I have crashed on my cyclocross bike and got a dent on it's alu frame. I don't see cracks. How serious is this dent? Can it lead frame to suddenly fail?
If I interpret the images correctly, that dent is in the upper tube of a diamond frame. This tube only has to withstand pressure, not tension. (When you have a frame failure, it's usually the lower tubes that fail, they are literally torn apart.) As such, I don't think that dent is critical. I'd be more worried if the dent were in the lower tube that connects the bottom bracket with the front.