Slap Shots, Exploding Watermelons, and Strandbeests
Welcome to the Structural Mechanics Newsletter.
This newsletter will allow me to share interesting media and articles that are relevant to our course (ME309 at Boston University), but it may be broadly relevant to anyone interested in the structures that make up our world. I will send it out once a week, probably on Thursday evenings or midday Friday.
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.
Callback from class: Hockey Slap Shots
We talked about how surfboard fins and hockey sticks store energy by bending and then release it as kinetic energy - either to help propel a surfer down the face of a wave, or to increase the velocity of a hockey puck. Here’s a YouTube explainer video to give you a better understanding:
Pressure Vessels
Next week, we’ll be talk a little about pressure vessels. We will begin with a structural failure and tragedy in Boston that ultimately changed the face of structural engineering in America.
PORTFOLIO FODDER: In 2015, wind-powered Strandbeests strolled mechanically around Boston City Hall Plaza and along our beaches.
(many Strandbeests incorporate sails to harness wind power. sails are also pressure vessels…)
…to read more about these structures, start here.
Exploding Watermelons
The watermelon in the video above appears to have a radius of about 12cm, and likely has a rind thickness of about 2cm. This gives a radius-to-thickness (R/h) ratio of about 6. Thin-walled pressure vessels refer to shells with R/h > 10, making a watermelon “thick-walled”. When a pressure vessel has thick-walls, we can no longer assume that the stress is constant in the walls (this is what we assume when we model thin-walled pressure vessels), and the thin-wall approximation will underestimate the stress in the walls. If you’re interested in seeing if you can estimate the stresses in the watermelon before it exploded, you can start by learning about the mechanics of thick-walled pressure vessels, here is my lecture and my notes.
Inflatables and Soft Robots
Alex Alspach makes soft, inflatable robotic grippers: Bubble Grippers. Learn more by listening to this podcast interview with him: