Take a one foot square piece of 3/4" (18mm) MDF and of 1/2" (12mm) plywood
(3,5 ply) and place half of it in a vise and then push on the exposed half.
Then, with a quick blow of a hammer - both will shatter/split about equally.
Apply ever increasing pressure slowly and the ply will bend where the MDF
will crack and fail.
The answer depends on more information. What kind span are you talking
about, what will be the supports, is it dead weight being applied or a
moving force, what kind and how many plys in the plywood, etc.
What is the application?
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