POLSON, Mont. -

Day four of a trial surrounding a 2011 crash saw University of Montana Professor of Metallurgy Dr. William Gleason on the witness stand all day

Thursday's testimony focused on the steering knuckle, the part of the Hyundai Tiburon driven by Trevor Olson, that the plaintiff claims failed, causing the crash that killed three.

The parents of cousins Tanner and Trevor Olson are suing Hyundai for the fatal crash.

Gleason testified that the three fractures on the steering knuckle were caused by fatigue -- the metal being bent repeatedly over time.

At least two of the fractures, he testified, had already occurred far before the crash took place.

The testimony explained that these fatigue fractures likely occurred because the metal wasn’t properly cast and was not chemically as strong as it should have been.

“That didn’t break in a wreck. That broke over time. A little bit, a little bit, a little bit, then that one broke.” said Gleason.

The trial and Gleason's testimony continue Friday at 9 a.m. at the Lake County Courthouse in Polson.