AE321_Fall_2023_HW_06

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University of Illinois, Urbana Champaign *

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321

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Mechanical Engineering

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Dec 6, 2023

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docx

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AE 321 – Homework #6 Assigned: Friday, October 13, 2023 Due: Friday, October 20, 2023 Please submit the homework online Chapter 4: Material Behavior 1. Uniaxial tensile test question: Instructions: • This homework is based on the uniaxial tensile testing that was discussed in class in Section 4.1 of the lecture notes and that you did in lab. • Brief description of Procedure: The initial sample gage section diameter is first measured. This measurement is needed to obtain stress. The sample is a round bar machined with wider sections at each end for gripping, whose diameter is also measured. The bar is then mounted in the load frame grips, and the top and bottom grips are tightened. A device called an extensometer is attached to the middle of the gage section using two metal clips. The extensometer measures elongation over a known distance, and is used to provide axial strain in the sample. The loading device is started and the applied load, as well as the output from the extensometer, are recorded continuously on the computer as loading progresses. Sample necking (before necking the gage length has a uniform diameter, although smaller than the original diameter because of sample lateral contraction) can occur before ultimate failure in which the two parts are separated. The resulting load-displacement was plotted on the computer screen. • The homework is to be done individually by each student on their own data. Multiple data sets for tension tests of three different materials should have resulted from your laboratory work. The materials were 1045 cold rolled steel, 7075-T6 aluminum alloy and 6061-T6 aluminum alloy. If you were unable to attend, please work with the TA to download the datasets that correspond to your assigned group . • The data from each test come in a file which first lists the parameters used in each experiment followed by the data recorded in columns: Time, Extension, Load, and Strain. Detailed help on what is presented in these data files can be provided by the TA or testing website: Data: https://mtil.illinois.edu/data/Courses/Fall%202023/AE321/ Helpful Resources: https://mtil.illinois.edu/data/_DataAnalysisHELP/_Tensile_Example/ • You can further explore the files posted from previous testing by viewing images of samples that have been pulled to failure in tension in the directory “Fracture Pictures ”.
Questions on uniaxial tensile test: 1. Using the data for each specimen to complete the following: (a) Plot the uniaxial stress strain curve for each test. (b) Identify (i.e., clearly mark and label on your diagram) the following points in each of the curves: Yield point Point of onset of necking Failure point (c) What is the value of the following quantities in each case? Proportional limit 0.2% offset stress Ultimate tensile strength Failure strain (d) Calculate the modulus of elasticity ( E ) for Steel and Al (use both curves for Al). Explain the details of your calculation method. (e) Calculate the final (plastic) strain in each case if the material is unloaded after 3% straining. What is the new yield stress if the material is then reloaded to failure? (f) What are the differences between the stress-strain curves of the different types of Aluminum? 2. Short Essay Question: Write a short essay, between ½ and 1 page (Times 12-point font, 1 in margins, single spaced), comparing the tensile modulus and strength for the epoxy/carbon fiber composite properties used on the Boeing 787 Dreamliner summarized here (Condition CTA is measured at -65°F (-54°C), RTA is measured at 72°F (22°C), Ambient: Explaining three reasons/advantages of why one would use composite materials in the fabrication of the aircraft. Please document any sources you use as references. Some guidance for this question can be found on slides from the introductory lecture.
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