wk10-11_LAB-HANDOUT_mech-props-metals-2

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Mechanical Engineering

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Jan 9, 2024

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MSE 235: Mechanical Testing of Metals II 1 Instructions and Safety: This lab will meet in ARMS 2191. You will need to wear long pants, close-toed shoes, and safety glasses. You should also bring a calculator, flash drive (memory stick), and at least a few people in your lab session will need to bring their laptops. There are pre-lab questions (see below) that you must answer before the start of the lab session. Instructions for your lab report are included – you should carefully read this section so that you know what observations to write down and data to collect during the lab activity. This is a more independent and challenging lab than the labs you have completed in the past. Please read this lab handout in its entirety before coming to lab – There is a lot to do in this lab activity, and you need to be prepared in order to finish the activities in lab. You will be working closely with one or two lab partner(s) to collect and compile all of the required data. Your TAs will provide an initial orientation to the mechanical testing equipment. Then they will help answer questions and give pointers, but they will NOT give you step-by-step instructions for collecting and compiling data, analyzing results, etc. After lab, you will share your data with your labmates. Background Reading: Callister & Rethwisch textbook, Chapter 6: “Mechanical Properties of Metals”, Sections: 6.1 – 6.10; Chapter 7: “Dislocations and Strengthening Mechanisms”, Sections 7.1, 7.2, 7.6, 7.9 (Solid -Solution Strengthening), 7.10 (Strain Hardening) Materials : Safety attire, calculator, digital calipers, copper or brass tensile bar, bench-top mechanical tester with extensometer attachment, permanent markers; each small group will also need to provide a laptop & memory stick to use for compiling data. Objectives: Upon completion of this lab activity, the student will be able to: • list and explain the experimental steps that are required to perform a valid and accurate tensile test, • determine stress-strain curves from force and elongation (or time) data collected during a tensile test, and • calculate and/or identify the characteristic features of stress-strain curves, including the elastic region, plastic region, Young’s modulus (E), yi eld strength (  y ), ultimate tensile strength (UTS). Background: Figure 1 is a schematic of a typical mechanical testing load frame configured for tensile testing. The gripping system holds the tensile samples in place (here, “dog - bone” samples are illustrated) while a crosshead moving at a constant rate is used to apply a load. The load cell is an electronic device that measures the force (typically in Newtons, N) being applied to the sample. Knowing the sample ’s cross-section dimensions allows one to convert from applied force to applied stress. Measuring sample displacement based on the movement of the crosshead is inaccurate for very small values of strain (i.e., in the elastic region) because not all of the crosshead displacement results in sample displacement. Examples of crosshead displacement not related to sample displacement include any slack in the grips, grip slippage, and any displacement of the load frame itself (imagine the load frame is made of rubber instead of steel). This effect is particularly important for measurements performed at small strains like Young’s modulus and the yield stress . Therefore, an extensometer Figure 1: Illustration of a “dog - bone” specimen for tensile testing.

MSE 235: Mechanical Testing of Metals II 2 – a device that accurately measures small displacements – is attached directly to the sample to measure sample displacement. Normalizing the measured displacement by the initial span of the extensometer allows one to determine the sample strain. For large strain measurements like % elongation, simply measuring the gauge length of the sample before and after testing is sufficient to produce an accurate result. Activity Instructions: 1. Each lab section will split into smaller sub-groups of 2-3 students (should have 4-5 groups per lab section). 2. Each sub-group will receive a sample of 1 copper specimen and 1 brass specimen (provided by their TAs). Work together with your entire lab section to come up with a consistent numbering scheme for each specimen (e.g., “C2” for copper specimen of sub -group 2). 3. Write the specimen number directly onto each specimen using permanent marker. 4. Measure and record the initial gauge length and gauge cross-sectional area for each specimen. 5. Use the Mtestquattro load frames to conduct a tensile test on your specimen. Use the extensometer (i.e., distance the crosshead on the load frame has moved) to obtain a force-displacement curve for each specimen. a. NOTE: The extensometer is fragile and can get damaged during fracture, so you MUST REMOVE the extensometer after plastic yielding occurs. The crosshead displacement will continue to be recorded even after the extensometer is removed. 6. Measure the final gauge length for each specimen after fracture. 7. Work together with the other sub-groups and your TA to share data within your lab section – so you should end up with approximately 4-5 data sets each for copper and for brass. Remember you need to share the data files obtained from the tensile tester and the gauge length/area measurements. 8. Analyze the tensile data and calculate the relevant mechanical properties (see “In - Lab Data Analysis” subsection below). In-Lab Data Analysis: If time allows, calculate the following from the engineering stress-strain curve obtained during the tensile test: (a) Young’s modulus (only for tests that used an extensometer) (b) Tensile strength or Ultimate Tensile Strength (UTS) (c) 0.2% offset yield strength (σ y ) (d) % elongation at fracture When analyzing your data, how many significant figures should you report in your data tables and why? Compare your calculated values of mechanical properties with reference values from the Tables in Appendix B of Callister. Your values may differ from the reference values by 10-20% (or even 30- 50% for Young’s modulus). However, if your values are different by more than a factor of ten, there is a good chance that there were problems with either the data acquisition or your calculations. In-lab is the time to sort these issues out, and we will further discuss some potential issues in the post-lab lecture and the pre-lab lecture for next week. Please refer to Mechanical Testing of Metals I Lab Handout for general instructions for using the Mtestquattro materials testing system and for lab report instructions.

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