I consider myself the caretaker on my elderly 1986 VW Cabriolet. The aluminum bumpers have the original ABS end caps.
Spot the small crack? I stop drilled the stress crack with a 3/32 " bit. Hopefully, that will keep the crack from migrating. Still needed to fix / reinforce the area. Ugh. Those stone pecks show up really well.
Now comes Ace Hardware's house-brand ABS 5-minute epoxy. Backtracking here to my aircraft construction for just a moment... Composite aircraft -fiberglas, E-glass, kevlar, vinylester, carbon fiber, etc- frequently need additional reinforcement at joint lines and corners.
Enter a material known as Cotton Flox. Visualize, if you will, a cotton ball that has been sent through the food processor on the puree setting. That's flox. A one-pound bag of this lightweight stuff takes up a lotta space. When blended with epoxy resins for lamination or epoxy adhesives, the flox becomes a reinforcing matrix of cross-linked fibers. Barney style, it's stronger than your momma's love.
I fixed the Cabriolet fender cap with the Ace ABS adhesive and a bit of cotton flox. A built up area on the backside of the fender cap did the trick. A suitably sized piece of heavyweight fiberglass cloth would work as well. The end cap break is now very solid. Couple links to suppliers who maintain web catalogs and sell cotton flox and f'glas cloth.
And this is the stuff. I would suggest not using the material on the exterior surfaces unless absolutely necessary. Wipe off any adhesive squeeze out before the material cures. That should camouflage the area nicely.