A crack was not the root cause in this case. The longeron spec called for it
to be 0.1 inches thick, some 182 aircraft were found to have the thickness vary
between .039 and .073 inches. If its not built to the required standard of
course its going to crack/break.
The plane had reached higher Gs before without failure and the longeron thickness didn't affect flight performance. The safety factor was still greater than 1. This was the extended crack that reduced the aircraft residual strength well below the limit load. This is mechanics (fracture mechanics, indeed) of this failure. The crack was the immediate cause of the aircraft disintegration.
However, if you like science-like sounding words, yes, the root cause of the accident was the longeron thickness deviation, that caused the crack initiation in an unexpected location or crack growth with higher rates, that caused technicians to miss the crack on inspection and so on...