Make a piece of a string same diameter, one sinew one yucca and test out.
Or take a slat back it with sinew and another with yucca same thickness.
I think I can estimate what the result is ...
Simson
I wasn't comparing sinew as a backing to sisal/yucca, but other wood. If wood is strong enough as a backing (in a self bow), then sisal/yucca (which aren't the strongest plant fibers) surely does the trick too.
And as far as rupture under a load is concerned: there's lots of actual data on this stuff, performed under controlled conditions. Sinew is indeed exceptional in that it can stretch up to 15-30% before breaking (at high MC), but past 5% it has nonelastic deformation (tension set). So past this stress this extra tension as a bow backing is irrelevant. (see
http://paleoplanet69529.yuku.com/sreply/519341/sinew#.Vq-3UIXhBD8)
As a backing on a self bow (no horn belly), sinew will hardly ever be asked to stretch more than 1.5 let alone 2%. It takes less force to stretch a strand of sinew for 2% (150 MPa) than to stretch a strand of sisal for 1% (220MPa) (though it can safely take 2%). So even though you're right to state that the same diameter sinew string will break at a higher tension load (at c. 1200 MPa vs 800 Mpa in the best case for sisal), for a bow backing this crazy elongation of 15-30% is irrelevant.
So in the "take a slat" example the yucca (or at least sisal-backed) bow would be stronger. Less elastic (but elastic enough to back any self bow), but stronger.
You could also use pineapple fibers. They are very strong too (comparable to hemp and flax).
For those interested, I compiled a database on all these properties for all kinds of woods and natural fibers (animal and plant based). Available upon request (and a work in permanent progress).
