Sorry for the slight creep but I recall seeing an in depth technical article about the alloy degrading in the spars, although it seems to have vanished from online.
It explained the concept of safe life as opposed to fail safe design and how the industry changed philosophy afterwards.
I understand the only thing that kept a on wing on one jet was the steel cross-brace in the wheel well.
Found one similar for those interested-
https://www.flightglobal.com/FlightPDFArchive/1955/1955%20-%200807.PDF
https://www.flightglobal.com/pdfarchive/view/1951/1951%20-%202503.html