Why does thicker material or a small hole in the middle of strip require more stripping force than thin materials or hole towards one of the edges in the sheet metal operation?
You don't describe your measurement technique, so I'll assume that it involves some kind of deformation in the process of removal. If so, then you're making a complex measurement that involves both the deformation of the materials and the debonding of one material from another.
In the pressure sensitive adhesive world, this has been studied extensively -- see Ghent from Univ of Akron back in the '80's and '90's. Ghent was the first to note that the mechanical properties of the highly flexible tapes figured significantly in the measurement of apparent adhesion to the substrate. Further experimentation showed that the deformation of the substrate could factor in as well. Bottom line: the majority of energy exerted to debond a tape from a rigid substrate goes into deforming the tape, rather than in the breaking of adhesive bonds with the substrate.
So I suggest you look carefully at the mechanical properties of your two components separately, and carefully measure the forces involved in the kinds of deformation you're using for "adhesion" testing. I think you'll find, as Ghent did, that most of your efforts are going into deformation of the components, and only a minor part into debonding.
In addition, the differences you cited regarding hole location and material thickness are also related to relative thickness. It's important to measure your thicknesses very carefully for both components over the entire surface that you're measuring. It's very likely that the thicknesses are varying far more than you might think.
Then determine the mechanical properties as a function of material thickness.
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