On Thu, Jul 03, 2003 at 04:13:49PM -0700, Deborah Harrell wrote: > Having read somewhere that "spider silk has greater tensile strength > than steel," I looked up a few articles. It seems that the properties > of spider silk that allow it to be both strong and resilient/elastic, > while still not understood, involve sheets of 'stiff' alanine chains > coupled with more flexible/elastic protein sequences, twined together > in different ways for different types/uses of the silk. It is > comparable to Kevlar (one article said 'tougher'), but much lighter.
.... > So, could the structure and properties of dragline silk be helpful in > the design of carbon nanotubules for a space elevator? (Of course, I'm > guessing you wouldn't want it to be that elastic, but the structure is > intriguing. Here are some graphics:) I don't really know enough about practical building considerations to answer your question, but I think it is an interesting subject. I just did some reading, and I mostly found that it is a much more complex subject than I initially thought. Especially when you get away from basic materials properties and start looking at weaves and composite materials (a composite is a material made up of more than one constituent, a matrix, like epoxy or concrete, filled with something like steel rebar or carbon fibers). Anyway, here are a few observations. Spider silk is much more rubbery than kevlar. Silk can stretch by 30% to 40% before it breaks. Steel can stretch, too, but most steel structures are designed to operate safely below the "yield point" which is where the steel starts to get very plastic. In the links below, note how steel has very low strain (little elongation) up to a well-defined yield-point where it starts to stretch, whereas silk does not have a well-defined yield point and it follows a "smoother" elongation curve until it breaks. http://www.umeciv.maine.edu/cie111/tension/default.htm http://www.nature.com/nature/journal/v410/n6828/box/410541a0_bx1.html "Average data (mean standard deviation, s.d.) for silk collected at 20 mm s-1 and 25 �C from an adult Nephila edulis female are: silk diameter, 3.35 0.63 um, tensile breaking strain, 0.39 0.08; breaking stress, 1.15 0.20 GPa; initial modulus, 7.9 1.8 GPa; yield stress, 0.15 0.06 GPa; and breaking energy, 165 30 kJ kg-1 (after ref. 63). Comparable data for Kevlar 81 high tenacity yarn 98 are: diameter, 12 um; breaking strain 0.05; breaking stress 3.6 GPa; modulus 90 GPa; no yield stress as yarn has a single modulus; and breaking energy around 33 kJ kg-1 (A. Grandus, personal communication). Thus Kevlar is 3 times stronger but spider silk is 5 times tougher because it is 8 times more extendible" The article I posted before quotes 60 GPa as the necessary number for a space elevator. So spider silk by itself is much too weak. But as a component in a composite? Maybe it could be helpful, I don't know. Obviously, when you are designing a structure like this, you want to build in a safety factor so that the worst stress that you expect to be applied is say, 40% lower than that which will break the structure. With steel, this is relatively easy to design since the steel has a well-defined yield point, and as long as you stay below that, the steel has virtually no permanent deformation, so you can count on your structure staying the same size and shape. But if there is an impact, with a short but very high stress applied, the steel may break. With something like spider silk, which stretches better, it may be possible to handle a very short but extreme stress better than steel (toughness). On the other hand, it looks like a very challenging design problem. Can you imagine if steel elevator cables were replaced with bungee cords? If a bunch of really large people got into the elevator, would the elevator still line up with the floors at each level? Would the space elevator wiggle back and forth like an anemone? It may be possible to design a space elevator with a rubbery material, but I think it makes for a very complex design that would require decades of study and testing to fully understand the implications. Here are a couple links that only begin to get into some of the complexities. http://fiberarchitects.com/reading/rebars.html http://calcul.com/ian/thesis/node47.html -- "Erik Reuter" <[EMAIL PROTECTED]> http://www.erikreuter.net/ _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
