I showed my wife the video where the GF does some etching on a pair of Jeans. Has anyone tried that or on a tshirt? She asked me this am if that was possible and I didn’t know. I imagine it could be possible but haven’t seen anything posted here.
Anyone try it yet?
If you do a search for Denim, you will see several examples of things people have tried.
Several people have done denim. The laser damages the threads in a fabric. Denim is a heavy fabric so you can do a little damage and no big deal. I suspect the problem with a t-shirt is that the lighter material won’t weather the laser as well and your settings would need to be dialed in perfectly. If it will work at all.
If you don’t have to laser the t-shirt, you can make a stencil and use fabric paint. I recall a couple different methods including making a “silkscreen” from mylar. I think this would be the way to go with t-shirts.
Another option is to cut a pattern into a poly-blend (the polyester seals the edges so no fraying) shirt and then wear it over another shirt.
I haven’t done T-shirt’s but do a lot of muslin cotton bags. And you’re right. My early experiments resulted in some very nice engraves - that promptly ripped out with any movement. 
That could be downright adventuresome with a T-shirt. 
Thanks for the feedback everyone. I sent my wife the link above.
The laser doesn’t magically make stuff disappear or change color, it burns it. A heavy multi-ply fabric like Denim will probably survive no worse off than if the cotton had been worn away mechanically. You’re only damaging the top layer. Demin’s ability to tolerate surface abrasion is why it’s been so popular since it was invented. But a thin low or single ply fabric like a Tee Shirt doesn’t have any backup structure to support the material you burn with the laser. It can’t help but fall apart, it’s been toasted.
That sounds like a challenge. 
Sounds like a nudist camp challenge!