I think Jim was talking about general shear in the MDF/plywood, not necessarily the edges. It's easy to peel apart the grains in a sheet of plywood, but takes much more force to break a grain if you're pulling on it from the two ends, in other words the directional nature of wood. Since MDF is a very nondirectional, uniform material, you don't get the advantage of longitudinal tensile grain strength as in a laminated plywood or fibreglass.
I agree, I wouldn't expect that your joints would ever come apart at the edges they way you've bonded them. I read that MDF has a tensile strength / modulus of rupture between 20 - 80 MPa (2900 - 11600 PSI). The actual force would depend on the size/orientation of the piece. |