MS43010 - N OVEMBER 25, 2019 Comparing the results in Table 3, it can be concluded that the shorter the notch length, the higher the force needed to fracture. The obtained value of fracture toughness shows that aluminium 7075-T6 is a brittle material compared to other metals. Steel 4340, for example, has a fracture toughness of 50 MPa √ m . Table 3: Fracture Toughness results Specimen Notch (mm) F (kN) W (mm) B (mm) a (mm) S (mm) K c (MPa √ m ) f (a/W) 1 8 12.4 29.7 15.02 9 100 24.3 1.51 2 12 4.5012 29.7 15.12 17 100 19.7 2 2.4 S-N curve estimation For the fatigue testing test, two specimens were taken, where one was notched and the other was smooth. A hydraulic fatigue machine with a maximum load capacity of 350 kN was used. The test required to fix the specimens between two chucks and apply fatigue load on the specimen. For both the specimens, the test was performed on 3000 rpm. The specimen which was smooth had 15.3 Nm applied to it, and the notched specimen had 7.9 Nm applied to it, so both the specimens have the same stress concentration. Due to fault in the fixing of the notched specimen, rotational values for fatigue testing of the notched specimen is found out to be unexpectedly high. For the notched specimen, our D/d ratio is 1.4 and and r/d = 0 . 1 , where r= radius of notch. Table 4: Data for S-N curve Number of cycles to failure (x10 5 ) Stress amplitude (notched) [MPa] Stress amplitude (smooth) [MPa] 0.089 400 405 0.539 351 349 1,06 166 410 1,50 150,8 280,1 1,52 249 299,3 1,84 350 305,6 3,07 257 300 Table 4 shows the data for the fatigue test done during the experiment and has data from the previous test results. The table has been created by adding the data generated through the experiment and data given on the Brightspace page. In general, it can be observed that stress amplitude for the smooth and notched sample is the same because the stress concentration applied on the samples was kept to be almost same. From Figure 11, it can be seen that the S-N curve follows the trend of the table represented in the Appendix F from the lab manual. The notched specimen, is expected to fail before the smooth specimen, but it did not. This is explained on the basis of faulty experimental setup which results in anomalies. For example, in this case, when the notched specimen was tested, it was tightened in a faulty way, which lead to exaggerated results. 10
MS43010 - N OVEMBER 25, 2019 Figure 11: S-N Curve plot for rotating bending test 11
MS43010 - N OVEMBER 25, 2019 A Appendix A.1 Fracture Toughness Shape Factor f a W = 3 ( a 0 W ) 0 . 5 h 1 . 99 - a 0 W 1 - a 0 W h 2 . 15 - 3 . 9 a 0 W + 2 . 7 a 2 0 W 2 ii 2 1 + 2 a 0 W 1 - a 0 W 1 . 5 (3) A.2 Critical Stress Intensity Bending K c = FS BW 1 . 5 f a W (4) A.3 Stress Intesity Factor K = σ * √ π * a (5) A.4 Critical Stress Intensity K = p E * G c (6) 12
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- Spring '18
- Dr. maarten bakker