2020(S08) Lecture:week 10
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Week 10 Tuesday
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 standard paperclips finds them reliably able 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? Reason 1: Reason 2:
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. When thinking about biotechnologies, what is akin to “material fatigue”? What situations might be considered uncharacteristic? Take the Eau d’coli project from the MIT 2007 iGEM team. These bacterial cells were designed to smell like bananas when they reach stationary phase.
