere near 210 MPa for the stress value, and approx. .06 (m/m) for strain. Given this information, and the equation shown below for calculating the modulus of elasticity, we can obtain a value for E. EQ. 3: E = 𝛔 / ϵ With our equation 3 (shown above), we can roughly deduce that our value for the Modulus of Elasticity is approximately 3.5e9 Pascals, or 3.5 GPa. Given our values for the modulus of elasticity, we can use this value along with our other given values from the machine to calculate the maximum deflection of the specimen, which is what is asked from us in the laboratory manual. The equation for the maximum deflection is as follows in EQ. 4: EQ. 4: 𝜹 =(FL)/(AE) For the maximum deflection we can achieve, we take the greatest value for F, which is the tension force applied on the specimen. Per the ZPM universal testing machine, the maximum F applied to the specimen is 1149.6 N, and when we plug this value into our EQ. 4, we get a maximum deflection value of .0016m. This deflection value differs from the theoretical value, primarily due to the limitations of the ZPM. Conclusion As mentioned in the abstract section, engineers are able to use this method to develop an integral understanding of a material's characteristics. By testing multiple small specimens, one can compare many materials at relatively low cost, safely.

REFERENCES [1] Khandaker, M. (2021). Laboratory Manual for Strength of Materials Lab . Department of Engineering and Physics (UCO).

Lab2

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