This is perhaps not the most auspicious post to mark my first post of 2008, but these words came up recently on DD, and I enjoy using them. Since, they're not interchangeable, and it's about time I learned the real difference! Call me inspired. I'll take it where I can.
Friable comes from the Latin "friare" for "to rub, break, or crumble into small pieces". Pretty straightforward to the current usage of "easily crumbled or reduced to powder". Must be because this word originated c. 1560, and not in the Middle Ages. What I find interesting is that it is related to "fricare" for "to rub", which gives us friction, but I'll save that for another time. Unfortunately, unlike the common word friction, friable doesn't have an easy association. Instead, it sounds like a word used to describe a cooking style or something having to do with the clergy, and not something which is broken through friction into small pieces.
Now, frangible comes from the Latin "frangere" for simply "to break", and is just as straightforward for its current usage of "easily broken, capable of being broken, brittle or fragile", even though it originated in the Middle Ages c. 1400. And just like friable, frangible doesn't give it's meaning away too quickly. Sounds like a pastry, even though I know with the "ible" suffix it is an adjective. Oh, well. Sometimes good words just need to be memorized.
So, with two straightforward etymologies to usage, the distinction is also blessedly straightforward. Even though friable has as part of its meaning "to break", in common with frangible, the difference is breaking due to the activity (rubbing) versus breaking due to the composition of the item (brittle). It becomes more obvious with usage.
Dry cookies are friable such that even milk can't revive them, but the glass the milk is served in is frangible. Children's toys are engineered to break in a frangible, not friable manner so that the child doesn't have an opportunity to ingest small pieces. Salt erodes concrete with friable results, back to it original sand and rock components. Snapping a pencil demonstrates its frangible qualities as well as the writer's total frustration. Ok, now that we've gotten the obvious usages out of the way. Because we have activity and composition at issue in the etymology of the words, the common usage is with tangible things. Forced to live in the South for too long, even her steely composure could be rendered friable. Perhaps a little too evocative and esoteric at the same time. You have to know that friable implies a rubbing element to understand that her composure was rubbed away. Not sure this works. Lashing out at a 4 year old may be a frangible result of holiday stress. Unfortunately, this usage also only makes sense if you know the brittle implication of frangible to understand that under the stress, the person snapped. Again, not sure this works, since no one would really understand what you meant to say. Plaintiff's counsel rubbed me all the wrong way with his friable personality. Ok, so it's a pun and it uses Plaintiff's counsel. Yeah, alright, next time I'll keep that one to myself. Plaintiff's counsel's friable client of the case crumbled under cross-examination. That's really just fragile, as well as requiring a colloquialism to get the point across, so, no. Whereas my frangible witness was subject to be treated as a hostile witness. Either that, or he'd be removed by a court officer. Yeah. Still not getting the meaning of this word from context, and my usages probably wouldn't motivate my listener to look up the word either. Friable ideas eventually yield to reason. Maybe, but just barely. Frangible ideas don't withstand even basic scrutiny. Also, maybe, but it leads me to believe that friable is a more versatile word. But I think it comes down to these words must be used with physically broken things in order to give your listener an opportunity to understand the word from context. If your listener is erudite enough to understand some nonstandard usages, then have fun!