Hello fellow riders - I have a carbon bike and I’ve attached a rear rack. 
As background for the curious, I bought this bike (a Giant Avail) years ago and I barely ride it. My ‘real’ bike, a standard commuter with well-used racks, was stolen and I’m determined to make use of this bike before thinking about another purchase. I realise it’s not designed for a rack. 
The rack (a Thule Pack n' Pedal) is tightly strapped to the seatstay - see photo. Carbon is clearly strong material (bikes are made from it) however I’d like to be prepared for any future dilemmas. 
I’d like to know whether the seatstay is likely to buckle as a result of the tight rack straps? And if so, would it happen gradually or suddenly (should I keep an eye on it)?
How long could it  take before I see/experience damage?
Should I carry loads as light as possible to reduce the likelihood of damage? I ride everywhere including the supermarket and sometimes carry heavy loads but I'd wear a backpack if weight carried is an issue. 
I appreciate anything you can offer. I realise nothing can be guaranteed and I do declare that no one will blamed or questioned as a result of comments made. I’m asking far-and-wide to be as informed as possible.  

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Good find snowy, that looks like an excellent solution.

The two upper arms are greatly multiplying the force needed to stabilise the rack because they are a long way from being horizontal, this means the bending moment on the seatstays could be considerable even with a light load and avoiding bumpy roads. Being carbon the stays would have been designed to take a mostly axial load - the rack is loading them at 90 deg to this (ie very unfavourably).

My humble suggestion based on a modicum of education in such things is to take this rack off and mount a conventional rack using Snowy-5's suggestion below. If you cannot locate an aero seatpost adapter it might be possible to double-up on the existing seatpost clamp bolts - I have done this on one of my bikes so that the nuts retaining the rack do not affect the seatpost clamping torque.

My carbon frames have no metal at the seat post clamp.  I'm a little concerned as to what forces the carrier may apply to the clamping area of the frame

I have a carbon roadie and I will never use a rack on my carbon bike but only on my alloy flat bar bike.

My carbon roadie have an alloy seat post clamp.so do other carbon roadies have it.

If you want the rear rack on yours, then do it at your own risk and don`t use heavy loads on it.

There`s a seat post clamp with rack holes as per pic below from Pushys for round seat post and for proper size as there`s 2 different sizes, search for it on google to find the best price.. For Aero seat post, then good luck on finding it as there`s none on google search.Unless I`m wrong.

Use this and the other QR adapter rack mount from my other comment to go with it.

Yes that's a good point, I suppose those frames are relying on the additional strength of the seatpost itself and a precision fit into the seat tube to make that area reliable.

From the photo above, it looks like the clamp is very broad, and also the aero profile adds depth in the right direction, so it might be ok for the rack mount.

The seatpost clamp I adapted was on an alloy frame by using a long through bolt (ie threaded both ends) with a nut each side for the clamp and then another nut each side for the rack arms. The job might need spacers if the clamp has recessed bolt head or nut holes. Acorn nuts can be used on the outside for nice finish; this was years ago when I couldn't source one of the neat adapter clamps in the size required.

I pondered this long and hard, I went for a TI frame as I am not convinced about bending resistance in carbon in forks. they are not designed for such loads. Great for racing

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