MENG_3331-Fatigue_Lab-group_report_Resubmission

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3331

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Feb 20, 2024

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MENG 3331 Materials Science Department of Mechanical Engineering Sept. 27, 2023, Statesboro, Georgia, USA Technical Report 5 FATIGUE LAB Bryce Cone Georgia Southern University Sparks, GA, United States Dylan Butler Georgia Southern University Albany, GA, United States Scott Rhodes Georgia Southern University Warner Robins, GA, United States Lawrence Almeter Georgia Southern University Dearing, GA, United States A BSTRACT Fatigue testing is a test conducted to study how a material reacts to weight being added to the end of it while being rotated at a high rpm (rotations per minute). In the following lab four samples of materials are tested and studied how they react. Each of the materials had a channel, or groove milled in the center. This allows the test to be conducted more accurately and more controlled. Each one of the materials had a different weight applied to the end of the rod. It is discovered that as more weight was added to the rod, the less amount of lifetime rotations would be made. The amount of weight applied to the bars started at 50 in-lbs and went up in increments of 15 in-lbs, thus making the weights used 50 in-lbs, 65 in-lbs, 80 in-lbs, and 95 in-lbs. After taking note of observations and results, the stress acted upon the material can then be calculated. N OMENCLATURE d Diameter, inches or meters in-lb Bending Moment, inch-pound Nm Bending Moment, Newton Meter M Moment (Bending Torque), in-lb or N*m Pa Pascal, N/m^2 σ Stress, Pascal INTRODUCTION In this lab Fatigue testing will be conducted to determine the maximum stress leading to failure. The Origins of this test can be traced back to the first documented cases of metal fatigue failures in 1838 documented by Wilhelm Albert. By the 1850’s fatigue testing was a standard practice for companies attempting to correct fatigue cracks in horse drawn carriages and railroad car axles [1]. August Wöhler, another German scientist conducted a 20-year load cycles to failure test which led to his popularization of the S-N Curve which is a plot of the Stress versus the number of life cycles in logarithmic scale [1]. The primary uses of fatigue testing today are to generate data on crack propagation, identify critical locations of samples, and demonstrate safety of certain designs. Cyclic loading causes the sample’s surface to go through very fast changes of tension and compression with varying loads allowing for the observation of failure. MaximumStress = σ = 32 M π d 3 = Pa Standard Deviation = σ = ¿ 1 ∈ – lb = 0.11298482933333 Newton meters 1 inches = 0.0254 meters (1) (2) (3) (4) EXPERIMENTAL METHODS The fatigue test will begin with selecting 4 metal samples that have been machined specifically for the test. Use a caliber to first measure the middle diameter to obtain the radius for each sample. Once measured, slide each end of the sample into 1 Copyright © 2023 by ASME

the fittings and secure the threaded ends of the fittings to the machine using the two wrenches. Secure the back end of the fitting by sliding the arm to the bolt to make sure the sample does not break instantly. Once the sample is connected to the fatigue testing machine the inch pound scale will need to be adjusted. For the first sample an inch poundage of 50 will be used. Now that the sample is secured, and the inch poundage is set it will be time to turn it on. Flip the switch and slowly adjust the rpms until it reaches ~4000. Once 4000 rpms have been reached turn the wheel on the life cycle until all digits reach zero. As soon as they reach 0 release the arm from the back fitting to allow the sample to now be undergoing the test. Wait until the sample has broken, record the number of life cycles it endured and remove the now broken sample from the fittings. This process will be repeated 4 times in total with the only change being a 15 inch-pound increase on each sample starting with the 50 inch-pound sample. DATA AND R ESULTS Table 1 is a summary of the experimental data gathered during testing. There were four samples each under an applied bending moment of increasing magnitude. The measured radius was extremely consistent between the four samples which will help keep the rest of the data consistent as well. The number of life cycles that the samples survived were as expected. The samples under less load had a much longer lifetime while the samples under heavy load had an extremely short lifetime. Table 2 shows the conversion of Table 1 into SI Units from English Units using Equation 3 and 4. It also shows the stress amplitude which was calculated using Equation 1. Figure 1 is a graphical representation of the stress amplitude versus the number of life- cycles the sample was able to reach. This Stress-Life curve visibly shows how the lifetime of the sample improved when under less stress. Table 1. Collected Experimental Data Sample ID Bending Torque [in-lb] Min Radius [in] Life Cycles x100 1 50 0.1355 320 2 65 0.1355 40 3 80 0.1355 32 4 95 0.1355 12 Table 2. Conversion of Table 1 to SI Units for Experiment Results Sample ID Bending Torque [N*m] Min Radius [m] Life Cycles x100 Stress Amplitude [Pa] 1 5.6492 0.003442 320 176.4336 2 7.3440 0.003442 40 229.3636 3 9.0388 0.003442 32 282.2937 4 10.7336 0.003442 12 335.2238 Figure 1. Stress-Life Curve of Samples DISCUSSION After conducting the following experiment, it was discovered that each piece of material broke. Those pieces of materials with more weight added onto the end broke much more quickly than the lighter ones. The weights used in the experiment were 50 in-lbs, 65 in-lbs, 80 in-lbs, and 95 in-lbs. These weights then had to be converted into N*m making the values 5.6492 N*m, 7.344 N*m, 9.0388 N*m, and 10.7336 N*m. The sample that was tested with 5.6492 N*m made a total of 320 lifetime cycles *100. The stress amplitude calculated for this sample was 176.4336 Pa. The next sample was tested using 7.344 N*m and made a total of 40 lifetime cycles *100. This made the stress amplitude acting upon it to be 229.3636 Pa. The next weight used for the experiment was 9.0388 N*m and it made a total 32 lifetime cycles *100. The following amplitude stress for this sample was 282.2937 Pa. The final weight used for the experiment was 10.7336 N*m and it made only 12 lifetime cycles before breaking. This made the total amplitude stress for this material to be 335.2238 Pa. the standard deviation of the stress amplitude is 69.5008. It is believed that the tests are accurate, but there can be errors in the material being used since it is not clearly stated what material was being provided. CONCLUSION The fatigue test is a test in which the metal sample undergoes a cyclic load while going a set number of rpms until failure. The results obtained from the test are under the 50 inch- 2 Copyright © 2023 by ASME

pound force the sample underwent a maximum stress of 176.4 MPa. Under the 65 inch-pound force the sample underwent a maxim stress of 229.4 MPa. Under 80 inch-pound force the sample underwent a maximum stress of 282.3 MPa. And under 95 inch-pounds the sample underwent a maximum stress of 335.2 MPa. R EFERENCES [1] Anderson, G. (2021). Fatigue testing, pt. 1: History, methodology, and role in Lightweighting . Engineering Plastics Resources. https://etp.teknorapex.com/blog/fatigue-testing- history-methodology-role-in-lightweighting 3 Copyright © 2023 by ASME

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