commas

06:10 Dec 21, 2008 

"The plastic zone is substantially reduced" is correct here.

16:14 Dec 20, 2008 

reduce - to become reduced, to become lessened

11:17 Dec 20, 2008 

It has been established that as the crack length decreases, the main plastic zone at the crack tip is substantially reduced.

Normally not comma after "that". Example: "It has been established that we don't know where to put the commas". The only reason to put a comma after "that" in the sentence in the question is that you want to pause after that word, so you put in a comma because you feel for it, not because of any comma rule.

Let's look at the rest of the sentence: "As the crack length decreases, the main plastic zone at the crack tip substantially reduces." There must be comma because we have an introductory element starting with "As", so comma is inserted between the introductory element and the main clause. Introductory elements starting with after, although, as, because, if, since, when or while are followed by a comma before the main clause.

You can't say "The plastic zone reduces". You can say "The plastic zone is reduced" or "The plastic zone decreases". If you say "The plastic zone reduces", I will ask: "What does the plastic zone reduce?".