I think that it was probably the mass cantilevered off the suctions cups along with the G's Axel was pulling out there around the pylons put too much stress on the plexiglass already softened by aero thermal heating.
On a more serious note:
I looked up the Tg for Poly(methyl methacrylate) (AKA Plexiglass). It is the temperature at which the material starts to transition from the "glass/hard" state to the "soft/malleable/plastic" state.
The following is excerpted for a Wikipedia article on Poly(methyl methacrylate),
https://en.wikipedia.org/wiki/Poly(methyl_methacrylate).
"The glass transition temperature (Tg) of atactic PMMA is 105 °C (221 °F). The Tg values of commercial grades of PMMA range from 85 to 165 °C (185 to 329 °F); the range is so wide because of the vast number of commercial compositions which are copolymers with co-monomers other than methyl methacrylate. PMMA is thus an organic glass at room temperature; i.e., it is below its Tg. The forming temperature starts at the glass transition temperature and goes up from there."
I am not a materials expert but I have spent a lot of time in investigations as to why things like this happen and working with some very smart scientists and engineers trying to find the cause and corrections for problems like this.
Here are some things to keep in mind:
- There are aircraft parked all over the southwest with and without covers during the summer months and we do not see the canopy material reforming because of the stresses from the suction cup mounts or the stress areas from the canopy mounting structures.
- Temperatures inside a cockpit on a aircraft with a sealed/non-vented cockpit with a canopy similar those on RV's has been measured at over +160 deg F in direct sunlight on a mid July day in the Mojave desert. This data was taken from an extensive study on cockpit heating several years ago. No canopy material deformation was observed.
- I was in the pit area during the race week and I doubt that the temperatures ever got high enough to soften the material to the point where it would flow into a deformed state. Not saying that it did not deform under the conditions on the ramp there at Reno but I would be surprised if this was the root cause of the deformation.
With those observations in mind some other things to think about
:
- Could your canopy material be something other than plexiglass.
- I have noticed that some of the suction cups I have used have material that is loaded with plasticizers that appear to be used to keep the suction soft and compliant. Could this chemical compound have migrated into the canopy material itself causing it to soften, deform, or change its properties?
- Could the location/geometry of the suction cup have caused the sunlight to be focused, concentrated, or integrated over time causing a hot spot that was higher than the Tg?
Just some random things to consider. At a minimum I would give any suction cup a good bath in soap and water to at least remove most of the chemicals that may be on the surface before mounting them.
See you next year in Reno.
- larosta