Update to whip problem

I caught a bad input value in my example analyses of a whip from the last post. I used 0.5 GPa as a working strength for a carbon fiber tether. That might be equivalent to the operating strength for a multidirectional composite laminate using current technology, including a lot of reduction factors normally considered. But for the theoretical strength of a pure tension tether under idealized conditions, it’s way too conservative. For some quick numbers, type “tensile strength carbon fiber” into Wolfram Alpha. It generates the following table for fiber products ranked by tensile strength.

Having spent a lot of time trying to gather together composite data into a handbook (MIL-HDBK-17), I’m pretty impressed that Alpha can now pull together this information.

I’ll assume these are mean values for the fiber strengths. There is typically a wide statistical variation in carbon fiber strength which makes the allowable strength much lower. And there should be a factor-of-safety. For the study, I’ll be ambitious and use a working stress of 3000 MPa.

The whip study was updated using the new strength. Obviously, the new value makes the release velocity of the projectile significantly higher, and thus improves the viability of the idea.

I’ve only been away from composites for three years and already it’s possible for me to write down a bogus material property. I’ll blame the metric system. American structural engineers (at least of my age) think in English units. There’s always the danger of not spotting a bad number when converting the units.

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