Solar Sails, Soft Robots, and Building Things with Rocks and Strings
Pressure vessels come in all shapes and sizes: tanks, sails, tents, kites, parachutes, ear drums, fish fins, bat wings, and pteranodon wings.
Sails: Ancient Technology Made Relevant Again?
“The shape of the sail was perfected by the experience of centuries. The latest advances in the field of stretched membranes have opened new possibilities, but the time of the sailing vessel is past. It remains to be seen whether the sail will still be of importance in the future.” Frei Otto, Tensile Structures (1973).
At the moment, our closest technology to achieving space flight that approaches even a fraction of the speed of light involves the use of solar sails. Light has momentum, and so when light shines upon you it imparts a (very tiny) force. Designing extremely thin and deployable sails to use the light from the sun, or from an array of high-powered lasers, may be fundamentally change space exploration - see how:
COMSOL: Finite Element Analysis
In class, we discussed the stress analysis and yielding of a pressure vessel modeled in COMSOL. Spend some time with the model to explore how changing the shape and material properties affect its performance.
Here’s the code and the manual. Can you estimate the stresses in the tank that failed during the Great Molasses Flood in Boston’s North End? If you’re serious about getting into structural mechanics modeling with COMSOL, this reference is a great place to start.
We also talked briefly about the manufacturability of nearly spherical caps for pressure vessels. This is serious engineering, check out this hot forming press making the head of a pressure vessel:
Soft Robots
We used our intuition to sketch a graph of the pressure inside a balloon as it is being inflated. Understanding the nonlinear constitutive behavior of balloons helped us rationalize a simple, counterintuitive example: if you connect two balloons inflated to different amounts together and then allow the pressure to equalize, the inflated balloon may actually inflate more. Here’s an example pressure vs. stretch curve to help guide your intuition (the left red dot corresponds to the small balloon, while the rightmost dot corresponds to the big one):
As suggested by one student while we sketched our ideas for building a soft robotic actuator, this interesting demo can actually be used to actuate soft robots. Here’s some recent research that does just that:
I also demonstrated some basic soft robotic tendril designs that built off of the “PneuNets” that helped transform the field of soft robots over a decade ago. These demos were built in Prof. Ranzani’s “Morphable Biorobotics” lab. All of these mechanisms work by incorporating a strain limiting layer. Essentially, you selectively add structural elements that resist stretching by making them thicker or from a stiffer material. The orientation of the strain limiting layer dictates how the actuator bends. You can learn more about those designs, which were pioneered by Prof. George Whitesides, by reading the original research paper in which they were developed.
Better yet - build your own! The Soft Robotics Toolkit is an incredible resource library with CAD models and clear instructions on what to 3D print and how to mold and bond these structures together. We have materials available for use in the MOSS Lab and plenty of fabrication options available to you in EPIC.
The Surfside Condominium Collapse
It’s helpful to juxtapose the molasses tank failure with a more recent structural engineering failure that occurred at a 12-story beachfront condominium in the Miami suburb of Surfside, Florida. The Surfside building collapse rattled the structural engineering community, and this discussion helps answer some of the questions of what happened and what role do structural engineers play following a structural failure.
Give this episode a listen, and come to class with questions on the terms and concepts that may be unfamiliar to you.
Building Things with Rocks and Strings
If you combine rocks and strings in just the right way, you can build load-bearing structures from the everyday materials that are all around us. Recent work in my lab, lead by Dr. Arman Guerra, has demonstrated that these structures are quite stiff, morphable, and capable of forming beams, columns, arches, and walkways without any adhesive or binding agent.
The simplest way to build a rock-string structure (we refer to them as elastogranular, as they combine elastic elements with granular matter), is to take a hollow cylindrical form (you can just use a sheet of paper wrapped into a cylinder), and fill it layer-by-layer with rocks and loops of string. If the loops are close enough together, the column will stand and be strong enough to hold you up! These students built one in class:
But, why on earth am I telling you about rock and string towers? Because the structures are stabilized by loops of string that bear the load as a hoop stress in a similar way that cylindrical pressure vessels behave. The grains are restricted from flowing outward by an intricate balance of friction, string rigidity, and the strings ability to withstand a large hoop stress.
I can’t do this on my own. If you come across interesting content related to our course, please share it with me via email — pictures, videos, Tweets, articles, TikToks, scientific publications, questions, GitHub code — anything relevant, send it along.