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what about mono leafsprings?


Same. On a multi-leaf the main leaf is doing most of the work. It's thickness and how much support it gets from the second leaf will largely determine how close to maximum stress it undergoes for a given load. On the cars we're talking about here, think about how the front half of the leaf springs also acts as a locating link. So the springs do a lot more than just carry load. The main leaf can be twisted into an 'S' shape by the torque of the axle when using sticky tires on a drag strip start or coming off a corner on an autocross.

If you switch material the spring is made of from steel to a composite, then the elastic limit of the composite will be different too. Off hand, I don't know whether they will last longer under the same conditions than a similar multi-leaf of good spring steel. However because they are susceptible to chips and impact damage, you'ld want to be checking their condition regularly. For competition, they will save weight and a little more scrutiny should be a normal part of any competion vehicle. Mitch and others have posted about their experience using fibreglass monoleafs if you do a little searching you should find those posts.