The test frames were run to 2X cycles and hours to validate the design.
But issues are found. Take the A320 wing corrosion issue that ended up effecting 7% of the fleet (hundreds of A320s):http://i.stuff.co.nz/business/3402061/C ... rbus-wings
It is possible, due to the tiny number of issues found, the pickle fork issue is a poor batch of aluminum, heat treat, coating, or damage upon install (not following process). If you see my link above, only 3 aircraft found with the issue.
Corrosion is tested to determine material properties. Sometimes it is a bad grounding path creating weaker parts as there is a ground loop.
I didn't get excited about the A320 issue (far more minor than the A380 incompatible thermal expansion issue that requires a titanium part put in between at a high cost per airframe).
Even the MD-80 was found to need doublers. Problems are found in every aircraft. For example, the C-series, errr... A220 needed a few dozen brackets in the tail replaced. Early 767s need new pressure bulkheads, but at cycle lives beyond what their Airbus competitors are certified for.
Another example, the 737 needs a brutal bulkhead inspection at 85,000 cycles. I was shocked to find out 733s were getting that inspection. But hey, if a heavy maintenance on an A320 is only good for 20,000 cycles (a third of certified life), having 25,000 cycles to go was enough to have enough economic benefit.
It takes a while to really find the root cause and an economical repair for these discoveries. For example, the E190 wing spar cracking was a terrifying find:viewtopic.php?f=3&t=1403321
21,000 cycles and 33,000 hours for that set of three E190. So even earlier, but other than time and cost to impliment an inspection and fix, a non-issue.
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