On a troop yesterday i experienced the dreaded cracking of one of my ABS shoulder straps! As many of you know, these poor little straps take a ton of abuse due to being bent in a curve as well as getting pushed around by the chest and back plate when the come in contact. What was happening was small cracks were forming along the edge of the strap and several spots were starting to "crease" and form small square angles as the plastic bent.
To fix this, i heat formed a second set of shoulder strips out of spare .080" ABS i had. I put them in the oven for about 10 minutes @ 200F to let them warm up fully. I then pulled the new strips out and formed them to the exact curvature of my existing shoulder straps. Once cool, i checked that they had the proper curvature and final trimmed them to length. Next, i used some plastic welder to repair the small cracks along the edges of the strap. Then, i glued the new strips to the underside with a generous helping of E6000.
I am now happy to say that the proper curvature has been restored to the straps (the new strip pushed the sharp angles out) and now they are quite stong (since they are about .160" thick!)
I will post up some photos of the repair when i get home. I hope my experience will help some other TKs out who run into this problem. I think you could probably even fix a fully broken strap with this method, though it would likely take some more glue.