Over the course of the summer our goal has been to manufacture shells and analyze their motion and sound as they “pop”. This popping motion occurs when a shell is turned inside out, creating instability. To regain stability, the shell pops or returns back to its original shape. Throughout this process, the popping sound can also be characterized to have a certain frequency.
Each shell can be described by three parameters, radius, height and thickness. As these parameters change different shells are created. We have characterized our shells into three categories, mono-stable, meta-stable and bi-stable. Mono-stable means the shell is only stable in one state. For example, the shell cannot be turned inside out because it is too flat or too thick. Meta-stable means the shell will snap back into its original position. For example, when the shell is turned inside out it will return back to its original shape. Bi-stable means the shell is stable in two states. For example, when the shell is turned inside out it will remain in this state. For our research, the meta-stable state is the most intriguing because the popping motion occurs for this state.
3D printed shells, meta-stable
Our biggest challenge so far has been to create a shell of uniform thickness. We have developed two techniques. The first is to print a mold from our 3D printer. This mold is designed in AutoCAD and then converted to code for the 3D printer. Our mold is printed in two separate pieces so material can just be poured into the mold. To ensure uniform thickness, the top is screwed into the bottom part by four screws. This method has proven quite successful in ensuring uniform thickness. The only downsides include the large amount of time it takes to print the mold. Also, the mold is not smooth and therefore creates a rough shell.
3D printed mold
The second technique utilizes a very unique method. We built two square airtight boxes that both have a circle cutout in one face. Over each circle cutout is a thin sheet of plastic. One box is positively pressurized and pushes the thin film outward into the shape of a dome. The other is a vacuum and sucks the film inward into the shape of an upside down dome. Material is poured in between these two films and allowed to cure creating a shell. This method has also proven successful in creating a very smooth shell in a small amount of time. The only downside is the fact that we have not quite mastered the amount of pressure necessary and our shells come out with a large radius and a small height, resulting in a pretty flat shell. Another downside is the fact that our thin films cure onto the shell and are not able to be removed, adding thickness to the shell.
Evolution of shells created over the course of the summer
While we were using the 3D printer, we noticed that both the left and right extruders failed to work. This is a fairly common issue with 3D printers; therefore, we have created a checklist of things to do in order to potentially fix the problem.
1. Make sure that the MakerBot is working efficiently by doing a quick maintenance check. Click on the link below to go to the MakerBot troubleshooting webpage. Once there, click on the red text titled ‘Maintenance’. This should bring you to a video on how to make sure the MakerBot is working properly.
2. Make sure that the PLA filament is not jammed. Click the following link to follow a step-by-step video on how to clear a filament jam. Once you have clicked the link and appeared at the troubleshooting page, click on the video titled ‘Clearing a Filament Jam’.
3. Do not fiddle with the 3D printer properties. Always use the default settings. For example, do not change the fill rate or the fill percentage. Changing these settings can lead to printer trouble (i.e. the extruders may become jammed).
4. Make sure all parts on the 3D printer are the correct parts. We found out that the screws which hold the cooling fans on the front of the extruder base were not the correct length. Because of this, our extruders would not work due to unstable parts. By replacing the incorrect short screws with longer screws, we were able to stabilize the printer parts and continue printing successfully.
This is a video on shear thickening that we made for our fluids class last fall that Dr. Holmes asked me to post. The assignment was to make an educational video about a fluid property that was aimed at a high school audience. I hope you enjoy it:
When it comes to research, it can always feel like the pressure to produce results is relentless. Here’s a really interesting article about the importance of valuing engagement with the material over output.
A vascular surgeon described a surgery during which the aorta above the vessels that feed the liver can only be deprived of blood for 30 minutes, or the patient will die. His advice to residents: “Slow down to speed up.”
Here’s a fantastic video showing the explosive fracture of Prince Rupert’s Drop:
Here’s a fantastic video explaining some ideas of differential geometry and elasticity using a slice of pizza.
You can read more here: http://ed.ted.com/lessons/pizza-physics-new-york-style-colm-kelleher
Dielectric elastomer fibers are constructed using PDMS tubing as a dielectric and carbon grease as a compliant electrode material. Fabrication is as follows:
- PDMS Tubes are cut to 6cm in length
- A syringe filled with carbon grease is used to inject the inner electrode into the tube. The inner coating in injected 4cm into the tube. This leaves 2 cm of the tube empty.
- A copper wire electrode is inserted ~1cm into the tube on the same side that contains the carbon grease inner electrode.
- The outer coating is applied by dragging the tube through carbon grease with the aid of a paint brush. The outer coating is applied on 4cm of the outer surface opposite the side of injection. This is important because if the outer coating is too close to the wire electrode, the system will short during the test!
Testing of the fiber electrodes is conducted using LabView. The LabView program controls the high voltage power supply that applies the voltage to drive actuation of the dielectric elastomer fiber. It allows for different voltage ramp conditions to be tested and records images, voltage, and current during the test.
Example Image Output:
Note that this is NOT an optimal image since the center of deformation is not the center of the image.
What we want:
Given a series of images and voltage data from an actuating experiment, we want to find the deflection of the center point of the fiber and match that to the input voltage.
In this code we are assuming the center point of the fiber is the center point of the image. Therefore, it is important to center the fiber in the image so that the location of actuation corresponds to this location. Future changes could be implemented to track the entire length of the fiber.
How it works:
The program uses a loop to analyze each image in the file in which the test data is stored. This file will contain images and a .csv file containing the raw data. Each image is analyzed by inverting the B&W image so that the fiber is white (white = 1, black = 0). The center column of pixels is then isolated and the first and last nonzero values are removed until only one remains. This point becomes the center point of the fiber. Each image’s centerpoint is compared to the initial position to give a deflection.
To match the deflection with the voltage data the timestamp of the image must be matched to the time in the .csv file. The time in the .csv file is relative to the start of the test. Therefore, a relative time is defined that corresponds the image time to the test time. Image times relative to the start of the test are then compared to the relative time in the first column of the .csv file. Once a match is found, the voltage data is extracted from the .csv file and paired with the images deflection.
Finally, with all the deflections from each image collected and the voltage paired for each image, a plot of voltage x deflection is constructed.
The source code can found here.