Tires: broken tire?, rubber dust, heat generation

Question
A couple of weeks ago I had a mostly if not completely flat tire and didn't know it, ended up driving on it for some miles. I got the hole plugged but later noticed an unrelated wear problem on another tire - I took the car back and, in case it was almost time to change more than one, I asked my mechanic to look at all of them. He pointed out the tire that had been plugged for the flat; half of the outermost side wall of the tire, where the brand name and spec of the tire is written, had been gouged off maybe 1/8" deep. He said that by driving it flat I had "broken" it and it should be replaced. I'd never heard about a tire becoming "broken". Is his assessment plausible?

Answer
Paul,

The mechanic wasn't very eloquent, but he was correct.  A tire that has been run low enough that the sidewall shows abrasion, is in danger of failing.  Yours shows more than abrasion.  It shows a significant amount of wear.

When tires are properly inflated, they run at certain levels of stress and heat generation.  This results in using up the amount of durability that is built into a tire.  Think of it as a bucket of water and that you are dipping out the water a teaspoon at a time when the tire is properly inflated.  When the bucket id empty, the tire has used up its durability and it fails.  Normally the tire will wearout before it is even close to failure.

Tires that are operated low in pressure generate higher levels of stress and heat and the tire fails sooner.

Depending on how low the tire is operated, the "water dipper" could be a tablespoon, a cup, or a large bowl.  It will not take long to empty the bucket with a large bowl - and that's what you were using.  

The real villain in this is the guy who plugged your tire.  First, a plug is a temporary repair - only to get you where you can get the tire properly repaired.  Besides not knowing what damage may be been inside the tire (and I'll bet there is rubber dust inside- meaning the tire has lost some of its ability to hold air), plugs are prone to leak.

Second, he should have looked at the tire and pointed out the sidewall abrasion and recommended you replace the tire immediately.  Instead, he took your money, gave you a substandard repair, didn't tell you about the danger you were in - all the time knowing he would probably never hear from you again - even if something bad happened.