Cal Poly San Luis Obispo Mechanical Engineering ME 329 Intermediate Design Winter 2007 Lab #2: A Tensile Testing Machine The laboratory project for this quarter will be the design and fabrication of a small hand-operated tensile testing machine. Such a machine will be used to test material samples in future materials and beginning design classes. The need for the machine comes from the need for engineering students to ex- perience ﬁrst-hand the behaviors of materials loaded to tensile failure. Therefore, the machine should operate in a manner similar to that of a typical tensile testing machine such as those manufactured by Instron. A specimen will be clamped in the machine and pulled apart as the load and deﬂection are measured. Because we want students to experience this test in small groups (rather than having a large class just watch a test conducted by the instructor), we will need many testing machines – about 12 to 15. This means that each machine must be small and inexpensive. Also, each machine should be so simple to operate that Freshmen can use it, and it must be so safe that Freshmen won’t be hurt by it. Your tensile test machine will use the electronic circuit which you have used to test the load cell. In addition to measuring force with your load cell, the circuit will also measure the displacement of the specimen. This will be accomplished through the use of a pulse counter. The pulse counter senses the movement of a magnet with a digital sensor – either a Hall eﬀect switch or a reed switch. These sensors cause a digital pulse whenever they are in the presence of a strong magnetic ﬁeld such as that produced by a permanent magnet. Because the extension of the specimen is measured through the detection of large-scale motions of a magnet, it will probably be most practical to use a power screw to pull on the test specimen. 6 in. 1 in. grip area The tester should be designed to pull apart a variety of test specimens. The ﬁrst type of specimen will be a sample of material from a beverage container (it could be aluminum or PET plastic). Approximate dimensions for a specimen made from thin, ﬂat sheet are shown in the diagram above. The width of the specimen’s end pieces will vary – stronger materials will have to be made into narrower specimens so they will fail at about 50 pounds – but about one inch is the maximum specimen width. It is expected that some specimens of wire of diameters between 0.05 inches and 0.125 inches will need to be tested, so the gripping mechanism you design should be able to accommodate wire if possible. Interchangeable grips are acceptable, as long as they’re not too expensive. Some of the materials to be tested are very ductile; the tester must accommodate at least 100% elongation before failure, and 150% is better. The Assignment and Deliverables The development of your tensile testing machine will take several weeks; there are several deliverables along the way. Week 4 As with most designs, your ﬁrst step should be to write a detailed set of speciﬁcations. This time your speciﬁcations need to be much more extensive than those for the load cell. Consider the forces which the tester must sustain, ease and convenience and speed of use, accuracy of measurement, cost, and above all safety. You should create a list of at least 20 or 30 speciﬁcations, most containing measurable numbers. Type them up in a spreadsheet and save the ﬁle. Then research the design of some other tensile testing machines. Although you want to create a new and innovative design, you don’t want to reinvent the wheel. There are a few links on the course web page (Library section), but search engines are your main tool here. Then come up with several alternative designs (at least three). Document each with clear sketches and descriptions, just as you did for the load cell. Even at this stage, think about how you’re going to build these things. Deliverables: Due at the beginning of lab next week will be a page or two with your speciﬁcations and your preliminary design sketches with descrip- tions. On top should be a short memo which states which of your preliminary designs you have chosen to develop further. You should also make a sim- ple presentation showing your design which you can show to the class in a very short design review (see below). You can use PowerPoint, Flash(?), transparencies, scanned napkin sketches, or whatever format you prefer to show your design to class – but it should be something which communicates your ideas clearly and is worthy of presentation. Week 5 At the beginning of lab, each group will present its chosen preliminary design for a very brief design review. We will discuss each other’s designs and try to learn from others’ ideas. Think of this activity as a cooperative eﬀort to surpass the other lab section’s designs. Who do they think they are anyway? Then do detailed design work on your chosen design. Your goal is to design the tester so that it meets your speciﬁcations for strength, stiﬀness, safety, and so on. Deliverables: None this week. Week 6 This week you should complete the tester’s design. The design includes a set of part drawings (one for each unique part in your design which must be fabricated rather than purchased), an assembly drawing, and a bill of materials. The bill of materials should list the parts which must be fabricated as well as those which must be purchased, such as nuts and bolts and gallon containers of JB Weld. There should be estimated costs for the materials. Any parts which must be ordered need to have been ordered by now so that you don’t have to wait a long time to begin construction. Deliverables: Due at the beginning of class next week (week 7 of the quarter) will be a short memo to which is attached your design document containing part drawings, assembly drawing(s), and a bill of materials with cost esti- mates. Week 7 This is the week for fabrication of the tester. It is expected that some reﬁne- ments will be made to the design during the fabrication stage; that’s OK, as it indicates that you’re learning (by doing!) about better design. Make sure to document your changes, as the changes will be an important topic in your project report. You also will write a test plan. This plan should contain instructions on how to test every one of your testable speciﬁcations – and a justiﬁcation for not testing any speciﬁcations which won’t be tested. Be ready to do the tests in your plan next week. Deliverables: At the beginning of lab next week, you should have your tester ready to use (or at least ready to assemble). You will also turn in your test plan at the beginning of lab – and then execute the plan during lab. Week 8 You should have all the parts of the tester built by this lab session. In this lab you will test the tester. There will be some material samples which can be pulled apart available; you’re welcome to bring in some other items to test (beverage containers, samples of various metals, parking tickets, etc.) Bring what you need to perform the tests speciﬁed in your test plan. If the unit is speciﬁed to operate underwater, bring a pool. Deliverables: The ﬁnal report on the tester will be due next week. The format of this report – a full design report, not a memo – will be described in a separate document available on the course website. There will also be a presentation to the class, so use your favorite presentation tool to put together a ﬁve to ten minute talk about your tester. Week 9 This week, you will present your ﬁnal tensile tester and its design, and turn in your ﬁnal report.