Week 10 Tuesday
Accidents waiting to happen
Not all engineers are pessimists but since good designs anticipate failure modes, many engineers must at least consider Murphy’s Law (what can go wrong will go wrong) as they flesh out the details of their designs. Like Daedalus’ wax wings flown too close to the sun, even designs that work well have limits and breaking points. If an engineered object is to work reliably, then the designers will have to carefully examine its multiple points of failure, including the three that today’s challenges address: extreme forces, performance variations, and human fallibility.
Challenge 1: Reliable materials/use
modified from a lesson described in Henry Petroski’s book, To Engineer is Human
Boxes of paper clips don’t usually come with “ a money back guarantee” since nearly everyone in the world who uses paperclips finds them a reliable way to hold a few pieces of paper together. But bend the paperclip wide a few times and it’s likely to break. How many times will that be? We’ll do a quick experiment to find out. Each of you will bend a paperclip back and forth until it breaks and we’ll plot the data on a histogram.
Did all the paperclips break after the same number of bends? If so, why? If not, why not?
Mapping these ideas to your project
This paperclip challenge spotlights two modes of system failure, namely
- fatigue of the materials that comprise the device and
- application of uncharacteristic forces.
As you've seen these affect each paperclip to differing degrees. When thinking about biotechnologies, what is akin to “material fatigue”? What situations might be considered uncharacteristic? How cell to cell differences can be taken into account is touched on in the next challenge but here let's apply material and use variations to the Eau d’coli project from the MIT 2007 iGEM team. Recall, these bacterial cells were designed to smell like bananas when they reach stationary phase.
- In your groups imagine at least two ways that the genetic material in this system might "fatigue" and how you'd know.
- Next define at least two environmental conditions that would derail the system and decide how likely these conditions are.
Challenge 2: Reliable performance
The first challenge today addressed catastrophic failure--if breaking a paperclip can be called “catastrophic.” The activity makes clear how engineers can enhance reliability by anticipating then obviating failure modes. Certainly, weaknesses can be tested and quantified, leading to reliability terms such as the "MTTF" (Mean Time To Failure). And certainly there are simple things to try that lengthen “MTTF” such as specifying operating conditions (“don’t repeatedly bend the open clip back and forth") or including redundant functions (“use two paper clips when in doubt”). But catastrophic failure is only one way to fail. Today’s second challenge will consider another mode of failure, namely unreliable performance.
Challenge 3: Reliable sources
Week 10 Studio
Part 1: Eau d'coli early test/debug
Part 2: Project Redesign Day
Week 10 Thursday
==Challenge: Ownership and Sharing