Given that the plate is 40mm thick and the length of the long side of the material is 500mm you may consider running the FEA as a shell or thin plate. The general rule I've read indicates a plate can be considered "thin" when the thickness is 1/10 or less of the long dimension on the plate. I...
I would also recommend looking at MIL-STD 810 rev G for the vibration levels of off road transportation. This will give you an idea of the accelerations the load on the trailer may have. The spec can be found on everyspec.com. I have noted down that "unimproved roads" and "unprepared terrain"...
