In many Engineering Mechanics and Materials classes, “dog bone” tensile test specimens are required at some point, as part of determining the yield strength and ultimate tensile strength of various materials. For each of these materials, the test specimens are often milled one at a time in the university’s machine shop by a certified lab technician, until there are enough for the entire class. What if there were an easier way that didn’t require a machine shop or certifications, and could even be used as a teaching tool?
Desktop CNC mills are rapidly becoming invaluable assets inside the classroom. Not only are they simple and safe enough to use that certifications are not required, but they can even be used by students. Designing a dog bone in CAD and then physically milling it on a CNC machine provides a rich experience for students. We made a video that gives an overview of such a process:
Going even further, students could test the specimen to failure, calculate its properties, and then design and fabricate a structure, such as a truss, out of that same material. They could then test that structure to failure, using the values calculated in the previous tensile tests.
Milling a test specimen and then testing it in an Instron.
Milling truss members from a sheet of aluminum.
Assembling and then load-testing a truss made from milled components. Images are from our Truss Lab Experiment.
This is also particularly useful if a shop has piles of spare metal whose alloy is unknown, as is often the case with aluminum. By milling a dog bone and testing the metal, the properties can be determined, and then students can build structures and predict their performance accurately.
Our Truss Lab Experiment is a 2-hour Statics lab for Engineering Mechanics and Materials courses. It comes with all the math and design files needed to describe and fabricate the project, as well as ideas for future projects.