The official Air Ministry "Behaviour of Skin Fatigue Cracks at the Corners
of Windows in a Comet I Fuselage By R. J. ATKINSON, W. J. WINKWORTH and G. M. NORRIS states on
page2 para 2.2. "Local Structure at Windows and Escape Hatches. With the exceptions of the two forward escape hatches, which interrupted a circumferential frame (Fig. 6), the windows and escape hatches were positioned between the frames.
The windows and escape hatches were rectangular, their relative sizes being:
Window: 16.6 in. wide x 14 in. high, corner radii 3 in.
Escape hatch: 19.0 in. x 21.5 in. high, corner radii 4 in. (see Fig. 5).
The apertures were reinforced by peripheral members of zed section bonded to the skin with
Redux adhesive and additionally riveted by ~ in. countersunk-head rivets at the corners.
A 3 inch radii is not 90 degree as I stated in post #7 but is close enough not to matter in this context.
On page 5 it states "5.1. Origins of the Fatigue Cracks. All the fatigue cracks originated at the counter-
sunk rivet holes in the skin at the window and escape hatch corners. Those cracks which eventually
became catastrophic started at outer-row rivet holes. The few cracks that originated at holes in the
inner row grew inwards to the edge of the aperture and did not become catastrophic"