I have Carbide grade tools like router bits, saw blades, etc. like everybody else. There are grades C2, C3 and C4, most of my tools are C3. They do say that it is brittle and if it hits the ground or something it shatters. I have dropped it a few times by accident but they never have broken before. Which makes me have the question as to how much it takes to break the carbide of the tools. Does it take a certain amount of pressure (like pounds or something) of impact for the carbide to shatter? Or is the cracking so small that I have no idea because it is so hard to see?
It depends what it falls on :)
When I worked with a CNC mill, letting the tool drop 30cm from the spindle onto the steel worktable would practically inevitably chip it.
OTOH they would occasionally roll off the utility table about 120cm tall, onto linoleum floor and I never chipped one that way.
Also, the damage is usually small and localized; a ~1mm chipped corner or such - hard to notice unless you inspect the tool closely. It matters a lot with precise machining of small metal elements, but may go unnoticed with large tools for woodworking.
edit: (I finished reading the question.) The tools are very durable against constant pressure type stress, very hard and difficult to break. They are vulnerable to shock though - impact against a hard object, and with carbide tools "hard" needs to be really hard - brass counts as "soft".
"How brittle is carbide?" is a little like asking "How rubbery is rubber?" It depends on the rubber how firm or rubbery it is, same in reverse for carbide. The information provided in your previous Question, Tungsten Carbide grades would allow you to Google the technical specs for the various grades.
One way of answering the question on a practical level is that carbide is innately brittle, it will crack or chip before it will bend or fold as you can get on many equivalent steel tools or components. As such every care should be taken not to drop it on a hard floor as it can and does fracture, possible rendering the blade or bit unsafe for use (think shrapnel).
I have dropped carbide router bits on the floor a couple of times and nothing has happened. I once dropped my router with a straight bit installed in it and one of the carbide cutters cracked (Murphy's law). The router did not suffer any damage at all, but added momentum for the impact. Fortunately the crack was easily visible and I could discard the router bit before I was exposed to a projectile incident. I recommend careful inspection of carbide cutters with a magnifying lense if they hit against something hard.
I agree with the other answers... carbide will break or shatter, but only if hitting something extremely hard.... usually metal in my experience. I've dropped carbide tool bits and end-mills before and they've bounced off a wood floor, chipped concrete, but dropped 8 inches onto the metal ways of a lathe, chipped the end off. I think it all depends on the way they fall and hit the other surface. hope that helps your understanding.