wk10-11_LAB-HANDOUT_mech-props-metals-1 (1)

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Jan 9, 2024

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MSE 235: Mechanical Testing of Metals I 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). Pre-Lab Questions (answer before coming to lab): 1. What equations are used to calculate engineering stress and engineering strain ? Define each parameter and include units. 2. What equation is used to calculate %-elongation at failure? Define each parameter and include units.

MSE 235: Mechanical Testing of Metals I 2 3. Draw an example stress-strain curve for copper alloy C26000 (also known as cartridge brass) that was cold worked (H04 temper). Label the axes with approximate values and units. Also label the regimes of elastic and plastic deformation as well as the 0.2% offset yield stress. Appendix B in Callister will be helpful for this question. Properly answering this question and knowing an approximate value for the yield stress will help you to know when to remove the extensometer from the specimen during the tensile test (explained more on pg. 3). 4. Using the stress-strain curve in Question 3, illustrate how you would calculate the Young’s Modulus of copper alloy C26000. Look up and report some accepted Young’s Modulus values of copper from the published literature. How do the units of Young’s Modulus compare with the units of yield stress?

MSE 235: Mechanical Testing of Metals I 3 Background: Mechanical behavior is a generic term for the response of a material to applied force. This response is measured by shape changes that the material undergoes as force is applied. The simplest means for quantitatively measuring mechanical response is a tensile test . Force is applied to a test specimen of a well-defined geometry (typically cylindrical or rectangular). As the applied force increases, the specimen first undergoes elastic deformation with an accompanying change in shape. Shape changes associated with elastic deformation are recoverable, and if the force is removed before the specimen is loaded beyond its elastic limit (its yield point), the specimen returns to its original dimensions. If the specimen is loaded beyond its elastic limit, the mechanical behavior depends on whether the material is ductile or brittle . Ductile materials exhibit plastic deformation that occurs via the movement of atoms (via dislocation motion and slip). Shape changes occurring during plastic deformation are retained even after the applied force is removed. Brittle materials do not plastically deform at room temperature because the ability of their constituent atom or ions to move in response to an applied force is very limited. Therefore, when the applied force is sufficiently large, brittle materials will fracture. 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. 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 crosshead displacement (i.e., distance the crosshead on the load frame has moved) to obtain a force-displacement curve for each specimen. 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). Figure 1: Illustration of a “dog - bone” specimen for tensile testing.

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MSE 235: Mechanical Testing of Metals I 4 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. General instructions for using the Mtestquattro materials testing system: 1. With your TA, review the method of operation for the Mtestquattro machine. 2. For your specimen (copper or brass), make necessary measurements to calculate the cross-sectional area. 3. Estimate the applied force needed to reach the ultimate tensile strength using the specimen dimensions and property values in Appendix B of Callister (assume your brass sample is made from copper alloy C36000). Check your answer with the TA. Your TA will select the appropriate crosshead speed. 4. Mark the initial gauge length on the specimen and measure the length using your calipers. 5. Load the specimen into the grips and attach the extensometer. Why is the extensometer used? 6. Start the test and at some point past yielding (your TA will tell you when), pause the test and carefully remove the extensometer without significantly disrupting the specimen. Why do we need to remove the extensometer? 7. Restart the test without the extensometer and test to failure. 8. After removing your specimen, measure the final gauge length using your calipers. 9. For each specimen that you test, please make sure you have recorded the following data: a. Crosshead speed (mm/mm) b. Initial gauge length (mm) and Final gauge length (mm) c. Initial cross-sectional area (mm 2 ) 10. Save the resulting data file as with a unique file name and import the data into an Excel workbook.

MSE 235: Mechanical Testing of Metals I 5 Lab Report Instructions: To prepare your report, a docx template is available on Brightspace; you should download and use/edit this document and then submit this document as a PDF through Brightspace using the appropriate assignment link. The report will worth a total of 60 points. Please note following: • all figures and tables should be properly labeled and include a descriptive caption, • all reported numerical values have an appropriate number of significant figures, • data is analyzed/summarized using appropriate statistical methods, • keep in mind the attributes of a well-designed, mindful figure that clearly communicates the required information, and • written text should be free from spelling and grammar errors, and please keep the elements of technical writing in mind (you may wish to review your lecture notes on voice, tense, conciseness and clarity or consult technical writing guidance online). Experimental Methods (10 pts): Write a short paragraph that fully describes the materials, machines, and methods that you used/performed during the tensile testing and hardness testing portion of this lab activity. Section 1: Tensile Testing Results Week 1 (20 pts) Please identify which metal (copper or brass) is mechanically stronger in tension by: • Creating graphs of engineering stress-strain data for the brass and copper sample, showing the full deformation response of the samples (using crosshead displacement to calculate strain). (5 pts) • Creating a data table that concisely summarizes the most important results (values) that you calculated. (5 pts) • Write a short paragraph (less than 200 words) that summarizes any relevant in-lab observations from the tensile test experiments as well as describes the important trends that are present in your data (e.g., how does the average yield strength of copper compare with the average yield strength of brass? etc.). Within this paragraph, you should directly refer to your figures and tables. (5 pts) Section 2: Tensile Testing Results Week 2 (20 pts) Please identify which metal (copper or brass) is mechanically stronger in tension by: • Creating graphs of engineering stress-strain data for the brass and copper sample, showing only the data from the samples’ elastic responses (using extensometer strain data). (5 pts) • Creating a data table that concisely summarizes the most important results (values) that you calculated. (5 pts) • Write a short paragraph (less than 200 words) that summarizes any relevant in-lab observations from the tensile test experiments as well as describes the important trends that are present in your data (e.g., how does the average yield strength of copper compare with the average yield strength of brass? etc.). Within this paragraph, you should directly refer to your figures and tables. (5 pts) Discussion (10 pts): Write two short paragraphs (less than 200 words each) that explain the following: First, explain the differences in your quantitative mechanical properties between weeks 1 and 2. Explain why the extensometer is necessary to obtain more accurate quantitative mechanical properties in some cases (and state which properties require use of extensometer), and why the crosshead is necessary to obtain more accurate quantitative mechanical properties in other cases (and state which properties require use of the crosshead). Second, explain the underlying reason for the trends that you observed in your tensile data (e.g., why, at the atomic level, are the yield strengths of brass and copper different? Think about various strengthening mechanisms in metals). Also, in your paragraph, report some reference values for brass and copper (see Appendix B in Callister & Rethwisch) and describe how your data compares to those reference values (note: determining the percentage of your data that is within ± 5% of the reference value could be a useful and concise way to communicate this information). Be sure to properly cite your sources.