cracked seatpost

The size and orientation of the cracks are important. Today many manufacturers use ud carbon, which is fine, however they use it on the surface. In other industries it would be sandwiched between woven plies to avoid crack propagation. It may be that the is just a separation between fibres on the top ply. Let's put it in context. Bikes are made in their thousands in China production lines. They do not have the same process controlled and inspection as other industries. Point is that all bike parts from carbon will have defects of some sort. Voids, wrinkles and delamination. Just need to judge how severe the cracks, if it is a cracking, is. I doubt very much that a carbon seat post would snap catastrophic.
Post a pic and we see how it looks.

The thing about carbon is once it's got a small crack, that's it gone really.

Metals can usually perform fine even with cracks in them (assuming the stress is below yield stress of the material - yes obviously then you get failure due to fatuige), they just won't last that long. 

I woud consider the carbon seatpost structurally failed with a crack in it, therefore the bin is the only palce for it.

I do see what you are saying though, suppose you cut it off above the crack (then treated the exposed fibres) before inseting to the original minimum insertion length, then you could reasonably consider it safe.

A cracked carbon post shearing a femoral artery or removing anyones perineum from usage (esp. your wife's) is NOT wise! Even if the current, visible crack will be hidden, the structural integrity is GONE! Replace any carbon posts with the 

https://www.bikethomson.com/product/masterpiece-seatpost/

and ride with your mind at ease. You CAN afford it-if you want to go cheap, buy the 

https://www.bikethomson.com/product/elite-seatpost/

which is still better than anyone elses post (except for Nitto for classic bikes).  

How big is the crack and in what direction? How much below the clamp would it be?

I've had a crack in the laquer to my carbon seatpost for the last 7 years, put a sticker over it and ignored it.  If the crack looks to be structural then bin it, otherwise it is probably fine.

Buy an alu seatpost until you can afford another carbon one, or an rsp carbon seatpost which are ridiculously good value.  

Do you actually like your Missus?

If you do, throw the cracked seatpost away.

Otherwise, yeah give her a dangerous seatpost without that might impale her if it breaks without warning.

Jeez.

Bin it.

Not a good idea - the crack could propagate.

Assuming the post of the same size, I'd still say that posts have a minimum insertion point for a reason. Although the clamp stops movement in two planes doubt it absorbs all the flex forces. And do you want to explain who is at fault to your wife if it breaks whilst she is on it...

Depends on whether your missus knows beforehand...

You do love to live dangerously don't you.

Why not get a short length of wooden dowel and jam it up the seat post, this will put your mind at rest until you can get a new one.  Incidentally, Planet X sometimes have decent seatposts at very attractive prices.

It might be safe, but it doesn't sound wise!