the following article
seems to imply that only low-cycle fatigue was involved, but the investigations clearly highlighted the stress raiser from the design as the crucial factor in making the structure prone to fatigue"This [the relevant piece of the fuselage] was recovered within a few hours of searching and showed, in the language of the coroner, the ‘unmistakable fingerprint of fatigue’. The fatigue crack was associated with the stress concentrations of the rather square rear ADF window cutout (stress of 315 MPa at edge of window), and with a bolt hole around the window"
A stress riser, as I said, can dramatically reduce fatigue life. But it is not in and of itself a cause of fatigue failure -- a cyclic load is required for fatigue.
What I see in the report is that after some 3057 combined flight and test cycles a crack was formed near the escape hatch, and repaired. After another 546 cycles the fuselage failed catastrophically. This is consistent with a poor crack repair, increased stress near the failure initiation point, and final fatigue failure.
I see a statement by a "coroner", but I do not see any clear statement of a microscopic analysis of the material by a metallurgist. Perhaps something is lost in the translation from British English to American English.
A crack is the quintessential stress riser. Ordinary stress analysis is not adequate for structural analysis in the presence of cracks and specialized fracture mechanics, with associated empirically derived fracture allowables for the material, are required for a proper analysis. I note that the report states that stress levels near the crack were only 70 MPa (about 10 Ksi) which is quite low. This suggests to me that there must indeed have been a very significant stress riser at the failure point. A sharp crack might well be the culprit.
Another effect of a crack is to form a point that is particularly vulnerable to corrosion. This can also be severe and in the presence of high stress in appropriate directions and a corrosive environment (a little moisture with free ions to give conductivity) can be quite rapid, as in the case of stress corrosion cracking of high strength aluminum alloys.
Nevertheless in order for fatigue
failure a reversing cyclic load is needed. The higher the amplitude of the applied load (which a stress riser will influence dramatically) the lower the number of cycles required to induce failure. http://en.wikipedia.org/wiki/Fatigue_(material
A stress riser can also cause failure by simply causing the local stress to exceed the allowable stress for the material. This is not fatigue failure, but the difference will be of little importance to a passenger on the plane that is now plummeting to the ground.