Homework 6

AE 321 Homework 6 Due in class on October 18 IMPORTANT: Each student must use their own group data that are identified by the machine they used.

For example: Group 3 that used Machine B should use the data files: A3B1045CR and A3B6061T6, for the 1045CR steel (DEMO) and the 6061T6 aluminum specimen, respectively. All data are posted at: http://mtil.illinois.edu/data/Courses/Fall_2013/AE321/Tension%20Testing/ and help files are available at: http://mtil.illinois.edu/data/_DataAnalysisHELP/ You must also read the online handout on Mechanical Properties of Materials.

Use the data you collected for the steel (DEMO) and Al specimens to: 1. Draw a schematic to show the main components of the mechanical testing instrument you used and mention the quantities (force and strain) measured by the particular components of the instrument. 2. Plot the engineering stress vs. strain curves for Al and steel. 3. What components of the stress and strain tensors your stress and strain data correspond to? 4. Use linear regression to calculate the elastic modulus of Al and steel. 5. Find the proportional limit, 0.2% yield stress, ultimate stress, and failure stress of Al and steel. 6. What was the maximum shear stress at the point of yielding (0.2% yield stress)? 7. What were the true normal and shear stresses right when fracture occurred? 8. How did the cross-section of the fractured specimens look like (cup-and-cone vs. flat), and what does it tell you about the type of stresses that make these materials break? 9. Calculate the specific elastic modulus, specific yield stress, and the specific maximum stress, as the ratio of the elastic modulus over the material density, the yield stress over the material density, and 3

the maximum stress over the material density. (Al  = 2.70 g/cm   and

steel

3

= 7.80 g/cm ). Which

material has the highest specific properties? 10. Which of the two materials would you recommend for an aircraft structure and why? In answering this question consider as your design criteria (a) the specific yield stress, (b) the specific maximum stress, and (c) the specific elastic modulus. Which criterion of (a), (b), (c) is the most important for reliable flight? 11. Tabulate ALL your results for questions 4-9, also making sure that you use the correct units.