Either one of these belts would most likely have passed a visual inspection a week before they failed. The rubber has dried out and weakened, but you can't see that. This is why a simple inspection of a timing belt is not a good indication of life remaining. Replace them at manufacture's recommendation.
This section of cogs stripped off due to fatigue, all of the other cogs on this belt appeared perfectly fine.
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Wayne's Garage
27 E 27th in Eugene
333 Q Street in
Springfield