This morning was especially exciting; I got to visit several different labs on Duke’s West Campus. Cambre, a graduate student from Atlanta currently studying at Duke, explained to me in detail each of the steps we took today towards a bigger picture. In the morning, we focused mainly on tensile testing, a process by which samples are pulled apart with a significant amount of force until they break. Of course, very detailed data was collected throughout the process. As you can see in the images, the hydraulic press we used for the tensile testing was rather substantial. Each of the silver clamps depicted weighs 50 pounds, and the grip holding each end of the sample exerts 10,000 Newtons of force onto the sample to prevent it from slipping. That’s right, 10,000 Newtons of force! The test is done by moving the upper grip up ever so slightly over time (fractions of a millimeter per second). As the distance between the clamps increases, the force exerted on the sample increases. That way, when it breaks the machine won’t shoot straight up since the top grip is only set to move up at an infinitesimal rate per second that nonetheless makes all the difference. The whole procedure was recorded by a professional camera called an extensometer, which has the ability to measure precision to the ten-thousandth of a millimeter. This helped to document the total displacement of the sample when tensile tested (how much it “stretched”). Unfortunately, I was not able to operate the extremely dangerous machine, but I did get to tape some broken samples back up together for later use!
Cambre also explained the specific shape choice of the samples. As you can see, the titanium alloy samples are made in a dog bone shape, and this is done for multiple reasons. First, the two ends are paddle shaped in order to be easily gripped by testing machines. Second, the middle, or the “gauge” is thinner so that when the samples are tested, all the force is concentrated into that area, forcing it break in that area and not anywhere else on the bone. The connection between the gauge and the paddle is also tapered to prevent any right-angled corners from being a stress absorption point during testing. In the lab today, we tested samples with solid gauges and those with a funky design in the middle called a gyro. The gyros provided a slightly porous gauge section and varied in wall thickness, from 0.25 mm, 0.5 mm and 1.0 mm. We found that the solid filled gauge dog bones required over 10,000 Newtons of pull to break, while those with 0.25 mm thick gyros only required a little over 2000 Newtons to break.
The whole reason for testing all of these seemingly plain dog bones is because we wanted to determine the quality of the laser titanium printer used to create the dog bones. That’s right, Duke has a 3D printer that literally prints titanium! I was lucky enough to take a peek at it after it was just cleaned, so I didn’t have to wear all the extensive safety equipment required to operate the machine. The 3D printing of the titanium is done through powderbed fusion, whereby an extremely thin layer of titanium powder is placed on a building plate in an inert atmosphere (filled with argon), and a laser melts the powder so it can solidify and reform into the desired shape. A sample can take up to 5 days to print depending on its complexity. Today, we were testing the reproducibility, or in simpler terms, the precision of the laser at different portions of the build plate. The laser in the printer is stationary, so when it shoots to a corner of the build plate, some of the energy may be lost when compared to shooting directly done because the distance the laser has to travel is increased. The tensile testing will determine whether or not samples printed at the corner of the build plate and at the center have any quality difference in terms of their strengths.
After Cambre analyzes the data we collected yesterday and determines how the laser’s distance affects the quality of the print, she will move on the printing more useful applications that can be used in medical procedures! She explained how the gyro shape used in some of the dog bones is actually crucial in the medical field because of its design and can be used in potentially many new applications.
In addition to our time spent in the compression lab, we also visited the Pratt Student Shop, a machinery-filled space for engineering students to complete their projects. Cambre happened to be mentoring an undergraduate student and dropped by in the shop to see how things were going. She showed me an extremely powerful machine called a EDM (Electrical Discharge Machine). This $200,000 gargantuan machine took up over half the room, all for the purpose of cutting materials with a thin wire using electrical discharge. Of course, it can cut metals of various strengths, and this is all done while submerged underwater. The student shop also had other massive tools, including several table saws, laser cutters, and rotary cutters.
Overall, today was a bit repetitive, but nonetheless, I learned much more about the mechanical side of engineering. I appreciated the amount of detail Cambre went into explaining the project and learned just how intricate testing can be. I am looking forward to another day at Chesterfield tomorrow, as I will be working with a new graduate student, Will!
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