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Is it possible to obtain (without modeling) the fracture strength of defect-free nanotubes or nanowires by tensile loading?

What boundary conditions would allow failure to occur in the gauge length and not at or near the clamps? One is not allowed (in suggesting ways of overcoming stress concentation at the clamps) to create defects in the nanotube or nanowire, to configure the region where failure will occur.  Thus, it is not possible (or is it?)  to create an analog of dog-bone specimens by, e.g., milling away part of the nanowire with a focused ion beam, etc., because this creates defects in the nanowire.

I ask: Must we be model-dependent, or is there a method to configure an experiment so that the force at failure alone (with knowledge of specimen geometry) allows one to state what the fracture stress is, in pure tensile loading?


Xiaodong Li's picture

Thanks a lot, Rod for posting this challenging question.  For a conventional engineering tensile test, we use a dog-bone sample to allow failure to occur in the gauge. Can one grow dog-bone samples directly (using bottom-up approaches)?  Now a lot of reach groups can synthesize 1D nanostructures like nanobelts, nanosprings, and nanonecklaces.  I hope that in the near future we can find a good recipe to synthesize dog-bone samples.    AFM three-point bending and nanoindentation tests are difficult to yield “pure” tensile fracture. Milling away part of the nanowire with focused ion beam is a novel approach (needs to make sure that milling process does not generate defects or alter the structure).  I agree that we badly need experimental data. Hope someone can come up with an innovative way to realize this. 


removed because I didn't note that i was, I think, quoting the original poster back at himself.


So to ask a different question, doesn't your work with Oleg Lourie  fit here

Thanks for comments, Xiaodong and Bill.

Any milling in my opinion is going to alter the fundamental structure of the nanowire or nanotube...growing dog bone structures is an interesting suggestion, but it does not address the other nanostructures that have the structures they happen to have.  If there were reliable methods to add material to the outer shell of the nanowire (or nanotube) over some fraction of its length, and actually have that "cladding" bearing load...but the cladding of course is not present in the region where we desire the stress to be highest and also sufficiently high that failure always occurs in the desired region, than perhaps that is a route to studying the fracture strength of the defect free nanowire or nanotube. 

Suppose we coat a SWCNT with a thin layer of a metal, for example...let the total length be 1 micron, and it is coated for 400 nm on the left and 400 nm on the right, so that there is an uncoated region of 200 nm in the center. If the load can really be distributed to the cladding as well as the underlying SWCNT (take a (10,10) nanotube simply for discussion purposes), then perhaps the failure would occur randomly in the 200 nm center zone, that is to say, essentially randomly at different positions within this 200 nm zone. 

So, perhaps adding material can work, but I am skeptical about removing any material by any method...our knowledge then of the exact chemical bonding would have to be so perfect that our "prior knowledge" would be so advanced...that if we were "that good" we'd probably not be needing to load to fracture anyway! Modeling would be so perfect there might be no need to do the experiment, if we could so well know the configuration and state of all chemical bonds in such a "notched" or "dog-bone like" specimen. And as mentioned, we'd no longer be studying the pristine, defect-free nanotube or nanowire as it actually was, prior to removal of some part of it.

Further thoughts welcomed.


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