Someone else might read this wondering how hot things need to be to damage an S5. I don't want people to think that they magically start to burn up as soon as the intake temperature exceeds 40°C. Accuracy is useful even when it's not necessary for the immediate task.
True but equally frightening. A miner burning up is a factor of a lot more things than intake temperature as you agree with the two S5's example. Normally we talk about "intake" temperatures because it simplifies the discussion down to a workable set, but doing so makes some basic assumptions, such as that exhaust airflow isn't constrained. As soon as someone starts violating those assumptions, everything changes.
It is possible(though not practical) to get a S5 to run safely with a 65 degree intake as well as to fail with a 15 degree intake. We do the readers no favors by glossing over this reality in my opinion.
I wasn't saying that air pressure did the bend, air pressure only influenced the direction of bend at best.
I posit that air pressure, primarily, caused the runaway temperatures localized to a certain point on the boards.
Specifically to this case... I looked up the coefficient of thermal expansion for PE and found from 80F to 135F you get ~0.5% expansion. Putting that in a right triangle gives you a bowing outwards of about 2.5cm from a ~17.5cm plastic strip. You said you measured 2cm at worst, so that's about right.
(Minus the expansion of steel and aluminum, which is much smaller.)
Most of the plastic deformation indicates a shear stress, not compressive stress. The only exception to this is the buckling that was observed in a small number of the most severely deformed plastic. The buckling of the plastic occurred entirely along the short axis of the plastic shields. If thermal expansion were the culprit, I would expect to see buckling along the long axis as well, especially along the line in between the front and back screws. That line is the least deformed part of the side panels. The bottom edge is more securely attached than the top edge as well, so if the main cause of deformation was thermal expansion, one would expect the bottom edge to show more buckling and deformation. The opposite is true. The greatest deformation occurred on edges which I expect had the greatest airflow (evenly along the top edge, plus the back and front edge near the holes for the tail exhaust and fan).
Remember when talking about temperature under cases where normal assumptions are violated, it is useless without also talking about specifically where the temperature is. The middle of the plastic sheets didn't warp because the middle didn't get so hot. The edges did because the edges touched the metal, a better heat conductor, and the exhaust edge specifically had higher heat. In the pictures you have provided, one perhaps 2 of the plastic have deformations on both sides. 6 or 7 have deformations on one side with far smaller or no deformations on the opposite side. Of all of the sheets in the picture, one shows minor top-edge deformations, one shows a lot of top edge deformation, and the only other one with top edge deformations is very close to the screws/my proposed hot spot. 6 show no top edge deformation at all.
Much of the deformation occurred on the top edge, above the highest screw attachment point. That edge typically is relatively straight, but it sags outward and downward. Since this edge was not being squeezed, and if it were it wouldn't cause the edge to sag like that, I don't think thermal expansion is a satisfactory explanation. Thermal softening combined with airflow is. The apparent buckling I observed on a few panels might actually have been fluid dynamic effects similar to ocean waves or sand dunes being created by wind rather than actual buckling.
Something just occurred to me. If the airflow was anterograde but slow through the whole intra-heatsink space, the air coming out the exhaust port would be very hot. The side spaces would not maintain their pressure as well, so airflow there would be more likely to turn retrograde. This would pull the hot exhaust air around 180° back in the side ports before flowing either out the top of the miner or through the gap in the panel between the screws. This could explain the pattern of deformation pretty well.
To be clear, I don't believe airflow played no part in what we see, that would be silly of me.